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Old Nov 24, 2006, 11:43 PM   #1
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Sometimes I want to know the subject distance when I shoot.

When I use irfanview, clicking on the "i" and then "EXIF Info" buttons gives you information in a child window. In EXIF info, focal length is listed. Below EXIF Info, the Maker Notes list "Subject Distance" about the fourth line from the bottom.

Today I put the rear edge of my camera about 39.75 inches from the wall. The calendar sticks out from the wall about 0.1 inch (I'm trying for 1 meter here). When I used maximum optical zoom, the "subject distance" was listed as 100.

Just one problem -- For all other zoom lengths, subject distance in the EXIF info varies without any pattern that I can determine. Frankly, even the "100" at 432mm might have been a chance occurrence.

So "subject distance" might be centimeters, but is altered by some other factor. Does anyone have any insight on this?

Does Canon have forums like AMD and various non-Intel motherboard manufacturers? I don't want to sound impatient, but they might have special technical insight.

Thanks in advance,


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Old Nov 24, 2006, 11:51 PM   #2
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Was focus locked when you took the photos? If not, then the information is not going to be reliable, even if the EXIF viewer can decipher it properly (and it may or may not be able to).

Newer camera models and lenses have the ability to determine focus distance (some newer lenses can report information to the camera that it uses for this purpose).

This information is used most often to help with flash strength calculations. But, not all lenses report the information.

Unfortunately, the Maker Notes section in the EXIF is proprietary to a manufacturer. They don't publish how they write to it. So, when you see EXIF viewers report this information, it's because they reverse engineered it, or information was "leaked" by an internal source.




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Old Nov 25, 2006, 12:13 AM   #3
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Was focus locked? A couple of times I just pushed the button down all the way without pushing it halfway first. Those are blurry, and "subject distance" in the Maker Notes can vary wildly from the very next shot, where I let the camera focus first (half-press). Ignoring the out-of-focus, I still can't find a discernible pattern between focal length (EXIF) and subject distance recorded (Maker Notes), keeping camera distance to subject constant.

I used Program mode with auto-focus (and the green focus-assisting beam) for all shots. ISO 400, I believe. Flash was used rarely, I don't think that would make a difference though. Polarizer was probably on (a mistake, but probably not a fatal one).

I waited for a clean image of the green lamp to come into the LCD before shooting. I think this is what you mean by "lock".

So far, the only thing I know is that $14 does not buy a good tripod. I should get a better one.

I suggest my next experiment will be to keep focal length constant (36mm or 432, which should I choose?) and vary distance to the subject. I don't want to "have to" reverse engineer this stuff, but I will if that's what it takes.

Canon doesn't have any forums, but there's always a way to ask questions. They don't have to answer, though.

If anybody has any ideas on how to extract this information, I'd LOVE to know it.

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Old Nov 28, 2006, 2:13 PM   #4
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I got a response from Canon technical support. Here are pieces of it:


"Unfortunately, our software does not calculate the distance of the object that you are focusing on."

"There is no way to actually determine the distance of which object based on the zoom setting that you have chosen for your pictures."

"For example, you may take a picture of a landscape scene, who is to say that you were focusing on the tree, the mountain, or the sunset?"


(Frankly, I only thought I would use this for simple portraits, and never more than 10 meters away.)


There you have it. When you use maximum zoom, and you are close to the subject (1 to 1.5 meters in my testing), the distance recorded in the EXIF is accurate, but it is just a coincidence, according to Canon's e-mail.

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