On Thu, 17 Apr 2003 12:48:15 +0900, Jim Davis <spammenot@someisp.jp> wrote:

>On Fri, 11 Apr 2003 21:40:50 +0200, "Pål Jensen"

><paaljensen@sensewave.com> wrote/replied to:

>>> If the digital SLR,s like my S2 only use the 2/3 center portion of the

>>frame

>>> then lense choice should be slightly "less critical" than film SLR,s

>>> because corner softness is a given on less expensive lenses?

>>> This woulds see another small benefit of digital SLR,s.

>>>

>>> My less expensive lense are now just a little better on the SLR?

>>No. It is usually the other way around. All things equal, the smaller the

>>format the bigger part the lens resolution provide of the total resolution

>>(lens + medium resolution). This is why lens quality is seldom an issue for

>>MF and LF photography as the lens resolution is a smaller contributor to the

>>overall quality compared to smaller formats. This means that if you have a

>>small digital sensor and a large digital sensor, lens quality differences

>>will be more apparent on the smaller sensor.

>Consider the image circle any lens projects onto the image plane.

>Where exactly is it always softest? Why, it's at the edges. These

>edges are cropped from the smaller digital sensor. Need I say more?

>Jim Davis

>Nature Photography

>http://www.kjsl.com/~jbdavis/

Uh, you're both "right"...;-)

While inferior lenses do tend to be worse at the edges

than at the center, they are often also worse in the center

at wider stops than better lenses. All lenses perform about

the same once they hit diffraction limiting, but for a

good lens, this may be around f5.6 in the center and f8-11

in the "corners" (at the edge of the image circle, appearing

in the full-frame rectangular image), while for the poorer

lens, diffraction-limiting may not appear until around

f11-16 in the center, and may never be reached in the

corners with the available stops... Also, for a given imager

resolution, doubling the lens resolution does not double the

final image resolution - the resolution of the lens and

sensor are interactive, with the resultant resolution likely

less (and never greater) than the lesser of the two (which

is why fairly poor lenses and really good lenses do not look

"wildly" different in the image - though they do look

different...;-).