I use a cheap Agfa loupe to focus on the ground glass of my Intrepid 4x5. For less than £10 it works pretty well. It doesn't give a very wide view of the ground glass as the lens distorts pretty quickly away from the center focus area. Since I am only spot checking focus this is sufficient. I have contemplated getting a much better loupe with better optics but they are comparatively very expensive.
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Agfa Loupe on Intrepid Ground Glass
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One difficulty I do face with this loupe is that it is impossible to check focus very near the edge of the ground glass. The is due to the wide clear plastic stand that keeps the lens the right distance from the ground glass. It is also due to the poor optics that, as I mentioned, means the best focus area is in the center of the image area. This is what lead me to consider a better aspheric version. It is also what lead me to this hack.
Since the loupe is cheap and plastic I reasoned that I could cut away a portion of the stand so it could overlap the ground glass frame allowing me to bring the focus area up close to the edges of the ground glass.
For this I simply measured the height of the wood frame above the glass which was about 5mm. I then took a small saw and cut away a portion of the clear plastic stand portion of the loupe. How much to cut was an interesting question. To get right into the corners I should cut away about 270 degrees. However the stand would not provide much stability so I reasoned that the cut should be less than 180 degrees. Below is what I ended up with.
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| Modified Loupe |
This has worked well in the field and is a noticeable improvement. Below I show how it works in different positions. The Intrepid has some challenges where the ground glass holder has various pieces of hardware that interfere in some positions; most noticeably the spring clips that keep the holder in place.
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Edge Position
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Corner Position
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Edge Position
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The solution isn't perfect but much improves the range over which I can check focus on the ground glass. It is cheap to do and the good news is these plastic loupes are cheap so if you make a mistake it isn't too expensive.
Comments
A cheap alternative for a high end loupe is just a simple 50mm F1,8 or faster SLR lens with a spacer or a slide projector lens. These lenses give a nice bright image and a flat field for next to nothing.
The aperture of the lens determines how big the picture looks so a fast lens with a lot of glass is more useful. A longer focal length needs more viewing distance, so i tend to use 50 or 90mm lenses.I mostly us a Leitz colorplan 90mm F2.5 lens from a slide projector, its sun hood gives the perfect viewing distance.