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Old Aug 4, 2007, 12:18 AM   #1
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It works, and is a fairly convincing way to show what is required for focus trap on the *ist DS (and maybe other models) and fully manual lenses.

I made a 'gadget' which consists of an aluminium ring (cut from the body of an old teleconverter) fitted with a lens mounting flange and a camera body socket. It looks and functions exactly like a fully manual macro extension tube.

The difference in this one is that I fitted an electrical contact to match with the 'A' contact on the front of the camera, if you look at the camera (with the lens off) one of the contacts is below the surface of the camera flange, it is the third contact counting from the lens mount lock.

I have found I can get focus trap with any manual lens provided this contact is grounded and although others have mentioned success using tinfoil I shudder at the thought of scraps of conductive tin foil getting inside the camera so I looked for another way.

It would be pointless me posting pictures of how I did it as anyone who would attempt the same would surely have the skills to figure out how to do it with whatever they have on hand. My contribution is to tell you how it works!



The total thickness of my adapter is 14.2mm which is really much thicker than ideal, it was however the thickenss of the piece of metal I had on hand! The consequence of extra thickness is loss of distant focus. My 400mm 5.6 Sigma prime has a maximum focus of about 15 metres, my 300mm Takumar f4 (as one would expect) cannot even focus that far. This might or might not be a problem depending on what anyone might like to use the focus trap for and I tend to suspect it will be more useful at close range that way out there.

This adapter goes directly onto the camera and grounds the camera's 'A' pin and the manual lens is mounted to the adapter.

There is only setting I have found the camera to give focus trap (even with this gadget) is with AF set (though the lens is not AF and the camera detects this apparently the switch must still be in the AF position. I have been into setup and set for continous focus and for spot focus, I have not experimented with changing either of these. I set the mode knob to 'Auto Pict' which is the green position.

With this setting and the 'A' contact grounded pressing the shutter button fully down does not cause an exposure unless something is in focus in the spot focus zone. This could be when the manual focus brings the subject into focus or when some object moves into the focus zone. I expect the latter is how most people would use focus trap.

Sitting with my finger on the button waiting for the elephant to come down the trail might get a little boring (especially around these parts) so I made a remote switch to do that.

The camera has a socket for a miniture phone stereo plug which has three connections. The tip contact is (I think) the shutter, the sleeve contact is focus and the outer is ground. I found an old switch which is exactly what I needed but anyone else might have to do something different. The requirement is that there is either two switches or a three position switch. The first position grounds the focus wire, the second grounds the shutter but it is just a little tricky in that the focus must remain grounded while the shutter is fired.

I have a lot of pictures of myself from testing this,:G especially when I set the drive mode to continuous and walked in and out of the focus zone. I was a little suprised to see these pictures to be accurately exposed (as shown on the histograms) as the camera was getting totally false information regarding the lens aperture and of course was powerless to stop it down.














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Old Aug 6, 2007, 4:10 PM   #2
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Ho hum... I thought someone would have had something to say.:roll:
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Old Aug 6, 2007, 8:55 PM   #3
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Sorry John. Your results are interesting but such an experiment is far beyond my abilities. I can't think what I would add.

Tim
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Old Aug 7, 2007, 1:17 AM   #4
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Thanks Tim.
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Old Aug 9, 2007, 5:53 PM   #5
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Is this similar to when I accidentally left my K10D in auto-focus mode with a manual focus KA lens, and it would only shoot when it was focussed correctly? I could twist the focus ring all day, but it would only release the shutter when it thinks it's in focus (which it generally was when it releases it).
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Old Aug 10, 2007, 3:12 AM   #6
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Yes, that is what I have been talking about. The camera is in a state where it will not take the exposure because it is not in focus, then something moves into focus and the exposure occurs.



I am trying to get a set up with my camera on a tripod, angled up into the sky and focused at a short distance. The I will open the packet of fish and chips, eat the fish and throw the chips into the air and hope for a seagull to intercept one in the focus zone!



Dont hold your breath though as these 'schemes' have a habit of not happening quite the way I would like!:lol:
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Old Aug 10, 2007, 4:10 AM   #7
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John Hill wrote:
Quote:
I have a lot of pictures of myself from testing this,:G* especially when I set the drive mode to continuous and walked in and out of the focus zone.* I was a little suprised to see these pictures to be accurately exposed (as shown on the histograms) as the camera was getting totally false information regarding the lens aperture and of course was powerless to stop it down.
John

Sounds workable if you have enough DoF. Would appreciate if you can post some pict to show the quality of shots with your setup. I suppose your set up will not electrocute the intended subject

Daniel
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Old Aug 10, 2007, 4:14 AM   #8
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Sounds like a plan!

There's lots of possibilities for this sort of thing, you could use it as a makeshift intruder detector, set it up with a mains adaptor, and it will take a photo of whoever walks past the camera... if only the things weren't so blooming loud...

I think I will have another look at this "feature" on the weekend, see what can be done with it.
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Old Aug 16, 2007, 10:30 AM   #9
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Just needs a switch mounted to the extension to turn it on and off...and a small LRD to indicate off or on. Mass market it starting with EBAY.

Dawg
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Old Aug 16, 2007, 4:19 PM   #10
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john.
not sure wh7y you posted this. is it a way to get focus trap to work with M42 lenses.
i can't get it to work using my 500mm/f4.5 and it has a K mount.

roy
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