Focus Distance Steps

[rhameed]

Hi all. I am a new member here and hoping you guys could help me out with some information that I need.

This is with reference to the discussion in following thread, about focus distance in canon exif:

https://exiftool.org/forum/index.php?topic=2934.0

It was suggested that the upper and lower focus distances reported by canon DSLRs in the exif correspond to the boundaries of a fixed number of focus range steps. And also that the number of these steps changes with each lens. For example someone in that thread suggested that the EF-S 17-55mm lens has 28 distance steps and other lenses might have as low as 10 or less steps. I am wondering if there is a resource which can help me find the count and boundaries of these steps for a given lens. For example the following link has some data but it is very sketchy:

http://magiclantern.wikia.com/wiki/Focus_distance

Will appreciate any help in this regard

[BogdanH]

Hi,

I have never seen a focus range/steps reference. I.e., I've got those values for EF-S17-55 by tryout, because I needed them to check something.

Bogdan

[rhameed]

Thanks for your response!. I actually just posted to an old thread in which you posted the numbers for 17-55mm lens. That was very helpful because based on your numbers this lens might have enough accuracy for my needs. However as I posted in the other thread I have questions about the data. Specifically how you derived the single distance numbers from the "upper" and "lower" boundary numbers and also if you still have the actual upper and lower boundaries for each steps instead of the single consolidated number.

[BogdanH]

To get these numbers, I have simply set lens to MF and after each taken photo slightly changed focus distance on lens. I repeated that until I got all focus value pairs connected. Here's my result:

   EF-S 17-55IS
   Focus distances reported to camera
   =====================================
   Lower   Upper       Lower   Upper
    0.33    0.34   /-->0.99    1.08
    0.34    0.41   |   1.08    1.18
    0.41    0.44   |   1.18    1.30
    0.44    0.47   |   1.30    1.45
    0.47    0.49   |   1.45    1.63
    0.49    0.52   |   1.63    1.87
    0.52    0.56   |   1.87    2.18
    0.56    0.60   |   2.18    2.62
    0.60    0.69   |   2.62    3.27
    0.69    0.74   |   3.27    4.34
    0.75    0.80   |   4.34    6.43
    0.80    0.85   |   6.43   12.33
    0.85    0.92   |  12.33  136.23
    0.92    0.99 --/  81.91  655.35


Note the lower value of the last pair (not a mistake). I assume, that the last pair (upper value, actually) only indicates "infinite" focus. As far known, these values are used by camera for calculating flash exposure -so, the precision of last two/three value pairs isn't all that important, though.

Bogdan

[rhameed]

Thanks a lot. This is very useful.

The accuracy is just about OK for my use. I am interested in using this for a computer vision project. Specifically the focus distance information from the lens will be used to form a rough estimate of the distance of an object in the scene. And for that I only need an accuracy similar to human vision. We humans can tell if something is roughly 0.5m away or roughly 1m away. But obviously we can't really tell if something is 600m away or 800m, so the accuracy goes down at higher distance. I would have been happier if there was another step between 12 and 136, but this might be good enough.