Clarifying my understanding of DOF

[Mr_Sukebe]

I was just looking to improve my understanding of DOF.

From what I can see:

DOF is directly related to the aperture and distance to focus point and indirectly related to focal length of the lens.

So does that mean that if I'm focused onto a target say 4m away, that the DOF would be pretty much exactly the same for a 100mm lens at F2.8 as a 50mm lens at F1.4?

 

[joescrivens]

100mm on a full frame at 2.8 with subject 4m DOF = 0.27

50mm at 1.4 subject 4m DOF = 0.54m

if your subject was only 2m away on the 50 then DOF = 0.13

to get the same dof at 50mm to the 100 the subject should be at 2m but aperture at 2.8 or 2.85m away aperture 1.4

http://www.dofmaster.com/dofjs.html

 

[HoppyUK]

I was just looking to improve my understanding of DOF.

From what I can see:

DOF is directly related to the aperture and distance to focus point and indirectly related to focal length of the lens.

So does that mean that if I'm focused onto a target say 4m away, that the DOF would be pretty much exactly the same for a 100mm lens at F2.8 as a 50mm lens at F1.4?

DoF is a function of image size (magnification) and f/number.

Both subject distance and focal length affect magnification (as does format) but if you maintain the size of the main subject in the frame, DoF remains the same regardless of distance and lens.

Other things change a lot if you do that though, like perspective and field of view.

 

[Mr_Sukebe]

My thanks chaps.
So in short, the calculation is non-linear. That helps.
Have to say that I was surprised to see that if you plug in the figures for say an 85mm and 100mm lenses, that there's not a massive difference between F2 and F2.8.

 

[HoppyUK]

My thanks chaps.
So in short, the calculation is non-linear. That helps.
Have to say that I was surprised to see that if you plug in the figures for say an 85mm and 100mm lenses, that there's not a massive difference between F2 and F2.8.

No, not linear - pretty much an exponential curve. Doubling the distance for example increases DoF, but doubling it again increases it a lot more.

You can do simple stuff in your head like shooting at 10ft with a 50mm lens will give the same DoF as 20ft with a 100mm lens, but I'm not sure how helpful that is.

Things that help me, for the kind of photography I do, is to remember that DoF gets very shallow at close distance - that makes more difference for me than fiddling with f/numbers.

The other thing to remember is that DoF is just a concept, based on a set of faily subjective parameters, that defines a certain level of sharpness - that says things on one side of a line will be sharp, and those on the other will not be.

Of course it's not like that - the transition is smooth and continuous and the definition of what looks sharp certainly doesn't fit all situations, or all viewers at all times.