Light entering camera via viewfinder

arclight

arclight

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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.
 
Good question, I was wondering that on seeing it too.... I've not noticed this effect with my astro imaging, and that's with having a laptop near the rear of the camera, and waving torches around at the rear too.
 
I don't think that it's a case of light falling on the sensor that causes the problem...as you say, the mirror is up at the time. It's light from the viewfinder affecting the metering before the shutter is activated that you need to guard against. A good deal of photography is performed with the sun behind the camera and it's good practice to shield the viewfinder for shots when you're using a remote shutter release in these situations.

Bob
 
jgs001 said:
Good question, I was wondering that on seeing it too.... I've not noticed this effect with my astro imaging, and that's with having a laptop near the rear of the camera, and waving torches around at the rear too.
Click to expand...

With this type of shot, the metering is fairly insignificant on super long exposures with little light.

Bob
 
I'm not sure re the Pentax, but Canon supply a little cover to block the viewfinder - I have forgot on several occasions to use this and not noticed a problem :shrug:

My other question would be with the newer generation of DSLR's allowing "Live View" with the LCD screen - should you use the cover then :shrug::shrug::shrug:
 
If your worried simply put your lens cap over the eyepiece, although many camera have a cap in the strap now. Wayne
 
I have just carried out a little experiment with my camera:-
Shutter set on B
Lens wide open on f1.4
Bright torch shone into viewfinder.
Shutter fired and held.
Looking into the lens I could not see any sign of light leaking through.

Tried that the other way around.
Bright torch shone into lens. No sign of light getting to the viewfinder when shutter held open.

Edinburgh Gary mentioned, I think, a purple blotch on his image if he did not blank off his viewfinder (His Nikon F3 has a switch for that) when making a long exposure with a ND filter. (Appologies if I got some of the detail a bit wrong, Gary)

When my 10 stop ND filter arrives I'll try a practical test.
 
Uneducated_Rick said:
Sorry Bob but light can enter through the viewfinder and affect the exposure even though the mirror is up

have a look at http://www.talkphotography.co.uk/forums/showthread.php?t=118089&highlight=light+viewfinder unfortunately the original picture has been removed but you can see the conclusion of the discussion
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I stand corrected Rick. (y)

My own tests were on faster exposures for outdoor macro and I found that once the shot was metered, there was no difference in exposure but the metering was definitely affected. I guess that there's a big difference in what can happen over 4 minutes versus 1/250th.

Bob
 
Canon Bob said:
I stand corrected Rick. (y)

My own tests were on faster exposures for outdoor macro and I found that once the shot was metered, there was no difference in exposure but the metering was definitely affected. I guess that there's a big difference in what can happen over 4 minutes versus 1/250th.

Bob
Click to expand...

Found the relevant post:-

Originally Posted by EdinburghGary
They are screw in. Have to remember to close the eyepiece too, or you get horrible purple blobs.
Gary.

So what causes this. Don't think that is a metering issue, folks.
 
In the old days, before AF and built in metering, mirrors used to reflect all the light so when the mirror was flipped up and against the light seals at the top of its travel, no light could get through and at the film. Nowdays, parts of the mirror allow some light through for AF and metering purposes so it (light) can end up where you don't want it.

The little eyepiece cover often gets lost (or just left behind) but anything that's relatively light proof can be used to stop too much light entering the viewfinder. I've normally got some insulating tape in the kit bag so have been known to use that to cover up. Even shading the viewfinder with a hand is (considerably) better than nothing.
 
Uneducated_Rick said:
Sorry Bob but light can enter through the viewfinder and affect the exposure even though the mirror is up

have a look at http://www.talkphotography.co.uk/forums/showthread.php?t=118089&highlight=light+viewfinder unfortunately the original picture has been removed but you can see the conclusion of the discussion
Click to expand...

