Teleconvertors and autofocusing - wide angles and DOF

Okay, I know photography pretty well and am a serious amatuer, but one question still seems to be lingering. Why do autofocus camera's lose their autofocus ability beyond f/5.6??? Since the camera focuses through the lens, I'd think that it is because of the light loss coming through more elements with the TC, but when I look through my Canon 300 f/4 with a 2x on it (f/8) it still seems like it's getting plenty of light.

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Another question is this, I have read that you don't need to use high apertures (f/16, f/22)with a wide angle lenses (20mm). Can anyone explain this reason to me. Thank you.

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Bill

Well, it's got nothing at all to do with the amount of light

coming through the lens (within reason, all AF fails if the light

gets <em>really</em> dim).

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It's more to do with the geometry of the optics used to image the

light onto the AF sensor which

drives the AF system. If you've ever used a split image manual

focus camera you will have seen the rangefinder part "black out"

with slow lenses (f8 and slower). The same sort of thing happens

with AF systems.

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You certainly *could* make an AF system that worked at f8, or f11

or evem f16, but it would become less accurate as the minimum

working aperture dropped. Similarly you could make a very accurate

AF system which required an aperture of f2.8 or faster (indeed,

Canon do this with part of the center AF sensor on the EOS-1 series

cameras). Why pick f5.6? I have no idea, except that it's about

the slowest "common" speed for consumer zooms etc. By chosing

f5.6, the AF system works with 95% of all lenses and does a pretty

good job of accurate focus.

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I don't understand your second question. You might want to rephrase it.

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BTW this isn't really a nature question, but I'll let it slide since

it probably is a point of interest for many nature photographers.

Thanks for the reply Bob,

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Sorry that it's not really a "nature" question, but I am a nature photographer and just wanted some general info. Maybe someday they'll have better light sensor's that can autofocus accurately at higher f stops.

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What I met by my question about the wide angle lens was in regards to landscape photography (or any photography where you want a large depth of field). With most lenses, you must set the aperture several stops higher than normal in order to get the close objects and far objects all in focus. My question was that I have read before that with a wide angle lens (such as a 20mm)you don't have to do this. Is this so, and if so, why? I was also looking at your chart on "hyperfocal" distance and find it helpful. It appears, that when using my 20-40mm zoom at 20mm (f/2.7)if I set the focus at 5.71m, then everything within 2.9m to infinity will be sharply focused. To have something closer to the lens appear in focus I will have to use a higher f stop.

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Actually, I have another question regarding landscape photo's. Is there an optimum aperture or shutter speed for sharp landscape photo's? In other words, would it be better to use my 20mm at f/2.7 with a fast shutter speed (and tripod of course), or would I get just as sharp images by using my 20mm at f/16 with a slow shutter speed?

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Thanks again

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Bill

Wide angle lenses can be considered to act as if they have a lot more depth of field

than telephoto lenses (not the exact wording here - "act as if"

is important!). As they are nornally used, you get more DOF from

a wide angle lens than a telephoto (though this is in fact due to the

fact that, as they are normally used, the subject magnification is

much less when using a wide angle than a telephoto lens and it's

really the magnification that sets the DOF).

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The optimim aperture is that which gets everything you want in

focus (based on depth of field considerations). Most lenses are

sharpest around f8 or so. Larger and smaller apertures tend to

be less sharp (aberrations increase at wider apertures, diffraction

effects make smaller apertures less sharp).

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I normally shoot wide angle lenses at f11-f16 unless I <em>really</em> need

a lot of DOF, when I will stop down to f22. Everything on a tripod

of course. If I really want the main subject to be as sharp as

possible and I don't need the DOF (which doesn't happen a lot the

way I use wide angle lenses), I'll shoot at f8. Only if I was

working in dim light without a tripod would I open up all the way

to get the fastest possible shutter speed - and I don't recall that

ever happening in the case of my nature photography.

Responding to the last question:

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f8 to f11 range of apertures is where most of the lenses perform best.

f2.8 and f22 seetings offer, unfortunately, more distorsion.

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Speaking about a zoom, the mid range of focal length is the best setting (i.e.: set at 50mm a 35-80mm zoom).

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Merging the 2 answers and if you use a 35-80mm zoom, best performance is at 50mm/f8.

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Hope this help.

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Vincent

Regarding the answer stating that zooms are sharpest in the middle range, this isn't true of all zooms. Many of the tele zooms are sharpest at their shortest focal length, from what I've read.

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Those that span the wide-tele range do, I believe, tend to be sharpest in the middle, but this may be due to the fact that at the short end they're retrofocus, while they aren't as they get longer, which probably makes the design more or less naturally favor the middle focal lengths.

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Regarding shutter speeds and landscapes, normally a slow shutter speed is fine. The one case where it isn't is when attempting to shoot fields or meadoes of flowers, with everything near and far sharp. The wind often blows in such meadows, which means you want a fairly high shutter speed. This means a fairly wide aperture, and insufficient DOF. A catch 22, solvable by lenses like Canon's tilt-shift lenses or by a view camera. These options allow you to align the plane of focus

with the plane of flowers, allowing a wider aperture and faster shutter speed while still keeping all the flowers in focus.

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Of course, by the time you figure out the movements on the view camera, the flowers have probably wilted. Or turned into suburbia :)