Today I have been reading the further info on loads of flickr photos to try and learn a few things about using this lens.
I have one question. To take a pic like this
http://www.flickr.com/photos/pixelstate/147474762/ how far away from the tiger would you have to be to get the sort of DOF at f2.8?
Same question about portraits, do you have to stand well back and use the 200mm end of the zoom to get small but not tiny DOF. I guess a face needs at least 5cm+ of in focus area to look ok?
Is there anything on the web to read/watch that will teach me a bit about this?
Andy S
The link above to DoF Master will tell you everything you need to know about depth of field, but the essence of it is this: DoF is affected by only two significant factors - image size, and f/number.
F/number is f/number, and that is constant between different lenses. But image size (or magnification) is the one that can be confusing. The bigger the image on the sensor, DoF is reduced, make it smaller and DoF increases.
Format is the first consideration and different formats (full frame, crop, compacts etc) yield different results because they obviously have different image sizes, but since that is fixed within any given camera we'll leave that as a constant.
The other things affecting image size are focal length and focusing distance. Focal length does not affect DoF per se (even though many people think that it does) but it obviously changes the image size. Same as moving closer or further back does. So if you fit a longer lens image size will be increased and DoF reduced, but if you then move back so that the image size is reduced again by a corresponding amount, DoF is increased once more and restored to the previous level. The net result of these two changes is that one cancels out the other and DoF is unaltered. And of course the same thing happens with a wide angle, in an opposite manner.
The rule is, if you keep the subject the same size in the frame, DoF will stay the same, regardless of focal length or focusing distance.
The other thing to consider with longer lenses is field of view, which can have a big effect on 'stand out' and subject isolation. As you tend to use long lenses from a greater distance, the angle of view is narrowed and the field of view much smaller, ie there is less background visible behind the subject. Less background usually means a much less cluttered and distracting background, which makes the main subject stand out more clearly.
With the 17-40 I feel I am fine technique wise but getting the 70-200 is proving to be a whole new beast to master. It's fun doing it though. 
