Originally posted by Marc Sabatella They tend to be lower in contrast and sharpness than other lenses, and the bokeh is weird (doughnuts-shaped). And while the physical aperture might be f/8, the amount of light they let through is even less, so shutter speeds are slower than f/8 might normally suggest - plus the viewfinder will be quite dim.
But there no other way to get a 500mm lens that fits in a typical camera bag for $100, so what are you gonna do?
All too true - although the Pentax reflex lenses can be sharp, they're not at all forgiving if out of focus. But one way to test the quality of a reflex lens is how it handles infinity. About 20 years ago I had a cheap reflex, and infinity was a donut disaster. Both the SMC 400-600 and the 1000 are much better in that regard. If one is fortunate in the multiple of the background to the subject distance, bokeh is pleasing; otherwise it can be very unfortunate.
The first photo is from the 400-600 (and I forget which zoomI had it on).
What I like about the reflex lens is the relatively light weight versus the equivalent typical lens, which makes it possible to track birds in flight, especially if the subject is distant (as is typical during a hawk watch).
Following is a sequence with the Pentax 1000mm reflex. The first is an adult snow goose from Middle Creek, PA; the second is one of the local great blue herons; and the third is an adult bald eagle. The latter is soft, but there's something about a bald eagle in flight that has its own type of power.
I blow a lot of shots with the 1000mm, but I've become a better birder.
Cheers,
--Chet