Is it possible to get smaller aperture with diaphragm in front of the lens?

Question

My camera has narrowest aperture of f8. To increase depth of field, could I place an opaque "filter" or lens-cap with a small hole, e.g. 12 mm for f16, on the lens? I realize that at short f.l. the filter would cause significant vignetting, but at 400 mm how far from uniform would illumination be? Tests with a piece of aluminum foil have been ambiguous, but that might be due to imperfections in the hole.

Answer 1

I think what will happen is:

• incoming light sees a smaller aperture
• then that will diverge a bit when hitting the lens
• then that narrower beam hits the internal aperture
• then that arrives on the sensor.

What you will have is:

• an improved aperture (but not that good as if you did it with the aperture blades)
• much narrower useable image (wasted sensor area)
• vignetting, but not with a distict contour (because of the Fourier transform and the out-of-focus qualities, you will see a soft ringing on the borders most probably).
• a kind of lens softness/halo.(The closer you put the small hole to the lens, the less this is.)

In general, the amount of light entering the system will be stopped down by the small hole, so you need a much larger exposure time.

Also note that any imperfections of the small hole will be visible on the image, especially around small light sources as flare.

You will have a better DOF on a smaller useable area of the sensor, and around it you will have this brighter/darker softness or ringing and then almost blackness.

Note also that the small hole will may not only cause brightness but color ringing as well.

Anyway, this is a great fun to play with. If you make some photos, just for the edification of all of us, please post some results :-).

Answer 2

I've seen a tip to use cutouts placed over the lens to give a shaped bocah. I have Cockin "filters" in a set that are the same thing, intended for vigentting. So, the effect you get will vary depending on the size of the cutout, the focal length, and the apeture used.

Why not take a series of photos with different apertures and the same cutout and make an animation of one effect morphing into the other?

IAC, why not just try it and see what happens? Then ask to analyse the optics involved and other ramifications, when you post the experimental results.

Answer 3

The apeture (F stop) is a fraction of the distance from the center of the lens to the film plane while focused on infinity. Therefore, in my opinion, this would not work for that reason.