Originally posted by yeatzee My main thing is working distance.... so whats the max. working distance with bellows @ 2:1
The working distance cannot be calculated. It depends entirely on where the lens designer put the node, the point that is where a simple lens would be placed to get the same focal length. If the designer has put the lens node outside the lens to the front, you get more working distance.
Ignore the long winded explanation below if you so desire. Also, all figures are rounded in the below discussion. Don't get hung up on decimal values.
A 400mm lens, such as my M 400/5.6, has a node that is 400mm from the sensor/film plane. It is a telephoto lens*, not a simple long focus lens, so the node is actually outside the lens toward the subject. My ruler tells me that my lens is 11 3/8" from mount to front surface of the front element, or 289mm. to that I add the mount register distance of 45.46mm and get about 335mm. The node of my M 400 is some 65mm in front of the lens when it is focused to infinity.
A macro lens, such as a 100mm lens, most often has the node outside the lens toward the subject. A really good lens designer will move the node quite a bit toward the subject to give more working distance than he/she would need to provide on a 100mm lens that is not designed to work at extremely close distances.
*Telephoto lens: A lens that has an optical design that puts the lens node outside the lens toward the subject. A telephoto lens is a long focus lens; not all long focus lenses are telephotos. What used to be called "girl watcher specials" are not telephoto design, and may have as few as two elements. These are very long, skinny looking lenses when you see them on eBay.