Longer Lens = More Background Blur? Yes and No.

I had this question bouncing around in my head for a while and no one could give me a good answer. I know the answer now so I figured I might as well write it down.

The question is “do longer lenses give better DoF blur?” The short answer is no BUT they do compress the perspective giving the feel of more blur. So maybe the answer is yes in a practical sense and no in a technical sense.

Example: I take a photo of an apple with a banana in the background 5 feet away. I shoot it at 50mm/F2.8. Next I shoot the same scene at 200mm/F2.8 being careful to fill the frame in the same way (e.g. the apple is the same size in the viewfinder). The shot at 200mm will have what appears to be shallower DoF due to the compressed perspective from the apple to the banana. The amount of blur is the same but the  distance between the two objects appears less.

Here’s a good article on the subject of perspective. I read this years ago and rereading it now probably should have been able to answer my question from what the writer says. For whatever reason it remained a point of confusion to me for a while.

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