Pentax 560mm F5.6 vs Sigma 500mm F4.5
Focusing
Next, let's take a look at how both manual and auto focusing works with the two lenses.
560mm distance scale detail
Focus Limiter
Since I was unable to balance the Pentax 560mm on a gimbal mount with the ARCA plates on hand, I did not shoot birds in flight or other moving subjects. The lens was on loan and even if I had a longer plate I probably would not have mounted the lens in a cantilevered fashion with the tripod mount not positioned above the gimbal support platform. Since the comparison of images was made using static images the advantage of the Pentax 560mm's focus limiter was not tested but is certainly a plus over the Sigma 500mm, which does not have a focus limiter. The lack of a focus limiter on the Sigma 500mm is annoying.
Switching Between Manual and Auto Focus
The Pentax 560mm has a simple switch that toggles between manual and auto Focus. I find this superior to the sliding sleeve toggle used on the Sigma 500mm. I rarely use manual focus with the Sigma 500mm and find it annoying when the sleeve accidentally moves to Manual Focus when the lens is handled or carried in the field. I set up for a shot and sometimes miss it if the sleeve has slid to manual focus.
The Quick Shift full-time manual focus override capability of the Pentax 560mm is another very desirable capability not present on the Sigma 500mm, which is designed much like an older FA* lens.
Auto Focus Speed
Auto focus is important for wildlife and action shots. Here's how I would rate three lenses with regard to auto focus speed: the DA* 300mm and the Sigma 500mm which I own, plus the Pentax 560mm.
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DA* 300mm– poor. The lens is very slow to acquire focus
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Pentax 560mm– fair. The Pentax 560 is faster than the DA* 300mm but still a disappointment for a lens targeted for wildlife and action shooting. The focus is quiet but not very fast.
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Sigma 500mm– good. The Sigma 500 really pops into focus thanks to a short throw. The only reason I rate it good versus excellent is the inherent noise.
