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01-30-2011, 11:22 PM   #1
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Teleconvertors and auto focus

Hi, i'm curious about teleconvertors, when i researched it for my canon camera i was told that they make the aperture number larger and also can cut off auto focus.
Does this happen with pentax and can you do something to prevent losing autofocus?
I don't mind the aperture thing but losing auto focus is not an option for me.

01-31-2011, 12:02 AM - 1 Like   #2
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It doesn't matter what brand your camera is - this is an optical issue.

Losing AF depends primarily on how fast your lens was to begin with. AF may fail to work if the lens+TC combo ends up being slower than f/8 or something like that - it really depends on the specific AF system of your camera model .

For example, a 1.4x TC adds one stop and a 2x TC adds two stops. So, if you start with an f/2.8 lens, a 1.4x TC will make it f/4 and the 2x TC will make it f/5.6. You should still maintain AF with each TC, as long as they are AF capable. But if you use a slower f/5.6 lens, its hard to say if AF will work with the 1.4x TC and it will most definitely not work with the 2x TC.
01-31-2011, 12:33 AM   #3
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QuoteOriginally posted by Laurentiu Cristofor Quote
It doesn't matter what brand your camera is - this is an optical issue.

Losing AF depends primarily on how fast your lens was to begin with. AF may fail to work if the lens+TC combo ends up being slower than f/8 or something like that - it really depends on the specific AF system of your camera model .

For example, a 1.4x TC adds one stop and a 2x TC adds two stops. So, if you start with an f/2.8 lens, a 1.4x TC will make it f/4 and the 2x TC will make it f/5.6. You should still maintain AF with each TC, as long as they are AF capable. But if you use a slower f/5.6 lens, its hard to say if AF will work with the 1.4x TC and it will most definitely not work with the 2x TC.
Hey, thanks, i'm guessing it would be better to buy a longer lens instead...because currently i only have the kit lenses so i'm guessing they'd lose auto focus.

Edit:sorry i wrote kit lens, the guy at the camera shop did a deal for me so i was thinking kit lens but my long lens actually isnt.

Last edited by lolliiy; 01-31-2011 at 12:39 AM.
01-31-2011, 12:56 AM   #4
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Yes, getting a longer lens is better. TCs are a compromise and they are best used with a fast lens where the IQ degradation still leaves you with something useful. Don't bother with them for slower lenses.

01-31-2011, 09:30 AM   #5
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Something else about TC's: Every telephoto lens has a TC optical unit built into it. This internal TC is matched to the other optics in the lens. Some premium longer telephoto lenses are sold with add-on TC's, also matched to the lens optics. All other add-on TC's are compromises. It may be necessary to try many different TC's with any lens to determine if they work together well, by which I mean, that IQ is degraded too much.

The situation isn't helped by pricing. AF TC's aren't cheap, and they're rare in the Pentax world. My opinion: If you plan to buy a premium long tele with matching TC, that's fine. That's a combination used by birders. But if you want to add a generic TC to an existing lens, good luck. You might get results suitable for surveillance or blackmail.
01-31-2011, 09:48 AM   #6
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Just as a quick follow up on RioRico's post.

Look at the sigma Screw Drive TCs

They only mechanically fit a small range of very fast long sigma lenses, and some pentax lenses. The TC requires that the lens have a recessed rear element, with 35mm diameter by 12mm deep clearance at the rear of the lens to accomodate the protruding front element of the TC.

There is a pentax rear converter that has even more interference (which is why the sigma TC will work on some pentax lenses)

These TCs work very well with long sigma lenses. and simply don't fit other lenses including sigmas.

The best overall TC I have found is the pentax 1.7x AF Converter, but again, it will only work on relitively fast lenses.

What must be remembered is that regardless of lens and TC, autofocus will only be retained reliably if the combination is faster than F6.7. this means for a 1.4 X TC, you need a lens faster than about F4.5, a 1.7X TC needs to be faster than about F4, and a 2 X TC needs a lens faster than F3.5.

Most consumer lenses at the long range are F5.6 or slower so you are simply wasting your time with a TC, unless you accept autofocus will not work and are willing to manually focus with a relitively dim viewfinder
01-31-2011, 10:59 AM   #7
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QuoteOriginally posted by lolliiy Quote
Hi, i'm curious about teleconvertors, when i researched it for my canon camera i was told that they make the aperture number larger and also can cut off auto focus.
Does this happen with pentax and can you do something to prevent losing autofocus?
I don't mind the aperture thing but losing auto focus is not an option for me.
What do you mean by losing auto-focus? I think any teleconverter will make the auto-focus slower at least.

I just recently wrote an article on the Kenko Pz-AF 1.5x Teleplus SHQ teleconverter that is available sometimes on the Marketplace and a few other sites that import from Japan. it works with screw-drive and SDM.
01-31-2011, 11:21 AM   #8
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QuoteOriginally posted by sjwaldron Quote
What do you mean by losing auto-focus? I think any teleconverter will make the auto-focus slower at least.

I just recently wrote an article on the Kenko Pz-AF 1.5x Teleplus SHQ teleconverter that is available sometimes on the Marketplace and a few other sites that import from Japan. it works with screw-drive and SDM.
see my post immediately before your's.

all camera makers are the same, they require lenses faster than F6.7 to gaurantee auto focus AT ALL

even at F5.6 speed is reduced and there nay be hunting, but above F6.7 they simply won't focus unless the sun is so bright that it melts your sunglasses.

01-31-2011, 02:46 PM   #9
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QuoteOriginally posted by Lowell Goudge Quote
see my post immediately before your's.

all camera makers are the same, they require lenses faster than F6.7 to gaurantee auto focus AT ALL

even at F5.6 speed is reduced and there nay be hunting, but above F6.7 they simply won't focus unless the sun is so bright that it melts your sunglasses.
Ah, I see. I just tried my slowest lens (the 50-200mm WR) at 200mm and it seems to auto-focus okay indoors and focusing out a window is fast. So that being said I think it would be usable, but results are another question. I use my Kenko with the DA* 300mm hand have had good results, but it does obviously not focus as fast (both due to the added range and drop in f-stop).
01-31-2011, 06:17 PM   #10
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Thanks everyone for the comments.
I asked because beside people shots, i also like going to places like healesville sanctuary, melbourne zoo and i'm real keen to try werribee open range zoo. The last one is the one where i will need a long lens most.

After reading the replies i feel it might be easier to just buy a longer lens....my longest at the moment is 50-200mm so i'm going to look at the ones that go up to 300mm, i really don't mind what it starts with whether it be 70mm or what ever.

Thanks again, from casey.
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