Manual variable aperture zoom on a digital Pentax

Started 3 months ago | Questions
alex_virt Senior Member • Posts: 2,404
Manual variable aperture zoom on a digital Pentax

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

ANSWER:
MightyMike Forum Pro • Posts: 41,181
Re: Manual variable aperture zoom on a digital Pentax
1

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

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OP alex_virt Senior Member • Posts: 2,404
Re: Manual variable aperture zoom on a digital Pentax

MightyMike wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

OP alex_virt Senior Member • Posts: 2,404
Re: Manual variable aperture zoom on a digital Pentax

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

I've just experimented with my film camera Z-1P. The F3.5 didn't change either.

However, the shutter speed decreases by 1 stop at the long end on both cameras.

MightyMike Forum Pro • Posts: 41,181
Re: Manual variable aperture zoom on a digital Pentax

alex_virt wrote:

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

I've just experimented with my film camera Z-1P. The F3.5 didn't change either.

However, the shutter speed decreases by 1 stop at the long end on both cameras.

You've jogged my memory, I seem to recall the A35-70mm F3.5-4.5 or was it the A28-80mm F3.5-4.5 or both having the same effect. It must be something to do with the simplicity of the aperture identification system and variable aperture zooms of the time.

That said, despite it reporting the wrong aperture wide open at the long end when setting it to an appropriate aperture F4.5 or higher it will maintain that aperture as you zoom. and despite the aperture not being correctly reported it seems the metering sill works. The difference between F3.5 and F4.5 is 2/3rds of a stop but I wouldn't worry if the camera adjusts the shutter speed by 1 stop, it could be how you have the camera setup or variability in the test or close enough to not make a significant difference.

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OP alex_virt Senior Member • Posts: 2,404
Re: Manual variable aperture zoom on a digital Pentax

MightyMike wrote:

alex_virt wrote:

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

I've just experimented with my film camera Z-1P. The F3.5 didn't change either.

However, the shutter speed decreases by 1 stop at the long end on both cameras.

You've jogged my memory, I seem to recall the A35-70mm F3.5-4.5 or was it the A28-80mm F3.5-4.5 or both having the same effect. It must be something to do with the simplicity of the aperture identification system and variable aperture zooms of the time.

That said, despite it reporting the wrong aperture wide open at the long end when setting it to an appropriate aperture F4.5 or higher it will maintain that aperture as you zoom. and despite the aperture not being correctly reported it seems the metering sill works. The difference between F3.5 and F4.5 is 2/3rds of a stop but I wouldn't worry if the camera adjusts the shutter speed by 1 stop, it could be how you have the camera setup or variability in the test or close enough to not make a significant difference.

If I set the aperture to F8, the shutter speed still drops by a stop at the long end. It means that the actual aperture at the long end is about F11.

Maybe it's because the KP and Z-1P were made to work with newer electronic lenses that communicate the actual aperture to the camera. I wonder how these lenses behave on older manual cameras ca. 1980s...

GossCTP Veteran Member • Posts: 5,657
What I've observed

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

Here's what I have observed. The lens communicates focal length so at some point it changes the wide open metering from f/3.5 to f/4.5. The lens itself though moves in a smooth continuum from one end to the other. You can observe this with DOF preview. Zoom in small increments with the lens set to wide open - there is no point where the blades ever stop down in the slightest. Therefore at the point where the lens jumps from 3.5 to 4.5 there will be an exposure error because the real aperture is at f/4 but at that point that the meter jumps 2/3 of a stop as the real aperture shifts very little.

I think the problem may be transposed even when stopped down, because the camera is stopping down with a fixed stop distances. In other words, if set to f/8, the camera stops down the lever by 2 and 1/3 stops. At the 3.5 to 4.5 threshold point it will change and stop down by 1 and 2/3 stops to get the same f8 at the long end. I haven't tested much on this second part as I ran out of time and interest.

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

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MightyMike Forum Pro • Posts: 41,181
Re: Manual variable aperture zoom on a digital Pentax

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

I've just experimented with my film camera Z-1P. The F3.5 didn't change either.

However, the shutter speed decreases by 1 stop at the long end on both cameras.

You've jogged my memory, I seem to recall the A35-70mm F3.5-4.5 or was it the A28-80mm F3.5-4.5 or both having the same effect. It must be something to do with the simplicity of the aperture identification system and variable aperture zooms of the time.

