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01-01-2013, 04:56 PM   #1
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Lens markings off on 35mm lens?

Bought a 645 A 35mm manual-focus lens from Keh. Seems to be working perfectly; testing it now on my 645N with a roll of film.

But noticed to day that the f-stop markings on the lens don't exactly correspond to the f-stop information in the viewfinder. For instance, if I turn the lens aperture ring all the way to the f/22 marking... to the last marked stop before going to the A setting... the viewfinder says f/19. The viewfinder seems to be 1/2 stop different all the way down to f/3.5. At f/3.5, the aperture ring marking and the viewfinder aperture are the same.

Here's the range... at left, the viewfinder aperture information... at right, what the little diamond is pointing at on the aperture ring markings.

3.5 -- on the f/3.5 mark
4.0 -- halfway between the f/5.6 and f/3.5 marks
4.5 -- on the f/5.6 mark
5.6 -- halfway between the f/8 and f/5.6 marks
6.7 -- on the f/8 mark
8.0 -- halfway between the f/11 and f/8 marks
9.5 -- on the f/11 mark
11. -- halfway between the f/16 and f/11 marks
13. -- on the f/16 mark
19. -- on the f/22 mark

If anyone else has a 645 A 35mm lens and a 645N, could you check and see if your lens & camera are doing the same thing?

Any insights on this would be appreciated. In real life I don't think it's a problem, and I'll trust the viewfinder display. But I'm curious. Did Pentax mis-mark the lens aperture ring? Is f/19 really the minimum aperture for this lens?

And, Happy New Year to everybody!

01-01-2013, 04:57 PM   #2
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I can only tell you that it does the same thing on my 645D
01-01-2013, 05:24 PM   #3
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Thanks Ed -- that's good to know. So... I'll bet that you trust the viewfinder readout when taking pictures... and haven't had any problems?
01-01-2013, 05:39 PM   #4
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To be honest, I generally expose using test shots and the histogram, so the actual read out makes little difference to me... On the rare occasions when I shoot simply using the metering as it stands, I haven't found a problem. But then I shoot raw. If I shot slides, where half a stop is critical, might have been more of an issue...

01-01-2013, 05:49 PM   #5
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I'm using negative film... 1/2 stop won't ever be a problem. Looks like this is a non-issue.

Will return to my New Year's resolution... getting Pentax to create a digital 'film' insert for those of us with film 645s! With a little LCD screen so we can also enjoy that histogram.

Thanks again for the fast help, Jon
01-02-2013, 04:52 AM   #6
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With the selector on my 645N body at A, the diaphragm numbers on my A 35mm correspond exactly to what is displayed in the viewfinder. No discrepancies.
01-02-2013, 04:41 PM   #7
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Thanks Smolk. Guess we still have a mystery. Anybody else with a 645 and a 35mm lens?

On mine, it will go to f/22 in the viewfinder if I push down the silver button next to the green A on the lens ring, and then gently turn the lens ring a little... but if I turn it just a little further, it clicks into the lens ring f/22 stop... and my viewfinder indicates f/19.

In practice, I think this is not too important. Went into town today, took a couple of pictures, sunny daylight, on full automatic, and the 645N picked 1/350 @ f/13, with IS0 400 film. And, from what I've read, the viewfinder's indicated 1/350 may not be exactly right... that the 645N has a 'stepless' shutter... so the viewfinder just shows you a rounded-off number that's closest. Works for me!
01-03-2013, 09:31 PM   #8
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IIRC, Ken R mentioned that problem on his web site as well.

01-03-2013, 11:01 PM   #9
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Bob -- I just looked there (ken Rockwell's site), couldn't find it... if you can point me in the right direction, would appreciate it!
01-04-2013, 06:43 AM   #10
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I just spent a few minutes looking for that comment on KR's site and a few others, I know I read about a lens doing that somewhere, just can't remember where.
When I find it I'll post the link here.
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