Light entering camera via viewfinder

Johnny1982 said:
I can't see it hitting the sensor. Its illuminating the chamber box causing a fog.
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I'm kinda struggling with this "fog" concept?

what fog is it causing? surely that would require there to be some "thing" in the air for the light to bounce off and cause the light bleed?

I can see light entering in through the viewfinder, bouncing around the pentaprism/mirror, through the focussing screen (where it could get dispersed somewhat) and through some semi-transparent parts of the mirror into the sensor area.

I don't hink much light is likely to follow this path, but on a long exposure, then there's lots of time for not much light to go there., esp if the viewfinder is facing a strong light source (like the sun, or a torch).

dave
 
fogging is just an old technical term for light leakage. It was used when light reactive paper becomes useles due to fogging.

light can be forced into the chamber but I personally can't see it returning to the sensor to become a direct sunspot.
It'll prob hit the rear of the lens element.
Anyway I'm not expert. I am just taking an educated guess.

I did a test too.

several shots with direct sunlight over the eyepiece for a F4/40 second exposure.

test1.jpg


same settings again but using a 580 EX on low power executed on a 5hz repeat for 40 seconds

test2.jpg
 
Johnny1982 said:
What have you done light wise to generate the above image?
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Turned my back on full sunlight so full sunlight enters the eyepiece. No PP apart from a resize to 600px and some jpeg compression. I got a similar result from shining a torch directly into the eyepiece - not as strong an effect but the pattern of the "image" is almost identical. Camera is a D700.

Just done the same experiment with the D70 and there's no sign of any leakage even on a 30 second exposure.
 
In some film slrs, that had the metering in the pentaprism this was an everyday problem.
Some got over it with two backward facing cells to compensate for stray light entering through the eye piece.

These days I only bother to cover the eyepiece, if strong light is falling on it, during tripod work. Though I have never had a problem when I forget.
 
Johnny1982 said:
again I have to explain..
You can't get direct light spill onto the sensor. light travels in straight lines only. It can't turn corners.
You can get spill through the upper chamber resulting in fog but not direct light.
I never said it wasn't possible I was explaining things a little differently.
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Light does travel in striaght lines, but not all straight lines are parallel to each other. With light entering the eyepiece at all different angles it will scatter into the body at different angles.

If it was fogging from internal reflection/leakage it would be all over the sensor surely? Having a clear line of different exposure shows that at least part of the light leakage is direct
 
In practise, has anyone ever noticed either light patches or fogging when NOT using an ND filter? Or are we worrying about a hypothetical problem? (Shining a torch into the viewfinder, or firing a flash gun, or exposing with the lens cap on, does not count ;) )
 
HoppyUK said:
In practise, has anyone ever noticed either light patches or fogging when NOT using an ND filter? Or are we worrying about a hypothetical problem? (Shining a torch into the viewfinder, or firing a flash gun, or exposing with the lens cap on, does not count ;) )
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Read Jambo's post (#12) - he noticed this when using an ND grad for a real shot (not as an experiment to prove something) and it was a vastly significant problem. I even linked the original thread where there were examples posted
 
arclight said:
I saw reference to this in a post relating to the use of ND filters for long exposures.
The posts referred to results being wasted unless the eyepiece was blanked off during the exposure. Some cameras have a facility to do this easily.

Can someone explain how light reaches the sensor from the viewfinder during an exposure?

I don't recall that being a problem when I had film SLR's because the raised mirror formed a light seal at the base of the prism to ensure that only light passing through the lens reached the film (otherwise it would fog).

If light can reach a DSLR sensor when the mirror is raised then why does it not affect all captured images to some degree.
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Hya,

Has happened to me ALOT. If I use my 10 stop filter, and leave the viewfinder open, I normally get an extremely bright splodge of purple, which bleeds randomly across the image. I suspect due to the 30 seconds to several minute exposures, the camera whilst normally fine, allows light to drown the chamber - extremely slowly though. I guess it's why they have the option of closing the eye piece.

Gary.
 
I have several examples at home. Shooting long exposures during the day is the best way to see how extreme this problem is.

Think of it like this.

10 stop over the lens. Nice and sunny outside. F22. You could in theory get a minute or so of exposure.

