I'm probably rehashing a subject already beat dead, and you'll probably think I'm an idiot, but can somebody explain briefly what all this talk about F/8 Autofocus is? Is there some reason a camera wouldn't AF at F/8? What am I missing here? Is this why I can't get clear focus from my 20D? lol.
F8 AUTOFOCUS
- Thread starter liongate
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Almost all DSLR's autofocus only functions when the lenses aperture wide open is f5.6 or brighter. It's for the same reason that slow lenses make the split prism focus screens on old manual focus SLR's go black.
Being able to AF, even with limited AF functionality with lens combos as slow as f8 was the preserve of the 1 series for canon shooters until the 5D3. Think of a 600/4 with a 2x TC, or a 100-400 with a 1.4x TC.
Your 20D focus issues will be unrelated. It will only attempt to AF with f5.6 lenses (or brighter). If it consistently misses, that's likely down to a problem (calibration or otherwise) with your lens or body.
Being able to AF, even with limited AF functionality with lens combos as slow as f8 was the preserve of the 1 series for canon shooters until the 5D3. Think of a 600/4 with a 2x TC, or a 100-400 with a 1.4x TC.
Your 20D focus issues will be unrelated. It will only attempt to AF with f5.6 lenses (or brighter). If it consistently misses, that's likely down to a problem (calibration or otherwise) with your lens or body.
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No, in this case, its referring to the maximum aperture a lens has. For example, a 100-400 has a max aperture of f/5.6 at 400mm, and so it will auto-focus on all Canon cameras. If you stop it down to f/8 or f/11, it will still autofocus. However, if you put a 1.4x converter on it, you've turned it into a 560mm f/8 lens, and it will no longer autofocus on most Canon bodies (1D series and 5dIII aside). The autofocus system relies on light and contrast to focus, and you've made it too dark a lens for the auto-focus system to work reliably.liongate said:
Now, technically, it will auto-focus at f/8 or f/11, but, odds are whatever you are shooting has moved on in the 5+ seconds it took to lock focus. And in some cases, it wont lock focus at all.
So, gaining f/8 auto-focus is more about the ability to use teleconverters on lenses that are already a little slow. A 2x on a lens that ends at f/4 or a 1.4x on a lens that ends at f/5.6. Generally its important for wildlife shooters, which is why it makes sense that a 7DII might have it (and say, a T5i wouldn't)
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liongate said:
When a DSLR auto-focuses it does so with the aperture wide open, regardless of the aperture to be used for the photo capture. Once focus is achieved, it stops down the aperture to the desired size and takes the picture.
Because of the physical construction of autofocus modules, it becomes hard to make them work well when "aperture wide open" is f8, so typically only high-end cameras have this extra engineering. When you add a teleconverter to a lens you increase the focal length, and therefore increase the "wide open" aperture. E.g. a 400 f5.6 might become an 800 f8 lens with a teleconverter. I.e., "wide open" is now f8. That won't work for a 20D.
We can help you better if you'll tell us what lens you're using with the 20D, and if you're using a teleconverter.
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Orangutan said:
This is probably the answer you were looking for.
For example, if you had an F2.8 lens and set the aperture for the photo to F16, the camera would auto-focus with the aperture at its maximum of F2.8, then close down to F16 for the shutter release. If the max aperture of the lens (or effective max aperture, if used in combination with teleconverters) is F8 or smaller, it will not auto-focus (except on some pro bodies and, hopefully, the 7DII).
This is my understanding, anyway. Take anonymous internet info for what it's worth!
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And, to further confuse the issue, the 70D will autofocus f/11 lenses. This is only possible while using live view, autofocusing of that 400mm f/5.6 lens with a 2X TC attached is allowed.
I'd expect to see this feature on the 7D MK II, but no live AF beyond f/11.
I'd expect to see this feature on the 7D MK II, but no live AF beyond f/11.
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neuroanatomist said:
Is it spec'd that way? I could not use AF at f/11 with my 1D MK III and live AF. Sometimes when pre=focused to be close, it would snap into focus. I never tried it with either of my 7D's. My 5D MK III acts much like the 1D MK III, is occasional snaps in to focus at f/11 when using live AF.
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neuroanatomist said:
I don't think it's spec'd at all. But the 7D + 100-400 + 2xII combo focused reliably, albeit slowly (even if IQ was nothing to write home about).
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I'm not expecting to get a 7D MK II, but I am interested in being able to reliably autofocus at f/11.
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Just some Real World experience to add to the mix: the 7D will AF usefully quickly for bird photography at f/8 (eg Canon 100-400mm and taped or non-reporting 1.4x TC) off the central AF points. Use it on the peripheral points, and it's actually not bad at all.
For example - 600mm, handheld, (Kenko non-reporting 1.5 TC), peripheral AF point on the bird's eye.
And another.
The 70D (mine, and that of at least two other users that I asked to test this) will AF at f/8 on all AF points. Again, it's not lightning fast, but it works.
For example - 600mm, handheld, (Kenko non-reporting 1.5 TC), peripheral AF point on the bird's eye.
And another.
The 70D (mine, and that of at least two other users that I asked to test this) will AF at f/8 on all AF points. Again, it's not lightning fast, but it works.
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The hardware may be capable in certain situations of working, but if the central AF point doesn't function, or the reliability doesn't meet the manufacturers stringent testing standards, it's much simpler for them to simply mask out this borderline operation via the firmware.Keith_Reeder said:
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Yep, fully aware of that, but it's beside the point: I'm letting people know that despite suggestions to the contrary, both the 7D and the 70D will work in circumstances which some on here have stated they won't work in.
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Keith_Reeder said:
But you need to use then a third party extender for fooling around the body concerning available max aperture. My opinion, the Canon extenders still deliver the best quality on Canon lenses. So, I would like to see a photo of the 100-400 + 1.4 extender on the tele end and taken on a 7D or 70D, as I have my question about IQ at that moment.
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rs said:
If there is sufficient of light then then 60D might do that, just as the 7D or the 70D could do it over there. there is however no guarantee that you can do it from Canon, as it is not build in in the design of the camera. I am not aware which firmware this even might block, but I think Canon could have done this. Did you look at the quality of your link to digital picture? even in the center the sharpness is gone when you compare the 560mm with the 400mm.
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