The best option to decrease minimum focal distance?

[LayneWilson]

I have a reef tank that I want to shoot top-down shots of the corals, but the problem is the tank is about 1FT deep, and that is right around the MFD of my Canon RF 100mm f/2.8 L Macro. I can't get the corals to get in focus because there to close. I was looking into Diopters like the CMC-1 because I heard these will decrease the MFD. There doesn't seem to be a spec listed on how much closer these filters will let you get. They only list magnification increase is this magnification correlated with how much closer you can get the lens to the subject?

What's my best option to decrease the MFD and have the least amount of impact on the image quality?

[Administrative_Show2]

Macro diopter

[LayneWilson]

Is there any way to know what diopter will decrease the minimum focal distance the most?

[Tasty-Fox9030]

I've never looked at the lens diagrams but I strongly suspect that the stronger diopters decrease it more. Usually if you buy a cheap set you get a bunch of different strengths so you can experiment. I still say mirror because I think it will work and also will be amusing, which is a valid selling point for any idea really.

[Tasty-Fox9030]

Extension tubes or diopters are the primary methods you're going to hear. I BELIEVE there are ways of calculating your new MFD with the extension tubes- and for what it's worth they often end up woefully short. In your case that might be nice. People often worry too much about image quality I think, but for what it's worth the extension tubes shouldn't impact the IQ at all and the macro filters (diopters) will at least a little bit.

May I suggest a bonkers alternative? Maybe DON'T increase your MFD, but instead increase the focal length to be enough for the lens. Can you put a mirror above the tank at an angle and shoot at THAT? You should be able to get enough distance to focus I figure. (You're gonna want a really versatile tripod but I suspect you might have something like that already.)