The prism contains a lot of “stuff.” It’s the top of the camera and houses the flash, the hot shoe for the external flash and inside, it contains the pentaprism which is according to Canon gives a vertical/horizontal coverage of approximately 100% (p253 of the manual). The prism also contains a lot of electronics and sensors like the AF sensors and the exposure control.
The pentaprism is designed so that the eye is 22mm away from the eyepiece. If your eye is too far, you will not cover the viewfinder and extraneous light will creep inside via the eyepiece.
For all of us, the defective people, aka the people wearing glasses (not contact lenses), there’s a huge gap. The gap is far more than the 22mm max. We are supposed to take our glasses off and use the camera without the glasses.
There’s a little dioptre adjustment wheel at the top right of the viewfinder to adjust so we can see inside without the need for glasses. The adjustment can be between -1 and +3. This is mostly for short-sighted people. You are supposed to activate any of the control to look inside the viewfinder and use the dioptre adjustment wheel to bring the viewfinder in focus for your eye.
The problems are:
- I’m not short sighted, I have astigmatism http://en.wikipedia.org/wiki/Astigmatism.
- I’m crazy and can’t use contact lenses. I’m deadly scared of putting contact lenses in my eyes. It’s completely irrational like all phobias. And what would I do with my glasses when I want make photos?
- There’s no marking on the adjustment wheel to show the zero to reset it. I tried and couldn’t reset it back to the original, the zero, setting. I had to get somebody with a “good” eyesight to help me reset it back to zero.
If direct sunlight or bright daylight shines between the glasses and the viewfinder, this will lead to the wrong exposure and poor or wrong focus. That’s why you are supposed to look through the viewfinder without glasses.
I took a look at the “Photo Frame” glasses from Hoodman. I looked terrible with it and it did not solve my problem with the dioptre. When I lifted the right eye lens to get the glasses of the way, everything else becomes out of focus because I have astigmatism.
BTW, this is not just Canon. The same also applies to: