A question on edge sharpness of lenses

Every time, I read a review on lenses, one part of the review deals with the edge sharpness of the lens.
This makes a lot of sense to me when the lens is used on a normal 35mm film camera.
However when used on a digital camera, like the K5, only a smaller inside portion of the lens seems to be used.
This seems to imply, that the edge sharpness is of no consequence anymore.
Is this correct?
Regards, Horst

Edge sharpness refers to the sharpness at the edge of the format frame, not at the edge of the image circle of the lens. The performance at the edge of our APS-C format images is just as important as ever it was. That's got nothing to do with film formats.

Now when we use a lens that can be used with film, we are only utilising the central part of the image the lens could produce. So it's likely that the lens will perform well at the edge of the APS-C frame. However, other factors will negate this advantage to some extent, such as the lens not being telecentric and such as poorer correction of chromatic aberration than would be the case with a digital lens.

Generally on digital I have found that digitally intended lens designs are better. One classic exception to this is the set of three FA Limited lenses.

Maybe the reviews are referring to full frame lenses when used on digital bodies as I still find it very important. I purchased the Da15mm Ltd recently and whilst the centre was very sharp the edges were disappointing when compared to the DA12-24...I sold the DA15mm as a result.

Simon

[[quote:3496ace15f="johnriley"]However, other factors will negate this advantage to some extent, such as the lens not being telecentric and such as poorer correction of chromatic aberration than would be the case with a digital lens.


Thanks John, I was mainly refering to full frame lenses. What is meant by "telecentric" and why should a full frame lens have poorer correction of the cromatic abberation?

Regards, Horst

Telecentric first. Especially with wide angle lenses, the light exiting from the rear element can be quite diverged, so the rays hit the film at an oblique angle. This is fine for film but digital sensors have each pixel sunk slightly in a pit. It is much better if the light rays are parallel and fall at 90 degrees onto the pits and hence onto the individual pixels. This problem is worse the nearer the rear element is to the sensor, so the K mount has a good back distance for lens design, whereas small CSCs and Leica rangefinders do not. In theory it is easier to make higher quality lenses for the K mount.

Chromatic aberration was not such a problem with film so corrections were not quite so high. It only really became a problem with long telephoto lenses and hence the increased use of extra-low dispension (ED) lens elements. With digital the effect is exaggerated, presumably because of the interaction of the light with the construction of the sensor causing diffraction effects at the edges of images.

Thanks John, for the detailed explanation.

Regards, Horst