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Old May 15, 2010, 9:11 AM   #1
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I thought about having a play with this today, figuring it would be helpful at times for macro photography. I remember playing with it briefly on my K10D and an old Pentax M 50mm lens and it worked great.

Knowing it wouldn't work with the Tamron 90mm, as it lacks a focus clutch (and it says in the Tamron manual, do not leave the camera in AF mode and the lens in MF mode) I thought of trying it with the extension tube. Sadly it was a complete failure!

Even with a shot in perfect focus, the shutter just would not fire. Wondering it it was just the camera, I slapped on the old 50mm M lens, and it worked.

Now, I thought the extension tube would have looked exactly the same to the camera as an old manual lens. Neither has any contacts, so as far as the camera should be concerned, it shouldn't be able to tell the difference.

Anyone know what the problem might be? It's not a huge issue if it doesn't work, but with the extension tube on, the Tamron is a nightmare to focus manually.
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Old May 15, 2010, 9:17 AM   #2
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Please delete, got it working now.

But it's rubbish, works brilliantly on the Manual old lenses, but terrible with the Tamron...

Last edited by Tachikoma; May 15, 2010 at 11:19 AM.
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Old May 15, 2010, 2:51 PM   #3
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With the Tamron, have you used a bit of aluminium foil to short-out the contact pins on the lens mount? That's how I force any AF lens to seem manual, to employ CIF.
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Old May 15, 2010, 4:47 PM   #4
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Well I tried it with the extension tube, figuring it would do the same thing, and it sort of works... in my computer room with tons of light it works probably 50/50, but outside where there is less light, it just doesn't trigger at all, even at f2.8
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Old May 16, 2010, 12:46 AM   #5
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I do not know the Tamron, so I can't speak to its idiosyncrasies. But I've used CIF with various lenses on tubes (and off) on my K20D and have only noticed that there's generally a cutoff at about f/8-11 (effective). Without tubes, CIF won't work with smaller f-stops. On tubes or with an extended macro lens, I have to consider extension vs effective aperture:

E= F * (M+1)

where E is effective aperture, F is nominal f-stop, and M is magnification.

So if the extension gives me 1:1 magnification and the lens is set to f/5.6, then E = 5.6 * 2 = f/11.2 which is beyond the CIF cutoff.

I've used CIF with M (full manual), A (auto-aperture), and FA and DA (autofocus) type lenses, all using the tinfoil trick. As long as I shoot with the lenses fairly wide-open and the subject sufficiently lit, it's no problem. I have not used any lenses with AF switches so I don't know if that's an issue. You might want to ask at http://pentaxforums.com about this.
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Old May 16, 2010, 3:08 AM   #6
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Thanks for the info!

The Tamron is 1:1 already, with the tube I think it gives about 2:1 (not checked)

I think it is as you said and a problem with not enough light, hopefully when the weather gets better it will let me use CIF out and about, but at the moment it's not much use.

Thanks again.
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Old May 16, 2010, 4:00 AM   #7
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Quote:
Originally Posted by Tachikoma View Post
Thanks for the info!

The Tamron is 1:1 already, with the tube I think it gives about 2:1 (not checked)

I think it is as you said and a problem with not enough light, hopefully when the weather gets better it will let me use CIF out and about, but at the moment it's not much use.

Thanks again.
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Yes !, lack of light gives less contrast, so AF is less reliable. The technique you're describing BTW is also known as "Focus Trap" ... Jack.
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