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07-13-2018, 08:53 AM   #1
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Pentax FA 28-105 F4-5.6 Pz not wide open?

I was testing my Pentax FA 28-105mm F4-5.6 (powerzoom version) and noticed my highlights wide open were nonagons (nine sided shapes) Turns out the aperture isn't entirely wide open, except at 105mm where it entirely disappears from view.
Here a pic at 28mm:


Is this normal behaviour for this lens or is it just my copy? I tried it with lens on or off camera. Makes no difference.

Pic of the lens:


07-13-2018, 09:11 AM   #2
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If you select each aperture individually on the lens and meter the same scene, do you see the correct changes in exposure for every aperture (including the maximum)?
07-13-2018, 09:24 AM   #3
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With many "Fixed" maximum aperture zooms the aperture iris opens and closes some as it is zoomed. I wonder if this is related. Perhaps they felt the f/4 opening gave better IQ than a slightly faster wide open option? I realize that most variable aperture lenses don't do this. I just wonder if this was part of this design.
07-13-2018, 09:39 AM   #4
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Wide open exposure is the same whether I use stop down metering or av mode. At 5.6 and narrower aperture the A-mode pics look a bit darker (but that is what I generally experience. Stop down metering a narrow apertures gives more exposed images than when using a-mode. If the lens were faultily stopped down wide open it would have under exposed pics wide open. But it doesn't. So it must be a feature and now we also know why the lens is so soft at 105mm.

07-13-2018, 10:15 AM   #5
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QuoteOriginally posted by D1N0 Quote
I was testing my Pentax FA 28-105mm F4-5.6 (powerzoom version) and noticed my highlights wide open were nonagons (nine sided shapes) Turns out the aperture isn't entirely wide open, except at 105mm where it entirely disappears from view.
Here a pic at 28mm:


Is this normal behaviour for this lens or is it just my copy? I tried it with lens on or off camera. Makes no difference.

Pic of the lens:
I believe this is normal. Are your exposures consistent across different aperture settings?

Adam
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07-13-2018, 10:16 AM   #6
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Normal as I have the same lens. You will see same behavior on other zoom lenses. For example, FA645 55-110mm has same behavior. This behavior is to ensure the designated wide open F stop at a given focal length. Just imagine how can one design a lens that, when right at wide open, exactly gives F value 4 (not 3.8) @ 28mm, 4.5 (not 4.3) at 50mm, 5.6 (not 5) at 105mm, etc.? Slightly adjusting aperture wide open position might be the simpler way.

Last edited by hyyz; 07-13-2018 at 10:21 AM.
07-13-2018, 11:20 AM   #7
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Well these lenses are not known for there terrific bokeh anyway. it's just I always test how bubbly they are and this lens gave me this:


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