08-19-2010, 04:07 AM
[quote name='wim' timestamp='1281991861' post='1827']
Under normal conditions you get (optical) vignetting simply because the image is stretched out further towards the edges than it is in the centre. Because the image is stretched out, it is logical that the resolution is also affected.
[/quote]
Hmm, not sure if I can see what you mean here. First of all, I don't think there's any more stretching of the objective plane on a non-retrofocus lens than there is on a retrofocus one.
Secondly, if the non-retrofocus design means the nodal point of the lens sits closer to the image/sensor plane than on a retrofocus design, then it means light has to travel a shorter distance... this should, on the flipside, reduce light fall off (without considering the sensor response w.r.t. AOI).
[quote name='wim' timestamp='1281991861' post='1827']
Also, the larger the angle of incidence of light (the more it diverges from the optical axis), the more the light will get refracted, and hence the spectrum of light broken up. This reduces resolution drastically, as it is very difficult to put all wavelengths of light back together again. This is why there generally is both more CA and less resolution towards the edges and corners of a frame then there is in the centre, where light essentially travels parallell to the optical axis of a lens.
[/quote]
I agree about the high refraction within the lens due to lens design... my question is how can such a lens perform better on film than on digital but show no increases in vignetting on digital?
[quote name='wim' timestamp='1281991861' post='1827']
However, I don't entirely know what the effect of relatively deep site wells is on reflected light and potentially the image, as reflected light might be refracted by the site well's edges, as well as light getting into the site well. I was speculating about this, to be very honest. I don't know for sure what the ultimate effect is. I do know that this is likely one of the reasons for the CA towards the edges of the frame with UWA lenses, especially non-retrofocal ones. I also know that PF is more apparent with digital than it is with film, which is an indicator that the sensor assembly contributes at least partly to PF. In an area that suffers from PF, the resolution is also a lot less than in surrounding areas. I reckon PF is caused at least partly by reflection or refraction of certain parts of the sensor assembly, and thereby is not only related to high contrast transitions (where it is blatantly obvious), but also to certain angles of incidence in a specific spot on the sensor, and probably to whether PF shows up or not. Again, speculation on my part, however.
[/quote]
Maybe we should think about what we haven't really talked about so far... for example film thickness and its role in reducing spherical aberrations, etc. I think even more so the reason is the magnification at which we look at digital images. You wouldn't print a 35mm slide/negative to the size of 86"x57" and look at from 2 feet away even if it was really that good, but every 12MP FourThirds image gets the same magnification on a regular 24" monitor when viewed at 100% <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='
' />
GTW
Under normal conditions you get (optical) vignetting simply because the image is stretched out further towards the edges than it is in the centre. Because the image is stretched out, it is logical that the resolution is also affected.
[/quote]
Hmm, not sure if I can see what you mean here. First of all, I don't think there's any more stretching of the objective plane on a non-retrofocus lens than there is on a retrofocus one.
Secondly, if the non-retrofocus design means the nodal point of the lens sits closer to the image/sensor plane than on a retrofocus design, then it means light has to travel a shorter distance... this should, on the flipside, reduce light fall off (without considering the sensor response w.r.t. AOI).
[quote name='wim' timestamp='1281991861' post='1827']
Also, the larger the angle of incidence of light (the more it diverges from the optical axis), the more the light will get refracted, and hence the spectrum of light broken up. This reduces resolution drastically, as it is very difficult to put all wavelengths of light back together again. This is why there generally is both more CA and less resolution towards the edges and corners of a frame then there is in the centre, where light essentially travels parallell to the optical axis of a lens.
[/quote]
I agree about the high refraction within the lens due to lens design... my question is how can such a lens perform better on film than on digital but show no increases in vignetting on digital?
[quote name='wim' timestamp='1281991861' post='1827']
However, I don't entirely know what the effect of relatively deep site wells is on reflected light and potentially the image, as reflected light might be refracted by the site well's edges, as well as light getting into the site well. I was speculating about this, to be very honest. I don't know for sure what the ultimate effect is. I do know that this is likely one of the reasons for the CA towards the edges of the frame with UWA lenses, especially non-retrofocal ones. I also know that PF is more apparent with digital than it is with film, which is an indicator that the sensor assembly contributes at least partly to PF. In an area that suffers from PF, the resolution is also a lot less than in surrounding areas. I reckon PF is caused at least partly by reflection or refraction of certain parts of the sensor assembly, and thereby is not only related to high contrast transitions (where it is blatantly obvious), but also to certain angles of incidence in a specific spot on the sensor, and probably to whether PF shows up or not. Again, speculation on my part, however.
[/quote]
Maybe we should think about what we haven't really talked about so far... for example film thickness and its role in reducing spherical aberrations, etc. I think even more so the reason is the magnification at which we look at digital images. You wouldn't print a 35mm slide/negative to the size of 86"x57" and look at from 2 feet away even if it was really that good, but every 12MP FourThirds image gets the same magnification on a regular 24" monitor when viewed at 100% <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='
' />GTW