Auto Focus Microadjustment - 5D III

[Eaglewing]

I am about to experiemnt with AFMA on my 24-70mm L f/2.8. I read from the manual that I need to adjust the wide end as well as the telescopic end separately.

Can anyone who has made similar adjustments please advise at what distance should this be carried out.

I read from various sites that the recommended distance is to multiply focal length with 50. So for the wide end, it will be 1.2m (50 x 24mm) and 3.5m (50 x 70mm) for the telescopic end.

And for the 70-200mm zoom, it will be 10m for the telescopic end (50 x 200mm). I plan to check out focus accuracy for this lens too.

I am also making a DIY GhettoCAL which I printed from the internet as against purchasing the DataColor SpyderLENSCAL.

This post is all about the question of using the correct distance when doing a focus test. 

Thank you in advance.

[diverhank]

Yes the recommended distance is 50 times the focal length...so you would do 2 sets of calibration, 1 for the wide end at 24mm and another for the 70mm.  You enter the two numbers in and the camera will interpolate for the FL in between.  I usually use the actual distance that I shoot with the most.  

One word of caution is that lighting condition (even the kind of light) as well as intensity of light will greatly affect the camera focusing system and therefore affecting your AFMA numbers so it is important for you to calibrate using the lighting condition that you will most often use.  For this reason, in the camera manual, Canon recommends calibrating before a shoot

Even though you didn't ask, I'd offer a good AFMA method called Dottune.  This is a very good and fairly simple method of AFMA.  You will need a very good, high contrast target for focusing.

https://www.youtube.com/watch?v=7zE50jCUPhM

http://www.testcams.com/DotTune/CrossHatch_Target.png

[Eaglewing]

Hi Diverhank,

Thanks for confirming 50x FL focus distance. I shall stay with this method for now as I do not have a "usual" distance that shoot with. Will give it a try once I have completed building the DIY GhettoCal; almost there.

I may not even have a AF problem to begin with but it is nice to have some sort of confirmaition that the lens is focussing correctly.

BTW I should take delivery of a new Sigma 35mm 1.4 DG HSM Art next week; looking forward to that. The Canon version is way over my budget, hence the Sigma and I have noted pretty good reviews.

The DotTune method seems complicating; but I shall get my head around it.

[RobertTheFat]

There's no reason you shouldn't give it a try. But I've yet to see an "L" lens that needed AFMA.

[Waddizzle]

---

@RobertTheFat wrote:

There's no reason you shouldn't give it a try. But I've yet to see an "L" lens that needed AFMA.

---

The lenses that I've bought from the Big Blue Box store are terrible.  I've had to return them.  The issues I had with a 70-200mm having a loose fit came from there.  It was in stock at the store.  I've since realized that the first 70-200 lens had a loose fit because was missing the rubber ring, for weather sealing, around the lens mount. The lens I finally wound keeping had a slight front focus issue.

The BB stores seem to have quality control or security issues.  They seem to have no reservations with selling returned lenses as if they were fresh from the factory.  I have scratched that vendor off of my list of places to shop.  I've had issues with other types of gear purchased from there, too, not just camera lenses.

As far as AFMA goes, I get the best results when calibration is performed more than once, and an average adjustment value calculated.  The Dot-Tune procedure advises you to initially manually focus the lenses to determiine a range of focus.  The entire procedure is conducted by manually focusing the lens, which raises some questions.

Be aware that the cameras record AFMA adjustment information in the EXIF data.  However, no AFMA infoormation is recorded when the lens is in MF, which makes me doubt if any adjustments are made during the calibration procedure, are actually carried out with focus adjustments.  Numbers can be entered into the camera, but it seems that no focus adjustments are made if the camera and lens are NOT in an AF mode, which actually makes sense.  Why should the camera apply a microadjustment when the lens is being manually focused.  Yet, Dot-Tune seems to assume and rely on the fact that focus adjustments are applied in MF mode.

[Eaglewing]

Thanks Waddizle for your input.

Honestly, I find Dot-Tune quite complicating and I shall leave it for now until I am able to get my head around it.

And since Canon has provided AFMA in the 5D III, I shall give it try.

I will also be taking delivery of the Sigma 35mm 1.4 together with the USB Dock next week; will see how it all pans out in terms of AFMA.

[diverhank]

---

@Eaglewing wrote:

Thanks Waddizle for your input.

Honestly, I find Dot-Tune quite complicating and I shall leave it for now until I am able to get my head around it.

And since Canon has provided AFMA in the 5D III, I shall give it try.

---

Edwin, Canon gives you the option to enter your adjustment numbers in the camera but NOT how to obtain the numbers.  You still need to do that yourself.  Dottune is actually the easiest method which takes the guess work out of the procedure.  Most common method is to focus on a point on say a ruler and look at the picture and see where the focus point actually is.  If it is to the back or front of that point then you'd enter in a value to try to get the focus point to be where you'd aimed at.  Problem with this method is it's hard to see your focus point at the 50x distances.

As some have discussed, you shouldn't need AFMA on a 24-70mm L lens.  I did them on mine just for fun and got 0/0 (no adjustment).   For critically shallow depth of field kind of lenses like my 100mm f/2.8L IS Macro or my 85 f/1.2L II...For the 100mm, I needed adjustment of -1 and  that made a huge difference at 1:1.  Similarly, for the 85mm, I needed adjustment of +2 and that made huge difference in sharpness at f/1.2

[Eaglewing]

Hi Diverhank,

Just completed my DIY GhettoCal and will give my lenses a check this weekend. I should be able to obtain the numbers using this DIY kit (free). Hence my initial post regarding appropriate focus distance.

On the formula of 50 x FL, I guess I will need to focus the long side of my 70-200mm at 10m.

Thanks again Diverhank for your input, much appreicated. My next prime is a Simga 1.4 Art and I plan to shoot at 1.4 most of the time; I shall see if this lens needs AFMA, I also bought the USB Dock for this lens.

[diverhank]

---

@Waddizzle wrote:

---

  Yet, Dot-Tune seems to assume and rely on the fact that focus adjustments are applied in MF mode.

---

Hi Waddizzle..focus adjustments are indeed applied in MF mode, at least where it is lighting up the green dot.  Otherwise this method would not have worked and it does...I've tried the traditional method and compared against dottune several times because I was a doubter...I'm no longer a doubter as you can see.

In this method, you started out with the correct focus and find out the focus range.  Since we are assuming there is an error in the camera auto focus, it's critical to make sure preventing the camera in focusing so you switch the lens to manual.  Luckily the green dot (telling that you are in focus) remains working when the lens is switched to MF.  The green dot will also take into account the AFMA correction so that's why this method works.

[Waddizzle]

"Luckily the green dot (telling that you are in focus) remains working when the lens is switched to MF.  The green dot will also take into account the AFMA correction so that's why this method works." 

That's the part that I'm not convinced about.  You can see the AFMA data in EXIF when the lens is in AF, but not MF.  In fact, it makes sense for it not appear in MF mode, because no focus micro-adjustment should be made.