More working distance in aquariums macros, good or bad?

[wetWolger]

I am trying to decide on a macro lens....I am pretty much between two

The canon 100mm f2.5 USM (this is the only macro lens I have had personal experience with, and I have been quite happy from the results, I just wonder if a longer lens might give even better results).

or the Tamron 180mm f3.5

I considered the sigma 150mm macro, but with the fstop only going to 22 instead of 32, i worry I may loose on the DOF (although I don't have any idea how sharp these other lenses will be between 22 and 32 anyways). I also considered the sigma 180mm macro, but I have concerns about reports on build quality. I worry about the build quality of the Tamron as well, but since few people have it, I have not heard a lot of first hand reports.

If anyone has any comments on these lenses, and what you recommend that would be great. But another huge question I wonder is about working distance when looking through the aquarium glass. Intuitively I would think being further away from the glass would produce less distortion. I also think the longer working distance may allow me to get closer to some of my subjects. But I also worry that by being further back it might start to show defects in the glass of the aquarium more.

Any thoughts on recommendations for a macro lenses like these? Thanks

 

[BlueCorn]

You honestly don't want to go above f/22 anyway. For normal shooing diffraction becomes noticeable above f/16, in macro work you can generally get away with f/22. Working distance has no bearing on sharpness, it just allows you to be further away and still get 1:1 magnification. If also has a side benefit of increasing the bokeh (the blurred background) behind your subject.

 

[wetWolger]

My concerns about working distance and distortion are because another layer of glass must be passed through (the aquarium glass). I wonder if more distortion is generated when your closer or further from the glass basically.

Intuitively I would think being further would be better because it seems like with normal viewing the distortion is not as great as macros with the lens up next to the glass. But I don't know how this correlates when using a longer lens.

Thanks for the comment about diffraction, I was not sure exactly when it would become a problem, but I was thinking maybe the f32 lenses handled the diffraction better since it allows a smaller aperture?

 

[BlueCorn]

Thanks for the comment about diffraction, I was not sure exactly when it would become a problem, but I was thinking maybe the f32 lenses handled the diffraction better since it allows a smaller aperture?

Diffraction is caused by the small aperture. Your sensor size, not the lens, determine the threshold before it's noticeable. Technically speaking it starts in above f/11 on crop sensors and f/13 on full frame. In most cases it's not really noticeable until you get up higher than f/16 or f/22.