Originally posted by THoog 
This is correct - the lens ID is communicated by the 'digital' data pin, which did not exist before the 'F' series lenses. 'K' and 'M" lenses have no data pins at all, and the data pins on 'A' lenses only provide codes indicating the smallest and largest aperture the lens is capable of.
Ok that was my memory. I couldn't remember how exactly the A part worked other than it just did. I actually think I only have one A lens but many M lenses.
In my own workflow, I do often forget to change the focal length when attaching M lenses. I've not necessarily noticed any negative impacts or if they are there I just attribute them to user error; I don't turn off shake reduction, so in theory I could see issues. I mostly just edit the Exif record at the end of the day to reflect the lens I've used when using an M or A lens because I rarely use more than one different one in a day. This method obviously doesn't work if you only use M lenses. In that case, I'd probably make a more conscious effort to appropriately set the focal length when I am turning the camera on.
The other consideration when dealing with M lenses is to see what focal length fields are filled with what data (when not using a FF camera). Many image files will show at least 2 focal length fields. One is the actual focal length of the lens, and the other is meant to be a 35-mm equivalent focal length (to obviously be taken with a grain of salt). The exact title for that field may be reported differently as I have mostly quit looking at it. But, if you are using an APS-C camera, it very well may report 50-mm as the actual focal length at 75-mm for that equivalent length. That could be critical if you are either setting the wrong number or looking at the wrong number.
As a final note, I remember being quite fixated on that 35-mm equivalent focal length. It helped because as I transitioned from a simple P&S digital camera to a bridge type camera to a K10d, the 35-mm equivalent helped me figure out what lenses I might need. But, man it also confused a lot of things too as I was a bit ignorant on what an APS-C camera really meant and that it wasn't an FF camera (it was the same size as my old film SLR) and the whole concept of focal length was messed up until I really got a grasp on what was different (i.e. FOV). Now I've been shooting APS-C for nearly 15 years, so I've quit worrying about equivalence of anything except when I pencil out what I might need if I ever buy a K-1 (or any other FF body).
Post #2 by Adam
Similar Threads 
