Variations in AF performance: any reason why?

[40d_dane]

The shots with the 40d/incandescent seem a bit front focused too. I don't think ma will help unless you plan to shoot only under incandescent light (if this theory were to hold true).

The 40D shots are indeed front focused - the difference between the 7D and the 40D shots is that the 40D front focus is to a large extend independent of the light source.

I did some quick ones myself today just with 7d/17-55 @ 2.8, 55mm, 35mm. The target was the old 40d box at 6' which we all know is red with white "Canon" on it. I shot tripod, no IS, sunlight and out of 7 shots, they were consistent (and in focus), matching LV shot for reference. The only time I didn't get consistent focus was focusing on the word 'Canon' when wide. Focus improved if I focused on the top of the box (focus square half on box, half wall). Not scientific but it's all I had time for. Maybe there's more to this 'shooting under incandescent' theory but it still doesn't explain photonuts' shots.

After shutting down my PC yesterday I noticed that Canon mention that there is chromatic aberration in the AF optics splitting the two phases used for focusing in the 7D brochure and that the 7D try to compensate for this. Maybe these AF issue are caused by problems with this compensation. I don't think that the 40D did any kind of compensation. I don't have time to investigate further now, but I will try to do so over the weekend. Changing the light source is just a simple way to change the color balance of the motive. I would guess that you can get the same effect from differences in color in the motive. And this may explain what photonuts experienced.

 

[sebaduh]

I am not goona comment on AF ,but i think it might not be a very good idea to post photos containing IBAN numbers on such a large visited forum or on any forum for that matter...

just your 2cents this time

 

[J A K]

I did extensive AF tests with 4 7D bodies shooting B&W resolution charts under artificial light. My findings were at 50X FL and the widest aperture of the lens I had nearly 100% in focus with my 50f1.4, 100f2.8 macro, 24~105f4LIS, 70~300DO and 300f4L. I experienced the worst performance on 2 bodies (5 out of 10 not in focus) with my 70~200f4LIS which has no problem on my 40 or 50D under the same testing conditions. I agree that there are some lenses that do not AF well with the 7D although I have no clue as to why.

Darn it, now you have me worried! I just bought a new lens and it is without camera. So anyway, I was thinking about a 7D because a couple of shooters I know are doing very well with that camera shooting BIF; however, to my knowledge they have never used the 7D at low levels of illumination. Do you recall what the shutter speed, aperture (I know you said widest), and ISO settings were for your tests? If you could provide that information I can get a rough feel for the illumination's EV by assuming proper exposure on your part. Just out of curiosity, when you shot the resolution chart was there a patch of 18% gray somewhere in the scene?

Hi Joe,

All the above tests were done under artificial (halogen) lights at EV 11 shooting at the max aperture for each lens at ISO 100. The 7D's that did not AF well were very early units (preorder) and were inconsistent with the 70~200f4LIS in daylight too even after extensive MA work. I need to say that the 7D I currently have from late December 09 AF's extremely well with all of my lenses. IMO Canon had an AF calibration issue with early batches of 7D's that I believe has been resolved. This was a standard USAF 1951 resolution chart and there was no patch of 18% gray in the scene.

An EV of "11" is very bright; that's almost like an overcast day. I normally perform my AF testing at an EV of "3" or "4". Let me make sure I'm getting this straight ... you had some early units that had problems acquiring focus at an EV of "11"?

There was a thread about the 1Dmk4's low light AF performance a while back. I don't know if this is fact but there were several posters who mentioned Canon was using a CMOS AF sensor in the 7D and 1Dmk4 instead of a CCD. They indicated this was a new direction for Canon and that the CMOS was not as sensitive for low-light AF as a CCD. This could all be BS too but the 7D from my tests is not as sensitive for low light AF as my 40D or 50D and there has to be a logical explanation.

The 1DmkIV white paper talks about a "new low-noise CMOS AF unit" but unfortunately there is no white paper (that I'm aware of) for the 7D. So, I think your "BS" conclusion is probably a good one.

Additionally, I believe "you" regarding the low light AF capability of the 7D versus the 50D/40D (AFAIC you are trusted source). Let me ask a question, do you still have your 40D and/or 50D available for further AF testing, TIA for your reply?

I do still have both but am looking to sell the 40D shortly as I never use it or the 50D for that matter but I still need a backup body. I can tell you that Imaging Resource found the 7D AF to be less sensitive than the 50D in low light as follows;

"The Canon 7D's phase-detect autofocus system was able to focus on the subject to just below 1/8 foot-candle light level unassisted, and in complete darkness with the AF assist enabled. That's not quite as good as most semi-pro SLRs. (The Canon 50D's AF system could focus unassisted to less than 1/16 foot-candle.) We were hoping the production-level Canon 7D would improve over the prototype in that regard, but unfortunately, it didn't. In Live View mode, the 7D's contrast-detect autofocus was able to focus down just below the 1/4 foot-candle level without assistance. That's fairly typical for contrast-detection AF."

This follows with a very rudimentary test I did where the 7D would hunt and never lock but the 40 and 50D both locked immediately. This was on a low contrast target indoor with natural light at EV 9.

EV "9" is also pretty bright for indoor natural light ... the max I get is about EV "6.6" under the skylight. I should mention that we live in Arizona and to help keep the house cool we use shadow screens that block about 80 percent of the light.