That said, despite it reporting the wrong aperture wide open at the long end when setting it to an appropriate aperture F4.5 or higher it will maintain that aperture as you zoom. and despite the aperture not being correctly reported it seems the metering sill works. The difference between F3.5 and F4.5 is 2/3rds of a stop but I wouldn't worry if the camera adjusts the shutter speed by 1 stop, it could be how you have the camera setup or variability in the test or close enough to not make a significant difference.

If I set the aperture to F8, the shutter speed still drops by a stop at the long end. It means that the actual aperture at the long end is about F11.

Maybe it's because the KP and Z-1P were made to work with newer electronic lenses that communicate the actual aperture to the camera. I wonder how these lenses behave on older manual cameras ca. 1980s...

Ok, well its not behaving the way I expected it to but my expectations are likely wrong and your lens/camera combo are working the way they probably were meant to. Seems like a strange omission on Pentax's part though.

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Mike from Canada
"I am not a great photographer! God is a great creator! All I do is capture His creation with the tools He has provided me."
'I like to think so far outside the box that it would require a telephoto lens just to see the box!' ~ 'My Quote :)'
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OP alex_virt Senior Member • Posts: 2,404
Re: Manual variable aperture zoom on a digital Pentax

MightyMike wrote:

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

I've just experimented with my film camera Z-1P. The F3.5 didn't change either.

However, the shutter speed decreases by 1 stop at the long end on both cameras.

You've jogged my memory, I seem to recall the A35-70mm F3.5-4.5 or was it the A28-80mm F3.5-4.5 or both having the same effect. It must be something to do with the simplicity of the aperture identification system and variable aperture zooms of the time.

That said, despite it reporting the wrong aperture wide open at the long end when setting it to an appropriate aperture F4.5 or higher it will maintain that aperture as you zoom. and despite the aperture not being correctly reported it seems the metering sill works. The difference between F3.5 and F4.5 is 2/3rds of a stop but I wouldn't worry if the camera adjusts the shutter speed by 1 stop, it could be how you have the camera setup or variability in the test or close enough to not make a significant difference.

If I set the aperture to F8, the shutter speed still drops by a stop at the long end. It means that the actual aperture at the long end is about F11.

Maybe it's because the KP and Z-1P were made to work with newer electronic lenses that communicate the actual aperture to the camera. I wonder how these lenses behave on older manual cameras ca. 1980s...

Ok, well its not behaving the way I expected it to but my expectations are likely wrong and your lens/camera combo are working the way they probably were meant to. Seems like a strange omission on Pentax's part though.

Conclusion: the actual aperture doesn't stay constant, but the metering remains correct, and the camera compensates for the aperture change by adjusting the shutter speed that reduces gradually from the wide end to the long end.

Probably when production of new zoom lenses with electronic contacts began, Pentax started to cut corners and reduced maximum apertures from 3.5-4.5 to 3.5-5.6

MightyMike Forum Pro • Posts: 41,181
Interesting
1

I have an SMC Pentax-A 35-135mm F3.5-4.5, its in my bargain bin because its got haze, I tried to reproduce what your issues are.

1) yes you can set F3.5 at the long end

2) yes it meters the shutter speed faster by 2/3rds of a stop and on occasion 1 stop

3) looking at the resulting images on the back screen of the camera the exposure (in aperture priority mode) looks effectively the same between F3.5 and F4.5, testing on in focus and completely out of focus targets (focused to infinity to get an even spread of light)

4) the histograms are very similar with the F3.5 having a broader range of tones, a close look reveals that the F3.5 photo has more notable vignetting than the F4.5 photo.

5) checking the aperture in bulb mode shows it actually stops down at the long end when set to F4.5 and stays wide open when set to F3.5, It also stops down at F4.0.

6) at 35mm it actually stops the aperture down just a pinch when set to F3.5 and stops it down typically for other apertures

7) in bulb mode when zooming the aperture doesn't change when set to F3.5, ditto for F4.5

8) in LV the aperture doesn't appear to open of close to adjust for different brightness's which I recall it doing with modern lenses.