Now over that minute, sometimes direct sunlight, and at other times indirect sunlight, can enter through the eye piece. I have no idea what it does from that point, but I promise, it DESTROYS the image, and in NO subtle way. You will find a massive purple blotch, with ZERO detail, and extremely bright. It twists and fades across the image in strands. It is horrific, and the simple solution seems to be, shut the viewfinder. I must have at least 10 examples of this problem.

Another thing I noticed. With the SCREW IN filter, and NO lens hood, the same purple light can be seen, in MUCH smaller quantities, around the edges of the frame (as opposed to in the middle). This means even the screw in filters are allowing light through, if the exposure is long enough. Solution? Lens hood :)

I will dig out some examples from home. This problem increases, the longer the exposure. I don't think I have seen it on exposures less than 5 to 10 seconds.

Gary.
 
HoppyUK said:
In practise, has anyone ever noticed either light patches or fogging when NOT using an ND filter? Or are we worrying about a hypothetical problem? (Shining a torch into the viewfinder, or firing a flash gun, or exposing with the lens cap on, does not count ;) )
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Don't think the problem has been fully bottomed out yet to enable a conclusion to be reached. The examples relate to long exposures with low ISO's.
Interested to find out at what length of exposure does the problem disappear and also whether using high ISO's causes it to appear at shorter exposures (seems likely).

It certainly has me wondering if there could be instances of slight image degradation that we have mistakenly attributed to other causes, e.g. a pic looking a flat or with muddy colours etc. Nothing drastic, but just not as good as it might have been.
 
arclight said:
Don't think the problem has been fully bottomed out yet to enable a conclusion to be reached. The examples relate to long exposures with low ISO's.
Interested to find out at what length of exposure does the problem disappear and also whether using high ISO's causes it to appear at shorter exposures (seems likely).

It certainly has me wondering if there could be instances of slight image degradation that we have mistakenly attributed to other causes, e.g. a pic looking a flat or with muddy colours etc. Nothing drastic, but just not as good as it might have been.
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I will do some tests, easy to replicate the problem. I suspect it's nothing to do with apertures as such, but more to do with how long the exposure is (which is directly linked to the aperture). I think I only see it at 10 seconds +, but can't remember. The brighter it is, the more the problem is apparent. I think in dull or overcast conditions, it might not be an issue at all. Also, I remember the problem on Skye. When the sun was behind me, it MASSIVELY increased the problem, shooting towards the sun? Much less of an issue. Same day, same location. Ruined two of my Skye shots.

Gary.
 
Just a thought, on long exposure the sensor will get hot, could this heat be causing issues?

In astro photography it is common to cool the sensor, but those are for really long exposures.
 
Martyn... said:
Just a thought, on long exposure the sensor will get hot, could this heat be causing issues?

In astro photography it is common to cool the sensor, but those are for really long exposures.
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Martyn, you get some hot pixels and noise from the heat, but I've shot for 20 minutes with the viewfinder uncovered on an astro shot, and not had this effect. But I've not shone a light at the back of the camera either, and there isn't enough ambient light incident to affect the shots.
 
EdinburghGary said:
No, I have done 30 minute exposures, and if the viewfinder is closed, no issues at all.
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jgs001 said:
Martyn, you get some hot pixels and noise from the heat, but I've shot for 20 minutes with the viewfinder uncovered on an astro shot, and not had this effect. But I've not shone a light at the back of the camera either, and there isn't enough ambient light incident to affect the shots.
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Thats OK then, always best to rule out all possibles.
 
Uneducated_Rick said:
Read Jambo's post (#12) - he noticed this when using an ND grad for a real shot (not as an experiment to prove something) and it was a vastly significant problem. I even linked the original thread where there were examples posted
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The original problem occured when using a very strong ND filter, not a grad. According to the evidence so far, in practise this problem only occurs when using very long exposures in bright ambient light. A situation that only really happens when you use an ND filter.

So the question is still valid. Has anyone definitely noticed this problem at any time when NOT using an ND filter, or are we worrying unnecessarily?
 
Using an ND filter does not make it a hypothetical problem nor unnecessary worry for everyone - only the majority who don't use filters very often ;)
 
Uneducated_Rick said:
Using an ND filter does not make it a hypothetical problem nor unnecessary worry for everyone - only the majority who don't use filters very often ;)
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So it doesn't apply to the use of a grad (as you claimed in post #49 ;) ) and it is only a problem when using an ND filter for very long exposures?
 
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