Daniel Browning
Well-known member
Some photographers mistakenly believe that a telephoto lens will have thinner Depth of Field (DoF) when framed the same way as a wide angle with the same f-stop. I thought it was just common among amateurs, but I recently found the same mistake in Cinematography, third edition. On page 24 there are three photos with the following caption:
I will outline several reasons that this error propagates. Most people understand the effect that capture medium has has on DoF (e.g. S16 vs S35). They know how focal length comes in to play. Where most people go wrong is understanding all the implications that focal distance has on DoF and especially background blur.
Magnification increases with longer focal lengths (framed the same as a shorter one by backing up), and when a slightly blurry background (or foreground) is magnified in relation to the subject, the blur becomes a larger part of the image. The error is mistaking this increase in blur for the increase in blur that occurs when DoF is truly changed, such as by changing the f-stop or focal distance (framing). In this case the background blur was changed without changing the DoF.
Another factor is that shallow DoF can be achieved by framing tighter, which requires a longer lens, so the photographer begins to think that the longer lens was the significant factor, not the fact that framing was changed. Then he begins to think he can keep framing the same and just change to a longer lens to get thinner DoF, which is incorrect.
A telephoto lens can often achieve a thinner DoF by framing tighter than a normal lens, which adds to the myth. But with a sufficiently close focus distance, a shorter focal length can actually achieve a thinner DoF, such as with macro lenses. However, the nature of most common subjects (people) and the desired perspective makes it such that changing the focal distance is more common.
Shooting with a very fast wide angle lens, like a 14mm f/1.3 makes this myth obvious very quickly. One can frame such that everything is out of focus except a person's face, then switch to an 85mm f/1.3, back up to get the same framing, and find that the face is still fully in focus (not just the eyes as one would expect if DoF had truly thinned). But most haven't used a fast wide because they're soft, vignetted, unpopular, and expensive.
I think it's important for photographers to be aware of the fundamental contributors to DoF and background/foreground blur; it's one of the key artistic abilities that RED is enabling at revolutionary price points. I think the myth contributes to a style of photography that I dislike: using deep DoF in wide angles and shallow DoF in telephoto shots. I prefer the reverse: wider angles have a much greater amount of foreground and background that may benefit from out-of-focus blurring, while telephoto shots often have so much background removed that they don't need the additional blur of a shallow DoF (I prefer the nose and ears to be in focus).
Is this mistake prevalent among more than just neophytes? What other reasons have caused it to spread so far and wide?
I will outline several reasons that this error propagates. Most people understand the effect that capture medium has has on DoF (e.g. S16 vs S35). They know how focal length comes in to play. Where most people go wrong is understanding all the implications that focal distance has on DoF and especially background blur.
Magnification increases with longer focal lengths (framed the same as a shorter one by backing up), and when a slightly blurry background (or foreground) is magnified in relation to the subject, the blur becomes a larger part of the image. The error is mistaking this increase in blur for the increase in blur that occurs when DoF is truly changed, such as by changing the f-stop or focal distance (framing). In this case the background blur was changed without changing the DoF.
Another factor is that shallow DoF can be achieved by framing tighter, which requires a longer lens, so the photographer begins to think that the longer lens was the significant factor, not the fact that framing was changed. Then he begins to think he can keep framing the same and just change to a longer lens to get thinner DoF, which is incorrect.
A telephoto lens can often achieve a thinner DoF by framing tighter than a normal lens, which adds to the myth. But with a sufficiently close focus distance, a shorter focal length can actually achieve a thinner DoF, such as with macro lenses. However, the nature of most common subjects (people) and the desired perspective makes it such that changing the focal distance is more common.
Shooting with a very fast wide angle lens, like a 14mm f/1.3 makes this myth obvious very quickly. One can frame such that everything is out of focus except a person's face, then switch to an 85mm f/1.3, back up to get the same framing, and find that the face is still fully in focus (not just the eyes as one would expect if DoF had truly thinned). But most haven't used a fast wide because they're soft, vignetted, unpopular, and expensive.
I think it's important for photographers to be aware of the fundamental contributors to DoF and background/foreground blur; it's one of the key artistic abilities that RED is enabling at revolutionary price points. I think the myth contributes to a style of photography that I dislike: using deep DoF in wide angles and shallow DoF in telephoto shots. I prefer the reverse: wider angles have a much greater amount of foreground and background that may benefit from out-of-focus blurring, while telephoto shots often have so much background removed that they don't need the additional blur of a shallow DoF (I prefer the nose and ears to be in focus).
Is this mistake prevalent among more than just neophytes? What other reasons have caused it to spread so far and wide?