9) when off the camera and manually adjusting the aperture the aperture stays wide open at F3.5 across the zoom range, it also stops down at both ends when set at the next click (F4.5 or F4.8)

10) in LV there is a 2/3rds of a stop difference in exposure on a neutral out of focus target between 35mm and 135mm when set to F3.5 and F4.5 and F8.0

So I'd suggest that at any point along the zoom choosing an aperture darker than F3.5 stops the lens down by that amount from whatever the wide open aperture is at that point in the zoom. So at 135mm F3.5 really is F4.5, F4.0 would be the same as F5.0 and F4.5 would be the same as F5.6, all you aperture values will be off by 2/3rds of a stop at the long end and lets assume 1/3rd of a stop somewhere in between.

BTW If you want this lens I'll let you have it cheap, the haze doesn't seem to have a substantial effect on the end result, worst case scenario you lose a little contrast.

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OP alex_virt Senior Member • Posts: 2,404
Re: Interesting

MightyMike wrote:

I have an SMC Pentax-A 35-135mm F3.5-4.5, its in my bargain bin because its got haze, I tried to reproduce what your issues are.

1) yes you can set F3.5 at the long end

2) yes it meters the shutter speed faster by 2/3rds of a stop and on occasion 1 stop

3) looking at the resulting images on the back screen of the camera the exposure (in aperture priority mode) looks effectively the same between F3.5 and F4.5, testing on in focus and completely out of focus targets (focused to infinity to get an even spread of light)

4) the histograms are very similar with the F3.5 having a broader range of tones, a close look reveals that the F3.5 photo has more notable vignetting than the F4.5 photo.

5) checking the aperture in bulb mode shows it actually stops down at the long end when set to F4.5 and stays wide open when set to F3.5, It also stops down at F4.0.

6) at 35mm it actually stops the aperture down just a pinch when set to F3.5 and stops it down typically for other apertures

7) in bulb mode when zooming the aperture doesn't change when set to F3.5, ditto for F4.5

8) in LV the aperture doesn't appear to open of close to adjust for different brightness's which I recall it doing with modern lenses.

9) when off the camera and manually adjusting the aperture the aperture stays wide open at F3.5 across the zoom range, it also stops down at both ends when set at the next click (F4.5 or F4.8)

10) in LV there is a 2/3rds of a stop difference in exposure on a neutral out of focus target between 35mm and 135mm when set to F3.5 and F4.5 and F8.0

So I'd suggest that at any point along the zoom choosing an aperture darker than F3.5 stops the lens down by that amount from whatever the wide open aperture is at that point in the zoom. So at 135mm F3.5 really is F4.5, F4.0 would be the same as F5.0 and F4.5 would be the same as F5.6, all you aperture values will be off by 2/3rds of a stop at the long end and lets assume 1/3rd of a stop somewhere in between.

BTW If you want this lens I'll let you have it cheap, the haze doesn't seem to have a substantial effect on the end result, worst case scenario you lose a little contrast.

Mike, thank you very much for this detailed and comprehensive answer!

I have the 35-105mm and 35-210mm, so I don't need another manual zoom in this range, but thanks for the offer

James O'Neill Veteran Member • Posts: 5,751
Re: Manual variable aperture zoom on a digital Pentax

MightyMike wrote:

alex_virt wrote:

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

No. See below.

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

Yes. See blow

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

I've just experimented with my film camera Z-1P. The F3.5 didn't change either.

However, the shutter speed decreases by 1 stop at the long end on both cameras.

You've jogged my memory, I seem to recall the A35-70mm F3.5-4.5 or was it the A28-80mm F3.5-4.5 or both having the same effect. It must be something to do with the simplicity of the aperture identification system and variable aperture zooms of the time.

On A series lenses the electric contacts tell the camera where the smallest aperture is (4 choices f/16, f/22, f,32, f/45) and how many stops the lens opens from there (8 combinations starting at 5 stops and going up.  See

http://kmp.pentaxians.eu/technology/k-mount/ka/

on some of the designs you can see which of the pins are 1s and which are 0s and decode the binary. This one obviously runs from 3.5 to f/22 or greater.

In practice fully when the lens is set to f/8 it may be  and f/9; f/16 is actually f/14 to f/18 and f/22 is really f/22-24. If you take an off-cameras light meter reading and it says "f/8 at 1/250th", you can only move up in +/- 1/3 stop increments so even with the biggest error you're half way between two exposures.

Metering with the lens wide open the camera might say "I am reading this brightness at f/3.5, I think the ideal is to close 2 1/2 stops closed from here to get to f/8" . 
Actually it might have been reading at f/4.5 and it's going to unwitting close down to f/11 but provided it closes down by the right number of stops from where it metered it doesn't matter what it thinks the numbers are. It might matter to you if you're metering off camera (e.g. studio flash) or if the d.o.f difference between f/8 and f/11 is significant.

