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I have this old soviet/russian "Granit" 80-200mm lens, connected to my Canon 550d DSLR through M42 adapter. I also have "modern" kit 18-55mm lens.

What I don't get is how aperture numbers work on these two, they don't seem to mean the same thing, so clearly I'm missing something.

Aperture number, e.g f/4.5 (wide open in Granit lens) should mean that opening diameter is 4.5 times smaller than focal length. So for 80mm I should get about 17mm opening, which sounds reasonable. But when I zoom that lens in, the same f/4.5 should give me wider and wider opening, which is not the case, opening stays exactly the same as it was at 80mm. It doesn't even seem to be physically able to give me this 44mm (f/4.5 at 200mm) opening, aperture housing just isn't big enough.

So by my understanding, after zooming from 80mm to 160mm, I should be at f/9? which should give me much darker photo, given the same ISO and time. But this is not the case, I can zoom in and out freely without correcting anything and still get correct exposure. Also, I don't think that this "silent" change of aperture number would be acceptable when this lens was brand new and all the photos were taken on film, using manual separate light meters.

On my modern lens (18-55mm, f/3.5-5.6), when I zoom it while holding depth of field preview button, I can see and hear electronically controlled aperture opening and closing a little, even when set to like f/9 which is available through entire focal range. So keeping f/9 requires change in opening size.

So what gives? Is this russian lens just lying to me (but why correct exposure then?), or isn't f number related to focal length at all, or at least is not directly related? Why does Canon lens change opening diameter, but Granit doesnt?

Also it seems that aperture housing is moving back and forth when zooming my modern lens (but its hard to tell by looking through lens), and I'm rather sure it doesn't move in Granit, since it's mechanically controlled and just seems to stay in place - this part I don't get at all, but since I'm not sure it really moves, maybe lets just skip that.

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  • Does this answer your question? Why doesn't the picture become darker the more you zoom in?
    – Michael C
    Mar 31 at 5:51
  • No, I believe main point I was missing was difference between physical hole in the iris and the the entrace pupil which is a projection of this hole through front lenses. this question is mainly about image brightness, which for me was just a symptom, not a real issue.
    – kao3991
    Mar 31 at 14:18
  • It seems from your question the reason your were suspecting your lens was lying to you was precisely because it wasn't getting darker as you zoomed....
    – Michael C
    Apr 1 at 14:57
  • ... and the answer to that question is precisely because it is the entrance pupil, not the physical diameter of the aperture diaphragm, that is measured when calculating f-number.
    – Michael C
    Apr 1 at 15:07
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f/4.5 doesn't mean that the hole in the iris is physically 1/4.5 times the focal length. It means that the size of the image of the hole, as viewed from the front of the lens through the lens elements in front of it, is 1/4.5 times the focal length. The image of the hole is the entrance pupil.

If you can, look at the image of the hole from the front as you zoom the lens. You should see the image get bigger, which is what matters for collecting light.

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  • Yup, guess you're right. I've looked into another 55mm lens and pupil size through front seems about the same as in canon lens at 55mm, while being physically much larger. Will check at another longer lens when i have the chance. Thanks a lot!
    – kao3991
    Mar 30 at 13:01
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As you know, the focal ratio (f-number) is calculated by dividing the working focal length by the working iris diameter. Therefore, these fundamental values are intertwined as to their influence on image brightness (exposure settings). The bottom line, as you zoom, image brightness rises an falls significantly. In fact, double or halving the focal length results in a 4X (2 f-stop) change in light level (exposure).

Such a drastic change in image brightness with the zoom is unavoidable and must be mitigated. There are several ways. A. mechanically change the diameter of the iris with the zoom. B. Change the exposure time with the zoom. C. Meter the change and adjust both shutter speed and / or aperture. D. Design the optics so the light gathered compensates.

Lets talk about D: The glass lens elements forward of the iris are figured so they are a stronger then needed positive (magnifying the view of the irs). The distance, iris-to-forward elements change with the zoom. This results with an appearance that the iris diameter swells and shrinks with the zoom. This arrangement migrates image brightness with the zoom. To correct the now too short focal length, the lens group after the iris has negative power which lengthens out the focal length. The result is a foreshortened lens barrel making this telephoto more manageable all the while making image brightness more uniform with the zoom.

It is possible to maintain image brightness throughout the zoom however this is a costly approach. More common, the maximum light gathering ability of the lens is a variable as the zoom progresses. Further, software in the modern camera alters shutter speed and or ISO with the zoom to compensate.

Foot note: You might have trouble understanding the drastic effect the zoom has on exposure. If you double the focal length, images of objects double in size but the area they occupy on film or sensor increases by 4X (2 stops). A 4X change in zoom magnification = a 64X change in image brightness = 6 f-stops. Its difficult to mitigate the light gain or loss plus keep distortion and aberrations in check.

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If all of the magnification takes place between the front of the lens and the physical aperture when you zoom, then the entrance pupil, which is the effective aperture that is actually used to calculate f-number, also grows by the same ratio as the focal length. As you zoom to longer focal lengths the size of the aperture as it appears through the front of the lens also grows, even though the actual physical size of the aperture diaphragm does not change.

This is also true of most variable aperture zoom lenses, except that only most of the increased magnification takes place between the front of the lens and the physical aperture diaphragm.

Take, for example, a 70-300mm f/4-5.6 lens. At 70mm and f/4 the entrance pupil is 17.5mm in diameter. At 300mm and f/5.6, the entrance pupil is 53.6mm in diameter. If the entrance pupil was still 17.5mm at 300mm focal length, the f-number would be f/17!

Or consider an 18-55mm f/3.5-5.6 lens. At 18mm and f/3.5, the entrance pupil is only 5.14mm wide. At 55mm and f/5.6, the entrance pupil is 9.82mm wide. A 5.14mm entrance pupil at 55mm would be f/10.7!

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