what comes to resolving power of 7d etc, it doesent make a big difference and you simply cant get the shots with a crop sensor that you can get with full frame
here is a sample pic of what crop sensor cant do
http://www.have-camera-will-travel.com/field%5Freports/full%5Fframe%5Fvs%5Fcrop%5Fsensor%5F-.htmlwhat this brings to portrait photography is that you can use wider field of view and still remain background blur. you cant get bokeh with wide lens when you want the whole person in the shot, because if you want the same fov you need to use wider lens on crop sensor and wider lens reduces the background blur, when you go full frame vs 1.6 crop, it has really big effect and 1.6 crop pretty much ruins it totally. when shooting some head + shoulder portraits, you can get the background blurred and still get the person in focus better because of the dof seem to change to bokeh faster(due to having to focus closer) because you need to use longer lens. if you just use lower f-number and move further away on a crop sensor to get the background equally blurred, the person starts to have bokeh on him because dof changes to bokeh on wider area(it takes longer distance for the sharp parts to go to total bokeh)
here are few examples:
if you want this wide with crop, the background wouldnt be as blurred
http://www.flickr.com/photos/kevin32832/5154076706/in/pool-canon5dif you would want the same background blur than this shot with crop sensor, you couldnt get the models face and that hairy thing around her in focus like that, instead the bokeh would start building on her face, slowly graduating to the same background blur making sharp parts on the edge of focused areas blurred. or if you want the same amount of sharp are on her face, whats still blurred on this shot, would be only half blurred with crop and the background wouldnt be as blurred
http://www.flickr.com/photos/bure/5164099581/in/pool-canon5dthis is shot with 50mm and f-1.4, you simply cant get background as blurred with crop sensor when the frame shows this much of the subject, if you move further away, the background wont get as blurred
http://www.flickr.com/photos/lorirask/5165186720/in/pool-canon5dthis is shot with 20mm 3.5(about same fov than with 12mm on crop), you simply cant get background blur with a 12mm lens on crop sensor(exif says 50mm f/2 because its a manual lens)
http://www.flickr.com/photos/santapolero/5164317821/in/pool-canon5di bet you get the idea. these things are something where a crop sensor simply cant match the full frame and if you want good portraits, you need a full frame(or at least 1.3 crop). 5d has pretty good iso handling even compared to new crop sensor cameras, not to mention that the viewfinder is much larger. anything that new crop cameras can offer doesent compare to what a full frame can do if you want to take good portraits
Very true and informative post. I didn't think about DoF with wider lenses.