12-28-2010, 10:52 PM
[quote name='Klaus' timestamp='1293457937' post='5230']
Sorry but results from different cameras are always non-cross-comparable - neither here nor elsewhere. The difference is that we tell so whereas others don't publish this aspect or even offer invalid comparison tools.
[/quote]
Not true.
If the vignetting is quantified by finding the point in the gray scale (of known density, positioned in the center of the image) which matches the luminosity of the corner of the image studied, then the resulting EV values are independent of the post-processing used in the camera and remain unaffected by any uniformly applied post-processing. Increase contrast? The vignetting gets stronger in visual appearance but with it increases the contrast of the subject. These always go hand in hand. Zeiss reports vignetting as a function of distance from the center, which is again a little more work but since how sharply the vignetting appears in the corners varies greatly, so one value is insufficient.
This procedure is a little harder to do, but it produces values that are immediately meaningful and comparable across sites and camera systems, when the experiment is correctly carried out.
What gray point is that? Which settings (i.e. tone curve) do you use in the camera? What is 1 (pseudo-)EV in terms of values in the file? (EV refers to the physical domain not the digital values). The pseudo-EV values are difficult to interprete if this mapping between real (i.e. subject) EV and the values given in your reports is not documented. Also, how large a patch of the image (as % of the image area) is used to calculate the vignetting value?
Sorry but results from different cameras are always non-cross-comparable - neither here nor elsewhere. The difference is that we tell so whereas others don't publish this aspect or even offer invalid comparison tools.
[/quote]
Not true.
If the vignetting is quantified by finding the point in the gray scale (of known density, positioned in the center of the image) which matches the luminosity of the corner of the image studied, then the resulting EV values are independent of the post-processing used in the camera and remain unaffected by any uniformly applied post-processing. Increase contrast? The vignetting gets stronger in visual appearance but with it increases the contrast of the subject. These always go hand in hand. Zeiss reports vignetting as a function of distance from the center, which is again a little more work but since how sharply the vignetting appears in the corners varies greatly, so one value is insufficient.
This procedure is a little harder to do, but it produces values that are immediately meaningful and comparable across sites and camera systems, when the experiment is correctly carried out.
Quote:The vignetting data is obtained at the same gray point peak so the exposure variation is less than 1/6th of an f-stop.
What gray point is that? Which settings (i.e. tone curve) do you use in the camera? What is 1 (pseudo-)EV in terms of values in the file? (EV refers to the physical domain not the digital values). The pseudo-EV values are difficult to interprete if this mapping between real (i.e. subject) EV and the values given in your reports is not documented. Also, how large a patch of the image (as % of the image area) is used to calculate the vignetting value?