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Old Apr 23, 2010, 8:37 PM   #1
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Default Catch-In-Focus

I have read about the "Catch-In-Focus" process for taking photos in darkened conditions. Can I perform this procedure using my K10d? If so, how, exactly, do I do it?
This is probably a dumb question, but it could be useful to me to know.
Thanks in advance.
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Old Apr 23, 2010, 9:06 PM   #2
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That's a good question OE, I don't have my K10 anymore but I don't recall having that option. it works pretty well on the K20.
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Old Apr 23, 2010, 10:10 PM   #3
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Quote:
Originally Posted by Old Engineer View Post
I have read about the "Catch-In-Focus" process for taking photos in darkened conditions. Can I perform this procedure using my K10d? If so, how, exactly, do I do it?
This is probably a dumb question, but it could be useful to me to know.
Thanks in advance.
Old Engineer
Hi OE,

Catch-in-focus AKA Focus Trap, works with all Pentax DSLRs. The Menu choice for the new cameras (I think) is to allow this feature to work with newer DA lenses with AF/MF switches on the lens, or to disable the feature if it's not wanted for some situation.

All you have to do is use a Manual Focus lens on the camera with the AF/MF switch on the body turned to AF-S, depress the shutter fully, then either turn the focus ring to focus -- or wait until something enters the frame at the preset focus distance that you've chosen. If you want to capture everything that might enter the frame at a preset distance over time, you can add a wired remote that can lock the shutter switch on, and walk away or manually hold the shutter button down. The camera will continue to take shots when it finds focus until you unlock the shutter or take your finger off the button.

If you want to use an AF lens, you have to short the data pin on the lens (the one closest to the screw drive, and farthest from the lens mount lock). Most have found that metal tape works best for this, but Aluminum foil works fine, though it's a bit fussy to mount the lens while holding the foil in place. With the pin shorted, the camera sees the lens as an "A" series lens, and AF will be disabled, but all AE functions will work. No harm will come from shorting the pin -- "A" lenses have no pin at this position, and the pin on the camera mount is shorted by the mount of the lens. The only possible problems would be form shredding the foil, and allowing pieces to get into the camera body.

The reason Focus trap works is that the AF mode on Pentax DSLRs is focus priority in AF-S -- the camera will not allow the shutter to fire if there is no focus confirmation, but if the lens is focused after the shutter is depressed, the shutter will fire at the first focus confirmation. I believe that this was discovered by users of Pentax AF film bodies, and Pentax has wisely chosen not to change it, and only recently has officially recognized it as a feature.

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Old Apr 24, 2010, 8:03 AM   #4
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Excellent description / instructions Scott.

I keep meaning to try this out but keep forgetting !
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Old Apr 24, 2010, 8:17 PM   #5
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Thank you. I kept meaning to look for how to do this.

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Old Apr 24, 2010, 8:48 PM   #6
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Thank you for that info Scott. I didn't know that.
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Old May 10, 2010, 2:23 PM   #7
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A few more cents:

Many PK-mount manual lenses have a nice shiny base that safely shorts-out the mount pins. Some bases are blacked, but that's easily sanded off. Many M42 lenses have blacked bases, and you might not want to sand them, but you can stick on a bit of thin metal tape intended for ducting repair. DO NOT PUT SUCH TAPE ON A PK-MOUNT LENS! The lens will be very difficult to remove from the camera.

Some M42 lenses, especially the old preset type, have very narrow bases that, even with metal tape, won't short the mount pins. To use these with trap-focus aka Catch-In-Focus (CIF), you need to put a bit of aluminium foil between the lens base and your infinity-focus adapter. I don't really like this because the foil tends to blow away when changing lenses.

NOTE: Cheap safe flanged non-infinity-focus adapters also cover the mount pins; if the adapter base is blacked, sand it. Such adapters are great when shooting close-ups or macros, where you don't *want* infinity focus. I often use such with a 200mm lens, where it brings the close focus in considerably, yet stays sharp out to 75m.

As previously mentioned, with CIF you hold the shutter down and either focus the lens or wait for something to come into focus. For instance, at a race or watering hole or sidewalk with predictable action, prefocus on a desired spot and wait for a subject to trigger the shutter. But wait, there's more! On my K20D I'll set drive mode to Continuous, plug a latching wired remote into the cam, and mount on a tripod. Aim the cam, prefocus where I expect a biker or bird or burglar to be, and latch the remote. Now, whenever something comes into focus, SNAP! And again, and again...

CIF ain't perfect. My K20D doesn't get focus confirmation in moderate light if the lens is stopped below f/11. I haven't tested to see if there's a specific EV cusp. I just know that with CIF, bigger apertures are better.
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