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05-30-2012, 12:13 PM   #1
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Question on AF on higher F numbers?

Hey guys I am a bit confused I have read many times when reading about tel-converters that you need a fast lens or your AF wont work, Is this different then setting my DA*300 to F8 or higher because it still auto-focuses for me.

05-30-2012, 12:16 PM   #2
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Your lens stops the aperture down at the moment of shooting, after the AF has locked. Until then, your DA*300 is wide open at F4 for the autofocus.

A teleconverter reduces the effective aperture size down, so a 2X teleconverter on your DA*300 will register as F8 while it is mounted on the camera.
05-30-2012, 12:18 PM   #3
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The lens always auto focuses and meters at maximum aperture. During focusing there is as much light coming through a lens at f22 as there is at f2.
05-30-2012, 12:21 PM   #4
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Your lens is always wide open until the actual shot is taken when it is stopped down. So how much light reaches the viewfinder (and the AF system) is determined by the maximum aperture of the lens. A teleconverter loses a stop or more of the light just being there -- the increased distance and the glass absorbing light. This will noticeably darken the viewfinder a bit when using it, and so therefore the AF system is getting less light, which means it doesn't work as well.

So to answer your question, no it is not the same at all -- what you set your lens aperture to makes no difference because it is always autofocusing at the maximum aperture. But if your lens is slow to begin with -- like a 5.6 on a telephoto zoom, the light loss of the teleconverter will make it so dark (now it is equivalent of f8 or more) that the AF will tend to hunt a lot and maybe never lock, or only work on contrasty subjects.

05-30-2012, 12:21 PM   #5
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TCs eat light. They reduce the amount of light hitting the sensor. If your DA*300/4 were on a 2x TC, it would only pass as much light as if the lens were manually stopped down to f/8. But you're probably using your camera to set your lens' aperture to f/8. While AF'ing and metering, the lens is still wide-open at f/4; it only stops-down to f/8 when the exposure is taken. Try this: Set your menu to Using Aperture Ring= Enabled (if you don't have it there already). Turn the aperture ring on your lens to f/8 or f/11. Now see how well it AF's.

EDIT: Ah, I see many of us came up with the same answer at the same time. Coincidence, or... ???
05-30-2012, 12:23 PM   #6
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QuoteOriginally posted by RioRico Quote
Try this: Set your menu to Using Aperture Ring= Enabled (if you don't have it there already). Turn the aperture ring on your lens to f/8 or f/11. Now see how well it AF's.
Still stays wide-open, doesn't it, just like an M lens? Only way to really stop down for focus is to use an m42...
05-30-2012, 12:23 PM   #7
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Both Jin and Jim are right

AF can theoretically struggle with stopped down smaller apertures, but the only lenses that are manual stop down are m42 so AF isn't an issue.
F8 (a 2x converter on an f4 lens) starts hitting problemn territory, but since you would be doing this in bright daylight it's not as big an issue (nightime will likely be a different story)
stick the same converter on a 55-300 and shoot at 300 wide open and it's be f11.2 more problematic certainly

Edit - boy it was a typing race apparently i'm too slow
05-30-2012, 12:28 PM   #8
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Keep in mind also that you need a teleconverter that allows AF to begin with. Not all of them do...

05-30-2012, 12:37 PM   #9
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Thanks for clearing that up for me, it all makes sense now.
05-30-2012, 01:22 PM   #10
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There are way too many Pentaxians sitting at a computer instead of outside taking pictures.................
05-30-2012, 02:32 PM   #11
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QuoteOriginally posted by jatrax Quote
There are way too many Pentaxians sitting at a computer instead of outside taking pictures.................
It's all a matter of self-preservation. If I go outside, the wild turkeys and squirrels will attack me, the deer will trample me, nearby bears and pumas will likely come for their share. And then there's the skunk... And my next stop is the dentist's chair. Life is hard here. [/me hears sad violin music in the distance]
05-30-2012, 02:32 PM   #12
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QuoteOriginally posted by jatrax Quote
There are way too many Pentaxians sitting at a computer instead of outside taking pictures.................
I'm at work
05-30-2012, 04:35 PM   #13
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QuoteOriginally posted by vonBaloney Quote
QuoteOriginally posted by RioRico:
Try this: Set your menu to Using Aperture Ring= Enabled (if you don't have it there already). Turn the aperture ring on your lens to f/8 or f/11. Now see how well it AF's.
Still stays wide-open, doesn't it, just like an M lens? Only way to really stop down for focus is to use an m42...
Oops, that's right. I was thinking of FA lenses -- and they stay open until shutter-click too. And M42s don't autofocus, though they exploit the AF system for Focus Confirmation and CIF. What I can say there is that FC and CIF seem to have a limit around f/9. They usually work if a lens is stopped down to f/8 or wider; they usually don't work if at f/11 or tighter; and the region between f/8-11 can be tricky.

Now, that's 'usually'. CIF works with some shorter lenses to f/14, and fails with some longer lenses around f/7. Adequate light is necessary. This correlates with using TCs on slower zooms. Don't expect AF to work if the effective aperture is f/10 or tighter.
05-31-2012, 12:39 AM   #14
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If you want to "simulate" AF on F8 or even higher apertures, you can try with ND filter attached.

On the other hand it is not a secret that even with fairly fast lenses, AF works better outdoors in broad daylight than indoors in dim light.
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