DOF and COC

[Jed Shepherd]

I understand what depth of field and circle of confusion are but what i dont understand is the need for calculations. I see many dof calculators etc but i dont understand what they do and why they do it. This also applies with coc as i also do not understand it either. Ive done some searching but i always end up on photography based websites which i dont know whether to assume what they say applies the exact same way to film.

 

Basically if someone could just explain the need for figuring out dof and coc and why its useful it would be much appreciated.

[Brian Drysdale]

The principle is the same for both photography and cinematography.

 

http://en.wikipedia.org/wiki/Circle_of_confusion

 

It used for a number of purposes which are important because the subjects are moving and there are dynamics in how a director and DP may want to tell the story. They may wish to know if two subjects in a scene are going to be in focus or if only only one is. Also, for model shots you usually want a large depth of field to simulate it looking like the real object.

 

1st A.Cs use the DOF calculator to quickly work out the DOF, although with modern lenses and digital cameras all may not be as it seems, to discover margins of error or if the DOF meets the scene's requirements.

 

Creatively, the two extremes are deep focus and shallow DOF, most productions lie between these two.

 

http://en.wikipedia.org/wiki/Deep_focus

http://en.wikipedia.org/wiki/Shallow_focus

http://en.wikipedia.org/wiki/Depth_of_field

[John Sprung]

As for exactly how the calculations work, here's a spreadsheet that does it all, and shows you the formulas:

 

http://www.auner.net/misc/DOF/DOF.xls

 

As a working DP, DOF might not often rank particularly high on your list of priorities and concerns. If you have to shoot long lenses in low light, it can get so bad that an actor's eye may be in focus but his ears and the tip of his nose are visibly out. Or worse, he leans back a tad, and it becomes a shot about the tip of his nose.

 

A lot of DP's just get a feel for it from experience rather than spending a whole lot of time on numbers and math. Pick the way that works for you.

 

 

 

 

-- J.S.