speters Loc: Grangeville/Idaho

blackest Loc: Ireland

If you understand focus it kinda drops into place.

Infinity focus is when the lens is at its closest to the lens, beyond infinity things get fuzzy again. Iif you want to focus nearer then the lens moves further away if you want to do macro its much further away. If you think of the lens as a movie projector and your sensor as the screen if the projector is further away from the screen the image circle of the lens is bigger and so the size of your subject is bigger too on the sensor.

The relationship between the position of the lens and the focus distance is logarithmic which makes the math hard except in some simple cases.

but the logs for focus distance in factors of 10

0.1 10x
0.01 100x
0.001 1000x
0.0001 10,000x

so for an 80mm lens to focus at 800mm the lens needs to be 88mm from the sensor (logically if not physically) a helicoid to move the lens out 8mm is pretty easy to build into a lens.

for a 100x focal length its just 1% of the focal length or 0.8mm for this 80mm lens
a 1000x focal length just 0.1% of the focal length or 0.08mm for us thats pretty much infinity at 10,000 x focal length you cant really move the lens much closer than that 0.08mm at a 1000x true infinity would be infinitely small, so in practice infinity is somewhere between 100 and 1000x focal length
Focusing closer the movement gets very large as you come in closer than around 10x and macro lenses will be moving lens elements to avoid the extreme physical movement, lets stop at simple lenses :)

so for a camera and lens the distance between the back of the lens optically is split between the lens and the body.
The camera's flange focal distance stays constant and so that is deducted from the lens for any given system.

when it comes to adapting lenses between systems the adapter has to bridge the gap between the flange focal distance of the new camera body and the distance the lens was designed to be from the sensor in the lenses native system.

because mirrorless cameras have a short flange focal length maybe 19mm and a lens designed for a SLR is designed to sit in front of a mirror box
Canon Eos is 44mm pentax k / m42 46.x Nikon 47mm its easy to adapt lenses to mirror less cameras. At least physically and as long as you can adjust aperture and focus manually it works, with converting between slr type systems it gets harder.

Canon is the shortest at 44mm so an adapter 2 or 3 mm thick is doable. Nikon being the longest has most problems to fit a canon eos lens to a nikon would require the lens to sit inside the nikon flange in order to get infinity focus. And it still needs to avoid hitting that swinging mirror , this isn't possible. On the other hand if you don't need infinity focus, you might adapt a lens designed for a shorter system for macro maybe on the longer system.

Adaptall lens had interchangeable lens mounts so the actual lens could be swopped between systems provided you had a compatible mount.

Image circle on these old slr lenses was made big enough to cover a 35mm frame when adapted to an aps or m43 frame that circle is the same size but the sensor is smaller which is why you get crop factors of 1.5 or 1.6 depending on system 2x on m43 systems.

The metabones speed booster adapters basically shrink the lens image circle concentrating the light, which effectively makes the lens faster and closer to its original angle of view. Teleconverters do the opposite and spread the image circle wider the density of light is less and the angle of view narrower.

It's possible to make a teleconverter which mates two camera systems and puts the lens at the right distance optically, usually iq is worse the lens is gathering less light and is slower. Because the image is so much smaller at the sensor the enlargement to a given size is increased and that tends to bring out the worst in a lens.

So while its counter intuitive you tend to need better quality lenses the smaller the sensor on a camera (up to a point) Because our eyes have limited resolution as long as the junky stuff is smaller than our resolution the image looks fine.

juanbalv Loc: Los Angeles / Hawthorne

Highly erudite, blackest, and wonder of wonders, I understood some 70 to 80 percent; my deficiency. I really appreciate your input. Thank you much.

blackest Loc: Ireland

would have helped if i had written that infinity focus is when the lens is closest to the sensor.

juanbalv Loc: Los Angeles / Hawthorne

I knew what you meant.