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07-08-2012, 04:29 PM   #1
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Is the K5 supposed to focus fast at F5.6?

Hello I have a question I am using a Kenko TC with my DA*300 and AF is kind of shit, like it has a hard time locking on and by the time it does the subject has moved on, in my case birds. Is this more the TC and lens combo or is it the camera. I am wondering because the 560mm that's coming is a 5.6 lens and if the AF is anything like my combo its going to be crap.


Last edited by coreyhkh; 07-08-2012 at 05:02 PM.
07-08-2012, 04:38 PM   #2
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QuoteOriginally posted by coreyhkh Quote
Hello I have a question I am using a Kenko TC with my DA*300 and AF is kind of shit, like it has a hard time locking on and by the time it does the subject has moved on, in my case birds. Is this more the TC and lens combo or is it the camera. I am wondering because the 560mm that's coming is a 5.6 lens and if the AF is anything like my combo its going to be crap.
You loose around 2 full stops with a TC on the end of a lens. Plus, if I recall correctly, the SDM lenses are a bit finicky about what TC they will work with, if any. The 560 will be inheriently a 5.6 lens and not suffer from elements added to one end that are not necessarily a good match for the lens, so it ought to work much better than a 300+TC at the lower f stops.
07-08-2012, 05:00 PM   #3
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QuoteOriginally posted by coreyhkh Quote
I am wondering because the 560mm that's coming is a 5.6 lens and if the AF is anything like my combo its going to be crap.
A 2xTC loses 2 stops, making an f/4 lens into an f/8. A 1.4xTC loses 1 stop. Assuming you used a 2xTC, the 560 lens should perform a better than your TC combo.
07-08-2012, 06:47 PM   #4
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QuoteOriginally posted by coreyhkh Quote
Hello I have a question I am using a Kenko TC with my DA*300 and AF is kind of shit, like it has a hard time locking on and by the time it does the subject has moved on, in my case birds. Is this more the TC and lens combo or is it the camera. I am wondering because the 560mm that's coming is a 5.6 lens and if the AF is anything like my combo its going to be crap.
You changed the focal length but the camera still thinks it's a 300mm (I think). The camera is expecting the focus response of the lens to be less than the response actually is, and as a consequence it's overshooting the 'focus' point - it's having to focus 3 or 5 times instead of just once.

Personally I didn't really love the 300mm with a TC for that reason. With the FA 1.7x the response is actually OK (focus was in the TC obviously) but with the Tamron (and presumably Kenko) it overshoots.

The 560 will focus MUCH better. It depends on the lens design (including but not limited to the aperture) but it'll prob. be close to the focus speed of 'one' focus cycle (before the lens reverses) on your 300mm f/4.

07-08-2012, 06:53 PM   #5
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QuoteOriginally posted by ElJamoquio Quote
You changed the focal length but the camera still thinks it's a 300mm (I think). The camera is expecting the focus response of the lens to be less than the response actually is, and as a consequence it's overshooting the 'focus' point - it's having to focus 3 or 5 times instead of just once.
Pretty sure it is just the loss of light, not confusion about the focal length. Even with a manual lens stopped down to f8 (like an m42 where it really stops down), you'll have trouble getting focus confirmation. Just not enough light. About f6.3 is the limit, and that is dodgy also in low light.
07-08-2012, 07:07 PM   #6
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QuoteOriginally posted by vonBaloney Quote
Pretty sure it is just the loss of light, not confusion about the focal length. Even with a manual lens stopped down to f8 (like an m42 where it really stops down), you'll have trouble getting focus confirmation. Just not enough light. About f6.3 is the limit, and that is dodgy also in low light.
Disagree. The 300mm and 1.4x Tamron goes past focus (overshoots) several times. In exactly the same condition the 150-500mm f/6.3 (admittedly a 1/3 stop or so faster) does fine.

I am acutely aware of the effect of aperture. Slower lenses than the 300mm (like for instance the 150-500 with polarizer) don't yoyo like the 300mm + TC.

