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03-21-2011, 05:47 PM   #1
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Simple question about the 67 105mm lens

The 67 105mm lens has a click stop between the marking for f/2.4 and f/4. I assume the click stop is for f/2.8? Can anyone confirm this as I have the lens but no manual.

03-21-2011, 06:11 PM   #2
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Yes, it should be F2.8. Pentax had a habit of doing that on a lot of lenses. ..

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Last edited by Parallax; 03-22-2011 at 01:21 PM.
03-21-2011, 07:46 PM   #3
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Adam, thanks.
03-22-2011, 12:53 PM   #4
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Edit: Ops, double. See below.


Last edited by Makten; 03-22-2011 at 01:13 PM.
03-22-2011, 01:12 PM   #5
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I don't think it's f/2.8. Look at the aperture from the front of the lens with the lens wide open and stopped down to the first click and then to f/4. The middle click is much closer in diameter to f/4 than to f/2.4, so I suppose it's f/3.1 or so. Which would make sense, since that's right in between f/2.4 and f/4 exposure-wise.

Edit: Oh, I had to do the math...

2.4*(sqrt2)^(1.5/2) = 3.1

Why? Because the difference in stops between f/2.4 and f/4 is...

2.4*(sqrt2)^x = 4 ... -> ... x = ln(4/2.4)/ln(sqrt2) = 1.5 stops
And half that is of course 1.5/2

Last edited by Makten; 03-22-2011 at 01:19 PM.
03-22-2011, 04:26 PM   #6
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Makten, that is why I am asking the question. Like you I am not sure the aperture is not in equal steps between f/2.4 and f/4. Does anyone have the manual?
03-23-2011, 04:56 PM   #7
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The lens manual shows the half click between f/2.4 and 4 to be f/3.4. I don't believe this is correct because that is a full stop. I would have to agree with Makten on this. Probably f/3.1 which is 3/4 stop past f/2.4. Also, the meter readings do not change a full stop when going from f/2.4 to the half click.

Last edited by desertscape; 03-23-2011 at 05:11 PM.
03-23-2011, 05:22 PM   #8
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Thanks folks. It looks like a mystery stop. I have a Pentax camera on the way and will have to test.

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