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I was playing with my Nikon D5100 yesterday and noticed that the exposure reading through the viewfinder was different to the reading on the LCD. I have never noticed this before. Is this normal?
For what its worth I was shooting indoors without much light, and with the smallest (highest) aperture that my camera would do. It was also on a tripod so the change was not the result of moving the camera.
I have no experience with the D5100 but I would be very confident that the two displays should be the same if they are in fact meant to be measuring the same parameters.
If the two readings are not made essentially exactly simultaneously then it is possible for a number of factors to influence the result. In low light conditions a very small change in the degree of obstruction of light from a stronger light source can make a noticeable difference in exposure. The act of moving one's head or arm or body even slightly may be enough to change readings depending on how far away the target was and the relationship of light sources.
In extreme cases, light entering via the viewfinder when your eye is not blocking it may cause the LCD reading to read as if there is more light on the subject than is actually there.
If it is not possible to read LCD and viewfinder values simultaneously by yourself you could use two people with one calling out the reading for the other person to confirm.
If the readings do differ under such controlled and simultaneous conditions then it suggests either that the camera is faulty (which seems unlikely) or that the two displays have somewhat different intended purposes (which also seems unlikely:-).
I'm not sure how to do this on a Nikon camera, but on Canon cameras you could use "AE lock" to check you assumption. I'm sure Nikon cameras offer the same functionality. From Using Auto Exposure (AE) Lock:
It “freezes” the camera’s exposure settings, so that if the camera is moved from one area to another, the auto exposure system won’t change aperture/shutter speed values.
That way you know the camera wont adjust the exposure settings in the time you change from looking in the view finder to looking at the screen.
I'm no pro, but in the manual of my Canon 500D I red that while using LiveView (operating the camera through the LCD display) I need to close the viewfinder. They explain that this is necessary to prevent light, that may enter the camera body through the viewfinder, from reaching the sensor and alter the image received through the lens.
Since the camera uses its sensor to detect the amount of incoming light and since you probably move away from the viewfinder to take a look at the LCD display, your situation could be explained by an overexposed sensor due to an open viewfinder.
Again, I'm no pro, just a guess :)
I think its due to the switching of electronic sensor when you switch to LCD for capturing the image however when you click from the viewfinder the optical sensor records and captures the image and that may be the reason. Because the samething happens with my Canon 77D
