Originally Posted by neuroanatomist
The standard ruler that comes with the LensAlign appears to be about the same length as the one with theSpyderLensCal, and that's fine for most lenses. But in some cases it's not long enough (i.e. the DoF of the focal length/distance/aperture being tested is nearly as long or longer than the Z-dimension of the ruler, i.e. the base of the right triangle where the ruler is the hypotenuse). Looking at your list of lenses, you'd be fine with either product when testing at 25 x focal length (which is what LensAlign recommends). But there are situations where you'd want a longer ruler - for example, wide angle but slow lenses on FF, a 400mm f/5.6 lens on FF or if you wanted to test your supertelephoto primes at a distance grater than 25 x focal length, such as the 50 x focal length that Chuck Westfall recommends). For those cases, LensAlign has a Long Ruler Kit that magnetically attaches to the standard ruler (and includes a larger focus target that also magnetically attaches).