Originally posted by kacansas03 About a telephoto lens magnification ability. I have several lenses from 135 mm up. I noticed that some will, as an example, have a focusing range like on one of my 200's that runs up to 200 feet just before being at infinity. I also have a 200 mm that goes up to 50 feet just before infinity. I do not think that this focusing range has anything to do with magnification but I am wondering why the much longer lenses do not go much higher in the distance of the focusing range? Yes, I know infinity is higher than any number on the barrel. I think that the focusing range has to do with placement of elements and this is where the designers come up with the ability of a say 300 mm lens to focus clearly at 2 feet, as an example. I think that same 300 mm lenses(just made it up people) would then have a short, say out to 50 feet, focusing range before reaching infinity. Would that 300 mm have less magnification ability than some 300 mm that had a focusing range out to 300 feet? I know this is confusion in my own head but I am trying to understand plus learn about some of the principles in lens design. Does anyone have an answer that could help me?
The near focus is set by the lens designer, but distance scale has a lot to do with the focal length and the change in depth of field as the focal length increases. For example, a 28 mm lens at f/8 has a hyperfocal distance around 10 feet on 24x36 mm film or sensor - focused at 10 feet with the f/stop at f/8, and 8x10 print will appear sharp from about 5 feet to infinity. On the other hand, the hyperfocal distance for my M 400 at f/45 (that's right 45, out beyond 32) is a bit over 300 feet, and will be acceptable on an 8x10 print as close as 200 feet.
The other factor about the focus scale is the amount of turn the focus ring has. The 400 has a focus turn of somewhere around 270 degrees which leaves lots of room to engrave numbers, so the scale reads all the way up to 300 ft and 100 meters. Another brand of 400mm lens may only have 180 degrees of rotation, which would leave less room to engrave the numbers on it. The 300/100 would run into the infinity mark.
In short, the distance scale needs a lot more numbers at 400mm than at 28mm. I hope this clears the mist up a bit for you.