What useful things can/can't you find out from exif tags?

Question

I'm aware that you can find out a lot from looking at the exif tags of an image, such as camera make/model and the various settings used to take the picture, but what non-obvious things can you discover about how a photograph was taken using the exif tags.

Almost as important - what are the limitations of the exif tags set by default in cameras? For example, I'm looking back through our wedding pics (we were able to obtain digital copies), and see on a picture that the Exposure Program was 'Normal Program' (as opposed to something like Aperture Priority or Landscape mode). Unless I'm misunderstanting, I can't tell if this means the image was shot using Program or Auto mode on the camera.

Answer 1

I usually look at EXIF if I found something wrong with the picture and want to learn from it. Plain obvious, but most useful are:

• aperture (is the DOF too deep/too shallow? does my lens really vignette so much at that aperture?)
• shutter speed (was it fast enough to freeze motion/cancel hand shaking? or was it slow enough to get the desired effect?)
• ISO (why is my picture so noisy?)
• focus mode in case of focus errors (didn't I switch to manual incidentally?)
• exposure compensation (didn't I notice the blown highlights?)
• lens used/focal length (do I like/dislike the field of view?)
• time/time between shots (how was the light and how did it change between shots?)
• anything intentionally set to custom value (white balance, metering, self-timer, etc)
• date becomes important later

For JPG shooter the picture settings like white balance/contrast/saturation/sharpness/quality are probably also very important things.

When you own multiple cameras then the camera itself also becomes important.

People who use flashes will probably care about whether it fired and what was the flash exposure compensation.

Answer 2

I used the EXIF tags over thousands of images to determine the focal lengths I tend to use, to try to help me decide which lens I should buy first when I upgrade.

Answer 3

Some cameras will write the focus information to the EXIF (e.g. focus point used, focus distance).

You can try KUSO Exif Viewer to see what it can reveal about your files.

The specification for ExposureProgram (0x8822) can be found at http://www.awaresystems.be/imaging/tiff/tifftags/privateifd/exif/exposureprogram.html and doesn't differentiate between auto and program mode (although maybe some cameras would put the scene mode chosen by the auto program, if their auto program fucntions like that).

Answer 4

I'm very keen on geotagging, and try to tag all of my pictures with a location. I find it's much easier to remember where I took a photo than to remember exactly when I took it, for some reason. So that's probably my most-used EXIF tag.

Some newer cameras have GPS and can do this automatically. For other cameras, you'll need to carry a separate GPS unit and insert the EXIF tags later - I've had good results with a Gisteq PhotoTrackr, which can be had for about £65.

Answer 5

Here's the C struct from a 2003 CodeProject:

typedef struct tag_ExifInfo {
    char  Version      [5];
    char  CameraMake   [32];
    char  CameraModel  [40];
    char  DateTime     [20];
    int   Height, Width;
    int   Orientation;
    int   IsColor;
    int   Process;
    int   FlashUsed;
    float FocalLength;
    float ExposureTime;
    float ApertureFNumber;
    float Distance;
    float CCDWidth;
    float ExposureBias;
    int   Whitebalance;
    int   MeteringMode;
    int   ExposureProgram;
    int   ISOequivalent;
    int   CompressionLevel;
    float FocalplaneXRes;
    float FocalplaneYRes;
    float FocalplaneUnits;
    float Xresolution;
    float Yresolution;
    float ResolutionUnit;
    float Brightness;
    char  Comments[MAX_COMMENT];
    unsigned char * ThumbnailPointer;
    unsigned ThumbnailSize;
    bool  IsExif;
} EXIFINFO;

I adapted the program to rename image files the way I wanted them but Nikon Transfer does the job for me now.

Answer 6

One of the potentially useful things I know of which isn't in the EXIF data on any camera that I'm aware of is the actual flash power used. As described in that question, the most obvious use here is trying to replicate a scene with automatic flash with manual flash (e.g. because you've moved the flash off-camera and don't have automatic control in that situation), but there are a number of other possibilities as to why this would be useful.

Answer 7

The most useful things I gather when looking at an image's exif are:

• The date the image was taken
• The camera make and model
• The lens used to take the image

Obviously exif information can be scrubbed and edited, but to me, those are the most important things I look at when looking at exif data.