DOF Calculation

egproductions

egproductions

Veteran
I'm trying to figure out what is going to give me a shallower DOF with the equivalent framing:

85mm @ 1.2

or

200mm @ 2.8

At the same distance to the subject the 200mm offers shallower DOF but what about when I frame up the same or similar shot like just the face?

Does anyone have a calculator that can calculate what subject distance is equivalent between two lenses?
 
Thanks Mino. Thats actually the DOF calculator I was using until I realized that the distance is going to change to get the same framing of the shot.

I guess my question is, how close do I have to get to achieve the equivelent framing of a 200mm at 10 feet away if I put on the 85mm lens? If the 200mm lens is 10 feet away would a 100mm lens be 5 feet away? Is it that simple?

I'm actually going to try a test my my 100mm and 50mm and see what it looks like visually....now wheres that measuring tape....
 
OK, its looks like the focal length is directly related to the distance between the two lenses. So a 200mm at 20 feet is the same as 85mm at 8.5 feet.

This means the 85mm at 1.2 is shallower than the 200 @ 2.8 (framing be equal)

Even more interesting, If you put the 85mm @ f2.8 to match the 200mm, You get the same exact amount of DOF as the 200mm (framing being equal)
 
Sorry for anyone that isn't interested in this but thought I would think outloud in case you are or can help me come to terms with how these calculations work.

I thought everything made sense until I went to Mino's link and put in the same values (ex. 50mm, f2.8, distance 10ft) and switched what type of camera the lens is on (full frame v.s. cropped) The calculator actually says that you get a shorter DOF with a cropped sensor than with a fill frame sensor. To me this isn't making any sense because it should be the same DOF no matter what body the lens is on. a 50mm lens @ f2.8 on a 1/3" video camera should have the same DOF as a 50mm lens @ f2.8 on a full frame body. The only difference is that the 1/3" sensor image will be more zoomed in. What am I missing?
 
The size of the sensor does effect the DOF. This is the reason that P&S cameras with very small CCD's have such a large DOF even at low f/stops. I don't know the technical reason behind it, but I do know that it is true.
 
uh oh...Many a DOF arguments start this way =). Don't want to say I am definitly right this is just my understanding of the topic:

The size sensor size effecting DOF is kind of misunderstood topic. What really creates the shallow DOF is that when you use larger sensors you are also using longer focal lengths in order to achieve the equivelent FOV of the smaller sensors. In reality when you stick your full frame lens on your cropped sensor body you get the same exact DOF but your FOV decreases. Its true this happens because the sensor is only covering part of your image circle but lenses that are designed for smaller senors are also created by producing smaller image circles to begin with.

Even if it where true the the DOF is shallower on bigger sensors, its still the oposite of what the calculator is saying happens.

If I had time I would love doing field tests and really break down and build up my understanding of DOF.
 
You won't get an argument out of me. :)

This actually sounds like a really nerdy test. I might play with it this weekend and see what happens.
 
And for what it's worth, I get the moving the camera to create the same FOV and thus changing the DOF.

And I also think reason should dictate the DOF would be the same if no factor is changing other than the size of the sensor. Camera position, subject position, focus point, lens, and aperture all remaining constant. I just know that science and often photography defy what my brain comprehends. There seem to be a lot of write-ups on the part we agree on, but I can't find much content for the part that we seem to understand differently.
 
As a photographer doing these "nerdy tests" don't mean much at first in regards to taking nice photographs but once it becomes second nature it really provides you with the tools or understanding of your tools to use them better and come out with better work.
 
Nice link. I've never been able to find a test shot like that. Like I said before, totally makes sense. I've just always heard otherwise. I agree it was well written, but I'm a little disappointed someone did the nerdy test before I had the chance to.
 
What I've really been curious about latley has been the difference between telephoto and wider lenses at the same aperture and the same framing of the shot. The amount of DOF distance is the same although the distibution of bokeh is heavier behind the subject with telephoto lense than with wider lenses.

I've also been craving the flattening and exagerated background size that telephoto lenses offer. Check out this planet earth shot (20 seconds in). I saw this shot on television and before they could cut to the wide shot I could tell this was an incredibly telephoto shot....so bold!

http://www.youtube.com/watch?v=UWmk6IGBTYs&feature=PlayList&p=F1FC4395D90FBB30&playnext=1&playnext_from=PL&index=14
 
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