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02-22-2012, 08:03 PM   #1
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DOF preview with M lens and Av/Tv mode

I usually shoot manual, so I haven't noticed this before. Today the light conditions were changing (sun-torrent-rainbow-sun within 5 minutes) so I decided to set the camera to Av mode (then Tv mode to see if the problem persisted there as well).

Today I was shooting with a 50/1.7 M lens.

I realized that in any mode other than M, the DOF preview doesn't work. I can't think of any technical or logical reason why this should be so, so I went through the settings, but can't find anything that's related.

Is there something I need to do to enable DOF preview on M lenses in Av/Tv modes, or it doesn't work as a fact of tough luck?
Would anyone have any idea why it would be so?

TIA
Argun

02-22-2012, 09:06 PM   #2
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What Camera, it work on my K7, I just tried it with my 50/f1.7 Horusbennu and A 50/f2.0 (in manual aperture) on both AV & TV

Is Digital Preview on in shoot menu 3?
02-22-2012, 09:16 PM   #3
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I was talking about the optical DOF preview.

Sorry, I assumed those were clear.
02-22-2012, 09:24 PM   #4
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The only DOF preview I know of, is switching the on/off switch the otherway, if this is what you mean?

02-22-2012, 09:26 PM   #5
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QuoteOriginally posted by bdparker Quote
The only DOF preview I know of, is switching the on/off switch the otherway, if this is what you mean?
Yes, that's the one I mean, and the viewfinder goes dark.
It does in M, but not in any other mode
02-22-2012, 09:39 PM   #6
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in the manual and on the camera it is called digital preview, so beside DOF, you can check histograph and dark/ligh areas
02-22-2012, 09:47 PM   #7
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On the K-5 there is no on/off for Digital preview. Once you go in there are check marks for Hist, B/D and Enlarge.
This is for the back LCD. I am talking about the optical preview through the viewfinder, for which I can find no settings.
02-22-2012, 10:41 PM   #8
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Check Rec mode (the first one), menu 5, button customization, preview dial. You can set it to either digital or optical preview.

You must have it set for Digital preview. I tested it with my K5. If set Digital Preview, in Av or other modes twisting the on-off button gives me digital preview. In M mode I get optical preview.

Oddly enough, I set it to Digital in Av mode, tested it, switched to M mode, found that Optical worked. Then went in to set it how it was originally and it was in Optical mode in all modes. The setting changed by itself.

Anyways, set it to Optical and your problem should go away.

02-22-2012, 11:24 PM - 1 Like   #9
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You cannot reliably use M lenses in Av mode- that's probably the problem. See the bottom of the first post for the reason:
https://www.pentaxforums.com/forums/pentax-lens-articles/110657-how-use-meter...k-x-k-7-a.html

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02-22-2012, 11:45 PM   #10
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I see that, but still don't get the logic as to why the camera wouldn't stop down the lens.
The only explanation I can think of is: if, in Av mode, with lenses which report aperture, exposure is not measured as is, but wide open, and then calculated as to what it would be with the reported aperture.

Of course how this would work with old A lenses which don't have any electronics to report their prime aperture is still a mystery. Or is it all mechanical with the lens' aperture arms are in specific micro positions for each prime aperture?
02-23-2012, 12:04 AM   #11
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I guess it's a design decision- since Av mode it stops the lens down to what's electronically set, and you can't set anything electronically with M lenses, they probably just blocked the mode for consistency's sake.

If you do want to use Av mode with manual lenses, try some M42 lenses- then it'll be very handy as the lens will always be stopped down

A lenses do report their maximum and minimum aperture to the camera, which can then calculate how much to move the aperture lever when exposing, based on a mathematical curve

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02-23-2012, 12:39 AM   #12
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QuoteOriginally posted by Adam Quote
calculate how much to move the aperture lever when exposing, based on a mathematical curve
Total respect to the designers of the cameras who did this before there were chips inside them.
02-23-2012, 05:34 PM   #13
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In Av, an M lens only shoots wide open. So the DOF preview gives you the DOF you wil have with a shot, i.e. the lens wide open. Makes sense to me (given the constraints).
02-23-2012, 07:21 PM   #14
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QuoteOriginally posted by Nick Siebers Quote
In Av, an M lens only shoots wide open.
Yeah, and that also explains the crazy shallow DOF I seem to have gotten with those f11 shots.
02-23-2012, 07:58 PM   #15
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QuoteOriginally posted by atekant Quote
Total respect to the designers of the cameras who did this before there were chips inside them.
1980'es technology! That's why there are quite a few electrical contacts on the mount - so as to have enough to encode the many combinations of smallest f/stop and f/stop range!
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