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01-28-2008, 11:15 AM   #1
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Trap focus, K10d, s-m-c TAKUMAR F1.4

First of all, let me say that I'm new to the Pentax world although I've been using Canon, Nikon and recently Panasonic for over 40 years I had all my gear stolen recently so when It came time to replace it, I looked very carefully at all the current DSLR's. I decided on the Pentax K10D, and, having used it for a month now, I am very impressed. I bought it with an 18-250 Pentax lens which works very well, particularly with the camera set to "Program line".

However, I recently got a Super-Multi-Coated TAKUMAR 50mm F1.4 lens and a Pentax screw mount converter. First of all let me say that the lens itself is wonderful, absolutely wonderful. And I was quite amazed at how well the old screw mount lens worked with the new high tech digital camera. Kudos to Pentax for a job well done.

However, I have read about the "Trap focus" functionality but I have been unable to get it to work. I have searched the forums but I'm still not sure why it does not work. Is it even supposed to work with this lens? If so, can someone please tell me what I might be doing wrong.

Thanks,

Mike

01-28-2008, 11:26 AM   #2
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Switch the focus mode to AF.S

Depress shutter button fully, rotate focus ring until shutter releases.


The shutter will only release when it has focus confirmation.
01-28-2008, 11:28 AM   #3
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For trap focus to work, you need to have teh shutter release interlocked with focus indication, I, You need to be in AF-S (i think) but the lens in manual focus, or a MF lens

Prefocus on a spot which youo want for the ohbject to be in sharp focus

Then you really need a cable release that can hold the shutter pressed for long periods. The pentax one can do this, and there are threads on this forum of making one yourself.

Whe something comes into view, the focus indication should then trip the shutter.

Note that for this period the camera is on and powered and battery life is not all that great. you may wish to consider the exteranl power.
01-28-2008, 11:29 AM   #4
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It's fairly simple. but the camera must be set to AF.S (auto focus). The shutter button is held down and then you focus the subject. When the cameras 'sees' the subject in focus, the shutter will fire. Great for fast moving subjects. Just focus to a spot the subject will move into and when it does and the focus indicator lights, the camera will fire the shutter. Great for macro and bug shooting as well.

You can do it with AF lenses as well but you need to place a shim or tape over the electrical contacts so the lens does not communicate with the camera.

01-28-2008, 11:31 AM   #5
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That is what I am doing. But the shutter does not lock. It just takes the out of focus picture as soon as it is pressed. I have tried every mode setting on the dial also, with the same results.
01-28-2008, 12:06 PM   #6
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So the Auto focus is turned on?? If so and you can't get this to work, do a quick test. Take some electrical tape (don't use anything like duct tape or other overly sticky tapes) and cover the electrical contacts on the camera mount. Then remount the lens and try it again. It's possible that the lens body is shorting the contacts and the trap focus will not work.
01-28-2008, 12:14 PM   #7
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Well, I have looked carefully at the lens and it seems that it does not short out any of the contacts. It is seems to be wide enough to just touch them but they are not shorted. By shorting them with a piece of aluminum foil, the trap focus works. I'm not sure whether the lens is not wide enough or if it just the paint on the lens acting as an insulator. I'm going to take a piece of 400 wet/dry and gently sand off a bit of paint and see if that helps. Otherwise, I'll have to fabricate a simple aluminum ring that can fit and short then out.
01-28-2008, 12:19 PM   #8
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I never use the focus trap connollymk, but some members here prefer to short out those contacts anyways saying that it helps with the metering.

So since it works why not try it for a few days and see if it's what you want.

Oh, sorry to hear about your old equipment getting stolen.
You did get a good replacement system out of it though.

01-28-2008, 12:50 PM   #9
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Well the sand paper trick worked. I just sanded off the black paint from the very edge of the lens and that was enough to short the contacts. And even if you look closely at the lens, it looks like it was designed and built with that little bit of paint missing.

Thanks for the quick responses. And, yes Stu, I am very happy with my new rig.

On another topic, I have been following the K20D threads with some amusement, particularly with respect to the FPS aspect. My amusement come from looking back 30 years or so when the MD-12 and 15 Nikon motor drives were the best. We had no trouble with them shooting car races, basketball, hockey games etc. And they were only 3 FPS. Funny from my point of view.
01-28-2008, 01:03 PM   #10
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Hi connollymk

So you are getting spot on focus now? I hve been trying for a month & I know others have & mine takes the picture beyond the point that I wish to focus on regardless of what the camers tells me.
01-28-2008, 01:37 PM   #11
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Spot on focus

Well, I just tried some samples and they went as follows. If I focussed too quickly, nothing happened. If I took my time and started from infinity and worked back, it seemed to work perfectly. But if I started focussing close-up and then moved out towards infinity, I had more pictures out of focus. However, I'm not sure how much that has to do with the camera and how much it has to do with me losing concentration and just moving past the focus point as the picture was being taken. In any case, it worked fine for me moving from infinity in. My guess is that if you get it close to in focus coming from either direction and then slowly focus in, it will work well either way. I'll play around some more later and post my findings.
01-28-2008, 01:47 PM   #12
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Here is how it seems to work best for me. I focus in quickly until it seems pretty close then press the shutter button half way down and just rotate the focus ring slowly until it fires. It seems to fire as soon as the red light goes on and the pictures are in focus.

I assume that you could move the focus ring faster if the DOF was greater but with the 50mm f1.4, the DOF is quite small. I'll try faster focusing with another lens when I get a chance.


thanks again for the quick responses.
01-28-2008, 02:39 PM   #13
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I also have a 135mm f2.8 that I paid $15 for including a hard case. So I tried it. It does not screw in all the way to touch the contacts so just sanding off the paint does not work. So I'll have to make up a little spacer. No problem.

Anyway, I tried it out with aluminum foil shorting the contacts. I set the aperture to f8 and sure enough, it was easy to focus much more quickly and have the trap focus shot turn out fine. The DOF was so much greater that using this lens this way will be a piece of cake.

I haven't had this much fun with the technical side of cameras in many years. I think I'm becoming a big Pentax fan.
01-28-2008, 07:13 PM   #14
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Thanks for the info; I'll try it later. It doesn't look as though a tripod will be an option with this lens though.
01-13-2010, 06:48 AM   #15
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Hey all,

Old thread, I know; but I just learned about this "trap focus" technique, so thought I'd try it out last night. I tried with a Pentax-A 50mm f2, and with a Sears 135mm f2.8, both wide open (my goal is low/moderate indoor light photos of a staged production).

Trap focus worked for me, but results were not as good as I'd hoped. I tried coming at the focus point from both far and near, both adjusting quickly and slowly. Because of low light conditions, I was not able to get a decently-exposed shot in less than 1/15th second, and was hand-holding, so was likely not able to stop adjusting focus and be still enough to get as good results as I could have; however, I had greater success with manually "focus bracketing". By focus bracketing I mean switching to MF, getting to where I think focus is best, backing off a bit, then, while holding down the shutter in continuous mode, moving focus to and past the point I thought was best. I fire off between 6 and 10 shots, and every time I tried this, I got one or two well-focused shots, which always were in better focus than the one I'd taken using trap focus.

My testing was limited to wide open aperture, in only one lighting condition, with only those two lenses. I will likely use the focus bracketing technique at a dress rehearsal of a play my kids are in, with the 50mm lens, next week.
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