Quote Originally Posted by DavidEccleston
The 200mm f/2.8 wouldn't just be for getting a high shutter speed, but also for isolation, which the which the longer 300mm lens wouldn't be as good at...But, the new 70-300 is f/5.0 @ 200mm, so 2.333 stops better for still subjects, but also 1.666 stops worse for isolation.

I'm assuming you mean for identical framing, e.g. 200mm f/2.8 at 10 m from the subject, or 300mm f/5.6 at 15 m from the subject. If you maintain the same distance to the subject, the isolation will actually be better at 300mm than 200mm despite the narrower aperture, since subject distance has a larger impact on DoF than aperture in these ranges. So, if you can get close enough to use the 200mm, and you have the 70-300mm zoom, you'll get better isolation with the zoom at 300mm f/5.6 than with the prime at 200mm f/2.8.


For example at 10 m distance to subject, 200mm f/2.8 will have a DoF of 26 cm. At the same distance, the new zoom at 300mm f/5.6 would have a 23 cm DoF - so, the extra 100mm of the zoom lens more than makes up for the loss of two full stops of aperture in terms of DoF.


Depending on what you're shooting and how close you are to it, you may actually need to stop down if you want to get the entire subject in focus - I run into this 'problem' with my 100-400mm, where at 400mm for a small, close bird I often need to stop down to f/7.1 or f/8 to get the whole bird in focus.


Here's hoping there are more lenses announced for Photokina - personally,I might be tempted by a 50mm f/1.4 II, butI'm really hoping for a 35mm f/1.4L II...


--John