With FA and later lenses the camera gets more data - lens model, current focal length, current nominal max and min apertures so the problem goes away.

That said, despite it reporting the wrong aperture wide open at the long end when setting it to an appropriate aperture F4.5 or higher it will maintain that aperture as you zoom. and despite the aperture not being correctly reported it seems the metering sill works. The difference between F3.5 and F4.5 is 2/3rds of a stop but I wouldn't worry if the camera adjusts the shutter speed by 1 stop, it could be how you have the camera setup or variability in the test or close enough to not make a significant difference.

The distances between the click-stops on the diaphragm don't change so (provided that metering is in camera) the exposure will be correct even if the number associated with each position is wrong.

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MightyMike Forum Pro • Posts: 41,181
Re: Interesting

alex_virt wrote:

MightyMike wrote:

I have an SMC Pentax-A 35-135mm F3.5-4.5, its in my bargain bin because its got haze, I tried to reproduce what your issues are.

1) yes you can set F3.5 at the long end

2) yes it meters the shutter speed faster by 2/3rds of a stop and on occasion 1 stop

3) looking at the resulting images on the back screen of the camera the exposure (in aperture priority mode) looks effectively the same between F3.5 and F4.5, testing on in focus and completely out of focus targets (focused to infinity to get an even spread of light)

4) the histograms are very similar with the F3.5 having a broader range of tones, a close look reveals that the F3.5 photo has more notable vignetting than the F4.5 photo.

5) checking the aperture in bulb mode shows it actually stops down at the long end when set to F4.5 and stays wide open when set to F3.5, It also stops down at F4.0.

6) at 35mm it actually stops the aperture down just a pinch when set to F3.5 and stops it down typically for other apertures

7) in bulb mode when zooming the aperture doesn't change when set to F3.5, ditto for F4.5

8) in LV the aperture doesn't appear to open of close to adjust for different brightness's which I recall it doing with modern lenses.

9) when off the camera and manually adjusting the aperture the aperture stays wide open at F3.5 across the zoom range, it also stops down at both ends when set at the next click (F4.5 or F4.8)

10) in LV there is a 2/3rds of a stop difference in exposure on a neutral out of focus target between 35mm and 135mm when set to F3.5 and F4.5 and F8.0

So I'd suggest that at any point along the zoom choosing an aperture darker than F3.5 stops the lens down by that amount from whatever the wide open aperture is at that point in the zoom. So at 135mm F3.5 really is F4.5, F4.0 would be the same as F5.0 and F4.5 would be the same as F5.6, all you aperture values will be off by 2/3rds of a stop at the long end and lets assume 1/3rd of a stop somewhere in between.

BTW If you want this lens I'll let you have it cheap, the haze doesn't seem to have a substantial effect on the end result, worst case scenario you lose a little contrast.

Mike, thank you very much for this detailed and comprehensive answer!

No Problem

I have the 35-105mm and 35-210mm, so I don't need another manual zoom in this range, but thanks for the offer

Regardless I'll send you my offer in a PM, you might be surprised.

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"I am not a great photographer! God is a great creator! All I do is capture His creation with the tools He has provided me."
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OP alex_virt Senior Member • Posts: 2,404
Re: Manual variable aperture zoom on a digital Pentax
1

James O'Neill wrote:

MightyMike wrote:

alex_virt wrote:

alex_virt wrote:

MightyMike wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

No. See below.

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

Yes. See blow

If set to a small aperture, e.g. F8, will the actual aperture remain F8 through the zoom range?

I'm confused...

Its an A-series lens, the camera therefore knows the aperture, the camera also therefore knows when the aperture is not F3.5 anymore when you zoom. So set the camera to F3.5 at the wide end, zoom the lens and watch the aperture change on the back of the camera (there is an off chance you may have to tap the shutter button to see the change) That said, when set to F4.5 its F4.5 regardless of what zoom position you're at, ditto for F8.0.

On my KP it doesn't change and remains F3.5 at both ends.

I've just experimented with my film camera Z-1P. The F3.5 didn't change either.

However, the shutter speed decreases by 1 stop at the long end on both cameras.