The 300+TC is not going to the stops like it might if it was just hunting for actual focus. The camera definitely acts like it knows where the focus should be, but the camera is just wrong with that calculation - and the lens is underdamped.
07-08-2012, 08:19 PM   #7
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Ok I just did some testing with 3 other lenses the 18-55, a sigma 70-300 and the 100mm macro. Results where surprising the af on all these works really good and fast with no hunting. This was done at night in my living room with dim lights. This means it's not the k5 but the da*300 and tc just don't like each other I think it's because it's SMD and if I am not mistaking all the tc that work with SMD are really just hacks and where not design with it in mind. Also of not the macro which is a 2.8 lens was the slowest of the 3.
07-08-2012, 10:28 PM   #8
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QuoteOriginally posted by ElJamoquio Quote
Disagree. The 300mm and 1.4x Tamron goes past focus (overshoots) several times. In exactly the same condition the 150-500mm f/6.3 (admittedly a 1/3 stop or so faster) does fine.

I am acutely aware of the effect of aperture. Slower lenses than the 300mm (like for instance the 150-500 with polarizer) don't yoyo like the 300mm + TC.

The 300+TC is not going to the stops like it might if it was just hunting for actual focus. The camera definitely acts like it knows where the focus should be, but the camera is just wrong with that calculation - and the lens is underdamped.
Then shouldn't the same thing happen with all lenses with a TC that passes through the focal length? It should mess up on all lenses fast or slow because the lack of light is not the problem, the incorrect calculation is. (Why have AF capable TCs at all if they can't function by definition?)

Anyway, that has not been my experience -- fast lenses on a TC autofocus almost as well as without the TC. Autofocus gets progressively worse the less light there is -- we know this to be true even if you are right so it is certainly *a* cause if not *the* cause. Expecting autofocus to work reliably at an effective f8 -- it just isn't going to happen. (Of course it is not just the stop value itself, but the actual light of the scene is very important also -- f8 may be OK on a bright sunny day.) But what evidence is there that the camera "knows" the scale of response? The focus throw is different for every lens, even of the same focal length. Does the lens transmit this information also? I've never heard mention of such a thing.

07-08-2012, 11:32 PM   #9
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QuoteOriginally posted by vonBaloney Quote
It should mess up on all lenses fast or slow because the lack of light is not the problem, the incorrect calculation is. (Why have AF capable TCs at all if they can't function by definition?)
Pentax had an AF SDM-compatible teleconverter on their roadmap years ago, then took it off. It's not clear why. Powerzoom (Pz) teleconverters, which have some SDM/HSM compatibility because the Pz power connector got re-used for SDM/HSM, have been discontinued, and it's not clear why.

Has anyone done a live view contrast detection AF vs. normal phase-detect AF performance comparison with Pz teleconverters and problematic SDM/HSM lenses? I've been curious if any issues are isolated to the phase-detect AF system.
07-08-2012, 11:38 PM   #10
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I was curious how a system like Canon's that has first-party teleconverters handles a similar sort of situation, and dug up this fascinating tidbit which describes how at f/5.6 with a 2x teleconverter phase-detect AF is disabled and contrast-detect is the only option on the tested body - Sigma AF 150mm f/2.8 EX DG HSM APO Macro OS (Canon EOS) - Review / Test Report - Tele Converter Analysis:

QuoteQuote:
...the Sigma AF 150mm f/2.8 EX HSM DG APO macro OS accepts both the original Sigma as well as Canon converters resulting in a 210mm f/4 (EF 1.4x) and 300mm f/5.6 (EF 2x) combination respectively... As mentioned the OS remains functional with these converters whereas (a slightly slower) phase detection AF only works with the EF 1.4x III (on the EOS 5D II). It's a bit strange that contrast AF (LiveView) is available with the EF 2x III though - this is mentioned in the manual of the lens.
07-09-2012, 02:40 AM   #11
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I had a 1.5x Kenko TC, probably we are talking about this one. It didn't work fast on my sdm lenses (did not have 300mm then). It is rubbish, but don't judge the upcoming 560mm for that, since it will be screw-driven.
07-09-2012, 06:46 AM   #12
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QuoteOriginally posted by vonBaloney Quote
1. Then shouldn't the same thing happen with all lenses with a TC that passes through the focal length?
2. It should mess up on all lenses fast or slow because the lack of light is not the problem, the incorrect calculation is.
3. (Why have AF capable TCs at all if they can't function by definition?)

Anyway, that has not been my experience -- fast lenses on a TC autofocus almost as well as without the TC.

