Originally posted by woodywesty It appears that I have been "confused" while reading all the lens descriptions on the B&H web site. Various descriptions like "equivalent to 859mm in 35mm format", "comparable APS-C focal length: 450mm" had me thinking that I would get 450mm of magnification instead of 300mm. Now that I see that I would get no more magnification out of the DA 300mm than my old Super Takumar 300mm I have no need for the DA 300mm. I know I need to have 500mm of real magnification and a 1.4 teleconvertor on on a 300mm will only get me in the range of 450mm. This is a "game changer" as it looks like the Sigma 500mm is the only real option.
Assuming your goal is to produce a subject that is pleasingly in focus...
Do not overlook the DOF gain of a 300mm lens cropped to 450mm (APS-C) vs. a true 450mm lens (FF).
For example, using a 300mm lens with a subject at 50 feet distant @ f/4.0, the DOF is 1.33 ft. The DOF of the 450mm lens (all else being the same) is 0.58 feet. This is a huge advantage for the lighter/maneuverable 300mm (crop 450mm), over the much heavier/clumsy 450mm (FF).
Fine tune the example to fit your anticipated circumstances, then calculate it yourself...
Online Depth of Field Calculator
Most modern higher density sensors can easily handle further image cropping during post processing. I've been shooting the DA*300/4 at wildlife subjects since 2008 starting with the K-20 and working thru to the K5IIs. Out of my thousands of successful hand-held images, I can count on one hand the number of times a longer FL may have helped (I have a Pentax 1.7 TC, too.).
...my 2 cents... M