You've jogged my memory, I seem to recall the A35-70mm F3.5-4.5 or was it the A28-80mm F3.5-4.5 or both having the same effect. It must be something to do with the simplicity of the aperture identification system and variable aperture zooms of the time.

On A series lenses the electric contacts tell the camera where the smallest aperture is (4 choices f/16, f/22, f,32, f/45) and how many stops the lens opens from there (8 combinations starting at 5 stops and going up. See

http://kmp.pentaxians.eu/technology/k-mount/ka/

on some of the designs you can see which of the pins are 1s and which are 0s and decode the binary. This one obviously runs from 3.5 to f/22 or greater.

In practice fully when the lens is set to f/8 it may be and f/9; f/16 is actually f/14 to f/18 and f/22 is really f/22-24. If you take an off-cameras light meter reading and it says "f/8 at 1/250th", you can only move up in +/- 1/3 stop increments so even with the biggest error you're half way between two exposures.

Metering with the lens wide open the camera might say "I am reading this brightness at f/3.5, I think the ideal is to close 2 1/2 stops closed from here to get to f/8" .
Actually it might have been reading at f/4.5 and it's going to unwitting close down to f/11 but provided it closes down by the right number of stops from where it metered it doesn't matter what it thinks the numbers are. It might matter to you if you're metering off camera (e.g. studio flash) or if the d.o.f difference between f/8 and f/11 is significant.

With FA and later lenses the camera gets more data - lens model, current focal length, current nominal max and min apertures so the problem goes away.

That said, despite it reporting the wrong aperture wide open at the long end when setting it to an appropriate aperture F4.5 or higher it will maintain that aperture as you zoom. and despite the aperture not being correctly reported it seems the metering sill works. The difference between F3.5 and F4.5 is 2/3rds of a stop but I wouldn't worry if the camera adjusts the shutter speed by 1 stop, it could be how you have the camera setup or variability in the test or close enough to not make a significant difference.

The distances between the click-stops on the diaphragm don't change so (provided that metering is in camera) the exposure will be correct even if the number associated with each position is wrong.

James, thank you very much for the scientific explanation!

Roland Karlsson Forum Pro • Posts: 29,605
Re: Manual variable aperture zoom on a digital Pentax

OK, the behavior for those A lenses seems weird.

I got a little curious and tested with my modern D FA 28-105 1:3.5-5.6

First I put it at wide and and F3.5. Then zooming back and forth the aperture was changing between 3.5 and 5.6.

Then I put it at the tele end and F5.6. Then the aperture stayed at 5.6 when zooming.

So, the camera (or lens) have some memory. It remembers where it all started. Because in the first case, when zooming from tele to wide, it changed from 5.6 to 3.5, but in the other case it kept 5.6.

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James O'Neill Veteran Member • Posts: 5,751
Re: What I've observed

GossCTP wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

Here's what I have observed. The lens communicates focal length so at some point it changes the wide open metering from f/3.5 to f/4.5.

There is a difference between A-Series lenses which have 5 pins which send just aperture info, and FA / DA / DFA lenses which send focal length and current min/max aperture at that focal length. You can see this in (e.g.) exiftool. for example with my smc PENTAX-DA 18-250mm F3.5-6.3 ED AL [IF] I see this.

Lens Focal Length : 100.0 mm
Nominal Max Aperture : 5.7
Nominal Min Aperture : 38
Max Aperture : 5.5

or
Lens Focal Length : 250.0 mm
Nominal Max Aperture : 6.7
Nominal Min Aperture : 45
Max Aperture : 6.2

The lens itself though moves in a smooth continuum from one end to the other. You can observe this with DOF preview. Zoom in small increments with the lens set to wide open - there is no point where the blades ever stop down in the slightest.

Yes! You have wide open which might be any value between f/3.5 and f4.5 and then 1/3 stop down, 2/3 stop, etc until you reach the smallest aperture.

Therefore at the point where the lens jumps from 3.5 to 4.5 there will be an exposure error because the real aperture is at f/4 but at that point that the meter jumps 2/3 of a stop as the real aperture shifts very little.

It doesn't really jump. What has actually happened is the meter will see a will think the lens is at f/3.5 and the light out side is say EV20, when it's really f/4.5 and the light is nearer to EV21. It might think it is stopping down to f/8 but but it's really going to f/10 but that's OK because the light is brighter than the camera thinks it is.

