Friday, December 14, 2012

Micro Adjusting - Absolute Bare Minimum Approach

EF 40mm STM Lens "Front Focusing" (Dec. 13, 2012)

Corrected (Dec. 14, 2012)

For all of the high tech bashing I've been doing lately, in favour of old technology (film), I must say that high tech digital cameras are offering certain undeniable advantages, especially the more expensive models. Yesterday I was on the moan about my new and expensive EOS 7D being slightly out of focus, with all lenses, all the time. I was suspecting a lens to camera micro-focus calibration issue, and happy to say just one day later, it looks promising this diagnosis was correct.

Yesterday, I spent some of my Christmas money on a new lens - the Canon EF 40mm f2.8 STM "Pancake". These are currently on sale pretty much everywhere, and I've been wanting one of these lenses ever since it was announced. So now I have one, and it is my first, and only genuine Canon Prime Lens (meaning, a single focus length of 40mm, non-zoom). All my other Primes are old Pentax, or Russian built M42 screw mounts which I use with an adapter. Then I also have two very old Canon EF Zooms - a 28-105 USM II and a 70-210 f4.

So, late yesterday afternoon I took a short walk with the Pancake to take some test shots. With one shot in particular, I saw a great opportunity to do up this micro-focus test once and for all. I took the top picture being very careful to set my single selectable focus point on the space between the 2nd and 3rd rock. Upon arriving home and carefully looking, I could see that the Auto-Focus actually happened on the face of the first rock. Again, I noticed that all of the other pictures were peculiarly soft, which is consistent with all of the other lenses.  I did some reading on EOS 7D Micro Adjustment, and discovered all kinds of high -precision methods available for doing this - here is a link to one of many. I found all of the methods to be quite overwhelming - I like keeping things real simple, and based on the bare minimum of what I need to know, which are:

  • Is my lens front-focusing or back focusing?
  • Which way do I need to adjust, + or - ?
  • How much do I need to adjust?


OK so it's very obvious from the first picture that my lens is front focusing, and by quite a bit. I reasoned that if this is the case, then  I would need to correct the camera's focal plane "backwards" - away from the camera. I looked up the procedure in my camera's manual, and, for information sake, this is all done through the camera's built-in firmware menu system - there are no screwdrivers required to make this calibration. When you come to the correct menu (read your camera's manual for details), you'll find a little pictogram scale showing that to adjust toward the camera for "back-focusing", you turn the rear dial toward "-" (negative) and for "front-focusing" you turn the dial toward "+" (positive), away from the camera. I started by setting it on a moderate "+8", went out and took the same picture, again being careful to auto-focus on the gap between the 2nd and 3rd rock. And, eureka, this seems to work exactly... got it right the first time! For now, I'm going to assume all my other lenses will benefit from this adjustment in the same way, although everything I read would seem to indicate this would probably not be the case. 

All I really want to do here is to get back the same performance I was enjoying with my old EOS 40D, which by the way did not have this micro-adjust feature - it just worked for all my lenses, with no friggin'. We shall see over time how it works out.

What I've done here is the bare minimum, in keeping with the instructions in the Canon 7D manual -

  • Adjustment is probably unnecessary
  • It works better with prime lenses than it does for Zooms
  • It should be done on the site of an actual photograph, (implying that making this adjustment with all the fancy lab charts is not necessary, and probably not as good)

I would really like to close off on this over the coming weeks, so I expect to be doing a lot more digital, instead of film photography, just to go through most of my lenses to see of my single global adjustment will indeed be good enough in real-world shooting. I'm expecting that it will be.

No comments:

Post a Comment

Reader's comments are welcome, and are subject to moderation by the author.