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05-15-2019, 07:52 PM - 2 Likes   #46
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QuoteOriginally posted by stevebrot Quote
The first two exposure should be uniformly dark across the frame. If the mirror is an effective blind, the third exposure will also be uniformly dark.
...and my results:

Control #1


Blind on, no light challenge. https://www.pentaxforums.com/gallery/photo-viewfinder-light-leak-control-1-57494/
Expected result is present!

Control #2


Blind on, light challenge. https://www.pentaxforums.com/gallery/photo-viewfinder-light-leak-control-2-57495/
Expected result is present!

Test #1


Blind removed, light challenge. https://www.pentaxforums.com/gallery/photo-viewfinder-light-leak-test-exposure-1-57496/
The test result indicates that light has leaked through the exposed viewfinder opening that would have been blocked by the viewfinder blind.

Oops!


Steve

05-16-2019, 03:26 AM   #47
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QuoteOriginally posted by M2358 Quote
If you look at the ground glass inside the camera, it typically will have light seal foam around it so that when the mirror flips up, it hits the foam. This serves two purposes: it reduces mirror slap vibrations so that a mirror lockup or mirror prefire feature may not be not needed in the camera, and it works in conjunction with the mirror to make a light-tight seal around ground glass, blocking stray light from the viewfinder window from reaching the meter or film.
The light meter on Pentax DSLRs at least is up by the prism. So mirror up will not prevent light from the viewfinder affecting the exposure. If i am using a tripod I always use manual exposure mode, taking a reading with my eye to the viewfinder. If there is strong sunlight behind me I will use my hand to shield the viewfinder during the exposure itself.
05-17-2019, 07:23 AM   #48
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There are two issues here:
1) Light leaks as demonstrated by @Stevebrot
2) Incorrect metering as noted by @pschlute

An image might see one or both of these depending on circumstances. Length of exposure in particular and also whether the viewfinder is shaded by you if standing close or there is sun or other light shining into it. For normal short exposure times it is rarely an issue, but with long exposures it must be accounted for in some fashion.

I really wish I could find a hot shoe mounted blind as shown earlier in this thread but I make do with a dark cloth.
05-17-2019, 07:31 AM   #49
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QuoteOriginally posted by jatrax Quote
1) Light leaks as demonstrated by @Stevebrot
Wasn't it you who actually demonstrated this in some landscape shots a few thousand posts ago?


Steve

05-17-2019, 08:03 AM - 1 Like   #50
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QuoteOriginally posted by stevebrot Quote
Wasn't it you who actually demonstrated this in some landscape shots a few thousand posts ago?
Yes, I was going to link to that thread but a quick search did not find it. I've managed to ruin a good number of long exposure shots by not paying attention to covering the viewfinder. This thread is from 2016 and just revived. I'm not sure when that landscape thread was posted.


Found it, here is that other thread: K-3 underexposes with ND filter - Page 2 - PentaxForums.com
6 Days Ago   #51
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With live view I don't really see the need for it, I focus through the viewfinder and then press the live view button.
6 Days Ago   #52
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QuoteOriginally posted by pentax360 Quote
With live view I don't really see the need for it, I focus through the viewfinder and then press the live view button.
There are two use cases. The first has to do with light leakage affecting meter readings when using exposure automation. Live view addresses that nicely. The second has to do with long exposures under daylight and certain other conditions where light leak through the viewfinder ocular and around the mirror into the mirror box are likely. The most common example might be use of ND filters to extend exposure times. Note that interval composite to accomplish the same goals as ND filters may produce the same risk. Live view does nothing to cancel this second case.

Read John's notes on his three example photos and then take a peek at the EXIF for the exposure times (cheat: 30 seconds) to see how real the problem is...

K-3 underexposes with ND filter - Page 2 - PentaxForums.com


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I didn't know that, good to know!
I even had one of those little viewfinder covers, and sold it. . . .

6 Days Ago   #54
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~~~~~~
They are mainly discussing the light coming in the viewfinder and upsetting the metering when your eye is away from the eyepiece.
~~~~~~~
Yeah, my reply was a bit hasty, but honestly, the only time Iíve even considered using an autoexposure mode with a camera on a tripod has been when it was to use off-the-film metering for TTL flash or a very long exposure metered in real time (with an Olympus camera supporting that).

In fact, if critical exposure is needed, establishing the exposure either with a handheld spotmeter or digital exposure histogram usually is best, and either of these techniques will normally involve shooting in manual mode anyway.
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