Archibald
Travel Guide
We often want to know what the magnification was of a macro shot we have taken. The Canon R7/RF 100mm macro combination shows distance and magnification information in the viewfinder when we are shooting. But back at home on the computer, that information isn't readily available. How to get that information?
While we can't directly learn the magnification, we can look up the Focus Distance that is recorded in the EXIF data in the raw and JPG files. The Focus Distance is measured from the sensor to the focused subject. Both Upper and Lower focus distances are recorded - not sure why there is a difference. Maybe it indicates uncertainty in the measurement. The two values are usually similar. The distance information can be recovered with software like ExifTool. It is also optionally reported by Lightroom Classic under Metadata. (Irfanview reports a "Subject Distance" but it is not reliable.)
With a simple lens, it is possible to calculate the magnification using the formula m = F/(u-F), where F is the focal length and u is the lens to subject distance. That formula doesn't work with a modern lens like the RF 100 because the focal length isn't known. That's because the lens is internal-focusing and the focal length becomes shorter with increasing magnification. We also don't know the value of u because it is measured from the front principle plane whose position varies.
But an empirical approach provides answers! We can determine magnification (by photographing a ruler) and distance (also using that ruler). Remarkably, the data gives a beautiful linear relationship between the reciprocal of the magnification and the recorded focal distance.
From this, I came up with the following simple equation to calculate magnification:
m = 1 / (11.319 * FocusDistance - 2.1438)
FocusDistance is the value reported by the EXIF or Lightroom Classic.
This works for the R7 and RF 100mm macro. I haven't investigated other combos (for now).
While we can't directly learn the magnification, we can look up the Focus Distance that is recorded in the EXIF data in the raw and JPG files. The Focus Distance is measured from the sensor to the focused subject. Both Upper and Lower focus distances are recorded - not sure why there is a difference. Maybe it indicates uncertainty in the measurement. The two values are usually similar. The distance information can be recovered with software like ExifTool. It is also optionally reported by Lightroom Classic under Metadata. (Irfanview reports a "Subject Distance" but it is not reliable.)
With a simple lens, it is possible to calculate the magnification using the formula m = F/(u-F), where F is the focal length and u is the lens to subject distance. That formula doesn't work with a modern lens like the RF 100 because the focal length isn't known. That's because the lens is internal-focusing and the focal length becomes shorter with increasing magnification. We also don't know the value of u because it is measured from the front principle plane whose position varies.
But an empirical approach provides answers! We can determine magnification (by photographing a ruler) and distance (also using that ruler). Remarkably, the data gives a beautiful linear relationship between the reciprocal of the magnification and the recorded focal distance.
From this, I came up with the following simple equation to calculate magnification:
m = 1 / (11.319 * FocusDistance - 2.1438)
FocusDistance is the value reported by the EXIF or Lightroom Classic.
This works for the R7 and RF 100mm macro. I haven't investigated other combos (for now).
