Subject Distance, S3 MakerNotes

[saturndude]

In Canon's MakerNotes, the "Subject Distance" is listed in every JPG created on my S3, and probably other Canons too (all DIGIC 2, or all Canons period?).

When my S3 is at full zoom, the distance recorded in MakerNotes appears to be in centimeters and is accurate (I checked with test shots and ruler).

But when I am not at full zoom, the "subject distance" listed is not the true distance. It is usually between 0.6 and 0.75 times the distance (in centimeters).

Does anybody know how to interpret this information? When I am not at full zoom (432mm), why does my S3 even bother to record numbers that aren't true?

Does anybody have any insight?

[BowerR64]

It knows the distance of the subject? no way, how does it know what my subject even is?

If it tells the focal length i can see but i doubt it can tell my subject distance.

If im shooting a picture of the moon and im at full zoom does it say 385000 km

[saturndude]

I've taken shots at 100cm, 125cm and 150cm (as close as I can, I don't know where the sensor is inside the S3). At full zoom, it is correct. Otherwise, it is not.

Subject Distance is in the MakerNotes, about the fourth line from the bottom. I don't think this will be very relevant in fast-action sports, or distances too far away (more than 10 meters?). But Canon chose to put it there, and if it can make me a better photographer, I'd like to at least try to use it. But first I have to decipher it (unless I am at full zoom).

[downtrodden]

Maybe you could post a picture and a screenshot of your exif data to demonstrate.. because some fields are filled in wrong. for instance, my ISO rating usually shows up in my flash field if flash did not fire. Sometimes it will be "Flash did not fire" and sometimes it will be 80, 100, or 200 (the ISO's i use)

The only way the camera could know distance to subject is if it had some sort of rangfinder in it.. and if it did i'd think there'd be a display on your LCD to tell you how far the target is.. this would help in manual focusing.. but it doesn't- and manual focusing can be tedious because of it... Doesn't make much sense to me, i'm with Bower on this one. Maybe try contacting Canon customer service and asking them what the numbers actually represent?

~Cory

[KSM]

Maybe you could post a picture and a screenshot of your exif data to demonstrate.. because some fields are filled in wrong. for instance, my ISO rating usually shows up in my flash field if flash did not fire. Sometimes it will be "Flash did not fire" and sometimes it will be 80, 100, or 200 (the ISO's i use)

The only way the camera could know distance to subject is if it had some sort of rangfinder in it.. and if it did i'd think there'd be a display on your LCD to tell you how far the target is.. this would help in manual focusing.. but it doesn't- and manual focusing can be tedious because of it... Doesn't make much sense to me, i'm with Bower on this one. Maybe try contacting Canon customer service and asking them what the numbers actually represent?

~Cory


It does show the distance. Focus and then press the manual focus button and the scale will show the distance it is set to correctly if your at full zoom. You can change it but it shows the distance at first. At any other zoom it shows an incorrect distance. Pretty much the same results that saturndude had to start with.

Why does it do this? I have no idea.

[saturndude]

I had already contacted Canon technical support. Here are pieces of their response:


"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?"**


*Nasty remark #1 -- maybe the hardware (auto-focus) does so, because SOMETHING put that EXIF data in there!

**Nasty remark #2 -- Frankly, I only thought I would use this for simple portraits, and never more than 10 meters away.

(I won't make those nasty remarks to Canon, but I do wonder about this data field.)

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.

Somebody at another forum suggested Canon does not publish how they use EXIF data unless you join their developer program. It is free, but I fear it would take more time (and brain cells) than I have.

Thank you all for your time. Maybe we'll get useful info from this field at some point, maybe not. But we tried.