How to properly focus using a Macro reverse ring?

Question

Fourth question from me about this topic.
I'm trying to digitize a large number of family slide films with a modified slide projector by taking photos of the slides (I replaced the original yellow light bulb by a cheap cool white 50W LED).
I'm using a Nikon D5000 with a 55-200mm lens that I reversed using a reverse ring.
I'm able to get quite decent shots (in manual mode of course) however my focusing process can really be improved I think

When looking through the built-in viewfinder I only see blurry colors
When watching the live view with the camera screen I can't see anything very sharp (but still better than the viewfinder).
When plugging the camera to my computer and using DigiCamControl to check the live view it's also quite blurry

Only once I take the shot I can see a clearer picture. So when I set up everything to digitize some slides I always have to guess where's the sharpest point, take several pictures and compare them to see which one is at the end the sharpest one.
Is there any way I can improve that? I really would like to be able to see through the built-in viewfinder directly to determine the exact sharpest spot.
Thanks for your answers

Mike Sowsun · Answer

If you achieve proper focus you will be able to see a clear image in both your optical viewfinder and your Live View display.
The fact that you don't see a clear image is just because you do not have proper focus.  It is just blind luck that when you take the photo, you see a slightly better image, and that is probably just due to Depth of Field.
Most people reverse shorter focal length lenses like a 50mm for Macro work.  My guess is you are too far away from the subject when using your 55-200 lens. Try shooting it at 55mm and move the lens closer so it is almost touching, and then very carefully increase the shooting distance until the image is clear in both your optical viewfinder and live View display.
If you are still not able to achieve sharp focus, it is because the focus point is actually inside the lens, and it will be impossible to use that lens reversed.

Michael C · Answer

In general, the best way to focus when using a reverser ring is to focus the lens at infinity, point the camera at your target, and then move the whole camera/lens assembly towards or away from your target until it comes into sharpest focus. This should be roughly at the lens' focal length from the optical center of the lens (which is not necessarily at the front of the lens).
Keep in mind that the camera must be perfectly square with the slide for the entire frame to be equally in focus. That is, the plane of the slide and the plane of the camera's sensor should be parallel, so that all four corners of each are the same distance from all four corners of the other.

How to properly focus using a Macro reverse ring?

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