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12-22-2014, 07:25 AM   #1
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Long exposure question

I've heard that the viewfinder on long exposure needs to be covered as stray light entering the viewfinder could mess up the exposure.

Is that because:
1. The stray light can confuse the metering?
2. The stray light can spill on to the sensor impacting the quality of the image (beyond metering)?
3. Both
4. Something I didn't consider?

I hope this doesn't look too much like a test.

Thanks!

12-22-2014, 07:57 AM   #2
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I found this article, petty good explanation.
12-22-2014, 08:09 AM   #3
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I think the answer is mostly #1 plus a bit of #2. It can vary between camera models.
12-22-2014, 08:28 AM   #4
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I'll check it out remseybuckeye

Thanks DeadJohn. That means that even if you meter it properly and set it to manual based on those readings, taking your eye away from the OVF, even while leaving the eposure setting in tact, still leaves the image vulnerable to degradation from the light leaking in through the viewfinder

12-22-2014, 08:30 AM   #5
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Number 2 isn't going to occur with any SLR since the mirror is up during the exposure and no light can reach the sensor from the eyepiece while the mirror is up except with leaky seals.

Number 1 May have a similar answer as there isn't any point in metering from the viewfinder during the time the mirror is up so I would expect that to be turned off at that time, but I can't confirm it.
12-22-2014, 08:49 AM   #6
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OK. So, based on this (I read the article too), it looks like if you set the camera in manual to the proper exposure, it won't matter if the the eyepiece is covered or not, assuming that the camera is in proper working order.
12-22-2014, 09:41 AM   #7
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QuoteOriginally posted by UncleVanya Quote
Number 2 isn't going to occur with any SLR since the mirror is up during the exposure and no light can reach the sensor from the eyepiece while the mirror is up except with leaky seals.
...
that is not exactly true. Some light can leak from the eyepiece onto the sensor causing ghosting, particularly when using ND filters and when there is some backlight present. This is documented in most brands of SLR (with an eyepiece).

I shoot long exposure landscapes, often with NDs, and when i've forgotten to cover or physically block my eyepiece, my images have been ruined.
12-22-2014, 09:53 AM - 1 Like   #8
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mikeSF - That was the kind of info I was looking for, even if I didn't want to hear that that was the case Thanks

12-22-2014, 10:28 AM   #9
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QuoteOriginally posted by UncleVanya Quote
Number 2 isn't going to occur with any SLR since the mirror is up during the exposure and no light can reach the sensor from the eyepiece while the mirror is up except with leaky seals.
There are reliable reports and sample photos of cameras from several manufacturers letting light get into the camera through the VF, lens mount, or body seams. Even a tiny light leak can degrade very long exposures with ND filters. It's also a concern for night photos if light from passing cars hits the back of the camera. Most users will never have an issue with a minor leak because it needs long exposures.

One way to test is a 30 second image at 12800 ISO with the lens cap on in a bright room or sunlight. The image should be black except for random noise. Bright spots can be light leaks or amp glow.
12-22-2014, 01:48 PM   #10
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My camera came with a little removable cover for the eyepiece, but I have lost it. I just lay a small black towel over the back of the camera & eyepiece when doing long exposures.
12-22-2014, 03:18 PM   #11
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I have that too Wingincamera. I was checking to see if it is necessary to use it if you use manual exposure. According to mikeSF, light leaking in through the OVF can act as a light leak on the image. So, I guess the moral is to cover your OVF if your eye is not planted on it, even if you are using manual exposure
12-22-2014, 03:40 PM   #12
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Here is a link to a series of posts I did on long exposure tests of my K-3 at various high ISOs:

https://www.pentaxforums.com/forums/29-welcomes-introductions/268543-hello-lo...profiling.html

Jack
12-23-2014, 06:40 AM   #13
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Did you cover the OVF? That's the question I was asking about wrt to long exposures. Thanks

QuoteOriginally posted by jbinpg Quote
Here is a link to a series of posts I did on long exposure tests of my K-3 at various high ISOs:

https://www.pentaxforums.com/forums/29-welcomes-introductions/268543-hello-lo...profiling.html

Jack
12-23-2014, 02:56 PM - 1 Like   #14
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As mentioned earlier, there is no guarantee that you'll have a problem with long exposures and leakage through the OVF. It would probably be good to test it out if you're concerned. Take a long exposure with the lens cap in some light and see what you end up with. If the image isn't black or has some light, then that is the leakage.

My K5 never had issues. I tested it a few times. I also rarely shoot long exposures out in the light. I'm not one to use a ND filter for much. Long exposures at night are rarely an issue because there usually isn't enough light to get into the camera from view finder let alone the lens.
12-24-2014, 11:23 AM   #15
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If you are not aware, the Pentax LX (film SLR) is the only camera ever made - by any manufacturer, that can aperture priority auto expose a scene for as long as it takes while monitoring the scene for changes in lighting and adjusting exposure accordingly. Since it takes it's exposure off the film plane, it doesn't require a cover for the viewfinder and stray light entering there does not influence the meter.

I have successfully conducted low light shots with very long exposures as shown below.

This one is on Kodak Ektar 100 for about 40 minutes . . .




This one on Kodak Gold 100-7 for more than 3 hours . . .

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