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04-06-2012, 10:30 AM   #1
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Zoom lens aperture selection

Hi everyone,

I'm new to film photography and have a very simple question for the forum.

I have a Pentax SMC M 40-80 mm f2.8 - 4 zoom lens with a manual aperture ring. I understand that the maximum aperture at 40 mm is f2.8 and at 80 mm is f4.

With the manual aperture ring, I can select f2.8 when the lens is at 80mm. Why is this possible? What would be the effects? I don't understand how f4 is the maximum when I can select f2.8 and continue to shoot.

Can someone please explain this to me? I think I'm not understanding this correctly. Perhaps with a DSLR and a modern lens, the camera might prevent me from selecting an aperture lower than the maximum at maximum zoom but with my film MG, I seem to override this. Or have I got this all wrong?


Thanks in advance.

04-06-2012, 10:45 AM   #2
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You can select f/2.8, but as you zoom in from 40mm to 80mm the aperture will open up only a teeny bit, until it reaches 20mm in diameter. (f=80mm, f/4 = 20mm) When you select f/2.8 on the aperture ring, it just means the aperture will stay open as wide as it can go for any given focal length.
04-06-2012, 11:26 AM   #3
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You cannot set that lens at F2.8 when you are zoomed at 80mm. If you look at the lens, above the aperture ring there are three markers, one red, one white and one green. The red marks the F-stop when using 40mm, the white when at 60mm and the green when at 80mm. If you look at the focal numbers they are color-coded also.

If you turn the ring all the way to fully open, the markers are (red = F2.8, white= ~F3.5 and green= ~F4.5) which should be the real F-stops.

The iris of the lens has a fixed size and does not change as you zoom in and out. It changes only with the ring. Since the definition of the F-Stop is Focal Length / Iris diameter you get:

At 40mm and F2.8, the iris is 14.3mm wide open and will stay at that while zooming.
When at 60mm the effective F stop will be 60/14.3 = F4.2
At 80mm it will be 80/14.3 = F5.6.

Mathematically it looks that the F4 and at 80mm is rather optimistic.

Last edited by demp10; 04-07-2012 at 09:03 AM.
04-06-2012, 07:42 PM   #4
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Thank you both for your reply.
This makes a lot more sense to me now. I was not sure about the tricolour index markers above the aperture ring and how to interpret them but I do now.

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