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09-24-2008, 08:54 PM   #1
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catch-in-focus

Like many others I was left somewhat disappointed by the fact that there was no k20d Super or K30D released at Photokina... now I am pondering the K20D.

One thing that might compensate for the low fps (for my applications) would be this "catch-in-focus" that I have heard about. I'm looking for some feedback from people who have taken advantage of this feature. Just how well does it work.

Suppose you were on the side of a freeway shooting (from a tripod) at 200mm F2.8 as cars were driving towards you and you had set manual focus at a distance that would fill the frame. What percentage of shots would come out sharp?

09-24-2008, 11:17 PM   #2
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Sorry, I have no idea about the scenario you've proposed but I do know that the feature works nicely for manual focus lenses. Too bad my Canon bodies don't have this feature.

JayT
09-25-2008, 01:04 AM   #3
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I STILL don't get what catch-in focus does, or where it applies, especially now that you mentioned "MF lenses"... damn man. Does it mean it only fires when the camera confirms a focus? SO you keep the shutter button depressed, and focus manually, and it will fire the shutter once the focus beeps in?
09-25-2008, 01:08 AM   #4
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QuoteOriginally posted by marlon Quote
I STILL don't get what catch-in focus does, or where it applies, especially now that you mentioned "MF lenses"... damn man. Does it mean it only fires when the camera confirms a focus? SO you keep the shutter button depressed, and focus manually, and it will fire the shutter once the focus beeps in?
Thats correct...

09-25-2008, 01:27 AM   #5
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Yes, it'll do just that but only with manual only lenses or if you place a piece of paper between the contacts of the lens and camera for af lenses.

As you turn the focus ring and hold the shutter button pressed the camera will fire only when the subject in the center will be in focus.

Disadvantages:

If you use very slow shutter speeds it might not work as when the subject is in focus, the camera fires, and as the shutter is still opened you might not react that fast and you still keep turning the focusing ring.

This won't work if you want your subject to be off the center because if you recompose you have to have the camera set to MF so that it would fire when the centerpoint is off focus.
09-25-2008, 02:15 AM   #6
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I thought the catch in focus was the af system doing a double check for accuracy, and this feature allows it to be disabled for faster af??

I have always referred to above as 'trap' focus, which all the pentax dslrs do (maybe I'm wrong?). Put a mf lens on, turn camera to af, hold shutter down (or lock with cable release) and when af system registers in focus it takes the shot. As to the question how well it would work for your scenario I cant answer, sorry.
09-25-2008, 04:46 AM   #7
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Same thing

Catch in focus is the same thing as trap focus.
09-25-2008, 05:24 AM   #8
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QuoteOriginally posted by clm Quote
Catch in focus is the same thing as trap focus.
learn something new everyday

09-25-2008, 07:22 AM   #9
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to answer OP question the closest experience I have is with manual focus 58mm VL Nokton and my young children running in the back garden

I set K10D Aperture Priority and f2.0 (shutter was about 1/180) AF was on. Prefocused about 3m away and let the kids run about, camera shooting whenever it judged good focus. Mostly it worked fine (certainly better than my unaided MF would be at f2.0) - in the frames where they were running toward camera pretty much every image the shutter tripped too late/slow and focus was behind them

My next experiement is to try to use trap focus to catch birds in flight or leaving perch - figure if I use flash and MF lens prefocused from the side it would work but face on the bird would have moved too far before shutter/flash fires

So my guess about the freeway scenario is shallow depth of field (if you use f2.8) and the slight (unavoidable?) shutter delay will prevent the camera getting good images unless you have a big working distance that wont fill the frame

would not be hard to do the math on vehicle speed vs depth of field and shutter

be interested to see how you get on

Last edited by LittleSkink; 09-25-2008 at 07:25 AM. Reason: typos
09-25-2008, 12:50 PM   #10
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hmmm... these are not the responses I was expecting. My understanding is that there was a specific "CIF" setting on the K20D that would somehow work different from the trap-focus you can do on pretty much any other Pentax camera (I don't know about the other manufacturer).

In my case I'm not actually going to be taking pics of cars on a freeway but I was curious how people thought that example would work because it combined a very large target moving at a high rate of speed - If people expected 90% sucess rate with CIF (using whatever fancy function the K20D has) in this scenario I would consider the upgrade. In reality I'll be taking pics of bikes which are much more challenging (not a large, solid object) but with careful technique should produce similar results
09-25-2008, 01:14 PM   #11
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QuoteOriginally posted by kunik Quote
hmmm... these are not the responses I was expecting. My understanding is that there was a specific "CIF" setting on the K20D that would somehow work different from the trap-focus you can do on pretty much any other Pentax camera (I don't know about the other manufacturer).
My understanding is the only thing new about the K20D here is that you can actually use the feature with AF lenses if they have a AF/MF switch on the lens and you have it in the MF position.

But it's not gong to magically give you a faster shutter speed than you'd get with any other method, so sharpness hardly seems a relevant difference. The only question would be how much lg time there is between when the camera notices a subject in focus and when it snaps the picture - if that's too slow, the car/bike might be gone already. That is, catch-in-focus wouldn't really improve sharpness in this scenarios - it would (ideally) help you get more pictures when the car or bike is actually visible. Sharpness would still depend on shutter speed relative to subject speed.
09-25-2008, 01:33 PM   #12
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Perhaps with a bicycle, the front wheel could trip the focus but by the time the image is captured, the handlebars or thereabouts--where I'm guessing would be ideal--would be in focus.

I've never played with a k20D, but they do provide for front focus/back focus adjustments. Perhaps you could create a front focus "error" that would correct for the delay between the autofocus sensing focus and the image being captured? You'd have to know the speed and distance of your subject, and the adjustments might not be large enough, but it is one possibility.
09-25-2008, 01:54 PM   #13
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QuoteOriginally posted by Marc Sabatella Quote
My understanding is the only thing new about the K20D here is that you can actually use the feature with AF lenses if they have a AF/MF switch on the lens and you have it in the MF position.
AFAIK, this also only works w/ Pentax lenses....not e.g., with Tamron lenses which also have an AF/MF switch...
09-25-2008, 03:13 PM   #14
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Probably because the Tamrons only disengage the focus gears when set in MF mode. The Pentax lenses are SDM if they have a focus switch.

Sigma has a few HSM lenses, someone tried how they work?
09-25-2008, 04:58 PM   #15
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This might be of help in understanding the process - its from the K200D user manual:


From this it looks like it triggers when the AF Spot area is regarded as in focus.

Hope that helps

Cheers
Chris
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