Can you estimate subject distance for depth of field when using a telephoto lens?
Asked 8/20/2021
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Most depth-of-field calculators need the actual subject distance, but with long telephoto lenses it can be hard to judge how far away the subject really is. Is there any way to convert an object's apparent size in the viewfinder into its physical distance? Also, if two subjects appear to be separated by a certain distance along the depth axis in a telephoto view, does the required depth of field depend on that apparent spacing, or only on their real physical spacing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
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What is perceived distance? If you know the physical size of the object you can determine its' physical distance by how much of the FOV it occupies.
The math is: (sensor dimension ÷ focal length) = (size ÷ distance)
Or, there's an app for that
Secondly, the DOF always has to be equivalent to the actual physical spacing of the objects ("y").
And thirdly, there is no such thing as perspective distortion nor compression. There is only perspective (relative distance) and it is the same for every lens at the same distance (the different lens FOV's only crop the scene before you).
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
4y ago
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Depth of field is based on actual subject distance, not how far away the scene appears in the EVF. A telephoto lens changes framing, but it does not create a different “viewfinder distance” you can directly use for DOF.
If you know the real size of the subject and how much of the frame it fills, you can estimate distance from similar triangles:
sensor dimension / focal length = subject size in frame / subject distance
So with a known subject size, focal length, and framing, you can work back to the physical distance.
For two subjects at different distances, the needed depth of field is determined by their real physical distances from the camera. Their apparent spacing in a telephoto view does not change the optical DOF requirement.
Also, the “compressed” look of telephoto images comes from perspective due to camera position, not from the lens changing perspective by itself. At the same camera position, perspective is the same; a longer lens just shows a narrower field of view.
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