Originally Posted by
jim10219
I think you misunderstood me. YOUR EYES need to focus on the ground glass grain, but that doesn't mean the loupe needs to be. Focusing on the top of the ground glass serves no purpose unless you want to look at your own reflection (or any guidelines you have there). And that's certainly not a suggestion I made.
Let's do an example. Take a pair of reading glasses. Do you need to keep the newspaper at a precise distance to keep the print in focus, or does it just need to be kept within a usable range? It just needs to be kept within a range, obviously, because your eyes can adjust their focus to compensate for the changing focal points created by shifting the distance between the glasses and newspaper. If you took the reading glasses, however, and built a camera that used a lens from those glasses, you'd discover that the image would only focus at one particular spot, and to change that spot, you'd either need to change the distance between the lens and newspaper, or the lens and film plane. That's because, unlike the adjustable lenses in our eyes (which can be pulled or relaxed by muscles to adjust their focal length), the film has no way to focus light on it's own. So the entire job of focusing light must be done solely by the camera lens (and aperture). That's why the lens on the front of the camera has to be focused precisely onto a specific point (film plan or ground glass), and the lens on the back of the camera (the loupe) just has to be focused within a usable range (because it's not the only lens in the system, since our eye is also a part of this optical system). Hence why you don't need to focus the loupe onto the ground glass grain for the ground glass grain to be in focus.
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