Originally posted by Just1MoreDave A particular f-stop should be a ratio between focal length and the diameter of the aperture opening. A 50mm lens set at f4 should have an aperture diameter of 12.5mm. A 25mm lens at f4 should have an aperture diameter of 6.25mm.
Originally posted by dmoon911 It looks like the formula is focal length/f-stop, if that's true it will help a lot.
The formula for the diameter of the stop is focal length / f-stop as Just1MoreDave said.
However the diameter you have to measure is NOT the diameter of the diaphragm as seen with the lens dismantled. The diameter you need to be concerned about is the entrance pupil diameter. The entrance pupil is the image of the aperture stop as seen from the front of the lens. Due to the magnifying effect of the front lens group(s) the aperture need not be the same size as the entrance pupil.
Originally posted by Just1MoreDave Then again, I only have one constant-aperture zoom I have where I can see the aperture while zooming - in other words, only the front elements zoom. I don't see any aperture size changes from 70 to 200mm. Who knows if this is right or not.
Do you mean that the size of the aperture diaphragm doesn't change as you zoom (do the blades move as you zoom) or the entrance pupil diameter doesn't change? The stop diameter doesn't have to change (the blades don't have to move) for the aperture to be constant as long as the optical arrangement causes the entrance pupil size to change as the lens is zoomed.
Originally posted by dmoon911 So here's my question after my long winded story. Does anyone know what size the aperture opening should be at each f-stop? Is it the same for all lenses or does it vary? The other thing is noticed was that if the aperture was at say f8 when I was at the 24mm end, when I zoom in the aperture looks like it opens up a bit. Is that how they make it a constant aperture?
I assume it's not a big deal on a dslr because the camera stops down the lens to get a reading and has no idea what the f-stop should be anyway. But I was hoping to use this lens on my new super program when it arrives. I assume I'll end up with consistent over or under exposure on a film camera?
Sorry that was a lot of reading to get to the point. Any help is appreciated.
Thanks -Dave-
If its like zoom lenses that I have dismantled previously then the aperture diaphragm will be widest open when the lens is zoomed to its longest focal length. If you zoom the lens to 48mm and set the aperture to it widest setting then you can adjust the diaphragm screws untill the aperture is just fully open and then lock everything up tight. This should be close to the right settings (although check that everything is at least close to the scribed marks to make sure it is not all way off).
Although with a DSLR in manual mode the exposure shouldn't be a problem as it stops down to meter, but if the aperture settings are too far open then you may get a lot of extra aberations etc ruining the images.