Started 2 months ago | Discussions thread
Bill Ferris
Bill Ferris Veteran Member • Posts: 6,447

mostlyboringphotog wrote:

LASR wrote:

This seems a very confusing way to present things. Why not just say that a 50 mm lens in FF has the same field of view as 50/1.5 = 33.3 mm (35 mm) lens in APS-C (if the crop factor is 1.5x).

Since the amount of light, depth of field, depends on the physical aperture (as BF said) the aperture of a 50 mm lens at f2.0 is 50/2.0 = 25 mm, the same aperture on a 35 mm lens corresponds to f1.4, 35/25 = 1.4, therefore, 50 mm f2.0 in FF has the same field of view and depth of field as 35 mm f1.4 in APS-C.

To be a bit more stickler -

50mm f/2.0 and 35mm f/1.4 may have the same ~25mm aperture diameter but 50mm f/2 has 1.5x more (deeper) DOF than 35mm f/1.4. However, Fuji APS-C is enlarged by 1.5x crop factor relative to the FF sensor, making the APS-C sensor 1.5x shallower DOF, which cancels out the deeper DOF of 50mm f/2 lens.

Same aperture (entrance pupil) diameter does not keep the DOF same (THE TRUTH)

With respect to LASR's post, both field of view and entrance pupil are stated as being the same. In that scenario, both the geometric and perceived depth of field (DOF) will be the same.

If we're focusing solely on a geometric depth of field - a range of physical distances within which the image appears to be acceptably in focus - you are correct that two different focal lengths operating with the same diameter entrance pupils at the same distance from the subject can produce images having different geometric depths of field.

However, there is an element of subjective perception involved in DOF. If we examine DOF as it is perceived by the viewer, then two photos made at the same distance from the subject, using lenses having the same entrance pupil diameters but different focal lengths can be perceived as having the same DOF.

Here's a post from last year in which this is demonstrated: https://www.dpreview.com/forums/post/64536294

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Bill Ferris Photography
Flagstaff, AZ

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