Depth of field - effect of focal length

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Henryti New Member • Posts: 17
Depth of field - effect of focal length

Hi,

Grateful if any of you technical, engineering sorts out there can help with this. My question is - how is depth of field affected by the focal length of a lens if you keep the subject (eg. a person) the same size in the frame?

My understanding is that, all else being equal,

(i) the longer the focal length the shallower the depth of field and

(ii) the closer you are to the subject, the shallower the depth of field.

Using a 85mm and a 50mm lens as an example, in order to keep a subject the same size in the frame, you would have to be closer to the subject with the 50mm lens compared to the 85mm. So at the same aperture on the same camera, which would have the shallower depth of field? Is there any kind of formula or guide if you want to compare different focal lengths?

Thanks.

Henry

 Henryti's gear list:Henryti's gear list
Fujifilm X-T2 Sony a7R IV Fujifilm XF 18-55mm F2.8-4 R LM OIS Sony FE 50mm F1.8 Sony FE 24-105mm F4
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