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Old Feb 18, 2006, 10:28 PM   #1
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i'm about ready to step up into the DSLR world from my trusty - and still quite capable - Panasonic FZ20. i'm considering a Canon 20D, but i've heard the AF on those cameras doesn't work at apertures smaller than f5.6. since many long lenses START at 5.6 at the full zoom end, that pretty much makes AF useless for a lot of wildlife or other outdoor shots using long lenses.

my question is, do all DSLRs do this, or is it a situation unique to Canon? i love the image quality and absurdly low noise the 20D produces, but not having a working AF above f5.6 seems to be a bit of a serious limitation...
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Old Feb 19, 2006, 8:00 AM   #2
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Pretty much all AF systems have the same problem. Low light levels, be it from a very slow lens or very low ambient light, makes it difficult for AF systems to operate.

On the plus side, if you are out shooting and at the long end of your zoom, your subject is most likely quite far away, so you can turn the AF off and just set the focus on infinity and shoot away.
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Old Feb 19, 2006, 8:16 AM   #3
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The camera will open the apature up for automatic focusing then stop it back down to what's needed for the shot. The only time you might run into problems in the real world is if you use a slower telephoto lens with a tele converter that reduces a stop or two. For example, if you try to use a 2X TC on an F/4 zoom lens, it probably won't auto focus reliably, where it would on a an F/2.8 lens. A 1.4 TC works fine on my EF 70-200F/4L.

That being said, the low light auto focus capability of the 20D is pretty good... better, I'll bet, than on your Panasonic.
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Old Feb 19, 2006, 4:44 PM   #4
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Wildman wrote:
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The camera will open the apature up for automatic focusing then stop it back down to what's needed for the shot.*
This is true, but a zoom lens that becomes f6.3, or f8 or slower at the long end is still limiting the amount of light reaching the AF sensors, even wide open, because wide open is now f6.3 or f8 or whatever.

Just something to keep in mind when you start to look at those super zooms.
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Old Feb 19, 2006, 9:35 PM   #5
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amazingthailand wrote:
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Wildman wrote:
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The camera will open the apature up for automatic focusing then stop it back down to what's needed for the shot.
This is true, but a zoom lens that becomes f6.3, or f8 or slower at the long end is still limiting the amount of light reaching the AF sensors, even wide open, because wide open is now f6.3 or f8 or whatever.

Just something to keep in mind when you start to look at those super zooms.
the zooms i'm looking at are typically f4 at the short end and f5.6 at full zoom. that's about what they were 20 years ago when i was shooting film SLRs. i just would prefer not to have the AF function inoperable if i choose, say, f11 for DOF reasons. i suppose anything that far away would turn out okay if i just set the manual focus for infinity and left it there, but i'm used to a lens with an "infinity" distance of thousands of feet at f8...


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Old Feb 19, 2006, 11:13 PM   #6
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The camera will keep the aperture wide open for focusing. That means that if you choose f/11 for DoF reasons, the camera will open it up to f/5.6 (assuming that's the max at the focal length you choose) during focusing, then stop down for the shot. AF won't be a problem as long as the max aperture is f/5.6 or larger at the focal length.

I've also heard (though have no personal experience) that the lens will sometimes trick the camera into believing it's an f/5.6 even when it's not, such as with the Bigma, which becomes f/6.3 at the long end.
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Old Feb 20, 2006, 4:37 AM   #7
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All SLR cameras focus with the lens wide open and stop down when taking the shot, so AF will work on any aperture that the lens is capable of as long as the maximum aperture is big enough.

Most DSLRs won't AF with lenses with a maximum aperture smaller than f5.6.

The Canon 1D series will still AF on lenses with a maximum aperture of f8, not sure about the Nikons.
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Old Feb 20, 2006, 9:43 AM   #8
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thanks, all... that tells me what i need to know. sounds like as long as i use a lens that opens up to at least f5.6, the AF will work fine regardless of what aperture it usesto actually takethe shot... question answered, no worries!


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Old Feb 20, 2006, 10:38 AM   #9
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Remember on 20D, you can use ISO400 without any problems, and with that even while shooting at f8, you will have high enough shutter speed for birds in flight shots. In good sun light according to sunny 16 rule, you will be 1/400 at f16, even overcast, you will be 1/400 at f8. FZ20 can't come anywhere near that. I know, I use FZ5 and have extensively used FZ1.

I use 1.4xTC on my 400mm f5.6 prime and still get very good shots. No problem with AF on my 10D which is not supposed to AF past f5.6. AF does slow with TC but without TC, AF has no problems at f5.6. Now this is using good quality glass, I have no experience with consumer quality lens,e xcept my 50mm f1.8.

Look at my gallery at http://bobbyz.smugmug.com , most shots with 100-400L and 10D at f6.7-f8.0.
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Old Feb 20, 2006, 11:38 AM   #10
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squirl033 wrote:
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... question answered, no worries!
Well . . . maybe not entirely. As you're selecting your lenses, you might try Googling for Canon 20D + "front focus" or "rear focus"; the 20D has gotten some bad press for its tendency (with some lenses?) to focus in front of or behind the subject.

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