Large lenses and small sensors

[OldNick]

I learned that digital cameras had to have _better_ lenses than 35mm format, as the smaller sensor (ignoring its own faults, noise etc) meant that it magnified the faults of a lens.

But I have just read a question about using old 35mm SLR lenses being _better_, as the sensor's being smaller measn that it will not see the edges of the lens , and therefore give a _better- image, with less edge softening.



IIRC, film plane "sees' the whole lens's picture, including edges, no matter its size. The only thing that stops edges softening is to shut down the iris and not _use_ the edges of the lens.



I can understand using a large aperure 35mmSLR lens on a small sensor would allow you to stop the lens dow and get the same F numbers. Is this what is being referred to?



Thanks for any advice,/refreshment on this one.

[scaramouche]

Greetings,

Every lens which has not been botched in manufacture is theoretically perfect exactly on the optical axis, i.e. right in the middle. All normal lenses, (not retrofocus wide-angle or telephotos, etc.) are equivalent to a single element lens of the same focal length. The problem with that single element is that even though it can produce an image with the desired magnification, that image carries with it some problems which the lens designers refer to as aberrations; chromatic aberration, spherical aberration, etc. To elminate these aberrations the lens designers introduce more optical elements into the optical path. This is an approach to the problem of optical quality which was worked out as early as the mid 19th century and can work veryy well. . What actually happens, however, is that the aberrations are not actually eliminated, but rather , shifted away from the optical axis, or middle of the image. This means that every lens, no matter how expensive or complex, will have a "quality gradient" associated with it. So, the better lenses will be better over a larger part of the visual field, measured from the center out.

Smaller than 35mm imagers are placed right in the middle of this quality gradient so what happens, with respect to quality, depends on the characteristics of the actual lens.

Hope this helps.

Richard S.

[eric s]

Another thing to add is that if you stop down too much you can introduce more problems. So while stopping down a bit can be beneficial, after a certain point a large f-stop starts to degrade the quality of the shot.

Eric

[OldNick]

scaramouche wrote:

Greetings,

Every lens which has not been botched in manufacture is theoretically perfect exactly on the optical axis, i.e. right in the middle. All normal lenses, (not retrofocus wide-angle or telephotos, etc.) are equivalent to a single element lens of the same focal length. The problem with that single element is that even though it can produce an image with the desired magnification, that image carries with it some problems which the lens designers refer to as aberrations; chromatic aberration, spherical aberration, etc. To elminate these aberrations the lens designers introduce more optical elements into the optical path. This is an approach to the problem of optical quality which was worked out as early as the mid 19th century and can work veryy well. . What actually happens, however, is that the aberrations are not actually eliminated, but rather , shifted away from the optical axis, or middle of the image. This means that every lens, no matter how expensive or complex, will have a "quality gradient" associated with it. So, the better lenses will be better over a larger part of the visual field, measured from the center out.

OK. Got that bit, and I was reasonably up with that.

Smaller than 35mm imagers are placed right in the middle of this quality gradient so what happens, with respect to quality, depends on the characteristics of the actual lens.

Now here I am still not sure whether a smaller sensor sees the whole lens image, or only part of it. My question was, if we place a small sensor, in the "sweet spot" of a lens, will it "see" only the "sweet spot", or will it still see the whole lens's image?

[Technophile]

eric s wrote:

Another thing to add is that if you stop down too much you can introduce more problems. So while stopping down a bit can be beneficial, after a certain point a large f-stop starts to degrade the quality of the shot.

Eric

Yes. At the edge of every image you get diffraction patterns, so the more you stop the lens down the greater the effect these pattersn have on the image. You get less CA though.