Canon 20mm f3.5 Macro Bellow Lens

SigmaTog wrote:

ken_in_nh wrote:

How'd you move your assembly for the stack? Step size?

Nice to see your lens and rig in action!

Thanks
The rig is getting too big & is vibrating with the slightest touch.
The steps were 19 into 1mm, ie 0.02mm.
My rig at present.
Tube length is 430mm flange to flange.
Is making the tube length 1/2 as long & using a 2x converter a useful arrangement ?

Absolutely. Your Canon lens is optimized for a relatively short bellows. It's not well corrected at 450mm, so you will have problems with chromatic aberration, spherical aberration, and curvature of field that you won’t have at 200mm.

And you're likely to pick up a decent quality improvement from controlling vibration better.

Of course, you don’t really need the teleconverter, because you don’t have the resolution. Are you familiar with the term "empty magnification"? That's when you try to operate past the "diffraction limit" of an optical system, so all you're doing is magnifying blur. Your 20mm f/3.5 is really only built for magnification up to 10x, and even then it's a bit "iffy".

I only used super-long extensions back in the day when I needed greater working distance. You really don’t want to build a high magnification system this way, because you're creating a bunch of optical problems. Extending a 20mm f/3.5 lens some 450mm (I've included the camera's internal depth) means you’ve got 21.5x magnification, which sounds great until you realize that the lens is about f/80 effectively. (3.5x430mm/20mm).

That means is a very strong "empty magnification" situation. The diffraction limit calculator says we're looking at an Airy disc of 107um at f80. I believe your Sigma has about 9um pixels, so the finest detail that can be resolved is about 5 pixels wide.

You could get the same, or better, pictures by dropping the tube to 200mm, cropping, and enlarging, with a lot more stability and controllability, and you’re in the range your lens was corrected for. You don’t need the teleconverter, because that would just be piling on more empty magnification.

What you really need, at 10x or 20x, is a faster lens. A much faster lens.

Microscope objectives are insanely fast. My favorite 20x objective is a Nikon CF M Plan 20x ELWD. It's an 11.1mm focal length (which doesn’t really matter) with a 10.5mm working distance (which does matter). It has a numerical aperture of 0.4, which converted to an f stop is f/1.1. That's over 3 stops faster than the 20mm f/3.5. More importantly, at 20x it's effectively f/22 instead of f/80, so you have an Airy disc of 30um instead of 107um.

Do I place the 2x behind the lens or in front of the cameras mount please ?

In front of the camera mount. A teleconverter is a negative lens (multi element, and pretty well corrected, but for analysis purposes it's effective a simple negative lens) of about -60mm focal length. If you put it near the positive 20mm lens, you'll get a net -40mm, and the system won’t focus light.

Interesting rail. Is it borrowed from your 3D printer?

And what's holding up the rear end of it?