On Mar 20, 2006, at 2:24 AM, Alexandru-Cristian Sarbu wrote:
Are the Pentax D lenses that precise in their image circle coverage?
In passing (somewhere) I've read of a "claimed" 1.3 crop factor.
You seem certain. Do you remember where you read it?
We know they're not that precise - but I wouldn't bet that *all* of
them can cover the APS-H format
The DA series lenses are designed around an image circle capable of
covering a 16x24mm format sensor. Some can handle a larger format in
part of their ranges, but quality at the edges/corners is a concern
for larger formats. Covering a 16x24mm format well requires a high
quality 14.4mm radius image circle.
The DA14mm is pretty tightly constrained. I tested it on a 35mm film
body and found that it would just barely cover a 24x24mm central
square of the 24x36mm format gate. That means its absolute image
circle is just about 17mm radius, and quality in the last 2-3mm of
that radius drops off by some at least.
A sensor with "crop factor 1.3" in a 2:3 proportion would have to be
about 18x27mm, requiring a 16.2mm radius image circle. That would
likely stress the DA lens design performance at corners/edges.
Godfrey
