DOF Question...

[msoskil]

Which will give me less DOF - My 100mm at 2.8 or my 50mm at 1.4? Or

are they equal. I know there are algorithms that can figure this

out, but I haven't learned them yet.

[brian_diaz]

How about a DOF calculator:

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<a href="http://www.dofmaster.com/dofjs.html">http://www.dofmaster.com/dofjs.html</a>

[msoskil]

Brian,

Thank you. That was exactly what I was looking for. I've bookmarked that site. Very helpful.

[jbq]

For a given magnification, the DoF only depends on the aperture, so the 100/2.8 will have more (about twice as much).

 

For a given distance (at normal magnifications, i.e. 1:10 to 1:20) the DoF shrinks as the square of the focal length, so the 50/1.4 will have more (about twice as much).

[steve_hovland]

But when it comes to applying these calculations, take them with a grain of salt.

 

I shoot interiors and recently made charts from the DOF calculators. I then tested the charts in room shoots and found that if I trusted the charts my pictures weren't as sharp as they should be.

 

My tests consisted of measuring the subject depth then consulting the charts for an f-stop and focus point that should have been "good enough." It wasn't.

 

I suggest looking for settings that have at least 50% more depth than the chart suggests.

[michael_barkowski]

When using a calculator, keep these things in mind:

 

- with the 100mm you'll probably be standing further from the subject.

 

- depending on the quality of the lens, you may be forced to prefer higher apertures for reasons other than DOF. For example, people say my 55/1.8 lens is "soft" up to F8.

 

So to sum up, the calculator is no guarantee. It is a great learning tool though.

[msoskil]

Thanks for all of the answers. I'm just trying to figure out which lens will blur the background better for pictures of my daughter inside during the winter. They won't be studio shots, just pictures to e-mail friends and family. Seems like the 50 1.4 is the way to go. Allows me to stay closer and gives less DOF.

[james_oneill]

For 35mm film you lets assume you want to do a portrait where the subject fits in a square 24cm x 36 mm. A 50mm lens will need to be about 50mm from the subject and a 100mm lens about 100mm from the subject. A 50mm Lens foucsed at 50CM has DOF +/- 1 CM @ f/2.8 and less than 0.5 CM @ f/1.4. A 100mm lens from 100CM has +/- 1CM @ f2.8.

 

Wide open the 50 has the least DOF, but they both have very little. At equal apperture the 50 will have more (you get more with the wider lens - some of which is lost by getting closer, but not all of it).