Manual variable aperture zoom on a digital Pentax

Started 2 months ago | Questions thread
OP alex_virt Senior Member • Posts: 2,400
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

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