Is longer or shorter focal length on a spotting scope better?
May 20, 2010 by
Filed under spotting scopes
I’m planning to buy a large spotting scope (100mm or larger). I’m looking at the various scopes from Meade and Celestron. It seems that for a given objective diameter, the Meade seems to have a longer focal length. Both companies offer telescope type interchangable eyepieces that offer different magnification. I’m actually trying to resolve a .3″ (30 caliber) hole at 1,000 and a .22 bullet splatter on white metal (or paper hole) at 250 yards. Thinking about one of the 125mm Schmitt-Cassigrain scopes that will double as a low power telescope. So, higher or lower focal lengths allow for better resolution? And, how much does magnification has to do this, given equal diameter objectives?
Resolution has nothing to do with focal length – it’s aperture.
This will help with your calculations:
http://en.wikipedia.org/wiki/Dawes_limit
Note that these are theoretical limits. Heat and air movement, dust etc will make it worse. You’ll have to do some trig to convert what you want to resolve at what distance to and angular measure.
The shorter the focal length of the objective lense the bigger the image is you see overall so it depends on the objeects you want to look at. (I’m prety sure this is right but I wouldn’t trust it too much)
The main drawback to using a long-focus scope for spotting is the limited field of view. with a 125mm f/10 scope, your maximum field of view (assuming a 1.25″ focuser) is about 1.3°, and the lowest power is around 40x. Might work OK for target spotting, but is less than optimal for wildlife. Maximum magnification will be limited by aperture (figure 50x per inch, or 2x per mm, at best) and atmospheric conditions. You’ll often find the highest theoretical magnification just shows the heat shimmer in more detail.