Manual variable aperture zoom on a digital Pentax

Started 3 months ago | Questions thread
James O'Neill Veteran Member • Posts: 5,750
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.

 James O'Neill's gear list:James O'Neill's gear list
Pentax K-5 IIs Pentax K-1 Pentax smc FA 50mm F1.4 Pentax smc DA 18-250mm F3.5-6.3 Pentax smc FA 43mm F1.9 Limited +3 more
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