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09-11-2010, 05:11 AM   #1
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Are there issues with video and Sigma HSM lenses?

Somewhere at the other forum I read that people have trouble with Sigmas HSM lenses when using contrast detection AF. Lenses would not lock focus at all in most cases.

Can someone confirm this? Or can someone confirm that HSM AF works properly in movie mode?

Thanks!

09-11-2010, 09:34 AM   #2
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Are you asking about continuous AF in video or AF single?

As far as I know continuous AF during video doesn't exists in Pentax DSLR's
09-11-2010, 01:49 PM   #3
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Both continous contrast AF as well as single contrast AF are said not to work properly. Here's the original thread I've read:

no focus with K7-Lifeview and Sigma HSM lenses??: Pentax SLR Talk Forum: Digital Photography Review

Can someone reproduce this AF behaviour?
09-11-2010, 01:52 PM   #4
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There is no AF while recording video regardless of lens, it's just prefocus weather you rely on AF or not

09-11-2010, 01:57 PM   #5
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Really? Not even by pressing the AF button? That would be a bit disappointing
09-11-2010, 02:10 PM   #6
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No AF in video DSLR's is standard, the 5DmkII, 7D, T2i, Kx, K7, D5000, D90, ect. they all do not have autofocus while recording video, you have to prefocus before pressing record and then rely on manual focus while you are recording

The only DSLR's that have continuous AF during video are the unreleased Sony SLT A55/A33 and the Nikon D3100, which are the first ones that can do that

There were some models that allowed you do single focus while shooting video, but it proved so ineffective that the "feature" was removed from the camera all together via firmware update or by changing the production models from the preproduction review models
09-11-2010, 02:33 PM   #7
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Thanks a lot for clarifying this. I wasn't aware of this. So it seems like camcorders still have a purpose
09-11-2010, 03:07 PM   #8
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Continuous autofocus, lenses that don't change focus when zooming, smaller sensors with more depth of field, no limits on recording time, using video codecs like AVCHD, zoom of at least 10x in one lens, and usually no slower than f/3.8, infrared recording, power zoom instead of physical manual zoom rings

there's still a lot of reasons for camcorders to be around, it'll be a long time before dslr's can replace camcorders for most uses. They are very useful for meticulously planned cinematic scenes though

09-12-2010, 01:09 PM   #9
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Some camcorders even seem to have a constant 1.4 zoom lens ...

But , aside from video, AF should generally be possible in live view on newer DSLRs, right? I mean contrast detection AF, not by flipping the mirror.

Will HSM lenses work with that?
09-12-2010, 01:29 PM   #10
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QuoteOriginally posted by Noisychip Quote
Some camcorders even seem to have a constant 1.4 zoom lens ...

But , aside from video, AF should generally be possible in live view on newer DSLRs, right? I mean contrast detection AF, not by flipping the mirror.

Will HSM lenses work with that?
Yes in theory it is possible, but contrast detect AF is so slow and ineffective that it would have adverse affects on the video. Nikon's new D3100 uses contract detect AF for autofocusing during video, there are no in depth reviews out yet so we have yet to see how effective it is. Nikons AF system and algorithms are generally pretty good so they might be able to pull it off, Pentax's Contrast detect (as well as their normal AF systems) are pretty miserable comparatively.

it is possible, and has been done before, but like I said the experience was so miserable that firmware updates removed the ability to use contrast detect in video, believe the T2i and the K7 pre-production review units (like the ones that DP review get) both had contrast detect AF in video but both were so terrible and inefficient that the ability was removed from the cameras firmware
09-13-2010, 01:52 AM   #11
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Usually current HD-DSLRs are lacking continuous auto-focus, because in DP world, there is no need for it. When shooting a movie, a shot cannot rely on the camera's focusing algorithm, the current focus is always determined by the director/DP and set by the 1st AC. Also, count in the hardware issue, it is much harder to do autofocus on an APS-C or 35mm sensor, than a very small 1/2" 1/3" sensor (because of the depth of field). As mentioned before, the K-7 could do C-AF in Contrast detect mode, but it was proven to be so bad, that is was removed from the final firmware.
09-13-2010, 06:21 AM   #12
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I have the Sigma 17-50mm F2.8, 50mm F1.4 and 70-200mm F2.8 II lenses, and with my K-7 (FW 1.10), neither the 50mm nor 70-200mm AF properly in LV. The newer 17-50mm does work however. This is in still photo mode, not video mode.
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