Depth of field: speed vs focal length

[gwg]

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Hi,

 

I am wondering about a new slightly longer lens and am between a faster 50mm (1.4 or 1.5) and a slightly slower 75mm (2.5). While I'd probably be able to shoot more hand-held with the 50, shoot in lower light and have frame lines on my M3, I was attracted to the 75mm because of the way it renders for portraits and blows out the background. A 90mm would probably not be ideal for me, since I prefer handheld shooting and don't shoot all that much in fully sun-lit outdoors conditions. (And my M8 isn't a high-ISO demon...)

 

Here's my question: Is there more depth of field with a 50mm 1.4/1.5 than with a 75mm 2.5 both at the respective maximum aperture?

 

Thanks in advance,

George

[Lindolfi]

Depth of field is independent of focal length at the same frame size in object space. So if you use f/2.8 with a 50mm lens framing a portrait, you get the same depth of field as with f/2.8 and 75mm framing the same portrait. The only differences are that the distance to the subject is 1.5 times longer with the 75mm and therefore the perspective of the subject. Also, although the depth of field is identical, the bokeh discs of highlights very far away are about 1.5 times larger in diameter with the 75mm lens than with the 50mm lens.

 

So it's your choice what you find important: perspective, depth of field or bokeh discs. That should determine your choice for the primary subjects of interest.

 

The sizes of the bokeh discs from highlights far away are about the same for a 75/2.5 and a 50/1.4 (slightly larger for the 50mm) at same focus distance, but about 1.5 times larger in the 50/1.4 at the same frame size in object space.

[mjh]

Here's my question: Is there more depth of field with a 50mm 1.4/1.5 than with a 75mm 2.5 both at the respective maximum aperture?

Yes, indeed there is, although the difference isn’t that big – at the same distance that is. If you adjust the distance to compensate for the different angle of view then you will get less depth of field from the 50 mm than with the 75 mm.