Aperture and depth of field
A question that came up in the forum recently was whether an 70-200mm f4 zoom would give you the same kind of look that an f2.8 zoom would.
For me, the f2.8 aperture is essential, especially with a telephoto zoom. For the same scenario, it gives me a higher shutter speed than the f4 zoom. Or I can use a lower ISO. More importantly, since I often bounce flash in large areas, the f2.8 aperture gives me more chance of successfully bouncing my flash than an f4 aperture would.
But what does the change in depth-of-field look like?
Keeping in mind that the how much the background appears to be blurred, depends on a few things:
– the chosen aperture. The wider the aperture, the shallower the depth-of-field will be, giving a more blurry the background. (This is the main factor that affects how much the background will be blurred.)
– how far your subject is from the background,
– your chosen focal length.
– The bokeh of the lens will also affect whether the background appears more blurred, or less. (Note that bokeh and shallow depth-of-field are not the same thing.)
The change in depth-of-field is incremental. If you had to look at a photograph showing amazingly thin depth-of-field, you wouldn’t be able to tell just from the photograph alone, whether f2 was used or f2.8 … but a side-by-side comparison will reveal the answer. An individual image won’t. However, it will be obvious when an image was shot at f 8 or f11 (on a 200mm lens for example), compared to either f2 or f2.8 … we should be able to recognize that a shallow depth-of-field was not used on the f8 or f11 image.
There are hordes of examples of how depth-of-field changes with aperture. But I thought it might be interesting to see with an overlay of images, just how much the depth-of-field appears to change …
Jessica and I braved the cold today to do a simple sequence of images with the zoom set to 200mm, and used at a variety of apertures. I used a tripod to keep the framing exact, and Jessica tried to keep as still as possible. The background is a clump of trees about 20 meters behind us. The lens used in this example is the Nikon 70-200mm f2.8 VR II (B&H).
Ideally, there should be a bunch of these comparative sequences done at different focal lengths and different distances between subject and background.
But just this one example should clearly show how the depth-of-field changes … and that while the change between (for example), f2.8 and f4 is distinct, it might not be enough reason to warrant the purchase of a more expensive lens. But that is up to personal choice and budget.