Does anyone know how to get more accurate subject distance numbers from their equipment. My 5d3 and 500 mmf4 either says 45.7m or 79.9 m or infinity on the long end. But theres a big gap between 45.7 and 79.9 etc.
Just wondering, these numbers are useful when picking keepers from a batch of say 500 shots
Where did you find the distance? "exiftool" doesn't report it for me, though it's obviously in the file because Canon's processing tool shows an uncalibrated dot on a near to infinity bar. Files from Nikon DSLRs do show distance with exiftool.
To a first approximation, to change focus from infinity to distance X a lens with focal length L has to move the elements about (L^2/X). What the lens tells the camera is not "how far is focus?" but "how far did I move internally?"
Because the measured quantity is the inverse of focus distance, the measurements will be less accurate at long distance. Alternatively, consider that you are seeing equal increments in reproduction ratio rather than equal increments in distance.
For example, if your lens is 500mm and the internal measurement has 1mm precision, the zero point is infinity focus, the first non-zero value corresponds to 250 meters, and the second is 125 meters.
The subject distance numbers have never been accurate with any Canon body or Canon lenses that I have owned. Like you write certain numbers can come up all the time.
Lars Johnsson wrote:
The subject distance numbers have never been accurate with any Canon body or Canon lenses that I have owned. Like you write certain numbers can come up all the time.
I guess thats about it then, another nit for Canon to address as well as the lack of a built in heater for the body to keep your fingers warm in the winter. If they can do it on a steering wheel why not a camera
PaulCal wrote:
I guess thats about it then, another nit for Canon to address as well as the lack of a built in heater for the body to keep your fingers warm in the winter. If they can do it on a steering wheel why not a camera
They can do it if you want to carry a car battery along with your camera.
Vox Sciurorum wrote:
Where did you find the distance? "exiftool" doesn't report it for me, though it's obviously in the file because Canon's processing tool shows an uncalibrated dot on a near to infinity bar. Files from Nikon DSLRs do show distance with exiftool.
To a first approximation, to change focus from infinity to distance X a lens with focal length L has to move the elements about (L^2/X). What the lens tells the camera is not "how far is focus?" but "how far did I move internally?"
Because the measured quantity is the inverse of focus distance, the measurements will be less accurate at long distance. Alternatively, consider that you are seeing equal increments in reproduction ratio rather than equal increments in distance.
For example, if your lens is 500mm and the internal measurement has 1mm precision, the zero point is infinity focus, the first non-zero value corresponds to 250 meters, and the second is 125 meters....Show more →
ExifTool will report it. Look in the "MakerNotes" section of the .cr2 file for the two tags: "FocusDistanceUpper" and "FocusDistanceLower" http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/Canon.html#ShotInfo
You are correct that there is no tag specifically for object/subject focus distance in the RAW file, but if you look in the "XMP" section of an image exported (as a tiff or jpeg) by ACR/Lightroom you will find an "Approximate Focus Distance" tag
It is not at all clear how Adobe comes up with that number. A logical guess, given the nature of focus movements, would be the harmonic mean of the FDUpper and FDLower, but that's not it, nor is it the average of the two. Sometimes their focus distance is not even in the interval between the upper and lower focus distances! Here are some tables I compiled for a few lenses. This is info provided to the camera by the lens. Is there some Canon guideline that requires 31 zones or just a coincidence?
