meeker
Established
I have a question that's been bothering me for a while, and I'm sure this is blindingly obvious to most here, but here it is:
Does DOF (at constant f-number) change with distance from the lens? If so, at
what distance do the DOF markings refer to on the lens barrel.
thanks,
Does DOF (at constant f-number) change with distance from the lens? If so, at
what distance do the DOF markings refer to on the lens barrel.
thanks,
erikhaugsby
killer of threads
Yes, the DOF of a lens changes relative to subject-lens distance. As the lens is focused further toward infinity, DOF increases such that more of the image is "sharp" relative to closer distances.
The DOF Markings are valid for all focus distances, as the DOF change is compensated by distancing the focus marks on the distance scale.
The DOF Markings are valid for all focus distances, as the DOF change is compensated by distancing the focus marks on the distance scale.
meeker
Established
Cheers Erik! (I knew it was obvious) Is there some rule regarding the DOF - double the distance, and double the DOF?
erikhaugsby
killer of threads
meeker
Established
I found a calculator on the web. For a 50mm/f2, it looks like every time you double the distance, the DOF increases by a factor of 4:
distance, total dof
1m, 0.05m
2m, 0.21m
4m, 0.84m
8m, 3.51m
distance, total dof
1m, 0.05m
2m, 0.21m
4m, 0.84m
8m, 3.51m
mfogiel
Veteran
colinh
Well-known
meeker said:
yes.
no.
The equations and derivation are about halfway down this wikipedia page
They're not really complicated. It's just similar triangles and a bit of substitution. It helps if you work through it yourself with pencil and paper (for a couple of real examples). If you do this you'll get a really good feel for it.
Most people are not all that interested
colin
Dektol Dan
Well-known
Rule of Thumb for DOF
Rule of Thumb for DOF
Whatever your apperature opening focus 1/3 in 2/3 out. For example, if shooting into a crowd focus on individuals 1/3 the distance into the group. Then those in front of your target will be in focus and those twice that distance behind it will be in focus.
Of course, if you have a fast lens wide open these distances in and out can shrink to inches so plan accordingly.
Just think exponential, not linear.
Rule of Thumb for DOF
Whatever your apperature opening focus 1/3 in 2/3 out. For example, if shooting into a crowd focus on individuals 1/3 the distance into the group. Then those in front of your target will be in focus and those twice that distance behind it will be in focus.
Of course, if you have a fast lens wide open these distances in and out can shrink to inches so plan accordingly.
Just think exponential, not linear.