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February 28, 2010

Depth of Field on Canon EOS 50D

The camera of 2010 is far removed from the camera of the late 1800's. A camera today is a pretty much a computer with an LCD display that happens to have a lens on the front.

The growing pains from this unholy alliance are numerous and painful. The people that built cameras didn't really understand what a computer could do for a camera. And the people that built computers didn't really understand how a camera worked to begin with.

Basically, they used the computer on the camera to automatically focus the camera, and to automatically set the aperture, shutter speed, and film speed enough to record a digital photo with a acceptable exposure. And that was it.

What they missed altogether was the Depth of Field calculation. The Depth of Field is based on a few things, including the camera sensor size, focal length of the lens, the aperture, and distance to subject.

You sensor size isn't something that's going to be changing. So, your DOF calculation is usually going to be based on three variables. Focal length of the lens, aperture setting (f-stop), and the distance to the subject. Now, these calculations are not something you can do in your head very easily. People do all sorts of tricks...they print out tables and laminate them or they make little spinning charts and attach them to their lens caps. Then they go into the field with their little crib notes and try to figure out what their depth of field will be.

They squint and push the DOF Prefiew button and pretend like that can judge the DOF when really this is fairly close to useless. All of this so that they can get some idea of the DOF when they're holding a very powerful computer in their hands.

The camera knows all of the information it needs to calculate the DOF to within a gnat's @ss. It knows the focal length of the lens and the lens aperture (f-stop) setting and even the distance to the subject. The 50D has two different LCD displays where it could display the distance to subject and the Depth of Field. But it doesn't display either. Nor does it save them. The distance is not stored in the EXIF data. It's thrown away.

And this is what I mean - the computer people and the camera people are two different groups and they talk to each other without communicating. Maddening. Just maddening.

There is some discussion that some distance information may be recorded in the EXIF Makernote field at 0x0004 as "FocusDistanceUpper" and "FocusDistanceLower".

I didn't see any like this in the EXIF data available through Adobe Photoshop CS4 or my Firefox EXIF Viewer plugin.
Now that I'm looking at it more closely, I see that I have the "Maker Note" information in my Firefox EXIF Viewer plugin data. But they say that the Maker Notes are "at least partially extracted", and they're in hex, so it's somewhat cryptic.

ExifTool by Phil Harvey claims to "Read, Write and Edit Meta Information". It also mentions support for 50D Makernotes. This one looks like the ticket. Now I have all sorts of data like:
Focus Distance Upper : 1.04
Focus Distance Lower : 0.93
Circle Of Confusion : 0.019 mm
Field Of View : 21.3 deg
Focal Length : 59.0 mm (35 mm equivalent: 95.5 mm)
Hyperfocal Distance : 33.49 m
Lens : 17.0 - 85.0 mm (35 mm equivalent: 27.5 - 137.6 mm)

Of course, now I'm just plain mad because, not only does Canon know the distance to the subject, they're calculating the DOF and the Hyperfocal distance as well. Show me the freaking data, Canon. I need this information in real-time as I'm taking the picture in the field. I don't want to have to come home and dig it out of the camera with some free DOS program off of the internet. Show me the DOF and distance to subject. Hello? I can't be the only one that wants this data.

This guy says "After further investigation with DPP and EXIFtool, I now believe my 40D and my lenses are saving focus distance properly in EXIF. DPP is able to read the information without a problem. "

Apparently, Canon has a software program call "DPP". I dug around and figured they were talking about "Digital Photo Professional". I've never even heard of it. So, I opened up my 50D box and sure enough, there's a CD in there with "Digital Photo Professional v3.5 so I installed it. But I don't see anywhere in DPP that I can view the Maker Notes. File - Info shows me some EXIF data, but not Makernote data.

So, it looks like maybe Canon is storing some distance-related information in the EXIF data, but in a proprietary format in the Makernote field. Hmmmm.

But now, the trick seems to be determining the actual distance from the "Focus Distance Upper" and "Focus Distance Lower", because the "Focus Distance Upper" and "Focus Distance Lower" value doesn't seem to be right, regardless of whether it's in feet, inches, or yards.

According to Phil Harvey, these fields are pretty much useless because they "are so innacurate".

OK. I just took a photo of one of the bird houses. I chose the center focal point and put in on the bird house and took a shot with the autofocus on. I used the 50D and the EF-S 17-85 f/4-5.6 IS USM lens. This is what I get for the upper and lower distance:

Focus Distance Upper : 17.45
Focus Distance Lower : 7.95

I measured the distance, and it was approx 51', or about 17 meters, which is very close to the Focus Distance Upper. Hmmmm.


Posted by Rob Kiser on February 28, 2010 at 11:51 AM


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