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Old Nov 28, 2010, 12:02 AM   #1
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Default Tamron lens question

1:2.5 135mm Close Focus 18 BBAR MC
Uses an Adaptall II mount.
1st the lens has an "A" setting (well it says "AE" on the Tamron) but the adapter does not seem to support this (no electrical contacts). Is that normal?

2nd the (for lack of a better term by a armature) Iris Lever, it seems to work backwards, what I mean is when I turn the aperture ring, it stops down right then. When you activate the lever (looking at the back at the adapter, camera side, clockwise activate it) the iris opens up. For some reason I thought the iris was suppose to be wide open until that lever was activated.

Can anyone enlighten this poor soul?

Thanks
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Old Nov 28, 2010, 1:39 AM   #2
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Hi fofa,

There are 2 different Adaptall 2 adapters for the K mount, the PK and the PK/A. The second has the contacts, and allows Auto Exposure and control of the aperture through the camera body. The "AE" position on the aperture ring is only relevant if you have an adapter that supports it.

The aperture lever is working correctly. It's quicker and more efficient to allow the return spring in the iris mechanism to close the aperture for exposure than have the camera's actuating mechanism do it, so the normal state of the blades with the lens off the camera is stopped down to the smallest opening, and open to the widest against the spring tension.

Scott
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Old Nov 29, 2010, 7:30 PM   #3
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Doesn't the camera actuate the lever when a picture is taken? If so then would it not take a picture with the fully open iris every time?
I also have a Vivitar 28-70mm and I can set the F stop to whatever i want, and the iris does not close until the lever is actuated, or just the opposite of the Tamron. But the Vivitar does not use any adapter, it is a PK/A mount.
In my feeble little mind it would seem to make sense to have the lens wideopen until the picture is taken (or the AV button) so it would be easier to focus, that kind of thing.
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Old Nov 30, 2010, 1:20 AM   #4
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Hi fofa,

"Automatic" lenses -- those that allow focusing and metering wide open, then stop down the lens just before the exposure do this, but manual lenses -- ones that need to be stopped down for metering don't necessarily.

Before the automatic lens, they made "preset" lenses (actually still do). With these lenses, there's a switch on the lens. You first set the aperture ring for the desired opening and meter the scene, then activate the switch, which opens the iris all the way to make it easier to focus. Before taking the shot, you'd switch back, snd the iris would return to the "preset" aperture that you originally set.

Instead of going into all the different ways that aperture actuation might be effected, suffice it to say that there have been different schemes that camera mfgs have used to control the aperture blades as systems evolved, and third party lens mfgs did what was easiest for them to adapt their lens designs to work with all of these systems. It's possible that different brands of third party lenses might work essentially differently from each other, but as long as they work with your camera, it really doesn't matter.

BTW, I think I found out what was the problem with your exposure problem and answered the thread in the other Pentax forum. . .if you haven't checked in on that thread.

Scott
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