I can't think of any better way to explain this than by using actual photographs. The depth of field is the same, the field of view is different. The crop body does not modify the depth of field.
Put the calculator away and take pictures.
Here is some feedback on the subject. Any comments are welcome.
From Joey Cassidy,
"I have a question regarding Gary Gray's field report on the depth of field of a crop vs full frame sensor.
Gary made the point very clear that both sensors have approx the same depth of field at the same subject distance, focal length and aperture. However, it seems more relevant (and what most people are referring to in the many forums) to compare the depth of field of the two sensors at the same field of view, as this is much more applicable when comparing two like photographs.
If you were to take the 5D and walk closer to give the same field of view as the 30D, would you not have a shallower depth of field?
If two shots were made at different distances with different bodies using the same lens, the depth of field would be different for each shot. Depth of field (at the image plane of the camera) is a function of the focal length/aperture of the lens and the distance to the subject. Change the distance, you change the depth of field. The body is irrelevant. There is a common practice to compare the 1.6 crop sensor to full frame in this regard, but I find it much simpler myself to treat the two the same. Mainly because the crop sensor cameras are designed the same way as the 35mm bodies are designed, to use the 35mm version of the lens. The only difference is the camera has the added feature of cropping off the outer third of the image for me. This isn't magnification, it's loss of image. One could apply the same theory upwards to medium formats / large format and even downwards to point & shoot sensor sizes, but for some reason the topic seems to hover around this particular frame size issue. On this theory, a point & shoot is your best possible solution for telephoto work. I think perhaps it gives amateur photographers the false belief that they are getting something extra when in fact they are always getting less with a crop sensor body. I don't blame them though, as I own a couple of crop sensor bodies as well. The wishful belief that my 500mm lens somehow magically becomes 800mm on my 30D sounds impressive, but in truth, it's still a 500mm lens with a large portion of the image missing from my view and my print. To make up for my discomfort, they cram more mega pixels into a smaller area, which also tends to make people feel warm and fuzzy.
Thanks for the feedback and visit us again.
Final notes...summer of 2010...
Since posting this article in 2007, the debates and discussions have continued to rage on and the confusion seems to be ever present on this article and subject. I've received countless emails on this article and quite honestly, I don't intend to respond to this subject any longer. The most common argument seems to be based on an assumption that two different bodies will give two different depth of fields for the same framing of a subject. The basic problem with this point of view is this, and again I'll try to explain it as simply as possible.
When you take a photograph, how many cameras do you generally use to take that photograph?
When you take a photograph, how many lenses do you generally have on your camera?
When you take a photograph, how many times to you take the same photograph using two different cameras?
Calculating a depth of field is the same, no matter what camera you use to take a photograph (assuming a DSLR.)
The body doesn't affect depth of field. Only the focal length of the lens, aperture and distance to subject affect depth of field.
Comparing two bodies is pointless unless you plan to take both bodies to a scene and take two shots of a scene from two different positions to obtain a similar field of view with identical dof. This never happens in the real world (I've never seen photographer do it in the past 10 years) A photographer takes a photograph with one camera and one lens, or maybe sometimes switches lenses. Sometimes moving around to get a different view. But taking the same shot with two different bodies to get the same look...naaaa, never happens.
There's a difference in trying to understand the effect of using two bodies on a similar fov shot and actually taking photographs in a real situation. Don't confuse them.
The Depth of Field Myth - Full Frame vs Crop Sensor
There seems to be a common belief that the camera with a full frame sensor will provide the photographer with more or less, (depending on which techno-wiz camera geek you talk to) depth of field than a cropped sensor camera such as the EOS 30D, which can be viewed either as an advantage or a drawback (depending on which techno-wiz camera geek you talk to.) On some internet forums you'll find never ending debates over this camera vs. that camera and the difference in depth of field one format sensor will provide you over another.
Forget about all that. If you're like me, I don't need nor do I want to take a ruler and a calculator with me when I'm strolling around with my camera, so I can calculate a miniscule depth of field change between camera bodies. What ever your need as a photographer may be, I am here to show you that the different camera bodies will provide roughly the same depth of field when using identical focal lengths and the same exposure. The more depth of field that the 5D supposedly gives you is a myth. Functionally, from a photographic standpoint there is no difference.
Below are two photographs, taken one right after the other, using the EOS 30D and EOS 5D with a Sigma 105mm Macro lens, pointing at a yardstick from the same exact distance using a tripod. I'm manually focusing on the 20 inch mark of the yardstick. The only obvious difference between the shots is the field of view (not to be confused with the depth of field.) The EOS 5D will give a wider field of view than the 30D using the same focal length lens. If you examine the depth of field provided in both shots by tracing the yardstick from the 20 inch mark, you'll see that the focus field is roughly identical.