Effective focal length of a reversed lens
I am googling this over and over, but so far I haven't been able to find an answer.
Quite often, and always for extreme macro purposes, I am using a reversed lens, either my 50mm or 24mm manual lens. Usually I reverse the lens on the body, before I have usually reversed the lens on another lens, such as the wide kit on 55mm or a telephoto lens.
In any case, the usual problem with macro shooting is the lack of light. Having flashes available, it is not a problem, but if I want to shoot something using natural light, it becomes really challenging.
Having in-body SR definitely helps, and having it available I would prefer it working at its optimum for such situations. However, I need to dial in a simple lens characteristic - its focal length.
Now, it is a 24mm lens, right, but it is reversed - how to find the effective focal of a reversed lens?
Most of the math I've seen tackles getting the diopter and magnification - but not the effective focal. Can it be derived from the data above? How to calculate it?
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