I wasn't happy with my estimate. As a sanity check, www.photozone.de's test example showed a little less than 1 EV falloff wide open at 135mm (0.91 EV). Given he's using real test equipment and what I'm looking at is some guess at the meaning of 8bit grayscale values vis-a-vis EV, it's quite reasonable to assume that I'm off by a factor of 2 ... his .91 EV is roughly 1/2 the slightly less than 2 steps I approximated, which are really "10% Linear Steps" not EV values, which are powers of 2... Kinda makes sense when you think about the math, actually. ;-)
I'd trust Klaus' numbers. http://www.photozone.de/8Reviews/lenses/pentax_50135_28/index.htm Godfrey On Nov 7, 2007, at 2:56 PM, Dario Bonazza wrote: > Thanks Godfrey. That's exactly what I was looking for. > Dario > > ----- Original Message ----- > From: "Godfrey DiGiorgi" <[EMAIL PROTECTED]> > To: "Pentax-Discuss Mail List" <[email protected]> > Sent: Wednesday, November 07, 2007 11:36 PM > Subject: Re: How to estimate vignetting on digital images? > > >> Photoshop shows values based on [EMAIL PROTECTED] 127 is the mid point, >> and it's about 25.5 points per EV step for the full range from Zone 1 >> to Zone 10. So 130 is right around Zone V and 80 is right around the >> boundary between Zone 3-4, so it looks like you are showing about >> 1.5-2 steps light falloff from center to edge wide open, a rough >> first order approximation. >> >> (Lightroom shows it to be about 51% brightness on center and 32% at >> corners, again showing a little less than 2 steps falloff to the >> corners. 10% on grayscale value is 1 step in Lightroom's scale.) >> >> Godfrey >> >> On Nov 7, 2007, at 1:53 PM, Dario Bonazza wrote: >> >>> Hi folks, >>> >>> Here is a white-wall shot taken with the DA* 50-135 @ 135mm F2.8, >>> where it >>> shows its highest light falloff at corners: >>> www.dariobonazza.com/public/Vignetting.jpg >>> >>> Do yo have any idea about how to measure that? I mean, how to >>> translate >>> Photoshop values (around 130 for each RGB channel at center, around >>> 80 at >>> corners) into f/stops? >>> >>> Thanks to anybody enlightening me ;-) >> >> >> -- >> PDML Pentax-Discuss Mail List >> [email protected] >> http://pdml.net/mailman/listinfo/pdml_pdml.net >> to UNSUBSCRIBE from the PDML, please visit the link directly above >> and >> follow the directions. > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above > and follow the directions. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
