A depth of field question

[inkybutton]

Hi all!
I came across DoFMaster the other day and I decided to play with its DoF chart thingy. I put in the values for EF 50mm f/1.8 II, a lens I'm thinking of buying (effective focal length of 80mm because of sensor crop on my camera; circle of confusion of 0.019mm.) The resulting table kinda surprised me. According to it at f/2, even focusing at something 2m away the depth of field would be 4cm (starts at 1.98m, ends at 2.02m). The figures sound more reasonable when in smaller apertures. But still... Am I interpreting it correctly? If so, is this true in real life or is the table screwed up?
Thanks!

 

[Plato]

Those values sound correct.

 

[robbie_vlad]

4cm sounds about right. When I'm shooting portraits with my 50 1.8, the average distance I am from the subject for a headshot is 1.5-2m. At this distance if I dont open the aperture to around f/4, only the eyes will be in focus while the nose and ears will be OOF.

 

[NateWagner]

ok, I just went to the links you mentioned... and that's not what it is showing me.

at 50mm and f/2 you're looking at 1.94-2.06 which is I guess 12 centimeters (or about 5 inches).

Edit: You're looking at the 80mm. The only problem is that it is only the perceived focal length that is about 80mm not the actual focal length. Thus the actual DOF for the 50mm with your camera would be about 12 cm when 2m away from the subject.

 

[Overread]

If the table is in meters for the distance then the depth of field will be measured in cms - so 1.94-2.06cm not inches - mixing the two is just asking for confusion

As for the distances I would not worry about them myself - sure they are good to know, but much of the math behind photography doesn't mean anything till you hold the glass in your hand and shoot with it in a real world setting. Then it gains meaning to you.

 

[NateWagner]

Well, I am the only one who mentioned inches... the 1.94-2.06 is indeed centimeters. I just converted to inches for myself as that's how I think (it's actually 4.7 inches).

I agree though, knowing the numbers won't really help you as much as just experimenting with the different apertures at different distances and figuring out what happens.

That's probably the case with a lot of things.