It was me who started this thread, without the viewfinder covered there was a horrible purple band but with the viewfinder covered it was fine which can only mean that light is leaking through the viewfinder and ruining the shot!! I'm at work at the minute but when I get home I'll post the original 'proplematic' pic (y)
 
Canon Bob said:
I stand corrected Rick. (y)
Click to expand...

it did take me about 20 minutes to find that old thread - wasn't quite ready to call you on something without having evidence :LOL:

When you see the original problem pic from Jambo it is obvious that there is a light leak around the mirror because the line between correct and incorrect exposure was so straight and quite well defined
 
Uneducated_Rick said:
it did take me about 20 minutes to find that old thread - wasn't quite ready to call you on something without having evidence :LOL:

When you see the original problem pic from Jambo it is obvious that there is a light leak around the mirror because the line between correct and incorrect exposure was so straight and quite well defined
Click to expand...

Good - at least that's an explanation now.
Maybe it has a greater or lesser effect depending on camera model.
 
arclight said:
Good - at least that's an explanation now.
Maybe it has a greater or lesser effect depending on camera model.
Click to expand...

Yes, surely some cameras will be better than others.

But I can't help thinking that this, in practise, is probably a problem almost unique to photography with an ND filter when the ambient light is actually quite high. At most other times with a very long exposure, the ambient light is very low so it shouldn't be so much of a problem.

<goes to see if the eyepiece cover is still in the camera box :D >
 
I've just tried a couple of experiments with a 5D2 and 50D at ISO800, body cap on and 20 second exposures.
With the viewfinder covered the histogram shows no light entered (as expected). With ambient light allowed to enter the viewfinder there was still nothing recorded on the 50D but the 5D2 was showing a slight leakage (just noticeable on the histogram). Shining a 9 LED torch into the viewfinder gave a slight leakage on the 50D but almost half filled the histogram on the 5D2

Bob
 
Did a similar experiment with a D700 - lens cap on, 10 second exposure, ISO 400.

Small torch shone directly into VF - slight band visible.
Ambient light allowed to enter VF - nothing visible.
Full sunlight allowed to enter VF - strong banding.
Same position but VF shaded with hand - nothing visible.
VF shutter closed - nothing visible.
 
Canon Bob said:
but almost half filled the histogram on the 5D2
Click to expand...

Hmmm, this is interesting. I never use my eyepiece cover (partly because I use a TP camera strap and never thought I might need the cover off my original strap), but have had horrendous long exposure problems as previously documented. I will try using it on some shots over the bank hols. (y)
 
light doesn't really affect the sensor. it bleeds into the metering system which allows the camera to make an incorrect light meter.

in regards to the actual sensor. I'd be guessing.
 
I tested this yesterday Johnny1982, and by shining a torch in the viewfinder when taking a longer exposure shot, there was a definite change in the resulting image from shooting with the viewfinder covered. But interestingly, I didn't notice much of a change using a 3 LED torch, compared to a torch with a normal tungsten filament bulb.... Whether that's down to the illumination delivery or the colour of the light, I've no idea...
 
Johnny1982 said:
light doesn't really affect the sensor.Yes, it does - try it if you don't believe me (and others...) it bleeds into the metering system which allows the camera to make an incorrect light meter. Yup, it does that too - and can have a few stops of effect on the exposures

in regards to the actual sensor. I'd be guessing.Yet in your paragraph above, you state that "Light doesn't really affect the sensor".
Click to expand...
 
I forgot to mention, that the angle of incidence of light on the viewfinder does have a bearing on the amount of affect on the resulting image. Which is probably why Bob, I've never noticed the effect before with my torche and laptop when astro imaging, as I've never shown them directly in the viewfinder...
 
If I may offer a conclusion... (and bearing in mind my experience of film cameras too from the dark/slow past that also let light in through the viewfinder)

Light shouldn't leak in, but it may on some cameras, and some more than others :(

Prior to a shot it may affect metering, but if leakage occurs during a shot it can affect the image as it does indeed reach the sensor sometimes :(

Pro spec cameras have viewfinder shutters - take it as read that if neither metering or the exposed shot wasn't affected, there would be no eyepiece shutter would there :shrug:

Conclusion...

On long exposure shots, either during metering or exposure - cover the damn eyepiece !!!

