Roger

You can email me at Roger@lensrentals.com Unfortunately I may be of little help there. The 7D in AI Servo mode is a place I just never, ever go.

Could I have your email address to write to regarding an focusing issue i have with my Canon 7D?
For the same low light level shooting condition, the camera’s AF system somehow just fails to focus in ‘AI Servo’ mode but achieves focus in ‘One Shot’ mode – even with a static subject!!!

From my limited experimenting and problem determination, this phenomena seems to occur for Single Point AF, AF Point Expansion, Zone AF & 19 Point AF, although it works slightly better in 19 Point AF mode with the center focus point locking in focus at times.
I would like to send you some images to illustrate my point.
Would you know what could be going on here?

For a semi pro (hi performance) camera that can work up to ISO 6400, one would expect the camera to be able to achieve focus under similar low light shooting conditions, be it ‘One Shot AF’ or ‘AI Focus AF’ for static or low movement subjects in conjunction with wide aperture lens e.g. Canon 50mm f1.4.
Wouldn’t you agree? Otherwise, Canon (Marketing?) should have advised accordingly.

As the AF achieves focus in One Shot AF but not AI Servo AF, I think the camera’s hardware is working OK, but something is not quite right (bug?) with the AI Servo AF code/firmware implementation.

Just so you know, I have contacted local Canon support regarding this but thus far have not had a reply from them yet. Canon tech i am liaising with tried on other Canon cameras (7D, 5Ds)and experienced the same phenomena.

Hope you can advise and assist with an explanation for this Canon AF phenomena I have stumbled on.

Enjoy your insightful articles very much.

Many thanks and very best regards,
Kenneth

i also tested my (cherry picked) 85 f1.8 which appears to have extremely consistent and accurate AF.

note that none of your consistent lenses have apertures larger than 2.8

Please accept my apologies for my presumptiousness (presuming you didn’t know LC).

Well, the cool thing is you have dealt with the electro-mechanical element i.e. the AF modules, and LC the practicalities and techniques of and for obtaining good focus. Between the two of you, you have got the subject of accurate focus completely covered in my view.

Now what I would like to see is a publication including your combined efforts. That would be the ultimate treatise on accurate focus.

Get to it guys!! The photographic community needs it!!

And if you “name and shame”, that is, publish a list of those lenses that work well with certain bodies and those that don’t, I will lay money on it that you will start to see Canon and Nikon rather quickly updatedating their lenses or at the very least improving their AF fine tune systems (they should allow AF fine tune of at least three apertures per lens to be of any real use). In London, oh I dunno, I think 20 or so years ago, the police started publishing a list of the cars that were most easily broken into and stolen (based on their figures as a result of call out, chases, and recoveries etc). The motor industry cried foul; amongst other things they suggested that the police were by definition engaging in product endorsement which they shouldn’t be doing. Anyway, the motor industry lost the argument, buyers began to gradually shun cars on the police blacklist and then very rapidly indeed car security improved. ;o)

Given your excellent study, and as you alluded to in one of your comments, really the camera manufacturers ought to be open and tell us all which of their lenses work best with which cameras (if they know, and I doubt that they don’t) instead of giving the impresssion (by saying nothing) that they are all pretty much as capable as each other (give or take an update here, and a tweak there).

Warmest regards,

And thanks again for the education.

plevyadophy

All the lenses were tested wide open to avoid any possible focus shift. The 70-300 you mention was f/4 when wide open at the focal length tested. But I totally agree with all of your points. I correspond with Lloyd Chambers regularly (and subscribe to his blog. He is, indeed, and invaluable resource and has helped me on many occasions. Not the least of which is “Lloyd’s already done that, so it doesn’t need to be repeated.”.

Best regards,

Roger

Additionally, I would like to add, perhaps, one minor, or major depending on one’s view, criticism; then a suggestion for further study and/or collaboration; and finally offer my take on the AF accuracy in low light issue.

Criticism
==========

In your study you seem to have overlooked the issue of focus shift. Have you over-looked it, or is it not entirely relevant to your study?

Anyway, what made me think you have overlooked it is your comment above where you say: “It might be that the 70-300 seems more accurate because it’s being tested at f/4 rather than f/2.8, but we’ve tested other lenses at f/4 and they weren’t this accurate.”

If a lens at it’s widest aperture, or near widest aperture is being tested against a lens that has to be stopped down one or two stops to make the same aperture, then it is understandable that the stopped down lens may show less focus accuracy. This may be so due to the issue of focus shift due to aperture change.

Collaboration
=============

To me, the leading authority, well the leading authority of the all the bloggers I have seen (and I have read lots) on the issue of focus shift is Lloyd Chambers.

The man is positively anal about lens/image quality. I don’t say that as a citicism of him by the way. If he says a lens is good then beleive me it’s good.

However, having read his numurous posts and after reading your excellent study what I find is that both of you are studying the same thing, namely focus accuracy but coming at it from different angles. Also, it does appear to me that there a gaps in your understanding, which he can fill and vice versa.

I really think you should collaborate with each other on a major study of focus accuracy. What the two of you have produced seperately is already excellent but the two of you working together would produce something awesome, that’s assuming the camera manufacturers don’t have you both assassinated first! )

His website is here: http://www.diglloyd.com/ and the subscription only section of his site, which to my mind is a MUST read is here (Making Sharp Images): http://www.diglloyd.com/index-msi.html

Declaration: I am an avid reader of his blog and a subscriber.

Comments
=========

Reading your study was an eye opener with regard to the problems that may be taking place as manufacturers try to make their AF modules work in lower light.

I think this is a classic example of “if it ain’t broke, don’t fix it” and “newer ain’t always better”.

According to Rob Galbraith and otehrs, the AF reliability, at least for continuous AF and tracking AF is/was far more reliable on the “old” Canon 1D Mark IIN than on the newer 1D Mark III and IV models hence why some sports shooters refuse to “upgrade”.

With that in mind and having read your comments, it seems to me that Canon, and maybe Nikon too, have had the solution to the problem for years but refused, for reasons best known to the themselves, to provide it. The solution is simple. In the case of Canon, all they had to do was leave the AF module of the 1D Mark IIN well alone and simply add an AF illuminator to the new Mark III camera body. To make it less intrusive they could have provided a laser type AF assist as Sony once did with their F series cams instead of a red or orange lamp.

It beats me why both Canon and Nikon seem to think that their respective 1D and D3 series cameras don’t need an AF assist lamp but must instead rely on a big lump of a flash gun atop the camera instead to provide AF assistance. Had Canon at least done this, that is provide an AF assist on the camera body, then maybe most of their 1D Mark III problems would have disappeared (that’s of course assuming the beam had long enough reach for sports shooters).

Warmest regards,

And thanks for the education,

Plevyadophy