The main question being explored here is: What is the trade-off between bokeh (e.g. subject isolation) and focal length, given similar framing of a subject. Or, in other words, when framing a subject in a similar manner, is the bokeh always the same at a given aperture regardless of focal length (obviously not). If not, then what provides the strongest gains in subject isolation, greater focal length or wider aperture?
This test has not been rigorously operated, but is fairly well controlled and set up in a real-world environment with a common subject (human). The subject, in this case my daughter, is in the exact same location with each shot. I did all the moving, attempting to reframe the subject exactly as before, but with a different focal length. I started at 24mm and roughly doubled the focal length to 200mm with each step, then followed with a 300mm perspective. I used a Canon EOS 60D, tripod and remote shutter release to capture each frame. Full-format cameras would produce somewhat different results, but the general conclusions drawn from this examination still hold. Also, different lenses would also likely produce somewhat different results, but probably not enough to affect the general conclusions. Do not read too much into the indicated focus distances as they are rough approximations--it is about time that these seemingly fancy digital cameras returned focus distance estimates with high precision and accuracy either in the viewfinder or on one of the screens.
Best viewed large. These composites are large files, about 19 MB, and it may require some time to download the full-resolution versions.
In this particular group of shots, I went for full-figure framing and only captured images at f/4. Next to each frame is a 50% crop of the image, to help show the background blurring more clearly. As the focal length increases, the trees in the distant background appear to gradually creep up on the figure. Through the use of telephoto lenses, the Ent can be created with ease! Keep in mind that the person remains the same distance from the trees with each shot. The five frames clearly demonstrate the extreme foreshortening that can be achieved at longer focal lengths.
What is also apparent is that at the same aperture, the amount of background blurring increases when the subject is framed similarly with each shot. Given that an easy way to create creamy bokeh is to move closer to the subject, and act that narrows the depth-of-field, and that the camera had to be moved further from the subject with each increase in focal length in the study, this is a clear demonstration that the focal planes of long lenses tend to be dramatically narrower than those of wide-angle lenses.
Among this group of shots, I would say that subject isolation is starting to get significant at 100mm, with more amplified results at 200mm and beyond. When strong subject isolation is desired, this series of images suggest that a good choice is to go with maximum focal length. This has the effect of requiring one to take photos further from the subject to achieve framing similar to what could be achieved at shorter focal lengths, and this may not always be possible given the constraints of the desired setting.