70-200mm as tube lens - finding true infinity focus

Beatsy · Post by **Beatsy** » Sat Jun 25, 2016 1:33 am

I've successfully used a Canon 70-200mm f/2.8 L IS USM II as a tube lens for a 5x Nikon Plan objective, so I'm happy it works well in practice. I set focus to "approximately" infinity, aperture wide open, and images are as sharp as I expect.

I have a 10x Mitty on the way (yay, at last) and wonder if the "approximation" of infinity focus (infinity is marked as a short line on the lens, not as a specific point) might degrade image quality at this higher power and resolution. What aberrations should I look for if the focus setting were not quite right? Or doesn't it matter much?

rjlittlefield · Post by **rjlittlefield** » Sat Jun 25, 2016 9:29 am

In basic theory, the main effect of something other than infinity focus is to add spherical aberration (SA), as a result of dragging the objective away from its designed focus point.

For small amounts of added SA, there will be a slight softening because the MTF curve will sag (without changing its cutoff point). Also some asymmetry will develop in the rendering of detail that is slightly in front versus slightly behind perfect focus.

For the particular situation that you've described, the amount of added SA will be very small. I would be surprised if it's detectable by eye, and shocked if it matters.

As always, test to be sure. Sometimes there are other issues that are not predicted by basic theory. This happens especially often away from image center, where other aberrations dominate.

--Rik

Pau · Post by **Pau** » Sat Jun 25, 2016 2:25 pm

Don't worry at all. With a 0.30 objective you even could play a bit with focus to tune magnification

Beatsy · Post by **Beatsy** » Sun Jun 26, 2016 4:28 am

Thanks both. I thought any effect would be negligible, but always good to tap the brains here for insights. Really looking forward to exercising the mitty as I've waited many, many years to get one! Any suggestions on how to speed up the postman?

Cheers