Yes, indeed: that's another possibility. I think the takeaway from this exercise, however, is that there is a healthy falloff wide open at 135mm, but that it is fully correctible in image processing. Whether it is .9 or 1.8 EV is fairly arbitrary as long as you know how to deal with it.
Godfrey On Nov 7, 2007, at 3:32 PM, Dario Bonazza wrote: > Hmmm... maybe estimating falloff on images also has to deal with > contrast > setting at time of shooting, which could change the perception of > light > distribution on images of the same white paper. > > Dario > > ----- Original Message ----- > From: "Godfrey DiGiorgi" <[EMAIL PROTECTED]> > To: "Pentax-Discuss Mail List" <[email protected]> > Sent: Thursday, November 08, 2007 12:12 AM > Subject: Re: How to estimate vignetting on digital images? > > >> 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. > > > -- > 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.
