How does lens AF micro adjustment work?

I have a question. I performed lens AF micro adjustment on all my lenses and camera bodies a long time ago. Very happy with results. I was discussing this with a friend and the question about exactly how it works came up.

It is my understanding that when AF micro adjustment is performed you are giving the camera body instructions on how to compensate the AF to get optimum sharpness of focus. Can someone explain exactly what goes on with the camera/lens, after AF micro adjustment has been performed, when the shutter button is pushed?

Thanks for any info.

A simplified explanation:  In a DSLR, the AF sensor is in a different place than the photo sensor.  So when light enters the camera, some of it is transmitted through a slight translucency in the mirror to a secondary mirror behind the primary mirror which bounces the light onto an AF sensor.  The camera then focuses the lens so that the light hitting that AF sensor is properly focused.  There are however manufacturing tolerances in AF sensor placement and photo imaging sensor placement so that when the light is properly focused on the AF sensor, it might not be properly focused on the imaging sensor.  When you do an AF-microadjustment (Canonese) or AF Fine Tune (Nikonese), you are telling the camera to compensate for those manufacturing tolerances by either focusing the lens closer than or beyond where the AF sensor thinks optimal focus is in an attempt to get the light to focus properly at the imaging sensor plane.  Other things such as extreme temperatures, certain wavelengths of light, etc can also have an effect.  BTW, when you use a DSLR in Live View you are using the photo imaging sensor to do the AF and this is therefore inherently much more accurate.  The camera does not utilize the AF adjustment values when in Live View.

As an aside, mirrorless cameras do all of their focusing on the image sensor so no fine tuning or microadjusting is needed.

Hope that helps!

Thanks, E. J. Your explanation is exactly what I was looking for.

To add to E. J.'s excellent explanation, there are also lens tolerances to consider, which is why not all lens have the same adjustment value.

jrhoffman75 wrote:To add to E. J.'s excellent explanation, there are also lens tolerances to consider, which is why not all lens have the same adjustment value.

Yeah, I was trying to keep it as simple as possible focusing on why you need to do this at all with DSLR PDAF systems

I wish you could focus using live view and then push a button and have the camera check and calculate the micro adjustment automatically. It seems like something that would be easy to program in software.

Wildflower-nut wrote:I wish you could focus using live view and then push a button and have the camera check and calculate the micro adjustment automatically.  It seems like something that would be easy to program in software.

That is basically how the new Nikon automated AFFT works (D500, D5, D850) except that it's all over the place and not reliable - but that's the concept.  Hopefully the accuracy will improve in future models and/or firmware.

Thanks for this explanation. So, I gather that my well calibrated lens/body combo will probably not be as accurate in the cold Northeast weather we are having now? I would hate to have to do a calibration out in this weather.

Ed Cordes wrote:Thanks for this explanation.  So, I gather that my well calibrated lens/body combo will probably not be as accurate in the cold Northeast weather we are having now?  I would hate to have to do a calibration out in this weather.

It generally doesn't change enough to warrant a recal unless you did your initial cal at an extremely hot temperature or with a sun/heat soaked camera.  I've never done a recal for super cold temps and haven't noticed anything unusual.

Of course as the world moves to mirrorless, you no longer have to worry about this at all

E.J. Peiker wrote: Of course as the world moves to mirrorless, you no longer have to worry about this at all

And yet, here's a thread which suggested that AF adjustment is still needed (it's a long one and it's on Olympus EM 1 which has on-sensor PDAF):

Micro Focus Adjustment


And what exactly is the advantage of mirrorless?? Smaller and lighter ? But is it that much different :

http://j.mp/2CJMD2m


May be the total weight is a bit better but it still takes up quite a bit of room unless it's with a smaller sensor. No?

Funny you would bring that up as I just wrote an article on that yesterday which I haven't published yet where I conclude that the analysis that most people do on overall size, due to the way Camera Size displays the results with an attached lens, is pretty much bunk because it completely ignores another very important dimension for packing and size of bag that you need:
http://camerasize.com/compact/#724.514, ... 8.107,ha,b

E.J. Peiker wrote:Funny you would bring that up as I just wrote an article on that yesterday which I haven't published yet where I conclude that the analysis that most people do on overall size, due to the way Camera Size displays the results with an attached lens, is pretty much bunk because it completely ignores another very important dimension for packing and size of bag that you need:
http://camerasize.com/compact/#724.514, ... 8.107,ha,b

If you're referring to the size of the camera bodies, then I agreed that it's there that mirror-less has the advantage. But as far as the lenses go, there's not much saving in space there. Not that there is none but it's not that much IMO. It ends up not that much of saving in total size and weight at all if I'm not mistaken, and so one still needs to carry large bags.

E.J. Peiker wrote:

Ed Cordes wrote:Thanks for this explanation.  So, I gather that my well calibrated lens/body combo will probably not be as accurate in the cold Northeast weather we are having now?  I would hate to have to do a calibration out in this weather.

It generally doesn't change enough to warrant a recal unless you did your initial cal at an extremely hot temperature or with a sun/heat soaked camera.  I've never done a recal for super cold temps and haven't noticed anything unusual.

Of course as the world moves to mirrorless, you no longer have to worry about this at all

Thanks E.J.  Appreciate your input - as always.