DOF findings

[karusel]

So I've been wondering what relation have aperture and focal length on DOF to a subject - frame of the same size. So I grabbed a DOF calculator and shortly after few entries pattern started to emerge. I've finished the chart regardless. Chart is valid for a 1.6 crop camera, the frame size is always 3.564m x 2.376m so basically a group of about 5 people with some head space.

FL-------F/num--distance--DOF
-----------------------------------------
35mm----f/1.4---3.5m----0.53m
50mm----f/1.4------5m----0.53m
70mm----f/2.8------7m----1.07m
70mm----f/4.0------7m----1.52m
85mm----f1.2------8.5m----0.43m
85mm----f/1.8----8.5m----0.67m
100mm---f/2-------10m----0.75m
135mm---f/2-----13.5m----0.75m
200mm---f/1.8-----20m----0.67m
200mm---f/2.8-----20m----1.06m
200mm---f/4.0-----20m----1.51m
300mm---f/2.8-----30m----1.06m
400mm---f/2.8-----40m----1.06m
400mm---f/5.6-----40m----2.13m

Surprising to me was that DOF depends on the f number alone (at the same frame size) regardless of the focal length.

[photofinish]

Here's a depth of field calculator you might find useful. :-)

DOF Calculator (http://www.outsight.com/hyperfocal.html)

[karusel]

There's a better one here (http://www.dofmaster.com/dofjs.html). ;)

[photofinish]

You are right. I like yours better. Thanks!

[ron chappel]

It's true!!

********DOF depends entirely on cropping and aperture*********

So
-NO MATTER WHICH LENSES YOU USE -
...if the cropping is the same and the aperture the same then the DOF WILL ALSO BE THE SAME


Of course it's impossible to compare different lenses when focused near infinity because you can't exactly move forward or back up to get the same cropping,but anytime you are doing closer subjects this rule applies easily.

Portraitists tend to use this rule exclusively ......while landscapers allways think in terms of "wide lens = big DOF,long lens = shallow DOF "





* Actually i should say that this rule isn't an ironclad optical given,but it's so close to being so that it's an extremely usefull way of visualizing and understanding DOF.

Certain lenses can be made to have slightly more DOF even at the same aperture/cropping.Macro lenses are like that i'm told.
Also some lenses have odd optical designs that definitely give less DOF than others (for a given aperture,etc) The canon 75-300 designs are one,the pentax SMC-M 50/1.4 another