:D

DD
 
I've noticed metering problems in the past with the camera on a tripod in daylight and the light from behind the camera. Got underexposures until I covered the eyepiece.... or kept my eye there for the shot and covered it that way.

edit. my post overlapped with DD. problem was obvious on normal daylight shots not long exposures.
 
RobertP said:
I've noticed metering problems in the past with the camera on a tripod in daylight and the light from behind the camera. Got underexposures until I covered the eyepiece.... or kept my eye there for the shot and covered it that way.

edit. my post overlapped with DD. problem was obvious on normal daylight shots not long exposures.
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Right from my earliest days over 30 years ago light always affected metering if your eye wasn't there, or the eyepiece wasn't covered; but I guess that's easier to forget at dusk/night-time and especially with these 'black' ND filters on where you can't see anything anyway sometimes

Basically, if it's on a tripod and you're using the camera's metering - cover the eyepiece

For longer exposures cover it anyway for light leakage

(y)

DD
 
Quote from Canon EOS 1000D manual p165

" If you take a picture without looking at the viewfinder, light entering the eyepiece can throw off the exposure. To prevent this, use the eyepiece cover (p.22) attached to the camera strap. "

Then follows a couple of illustrations with instructions showing how to remove the eyecup and attach the eyepiece cover. So it would appear that it is the exposure that can be affected.

Ray
 
gribb99 said:
Quote from Canon EOS 1000D manual p165

" If you take a picture without looking at the viewfinder, light entering the eyepiece can throw off the exposure. To prevent this, use the eyepiece cover (p.22) attached to the camera strap. "

Then follows a couple of illustrations with instructions showing how to remove the eyecup and attach the eyepiece cover. So it would appear that it is the exposure that can be affected.

Ray
Click to expand...

That would suggest that the problem may be worse when using live view since the viewfinder is totally exposed to ambient light.
 
yes but shining a torch and ambient light spill isn't exactly the same thing is it.

I always cover up my viewfinder when working off the viewfinder.

What I mean't by my comments was that direct light isn't bouncing onto the sensor. If anything its casting a fog in the chamber and thats whats over exposing the images.
 
gribb99 said:
So it would appear that it is the exposure that can be affected.

Ray
Click to expand...


Yes but what the manual doesn't point out is that with long exposures and bright light falling directly on the eyepiece, light can (and does) get through and make itself known on the sensor.
 
light affecting the metering system isn't affecting the sensor then is it!

I know light spill affects the metering system but that metering system causes faults with auto exposure bracketting. I was on about the ACTUAL physical sensor.
 
Johnny1982 said:
light affecting the metering system isn't affecting the sensor then is it!

I know light spill affects the metering system but that metering system causes faults with auto exposure bracketting. I was on about the ACTUAL physical sensor.
Click to expand...

Is it just the metering system that makes this happen then?
3485053815_8829e4920d_o.jpg

Lens cap on, 10 seconds exposure, 200 ISO, direct sunlight falling into eyepiece.
 
Johnny1982 said:
yes but shining a torch and ambient light spill isn't exactly the same thing is it.

I always cover up my viewfinder when working off the viewfinder.

What I mean't by my comments was that direct light isn't bouncing onto the sensor. If anything its casting a fog in the chamber and thats whats over exposing the images.
Click to expand...

You're right, shining a torch isn't the same as ambient light spill, but I was after a controlled experiment to find out the effects and it's far easier to force it with a torch as I got far more noticeable effects.

I metered with my at the viewfinder btw....
 
For me, the explanation that the metering would be affected when the mirror is up, due to light making it's way through semi-transparent parts of the mirror which are there for the AF sensors, holds water

I can certainly understand how the light would affect the metering, and the above seems an acceptable reason why it would affect long exposures too...

dave
 
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.
 
Ignoring the effects of gravity on light (pretty sure there's nothing massive enough in the prism or mirror box to cause that), no mirror is perfect and the focus screen can (and does) scatter light away from the direct path. This light is then reflected by the raised mirror (which is now facing the focus screen) and can (and does) bounce between the screen and mirror with some of it eventually hitting the sensor.
 
I can't see it hitting the sensor. Its illuminating the chamber box causing a fog.
 
If it was that, the whole frame would be affected.
 
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