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.