DOF question

[meeker]

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,

 

[erikhaugsby]

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.

 

[meeker]

Cheers Erik! (I knew it was obvious) Is there some rule regarding the DOF - double the distance, and double the DOF?

 

[erikhaugsby]

There isn't any "rule" that I know of, though you might find more here.

 

[meeker]

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

 

[mfogiel]

This link should be helpful:

http://www.dudak.baka.com/dofcalc.html

 

[colinh]

Cheers Erik! (I knew it was obvious) Is there some rule regarding the DOF

yes.

- double the distance, and double the DOF?

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]

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.