Quite often we want to check focus more carefully in a view camera . To do that, the obvious idea, goes about using something that enlarges the projected image allowing for more precision than with bare eyes to inspect image’s sharpness. Three problems generally arise when using common loupes and magnifying glasses :
- There is extraneous light falling on the spot in ground glass where we are looking at
- The loupe demand a proximity that we are not able to reach
- Common magnifying glasses need to be focused on ground glass, so then you have a second adjustment to do instead of one
One easy way to solve that is to mount an old lens in a tube cut with the right size to be always in focus. Good lenses for that are the ones in reprography and enlargers, but any photographic lens, in principle, will do. The main feature is a focal length rather long. Something between 100 and 150 mm works well, but it must be decided according to what is available and the camera in which it is intended to be used.
The loupe in this post uses a 150 mm reprography lens. Note that the front element faces the viewer and becomes an ocular lens. The tube must be cut in a way that when touching the ground glass it will yield a focused image. It is easy to assess that measuring the distance while looking to a printed paper or something alike and taking note of the distance. The most difficult part is probably about finding and adapting the right tube. In this case, I had a PVC tube that was heated up and fit perfectly on lens’ thread.
Below, this ground glass loupe in a Crown Graphic from Graflex.
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