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GossCTP Veteran Member • Posts: 5,657
Re: What I've observed

James O'Neill wrote:

GossCTP wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

Here's what I have observed. The lens communicates focal length so at some point it changes the wide open metering from f/3.5 to f/4.5.

There is a difference between A-Series lenses which have 5 pins which send just aperture info, and FA / DA / DFA lenses which send focal length and current min/max aperture at that focal length. You can see this in (e.g.) exiftool. for example with my smc PENTAX-DA 18-250mm F3.5-6.3 ED AL [IF] I see this.

Yeah, I realized that after posting. I don't have any A zooms, only primes.

Lens Focal Length : 100.0 mm
Nominal Max Aperture : 5.7
Nominal Min Aperture : 38
Max Aperture : 5.5

or
Lens Focal Length : 250.0 mm
Nominal Max Aperture : 6.7
Nominal Min Aperture : 45
Max Aperture : 6.2

The lens itself though moves in a smooth continuum from one end to the other. You can observe this with DOF preview. Zoom in small increments with the lens set to wide open - there is no point where the blades ever stop down in the slightest.

Yes! You have wide open which might be any value between f/3.5 and f4.5 and then 1/3 stop down, 2/3 stop, etc until you reach the smallest aperture.

Therefore at the point where the lens jumps from 3.5 to 4.5 there will be an exposure error because the real aperture is at f/4 but at that point that the meter jumps 2/3 of a stop as the real aperture shifts very little.

It doesn't really jump. What has actually happened is the meter will see a will think the lens is at f/3.5 and the light out side is say EV20, when it's really f/4.5 and the light is nearer to EV21. It might think it is stopping down to f/8 but but it's really going to f/10 but that's OK because the light is brighter than the camera thinks it is.

I get that the meter is working off the real wide open f/stop (t/stop really). But there is a point in any of these post A lenses where the lens tells the camera it just went from one max aperture to another. You turn the lens barrel 1mm and the value changes abruptly while the real aperture moves on a smooth continuum. The meter will see a very minimal difference between say 44mm and 45mm, wide open, but meter thinks it has shifted and will jump to the next aperture value For metering. For whatever reason all the Pentax cameras I have used seem to jump from 3.5 to 4.5 with variable aperture lenses. I think the test lens I was using was the 16-85.

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James O'Neill Veteran Member • Posts: 5,751
Re: What I've observed

GossCTP wrote:

James O'Neill wrote:

GossCTP wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

Here's what I have observed. The lens communicates focal length so at some point it changes the wide open metering from f/3.5 to f/4.5.

There is a difference between A-Series lenses which have 5 pins which send just aperture info, and FA / DA / DFA lenses which send focal length and current min/max aperture at that focal length. You can see this in (e.g.) exiftool. for example with my smc PENTAX-DA 18-250mm F3.5-6.3 ED AL [IF] I see this.

Yeah, I realized that after posting. I don't have any A zooms, only primes.

Lens Focal Length : 100.0 mm
Nominal Max Aperture : 5.7
Nominal Min Aperture : 38
Max Aperture : 5.5

or
Lens Focal Length : 250.0 mm
Nominal Max Aperture : 6.7
Nominal Min Aperture : 45
Max Aperture : 6.2

The lens itself though moves in a smooth continuum from one end to the other. You can observe this with DOF preview. Zoom in small increments with the lens set to wide open - there is no point where the blades ever stop down in the slightest.

Yes! You have wide open which might be any value between f/3.5 and f4.5 and then 1/3 stop down, 2/3 stop, etc until you reach the smallest aperture.

Therefore at the point where the lens jumps from 3.5 to 4.5 there will be an exposure error because the real aperture is at f/4 but at that point that the meter jumps 2/3 of a stop as the real aperture shifts very little.

It doesn't really jump. What has actually happened is the meter will see a will think the lens is at f/3.5 and the light out side is say EV20, when it's really f/4.5 and the light is nearer to EV21. It might think it is stopping down to f/8 but but it's really going to f/10 but that's OK because the light is brighter than the camera thinks it is.

I get that the meter is working off the real wide open f/stop (t/stop really). But there is a point in any of these post A lenses where the lens tells the camera it just went from one max aperture to another. You turn the lens barrel 1mm and the value changes abruptly while the real aperture moves on a smooth continuum. The meter will see a very minimal difference between say 44mm and 45mm, wide open, but meter thinks it has shifted and will jump to the next aperture value For metering. For whatever reason all the Pentax cameras I have used seem to jump from 3.5 to 4.5 with variable aperture lenses. I think the test lens I was using was the 16-85.

