08-16-2010, 08:51 PM
Hi geno,
[quote name='genotypewriter' timestamp='1281974177' post='1815']
Hmm I'm still puzzled by how resolution can be affected by the angle of incidence without any added vignetting.[/quote]
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.
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.
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.
That may well be the case here. Do note that I was really trying to speculate why this might be the case. I am very interested in this type of behaviour, because I don't particularly like (extreme) vignetting a lot myself as it also takes away from available DR if you correct it. The lens is rather long for a 50 mm, also because of the two static elements at the back for additional close focus and possibly other corrections. This means a long lens barrel, which may well cause more mechanical vignetting. This may not show up in the same way as with e.g. mechanical vignetting caused by a too small filter, as a lens barrel is completely defocused, but it will likely have an influence, and that influence may be rather large.
Yes, I have seen Juza's test, and I saw another one as well, not the chart test however. If you look carefully at the detail of the corners/edges in any of the photographs taken with a Sigma EX 12-24, you will notice the smearing of detail, on FF that is. And in high contrast situations, the CA towards the corners is truly of a magnitude you don't want to know about, although that may be caused by the same smearing effect (coma/astigmatism probably).
Do note that the winter landscape photograph Juza shows, has distinct smearing in the detail of the large rocks in the bottom corners. This at F/11, where these should be really sharp. I saw a similar photograph somewhere else, where it was even more obvious, because the person testing the lens also showed 100 % crops of the bottom corners. He was quite happy with it however.
Here are some of my results with the Sigma 12-24, deliberately taken under difficult conditions, whole image first, followed by a 100% crop of the top right corner, which shows its flaws best:
![[Image: Sig12-24-02.jpg]](http://www.lumifer.com/pz/Sig12-24-02.jpg)
![[Image: 12-24-01.jpg]](http://www.lumifer.com/pz/12-24-01.jpg)
Do note that this part of the image is well within DoF. Also note the smearing of detail, and the heavy CA. This photograph, and the whole test set I did to which it belongs, really freaked me out, and this was the best lens out of 3 I tried in the shop! It is also why I ended up with a Nikkor 14-24 first (good but awkward to operate if you want to choose a specific aperture other than F/8, wide open or fully closed), followed by a TS-E 17 <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' /> (better than the Nikkor, and probably the sharpest lens I own). And I couldn't be happier, as far as UWA is concerned. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='' />
Kind regards, Wim
[quote name='genotypewriter' timestamp='1281974177' post='1815']
Hmm I'm still puzzled by how resolution can be affected by the angle of incidence without any added vignetting.[/quote]
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.
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.
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:I'm not sure whether I agree with your first comment because as you said in the last part, a large image circle does help reduce the vignetting. Also there are many examples of the same focal length+max f-number combinations producing different levels of vignetting mainly because of design.
That may well be the case here. Do note that I was really trying to speculate why this might be the case. I am very interested in this type of behaviour, because I don't particularly like (extreme) vignetting a lot myself as it also takes away from available DR if you correct it. The lens is rather long for a 50 mm, also because of the two static elements at the back for additional close focus and possibly other corrections. This means a long lens barrel, which may well cause more mechanical vignetting. This may not show up in the same way as with e.g. mechanical vignetting caused by a too small filter, as a lens barrel is completely defocused, but it will likely have an influence, and that influence may be rather large.
Quote:Yes, it doesn't sound too different from what most people encounter with the 12-24, unfortunately. Not sure if you've seen the review below:
[url="http://www.juzaphoto.com/eng/articles/sigma_12-24_review.htm"]http://www.juzaphoto...2-24_review.htm[/url]
IMO, this is not the sharpest sample of the lens I've seen but it's still very good. There was another test from a lesser known guy somewhere on the net which was more of a chart test and the one he had was even more impressive.
GTW
Yes, I have seen Juza's test, and I saw another one as well, not the chart test however. If you look carefully at the detail of the corners/edges in any of the photographs taken with a Sigma EX 12-24, you will notice the smearing of detail, on FF that is. And in high contrast situations, the CA towards the corners is truly of a magnitude you don't want to know about, although that may be caused by the same smearing effect (coma/astigmatism probably).
Do note that the winter landscape photograph Juza shows, has distinct smearing in the detail of the large rocks in the bottom corners. This at F/11, where these should be really sharp. I saw a similar photograph somewhere else, where it was even more obvious, because the person testing the lens also showed 100 % crops of the bottom corners. He was quite happy with it however.
Here are some of my results with the Sigma 12-24, deliberately taken under difficult conditions, whole image first, followed by a 100% crop of the top right corner, which shows its flaws best:
Do note that this part of the image is well within DoF. Also note the smearing of detail, and the heavy CA. This photograph, and the whole test set I did to which it belongs, really freaked me out, and this was the best lens out of 3 I tried in the shop! It is also why I ended up with a Nikkor 14-24 first (good but awkward to operate if you want to choose a specific aperture other than F/8, wide open or fully closed), followed by a TS-E 17 <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' /> (better than the Nikkor, and probably the sharpest lens I own). And I couldn't be happier, as far as UWA is concerned. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='' /> Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....