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11-30-2007, 08:55 PM   #16
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QuoteOriginally posted by noblepa Quote
It seems to me that the distance from the nodal point of the lens to the sensor and the distance from the nodal point to the focusing screen MUST be identical. Otherwise, the focus would not be the same. (snip) For that reason, moving the screen one way or the other is simply not an option.

The "cropping" of the image to 95%, therefore, must be happening between the screen and the eye; that is, somewhere in the pentaprism/pentamirror assembly. Either that, or the screen is not the same size as the sensor. (snip)

Well, "moving the screen one way or the other is simply not an option" unless one changes the lens design, thereby changing the nodal point. Pentax is obviously now restrained somewhat, due to a number of factors, to a specific lens design, but remember we're using theory to explain reality here.

And your assumptions about where the percentage loss is occurring is likely correct, except that it might also be in both (a bit in the prism and a bit in the focusing screen).

I, too, don't understand why the camera makers do this. (snip)

It's not really all that mysterious, Paul. Big, bright, viewfinders with nice magnification sell cameras. Therefore, manufacturers seek to offer that and are obviously willing to sacrifice a small area along the edges of the viewfinder image in the process.

By the way, this discussion is now complex enough to suspect we're losing some along the way. For those individuals, here (Wikipedia) is a decent general description of the overall design of a modern digital SLR camera, complete with a diagram and a picture of the Pentax K10D as an example of that modern DSLR camera.



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