Is there any general set of rules or guidelines on when would the background compression generated by a long focal length look good in a picture?

Example : in a headshot, I really like the background compression (besides face flattening) from the 85mm vs 50mm

For reference, I shoot with a full frame and generally get a hard time deciding between 50mm and 85mm in outdoor situations. I can back up as much needed (open areas) and shots I am going for are 1/2 to 3/4th of full body. At times, full body too.

  • Are you possibly confusing background compression with bokeh? Compression just means that longer shooting distances (which normally accompany longer focal lengths) will make the distance between subject and background seem smaller. Which looks better is purely subjective. The difference in compression between an 85mm and a 50mm can be offset by changing the distance between the subject and background at the same ratio as the camera to subject distance. – Michael C Dec 14 '15 at 5:30
  • I originally composed an answer thinking the OP was asking about bokeh, but I deleted it because I now realize he may be talking about something else. – Mike Sowsun Dec 14 '15 at 6:54
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    I understand the differences between bokeh and background compression. The question here is about the latter. – Mayank Jain Dec 14 '15 at 6:58

I usually concern myself with background compression depending on if it will affect how I want the shot to look. If I want to emphasize the size of something in the distance I will go to a longer lens and move farther away from the foreground elements. That increases the apparent size of object in the far distance. An extreme example is the moon. Shoot a cityscape from far away with a 500mm lens and the moon on the horizon will look huge. Conversely if I want to shrink background elements or emphasize foreground elements I'll go to a wider angle lens. I've never had a rule like it sounds you are looking for. I just know that a 50mm on a full frame is about "normal" and as I deviate to either side it will affect my picture more. If you were doing a portrait like you mentioned but want to include a background element that you wanted to look bigger, I'd go much longer than your standard portrait lens. I guess one thing I've always considered is; will the compression make objects look too unrealistically exaggerated (either very large or very small)? And is that the look I am going for or am I trying to avoid that?

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