Long time exposures: For my camera, Nikon D7100,there is a small plastic piece to cover the viewfinder when you take night pictures in T or B mode. This is supposed to prevent light...

Elmerviking

For my camera, Nikon D7100,there is a small plastic piece to cover the viewfinder when you take night pictures in T or B mode. This is supposed to prevent light entering the viewfinder affecting the exposure. How is this possible when the mirror is up? I have never experienced any difference, with or without the viewfinder covered.
Any idea?
/Bosse

MT Shooter Loc: Montana

Elmerviking wrote:

For my camera, Nikon D7100,there is a small plastic piece to cover the viewfinder when you take night pictures in T or B mode. This is supposed to prevent light entering the viewfinder affecting the exposure. How is this possible when the mirror is up? I have never experienced any difference, with or without the viewfinder covered.
Any idea?
/Bosse

Its no prevent extraneous like from affecting the image by infiltrating through the viewfinder prism and seals. Usually not a problem except in very long exposures where the viewfi der might be exposed to an outside direct light source. (i.e. a flashlight while doing night sky shots.)

Longshadow Loc: Audubon, PA, United States

MT Shooter wrote:

Its no prevent extraneous like from affecting the image by infiltrating through the viewfinder prism and seals. Usually not a problem except in very long exposures where the viewfi der might be exposed to an outside direct light source. (i.e. a flashlight while doing night sky shots.)

So there is no light seal around the mirror when it is up?

BebuLamar

It's not so much for long exposure. It's for any time you don't use live view, you don't have your eye against the viewfinder and you use an autoexposure mode. In this case the light can enter the viewfinder and inflate the meter reading and thus give you wrong exposure. The light meter sensor is in the viewfinder.
Many said that's to prevent fogging but if the mirror foam is good there is no chance of fogging.

Elmerviking

Wouldn’t there be a problem if you have the sun behind you, the camera on a tripod photographing something in the shadows with a relatively long shutter speed, for example 1/30 s?

MT Shooter Loc: Montana

Longshadow wrote:

So there is no light seal around the mirror when it is up?

There is of course a light seal gasket, but they wear and don't always seal 100% so the longer the shutter is open, the more chance there is of infiltrating light to effect the image.

Elmerviking

BebuLamar wrote:

It's not so much for long exposure. It's for any time you don't use live view, you don't have your eye against the viewfinder and you use an autoexposure mode. In this case the light can enter the viewfinder and inflate the meter reading and thus give you wrong exposure. The light meter sensor is in the viewfinder.
Many said that's to prevent fogging but if the mirror foam is good there is no chance of fogging.

The light meter is actually at the bottom, beneath the sensor.

BebuLamar

Elmerviking wrote:

The light meter is actually at the bottom, beneath the sensor.

Nope! it's in the viewfinder. Down there is the AF sensor.

Longshadow Loc: Audubon, PA, United States

BebuLamar wrote:

It's not so much for long exposure. It's for any time you don't use live view, you don't have your eye against the viewfinder and you use an autoexposure mode. In this case the light can enter the viewfinder and inflate the meter reading and thus give you wrong exposure. The light meter sensor is in the viewfinder.
Many said that's to prevent fogging but if the mirror foam is good there is no chance of fogging.

Good to know about the mirror foam to stop light fogging.
The other concern being possible metering change from extraneous light without your eye against the viewfinder.

BebuLamar

Longshadow wrote:

Good to know about the mirror foam to stop light fogging.
The other concern being possible metering change from extraneous light without your eye against the viewfinder.

The meter changes a lot when you do not have the eye to the viewfinder. Put the lens cap on and read the meter reading on the top of the camera. It shows higher reading than when you put your eye to the viewfinder. That's even in normal room light. Out in the sun it's a whole lot more. That's why whenever I shoot without my eye to the viewfinder I always shoot in manual. Of course if you use live view you won't have that problem.

Longshadow Loc: Audubon, PA, United States

BebuLamar wrote:

The meter changes a lot when you do not have the eye to the viewfinder. Put the lens cap on and read the meter reading on the top of the camera. It shows higher reading than when you put your eye to the viewfinder. That's even in normal room light. Out in the sun it's a whole lot more. That's why whenever I shoot without my eye to the viewfinder I always shoot in manual. Of course if you use live view you won't have that problem.

rgrenaderphoto Loc: Hollywood, CA

Elmerviking wrote:

For my camera, Nikon D7100,there is a small plastic piece to cover the viewfinder when you take night pictures in T or B mode. This is supposed to prevent light entering the viewfinder affecting the exposure. How is this possible when the mirror is up? I have never experienced any difference, with or without the viewfinder covered.
Any idea?
/Bosse

Light leak will ruin long exposures with magenta/red streaks across your image. Try a 2 minute exposure with a 10x ND and see what happens. See example.

(Download)

Longshadow Loc: Audubon, PA, United States

rgrenaderphoto wrote:

Light leak will ruin long exposures with magenta/red streaks across your image. Try a 2 minute exposure with a 10x ND and see what happens. See example.

Bad mirror foam or what in this instance. Do you know where the leak is located?

John Maher Loc: Northern Virginia

Thank you for asking and for the discussion. I have one, too.

That's why I read UHH daily.

The more I learn, the less I know.

lamiaceae Loc: San Luis Obispo County, CA

BebuLamar wrote:

It's not so much for long exposure. It's for any time you don't use live view, you don't have your eye against the viewfinder and you use an autoexposure mode. In this case the light can enter the viewfinder and inflate the meter reading and thus give you wrong exposure. The light meter sensor is in the viewfinder.
Many said that's to prevent fogging but if the mirror foam is good there is no chance of fogging.

CORRECT ANSWER to my knowledge. My Pentax K2 DMD (film camera) has a switch to close a "window" instead of the slide on cover.