Need advice re long telephoto lenses

A 50-500mm is not quite as long as is claimed. The Bigma is an internal-focus lens, therefore gradually loses focal length at subject distances below infinity. Here are some numbers that one reviewer came up with:

At infinity ~ 485mm

• At 25m ~ 465mm
• At 7m ~ 435mm
• At 2.5m ~ 385mm

http://www.bobatkins.com/photography/reviews/sigma_50-500_os_hsm_review2.html

The difference between a 300mm f4 prime and 1.4X TC is 1/2 stop, f6.3 vs. 5.6, which matters to me. What I find is that I sometimes don't even have enough light for decent shutter speeds at f5.6, in the woods, or in twilight. The advantage with my 300mm primes is that I can remove the TC and I have a 300mm f4 lens again. OTOH, the Bigma is still f6.3 at 300mm, 1-1/2 stops behind. This versatility (longer & slower vs. shorter & faster) is something I need for my particular shooting conditions.

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Dan