I have a heavy weight champion Ektar and I want to place a drilled circular plate aperture in behind of lens. There is a rim at back of the lens and plate could be put in place flat and tight. But I read many times , place of aperture used to control aberrations. If I put it , does it kill the qualities of the lens ?
And How to decide the radius of the drill ?
Umut
Umut, this is actually a rather interesting question. A thought experiment occurs to me which may be useful. The starting point is to realize that lens aberrations are typically at their worst with the lens wide open, and stopping down from there tends to reduce them and improve the overall image quality (up to diffractive limitations, at least) due to clipping of marginal rays. With the internal iris wide and an additional aperture
behind the lens, the overall effect is still the clipping of marginal rays - but you are just not doing it as efficiently. Another way to picture it is that if the aperture you place behind the lens just meets the edge of the light cone which would have resulted from using a particular setting of the internal iris, then you are working at that same f/#. But the set of marginal rays you are blocking may be slightly different - some of the marginal rays that would have been stopped by the internal iris may now make it through the rear aperture, and consequently you would get slightly more aberration in the image. So based on this, I would think that at any particular f/# the aberrations for the rear stop configuration will likely be somewhat greater than if the internal adjustable iris were used, but I don't think it could be any worse than what you would get with the lens wide open. If that thinking is correct, it at least bounds the problem.
All of the above relates only to the 7 classical lens aberrations. Another image characteristic to consider is vignetting, and you can expect that you will only get
more of it due to the rear aperture placement. This can be minimized by placing the aperture stop as close as possible to the rear of the lens assembly. It depends on how much of the lens's native image circle you really need to work with too, so hopefully it will be a pretty minor effect for you. Another factor to consider is stray light. With the aperture stop at the "wrong" location for the lens, you may also find increased flare in your images. As always, using a lens hood may help.
When it comes to calculating the hole size needed for a given f/# for a rear aperture, I think it is actually very easy. Thinking of the cone of light reaching image plane and considering that there are no intervening optics, the f/# must be just =
L/
d (where
L is the distance between the physical aperture and the focal plane, and
d is the diameter of the hole). This gives a different result than the usual f/# =
f/
d equation (where
f is the focal length of the lens). I claim that this equation doesn't work here because the "
d" in that equation is technically the diameter of the
entrance aperture of the lens, not the diameter of the physical hole you are placing there. They are not the same in this case. I have encountered this discrepancy before, for example in working with and modifying Holga cameras. In fact, quite a few people seem to be off by about one stop in their estimation of what the Holga's "real" f/# is, or in their calculation of the new f/# when they modify their camera; and I think this is the reason.
Anyway, I hope that is helpful. Good luck!
Jeff