4. Autofocus gets progressively worse the less light there is -- we know this to be true even if you are right so it is certainly *a* cause if not *the* cause. Expecting autofocus to work reliably at an effective f8 -- it just isn't going to happen. (Of course it is not just the stop value itself, but the actual light of the scene is very important also -- f8 may be OK on a bright sunny day.)

5. But what evidence is there that the camera "knows" the scale of response? The focus throw is different for every lens, even of the same focal length. Does the lens transmit this information also? I've never heard mention of such a thing.
1. Yes.
2. It should.
3. There are AF capable TC's. I've used them with long lenses. I'm not sure if the K-5 does some funny things with the 300mm to improve the AF that it doesn't do with, say, the sigma 500mm f/4.5 + TC. Perhaps there's some algorithm embedded in the camera.

I'm not saying that this scenario makes sense to me. But it is without question in my experience that the AF of the 300mm works differently with TC than any other lens I've tried TC with. (Really only the sigma 500mm f/4.5 and the sigma 300mm f/2.8). At least two other posters have complained about an 'odd' overshoot as well with the Tamron + 300mm.

4. Of course.
5. Again, I don't have a good explanation so I'm defaulting to this 'maybe the camera is doing something weird'.


In summary - I don't expect this to be a satisfying answer to you. I don't know if I'd believe you or find you credible if our situations were reversed. If you have access to a 300mm, I'd suggest you try it - perhaps it's just a problem with my setup... but I doubt it.
07-09-2012, 06:47 AM   #13
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QuoteOriginally posted by RonHendriks1966 Quote
I had a 1.5x Kenko TC, probably we are talking about this one. It didn't work fast on my sdm lenses (did not have 300mm then). It is rubbish, but don't judge the upcoming 560mm for that, since it will be screw-driven.
? I'm guessing it will be SDM. I'm not sure if I saw that somewhere or I assumed it based on the mass and purpose of the lens.
07-09-2012, 06:59 AM   #14
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QuoteOriginally posted by ElJamoquio Quote
? I'm guessing it will be SDM. I'm not sure if I saw that somewhere or I assumed it based on the mass and purpose of the lens.
it will not be SDM. That engine is not strong enough for this lens. Maybe some other lens, but I think screwdriven.
07-09-2012, 08:53 AM   #15
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QuoteOriginally posted by ElJamoquio Quote
1. Yes.
2. It should.
3. There are AF capable TC's. I've used them with long lenses. I'm not sure if the K-5 does some funny things with the 300mm to improve the AF that it doesn't do with, say, the sigma 500mm f/4.5 + TC. Perhaps there's some algorithm embedded in the camera.

I'm not saying that this scenario makes sense to me. But it is without question in my experience that the AF of the 300mm works differently with TC than any other lens I've tried TC with. (Really only the sigma 500mm f/4.5 and the sigma 300mm f/2.8). At least two other posters have complained about an 'odd' overshoot as well with the Tamron + 300mm.

4. Of course.
5. Again, I don't have a good explanation so I'm defaulting to this 'maybe the camera is doing something weird'.


In summary - I don't expect this to be a satisfying answer to you. I don't know if I'd believe you or find you credible if our situations were reversed. If you have access to a 300mm, I'd suggest you try it - perhaps it's just a problem with my setup... but I doubt it.
My experience is limited to screw-drive AF lenses (and a limited number at that) although the 2x TC I have does have SDM (or zoom) contacts -- but never had an SDM lens to try it with. Possibly they are doing something more sophisticated with SDM lenses -- I figured they were just replacing one drive mechanism with another that wouldn't change the search pattern itself, but that may not be true.

All I know is that if I put the TC on a slow lens, it is hopeless. You can see this easily on a variable-aperture zoom where at the wide-end and bigger aperture it will work ok (but why would you want it to?), but at the long end where the max is 5.6 or 6.3 (without the TC) it hunts like crazy. It has been a long time since I tried it -- I think it locks once in a while, mostly falsely -- seems pretty random. (It will look soft even with perfect focus on a zoom so can't really tell.) I assume we are talking strictly about PDAF here -- the CDAF in live-view on the K-5 will falsely lock (usually front-focusing) on a slow lens even without the TC -- it's reliable limit seems to only f4.5ish.

Now, I did do some experiments where I blocked off the pin so that it wouldn't know the focal length and I could enter it manually (so I could enter the effective focal length with TC for the SR to work properly). I don't remember trying this with AF, but maybe I did. Blocking off the pin probably kills the AF function anyway.
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