Does the distance to the object affect the blur?

Question

Photo

Good night, such a question.
For example, we have an 85mm lens.
We set the aperture to 2, for example, and leave a picture in the frame where a person can be seen from the top of the head to the waist.
The second situation, we take a 35mm lens, also an aperture of 2 and leave the same picture from the head to the waist, but we have already come closer because of the 35mm.
And the closer we are to the subject, the smaller its width of field around the subject.
Does this point refer to increasing the blur of the same picture if we take 35mm and stand closer or will there be no difference?

Answer 1

Of course, you forgot to specify one of the conditions that is needed for calculations - the size of the camera sensor.
To shoot a person of average height at full height with an 85mm lens (we assume that this is a camera with a full-frame sensor), you need to stand about six meters from him.
To do the same with a 35mm lens, you need to stand at a distance of 2.5 meters.
Based on this, we can calculate that with a relative aperture of f / 2 (as indicated in the question), the depth of field for a 35 mm lens will be about 55 cm, and approximately the same for an 85 mm lens.
For the case if this is a camera with an APS-C sensor, the numbers will be, respectively, 9.5 meters and 4 meters, and the depth of field will be about90 centimeters in both cases. In fact, the numbers will not be the same, but in practice, it will be impossible to notice the difference.
If you calculate exactly, then the depth of field of a lens with a smaller focal length will be slightly larger than a lens with a longer one - for example, using the same conditions and an APS-C camera, a 210 mm lens will give a depth of 902 mm, and a 20 mm lens will give 941 mm. However, it should be taken into account that this calculation is idealized, and therefore, in reality, with a lens with a shorter focal length, you will have to move a little further due to the perspective effect.