Thanks for all the advice.
How valuable is the IS on the Mark I? And is the F4 IS good for indoor portraits on a crop camera? Logic would suggest that it would basically be giving me a f5.6 depth of field wide open on the 70D. is that correct?
if i remember correctly, 70-200mm f/2.8 IS version 1 has 3 stop of IS whereas 70-200mm f/4 IS has 4 stop. about the sharpness, to me 70-200mm f/4 IS is more better than 70-200mm f/2.8 IS version 1 even at f/4. i am not trying to say that version 1 is not good, i did own this version before upgrading to version II, and version I served me pretty well on my candid shots.
as many people suggestion on 85mm f/1.8 for in door portrait, i am on the same page with them. but i do not have 85mm f/1.8; i am using 50mm f/1.4 instead for everyday purposes (it is 80mm on crop.) ideal focal length of portrait photograhy is twice of your sensor size if i am right (i think i got this information from computer science 178 course offered by uc stanford). all i have to do is just zoom by feet. number of people say that this lens is tack sharp at 2.8, but i see it at f/3.2 even at low light with high iso upto 1000 on my 30d, 3200 on my 7d and slightly greater than 6400 on my 5d3. however, i am not hesitate to use wider f/stop or even widest if i have to... below are images that i have recently shot with this lens at f/2 with my 30d (note: the very first image were shot with PL linear filter to get rid some of reflection that i did not want with breaking rule of using PL filter.)
for some reasons, available light in-door portrait without IS on 70-200mm f/2.8 does not convince me much on golden rule of shutter speed (yes, i violate shutter speed sometimes, but not all the time
), especially you have not eaten breakfast, lunch or dinner before shooting
note: there is a pro protographer who is currently using 70-200mm f/4 IS and he is happy with it. his name is Tony Corbel. i am not a pro. photographer, i am just a learner and hobbyist giving out my own thoughts...