50 vs 85 1.8

Started 2 months ago | Discussions thread
Tom Axford Veteran Member • Posts: 6,063
Re: 50 vs 85 1.8

Aldone wrote:

I am not 100% sure how this works, so could someone explain which of these two focal lengths (50 or 85) will produce the most shallow depth of field, assuming the same framing? Of course the depth of field is shallower with the 85 if the two are focused at the same distance, but if I step back to achieve the same framing of the subject (ex. A person) the depth of field increases. Which of the two factors, distance and focal length has the greater effect, or do they balance out?

For the same framing, the depth of field at f/1.8 will always be about the same, no matter what the focal length.

However, I would guess that you are probably also interested in how much background blur you might expect to get.  That is not the same as depth of field.

For a background at infinity, the amount of blurring is proportional to the entrance pupil of the lens, which is the focal length divided by the f-number, i.e. 28mm for 50mm f/1.8  and 47mm for 85mm f/1.8.

So the 85mm lens gives the same depth of field, but much more background blur.

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