How do you know you've reached critical focus, unless you first, purposely, go PAST it?
You find the sharpest image you can get by shooting a subject at finity while focus bracketing, with the lens wide open. Once you've found that sharpest point, stop down to f8 and see if it gets any sharper. If it is about the same, you've find the infinity setting. Then you grind, or shim, or adjust the lens screws, to get that sharp setting adjusted to move to the infinity mark.
Here is a huge collection of adapter adjusting and some mirror cutting threads from two years ago. I can't seem to find my article on determining critical focus. Anyway, plow through them and come back with questions:
Don Clary wrote:
You find the sharpest image you can get by shooting a subject at finity while focus bracketing, with the lens wide open. Once you've found that sharpest point, stop down to f8 and see if it gets any sharper. If it is about the same, you've find the infinity setting. Then you grind, or shim, or adjust the lens screws, to get that sharp setting adjusted to move to the infinity mark.
I think I agree, up to the f/8 part...
Single lens assemblies are easy to adjust as they can be focused PAST infinity with the adjustment/grub screws, either under the front label ring, or under the rubber focus grip. End of story.
Its the floating element lenses that pose the challenge, as there are no adjustments. If 2 lens groups move relative to each other, then there is one position, relative to the sensor, ONLY. Therefore, the only way to adjust infinity focus is to change the thickness of the adapter used. This doesn't have to be difficult.
Most floating element lenses are wide angle, except for lenses like the Leica 75mm Summicron, Leica 50mm Summilux, and Canon FD 50mm L. If one had a lens that focused shy of infinity, take a little of the adapter. Monitor the center sharpness and corner sharpness. At the point where the center sharpness leveled of and the corner sharpness continued to sharpen, you found the sweet spot. Remember the focus plane is spherical. If you push the center optimum focus point past or into the sensor plane, you will pull the corner focus closer to "in-focus". That's the secret.
Don Clary wrote:
Here is a huge collection of adapter adjusting and some mirror cutting threads from two years ago. I can't seem to find my article on determining critical focus. Anyway, plow through them and come back with questions: ........
Thanks, Don. I read them all. Great list!
Still, my original question remains.
One idea written is to compare the lens, wide open, to the same lens at f/8 or f/11. That makes some sense, except what if your lens changes focus slightly when stopped down? This has been well documented with some lenses. I suspect that a lot of folks are using alt glass without having critical- focus at infinity.
I guess I'm going to sand some of my adapters down, the shim them back. No idea how the accuracy will fare with sanding, though. Sure wish I had your diamond-lapping machine! Maybe I can find a machine shop to grind them. -But I also wonder how that will work, since it will make the mount looser. Arrrghhh...
Luckily, my two Zeiss lenses have the helix adjustment built-in, so I can tweak THOSE properly, with no grinding. (thanks for that url!)
