К слову, а есть ли разница ощутимая разница между 70-200/4 L и 70-200/2.8 L
в цене так ощутимая - 300-400 баксов!
пишут, что блюр похуже у четверки:

The primary benefit the Canon EF 70-200mm f/2.8 L USM Lens offers over the Canon EF 70-200mm f/4 L USM Lens is a 1 stop faster aperture (2x the light). There is a significant cost to this extra f-stop - in size, weight and price. The Canon EF 70-200mm f/2.8 L USM Lens is about 22 oz heavier (47 oz vs. 25 oz), .8" longer and .3" wider than its smaller sibling.

Of course, this is a valuable f-stop. I found that I most often used my Canon EF 70-200mm f/4 L USM Lens at f/4 - and was often reaching for higher ISOs to be able to hand-hold the lens. F/2.8 is also helpful for isolating a subject from a distracting background. F/4 can blur the background, but f/2.8 can provide much more blur. The bokeh (background blur quality) is aided by an 8-blade aperture.


Но самое главное:

The primary benefit the Canon EF 70-200mm f/2.8 L USM Lens offers over the Canon EF 70-200mm f/4 L USM Lens is a 1 stop faster aperture (2x the light).

Он что, в ДВА раза светлее???