Spherical aberration

cavis

Hi there,
I'm having a little confusion regarding spherical aberration from some experiments I've been doing with a concave mirror. I've been directing the mirror so that it faces the distance (ie. objects at infinity) and then positioning a flat piece of paper so that an image forms on it. The image, even at is sharpest, remains somewhat blurry which is due to spherical aberration from the mirror, as far as I understand.

And yet, when I look at the distance directly in the mirror (without looking at the screen), I see an image that is perfectly crisp. What am I missing here?

Thanks in advance,

Chris.

Drakkith

What's the focal length, diameter, and F-Ratio of your mirror?

How far from the mirror are you placing your eye when you look in it? Very close to the focal plane, behind it, or in front of it?

It seems to me that your eye may only be getting part of the converging light cone from the mirror. If you take only a small piece of the light cone, the spherical aberration is greatly reduced.

sophiecentaur

That's a very astute observation. I agree with Drakkith about the reason. When you look at the image directly, you are effectively putting a tiny aperture in the way (your pupil), which means that each part of the image you are looking at is coming from only a small part of the reflector and the errors are small.
In cameras, the lens distortions are always less and the depth of focus is always much greater when using a small aperture. You could try putting a circular aperture in the way of your mirror and see how the projected image improves.