What I've noticed with the 18-250 is it doesn't record all the focal lengths, those go in steps, so it's fair to assume that the min/max aperture do too - actually in those samples above it seems "nominal max aperture is in what the camera displays (and it does include f/4.0) and "max aperture" is whatever odd value it happens to be.

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Jon Donahue Contributing Member • Posts: 792
Re: Manual variable aperture zoom on a digital Pentax
1

As basically an uninformed user, reading this thread I really, really appreciate Pentax maintaining backward compatibility... so that someone like me can just put on an older lens and it will work. That's a big deal, and from what you guys are posting, Pentax did a lot of serious engineering to make it happen!

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MightyMike Forum Pro • Posts: 41,181
Re: What I've observed

James O'Neill wrote:

GossCTP wrote:

James O'Neill wrote:

GossCTP wrote:

alex_virt wrote:

Can anybody explain to me how a manual A-series zoom with variable max aperture behaves on a digital Pentax camera?

For example, I have an F3.5-4.5 zoom.

Do I understand correctly that the camera doesn't know the actual aperture and treats the lens as a constant F3.5 zoom and underexposes at the long end?

If I set the aperture in camera to F4.5, will it remain F4.5 through the zoom range and become wide open at the long end? Or will it remain stopped down to the actual aperture of F5-something at the long end?

Here's what I have observed. The lens communicates focal length so at some point it changes the wide open metering from f/3.5 to f/4.5.

There is a difference between A-Series lenses which have 5 pins which send just aperture info, and FA / DA / DFA lenses which send focal length and current min/max aperture at that focal length. You can see this in (e.g.) exiftool. for example with my smc PENTAX-DA 18-250mm F3.5-6.3 ED AL [IF] I see this.

Yeah, I realized that after posting. I don't have any A zooms, only primes.

Lens Focal Length : 100.0 mm
Nominal Max Aperture : 5.7
Nominal Min Aperture : 38
Max Aperture : 5.5

or
Lens Focal Length : 250.0 mm
Nominal Max Aperture : 6.7
Nominal Min Aperture : 45
Max Aperture : 6.2

The lens itself though moves in a smooth continuum from one end to the other. You can observe this with DOF preview. Zoom in small increments with the lens set to wide open - there is no point where the blades ever stop down in the slightest.

Yes! You have wide open which might be any value between f/3.5 and f4.5 and then 1/3 stop down, 2/3 stop, etc until you reach the smallest aperture.

Therefore at the point where the lens jumps from 3.5 to 4.5 there will be an exposure error because the real aperture is at f/4 but at that point that the meter jumps 2/3 of a stop as the real aperture shifts very little.

It doesn't really jump. What has actually happened is the meter will see a will think the lens is at f/3.5 and the light out side is say EV20, when it's really f/4.5 and the light is nearer to EV21. It might think it is stopping down to f/8 but but it's really going to f/10 but that's OK because the light is brighter than the camera thinks it is.

I get that the meter is working off the real wide open f/stop (t/stop really). But there is a point in any of these post A lenses where the lens tells the camera it just went from one max aperture to another. You turn the lens barrel 1mm and the value changes abruptly while the real aperture moves on a smooth continuum. The meter will see a very minimal difference between say 44mm and 45mm, wide open, but meter thinks it has shifted and will jump to the next aperture value For metering. For whatever reason all the Pentax cameras I have used seem to jump from 3.5 to 4.5 with variable aperture lenses. I think the test lens I was using was the 16-85.

What I've noticed with the 18-250 is it doesn't record all the focal lengths, those go in steps, so it's fair to assume that the min/max aperture do too - actually in those samples above it seems "nominal max aperture is in what the camera displays (and it does include f/4.0) and "max aperture" is whatever odd value it happens to be.

Across all modern zooms the Pentax system only reports 16 focal lengths (can't speak for the absolute most recent lenses but I suspect nothing has changed).

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Mike from Canada
"I am not a great photographer! God is a great creator! All I do is capture His creation with the tools He has provided me."
'I like to think so far outside the box that it would require a telephoto lens just to see the box!' ~ 'My Quote :)'
http://www.michaelfastphotography.com/galleries/VP-BDI_3a.jpg
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