More on Lens Selection

You've probably heard all sorts of things regarding "normal lenses," which lens to use for portraits, and how a 50mm is not a good portrait lens due to the distortion it creates (or the opposite recommending a 50mm). And maybe you've heard that you can "zoom with your feet" by moving closer/further when using a prime lens.

All of this is basically wrong and it comes from not really understanding how lens selection affects things.

Here are three images all shot from the same position (+/- a little wiggle from being handheld) using three different focal lengths. 82mm, the often recommended portrait focal length. 50mm, the "normal" focal length on a full frame camera. And 28mm, the normal focal length for some crop sensor cameras. All images in this article were shot on a D800 using the 28-300mm lens @f/5.6 (the widest available aperture at 300mm)

 

82mm
50mm
28mm

They are all certainly different because they contain a different field of view. But the perspective is exactly the same in all of them! Here are the three images cropped to the same field of view. They are exactly the same other than less depth of field due to using a longer focal length. (hard to see at this size and due to using f/5.6).

Now here is the same image captured at the different focal lengths by changing the subject distance.

82mm
50mm
28mm

And just for grins, at 300mm.

I wasn't exact in keeping the people the same size, but it's pretty close. Here you can see the perspective has clearly changed due to the differences in working distance. The longest focal length includes the least amount of the background. The images also have approximately the same depth of field even though it doesn't look like it. Using a longer focal length from further away for the same subject size keeps the depth of field constant.

This is why you cannot "zoom with your feet." An image looks entirely different taken with a shorter focal length from closer than it does if taken with a longer focal length from further away.

For any image there is only one "ideal" focal length, subject distance, and aperture. It's all a balancing act. In fact, changing subject distance has a much larger affect on the depth of field than changing aperture does if the focal length is kept constant.

Think of perspective like judging the distances between cars parked on a street. The further away the cars are, the closer together they appear, and the harder it is to judge the distance between them. If you want to minimize apparent differences between things, use a longer working distance. That's why most "professionals" use longer focal lengths for portraits...it allows them to work from further away without having to crop away most of the image.

 

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