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Forum: Pentax SLR Lens Discussion
12-11-2010, 05:24 PM
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I like the second one, nice shot! The first one demonstrates the problem one often has with these kind of shots, AFAIC. The nose and ears are so out of focus that they exaggerate the depth of the face and thus create an almost comical impression.
Also, I find that such thin DOF almost mandates a "front on" pose, which has limited use. A bit of angle on the face and then having one eye in focus rarely works. I'm not saying ultra fast f-ratios don't work, but rather that their use is limited.
BTW, which lens did you use for these? Only using a tiny screen at the moment but they look very nice.
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Forum: Pentax SLR Lens Discussion
12-11-2010, 05:14 PM
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You cannot make such statements without providing further information. For instance, the same lens, using the same f/2.8, shooting the same subject from the same distance will show more DOF with an FF camera (because the APS-C image requires higher enlargement to yield the same output size, thus losing DOF).
That's why I said you either need the appropriate lenses on FF or you will be making different images (using less subject to camera distance).
Anyhow, I think for the sake of keeping the thread on topic we shouldn't continue this FF sidebar.
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Forum: Pentax SLR Lens Discussion
12-11-2010, 02:41 PM
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I fully concur but what are f-ratios that are still practical in terms of both a) usable DOF and b) usable IQ?
With an A* 85/1.4 or DA* 55/1.4 you might get the odd f/1.4 shot that works really well but with most lenses and most shots, we won't be using ultra fast f-ratios, or will we?
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Forum: Pentax SLR Lens Discussion
12-11-2010, 02:23 PM
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Not so clear to me. Also, sensors do not create shallow DOF, lenses do. In other words, if you haven't got the lenses, a format change won't help you (unless you want to take different images (with a different perspective) which also happen to have shallower DOF).
Also, whether or not the perceived desire for thinner DOF is reasonable or not is another matter.
I just read an old Amateur Photographer issue where the Canon 85/1.2 was tested. The f/1.2 sample portrait just looked weird. BTW, the lens has a lot of lateral CA which doesn't disappear when stopping down. Great glass to look at but photographically it seems to make less sense than its appeal suggests.
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Forum: Pentax SLR Lens Discussion
12-11-2010, 12:53 PM
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The term "fast" indicates the primary purpose of "lens speed" in the film days. They would have been called "shallow" or similar otherwise, right? So the purpose of allowing shorter shutter speeds is now better achievable through high ISO settings. I say "better" because one isn't forced to use very shallow DOF and doesn't need to use a lens at a settting where it has its weakest performance.
Pentax certainly thinks that we don't need as fast lenses anymore, otherwise the 55-135 - the equivalent of the common 70-200 of film days - would have had to feature a constant f/1.8. With the f/2.8 it has, we cannot take the same images as someone with a 70-200/2.8 on a FF camera.
I disput the claim that ultra fast lenses are better for AF. Such lens designs often feature significant aberrations which lead to focus shift. The AF determines the focus a) for one colour only and b) one aperture only so if the lens has high aberrations the real focus may be somewhere else for a given shooting aperture. I understand cameras try to compensate for focus shift but it doesn't always work perfectly. Ultra fast lenses also require more precision from AF systems as the margin of error is much smaller.
Having said all that there is still the purpose of allowing DOF control. While subject / background separation can also be achieved with PP it is easier and often nicer to do it with a fast lens. Often, in particular with longer focal lengths, I think f/2.8 will be sufficient though.
In conclusion I think that the hay days of ultra fast lenses are over and that the corresponding race has been replaced by the high-ISO race.
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