Focal length (tag)
From SnapMap
focal_length=<nnn>mm is the simplest way to specify a photo's focal length. If the camera is not a 35mm film camera, or a digital camera that uses the full frame area of a 35mm film camera, this must be converted into its 35mm equivalent.
For convenience, zoom=<nnn>mm is a synonym for focal_length.
Most digital cameras store the zoom setting in the EXIF data, so this is a good source for focal_length. But it probably needs to be multiplied by a "crop factor", depending on the sensor of the camera.
Contents |
Crop factor
Take care to discover the crop factor in any camera whose photos are imported. A crop factor higher than 1 is caused by a camera sensor smaller than "full frame", where full frame means 36x24mm, the area of film exposed in a traditional 35mm film camera.
Note that it's the ratio of the diagonal across the frame that is the factor to use. The diagonal in 35mm film is sqrt(36^2 + 24^2) = 43.27mm. Let's say we have a sensor of 20.7xcmm; its diagonal is sqrt(20.7^2 + 20.7^2) = 29.27mm. Thus we have a crop factor of 43.27/29.27 = 1.48. A Canon 400D has a crop factor of 1.6, meaning that an image with EXIF data reporting a 50mm focal length has a 35mm-equivalent focal length of 1.6 x 50mm = 80mm. This new value of 80mm is what should be used in the focal_length tag.
Cropped images
Quite apart from the virtual crop that takes place thanks to having a small sensor, we need to consider images that have actually been cropped by the photographer, or somewhere along the road to publication. The only ideal case is when the centres of the cropped and uncropped images remain identical, i.e. equal amounts are removed from either side, and equal amounts removed from top and bottom. In this case, a new focal length should be calculated using an easily determined crop factor, as if the sensor were smaller, comparing diagonals as above. But arbitrarily cropped images are not so neat. We have two choices:
- a) Pretend that the centre has not moved, and set up the focal length in the normal way, using catapult views or PhotoOverlays. Because this upsets the heading and elevation, we may be unable to get a perfect
- b) Record the horizontal and vertical fields of view, each with start and end angles. Difficult and unlikely to be supported by user agents, except Google Earth with PhotoOverlays.
In practice, many old images we come across lack any indication of how they were cropped, if at all. We are forced to choose (a).
Accuracy
We have read reports of cameras being somewhat inaccurate in their recording of focal length.
