Hitting the "sweet spot" with extension tubes?

[TobascoJackson]

I just ordered a REALLY cheap set of extension tubes on eBay, and they arrived today. They were like $7, because there are no electrical contacts. That means manual focusing and to set the aperture, I get to mount the lens to the camera, set an aperture, press DoF preview, remove lens while holding DoF preview, mount tubes, mount lens...

Anyway, my understanding of tubes is that the shorter the focal length, the closer the focusing distance. Obviously if I go too short, my focusing distance will be so far back that it's inside of the lens. So I have a 9mm, a 16mm, and a 30mm tube. Out of the selection of lenses in my signature, what do you think my best bet is? I'm shooting for absolute maximum magnification before lighting becomes really difficult due to working distance. Ideally I want to go quite a ways past 1:1.

Has anyone found a really good combination of tubes and a lens that yields intensely close-up macro shots?

 

[Flash Harry]

The 50 will be your best bet imo, though I don't do much macro at all anymore and when I did it was on film LF with double bellows extension and building exposure with multiple flash, still you will probably need extra light since you are attempting to go beyond 1.1, you'll know whats needed once you see the shot. The formula for correct exposure, is the amount of magnification +1 squared, the number you get is the number of flashes you need after using the light meter for the initial reading at a given aperture, Focus and DOF are the main issues, the greater the magnification the more you need to stop down, hope this helps, especially for those who think they have it all sussed. H

 

[TobascoJackson]

Thanks for the help... I ran some tests and it looks like with maximum extension, 50mm is about the closest I can get anyway before the subject intersects the outer lens element. With 55mm of tubes and a 50mm lens though, I did get slightly closer than 1:1.

So I want to get significantly closer than that... and it looks like lens couplers are the way to go. I have a 100mm macro; if I picked up a 35mm and reverse mounted it on top of the 100mm, it looks like I would get to nearly 3:1. Better. Now... what if I threw the extension tubes on, then the 100mm Macro, then the reverse-mounted 35? Obviously it would get me closer, but would it eat up all my working distance?

 

[Hardrock]

In order to go beyond 1:1 put them on your 100mm macro its your only lens that will allow you to go to 1:1 and adding extension tubes will help increase it(the magnification). Im not sure on the math so I dont know how much. Contact Overread or NateS they will have the answers for sure. Although NateS I believe is in school so you would have better luck with Overread.

 

[Derrel]

Yes, the 100mm macro + extension tube will provide a good very close-up shot. The 100 macro focuses closer than the 50 or the 135/2-L, and will probably have the flattest field and the best overall image quality of any of the lenses when used with an extension tube.

I think the 16mm tube will be the best overall choice, as you mentioned, due to lighting issues with extreme extension causing the subject to be doggone near right inside the filter threads!