Focal 135mm f/2.8 closer to f/3.5

Started Nov 2, 2012 | Discussions thread
OP math guy Senior Member • Posts: 2,476
Re: Focal 135mm f/2.8 closer to f/3.5

DrewE wrote:

That does indeed sound like some overly optimistic specification/labeling.

I guess I wasn't clear, but the lever I was referring to that may be misadjusted (and still could be, since the lens isn't giving consistent exposure results) is the one on the lens, not the one in the camera that connects with it. Pentax 'A' and newer lenses adjust the diaphragm by moving this lever, with a defined relationship between the distance moved and the number of stops below wide open that the lens is stopped down. Each stop ends up being maybe about 1mm or so of movement of the lever, so it doesn't take too much for it to be off enough to cause problems. (As others have mentioned, sluggish or sticky diaphragms can also cause inconsistencies in exposure.)

Incidentally, Pre-'A' lenses used the lever to open and shut the diaphragm, but didn't have a defined relationship for how much the diaphragm was stopped down when moving the lever to intermediate positions.

I think I'm convinced that it is, as you put it, optimistic labeling.

However, the next thing you said does make me wonder if I may have caused a problem myself. To remove the Ricoh pin, I had to take the mount off the back of the lens. Is it possible that I may have not aligned the lever you were talking about when I reassembled it? I tried to be careful to put it back exactly as it was, and there didn't seem to be another way to line up that lever. But now I wonder.

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-- Joe S.
"We make a living by what we get; we make a life by what we give." ~ Sir Winston Churchill

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