What tests are you looking for?

Various targets shot at an EV values around "0" and "1". Note, according to the 7D specification the AF metering range is from -0.5 to 18 in EV units. Just out of curiosity do you have a light meter to measure incident illumination?

If I'm understanding your indoor test results correctly and you were having problems at those fairly bright indoor illumination levels then there must be something wrong with either the 7D's AF system or your AF test target. Just out of curiosity what does your AF test target look like ... could you post a resized image of the scene you are shooting and maybe a crop of the actual target the camera is trying to acquire, thanks.

Oh, one more thing just to make sure my understanding is correct ... the object distance you are shooting at during your AF testing is about fifty times the focal length; is that right?

Regards,

Joe Kurkjian

Galleries: http://www.pbase.com/jkurkjia

--



SEARCHING FOR A BETTER SELF PORTRAIT

 

[schmegg]

Oh, one more thing just to make sure my understanding is correct ... the object distance you are shooting at during your AF testing is about fifty times the focal length; is that right?

Hi Joe

I know this is important for MF adjustment setting, but is there any reason why this is relevant to testing AF abilities?

Just wondering because I'm going to do some testing here tomorrow but I wasn't going to use 50x the focal length - just a subject distance that works fine in good light.

 

[photo nuts]

After shutting down my PC yesterday I noticed that Canon mention that there is chromatic aberration in the AF optics splitting the two phases used for focusing in the 7D brochure and that the 7D try to compensate for this. Maybe these AF issue are caused by problems with this compensation. I don't think that the 40D did any kind of compensation. I don't have time to investigate further now, but I will try to do so over the weekend. Changing the light source is just a simple way to change the color balance of the motive. I would guess that you can get the same effect from differences in color in the motive. And this may explain what photonuts experienced.

Sounds plausible. I am still trying to figure this out... Sigh...

 

[J A K]

Oh, one more thing just to make sure my understanding is correct ... the object distance you are shooting at during your AF testing is about fifty times the focal length; is that right?

Hi Joe

I know this is important for MF adjustment setting, but is there any reason why this is relevant to testing AF abilities?

Just wondering because I'm going to do some testing here tomorrow but I wasn't going to use 50x the focal length - just a subject distance that works fine in good light.

I documented the fixture/approach (link below) back in the 30D days ... suggest you read everything carefully, note the new AF test target, and document your "AF error" readings either for subsequent data manipulation and/or future reference. The objective is to quantify what's going on in a fairly repeatable environment. Note, for dynamic focus testing I use the family car because it doesn't change on a daily/weekly/monthly basis.

One of the things I do is take four shots with the lens initially set beyond infinity, average the DOF error, take four more shots with the lens set to near focus, average those results and compare the two averages to get an idea of the amount of dead-band present. In the end I average the two four-shot averages to compute "the" static AF error.

FWIW I've noticed that two of my lenses exhibited little of no hysteresis when they were new but after many years of use there is about a 0.2 DOF dead-band; AFAIC these lenses are still darn good and more than adequate to keep in the bag but I do check them out every four to six months or so when the opportunity presents itself.

Also note older (and more sloppy) lenses may be subject to repeatability issues; that's why I take at least four readings. If you see too much variation in your first four readings then consider taking eight shots prior to computing your average.

http://www.pbase.com/jkurkjia/canon_30d

Good luck with your tests!

Regards,

Joe Kurkjian

Galleries: http://www.pbase.com/jkurkjia

--



SEARCHING FOR A BETTER SELF PORTRAIT

 

[rwbaron]

An EV of "11" is very bright; that's almost like an overcast day. I normally perform my AF testing at an EV of "3" or "4". Let me make sure I'm getting this straight ... you had some early units that had problems acquiring focus at an EV of "11"?

EV 11 is a reflected camera meter reading not an incident meter reading. Yes, EV 11 is bright but the AF problem with both of those bodies were only with my 70~200f4LIS which AF's fine with the latest body. The problem was discovered trying to MA the bodies to the lens not test the 7D's AF.

EV "9" is also pretty bright for indoor natural light ... the max I get is about EV "6.6" under the skylight. I should mention that we live in Arizona and to help keep the house cool we use shadow screens that block about 80 percent of the light.

Again, this was not an incident meter reading but reflected referenced from the camera in evaluative. Our house has a lot of windows that are unobstructed so it is quite bright during the daytime if full sun. I just tried this again with the shades in the room drawn and at ISO 100 the reflected reading is f4 at 1/4 sec or EV 6. The camera has some difficulty with the 24~105 finding focus (sometimes hunts and won't lock) at 24mm using center point AF but improves using center point spot. The AF target is a seam in a medium brown lamp shade so it's very low contrast. This is interesting because I just tried the 24~105 on my 50D and it's also not locking but it did with no problem the last time I tried this (in January) when I had 2 7D bodies to test and both exhibited the same behavior. This is not a very scientific test, only one of interest because of the difference in performance I saw between 2 7D's and my 40 and 50D when first performed last January.

What tests are you looking for?

Various targets shot at an EV values around "0" and "1". Note, according to the 7D specification the AF metering range is from -0.5 to 18 in EV units. Just out of curiosity do you have a light meter to measure incident illumination?

I have an old Gossen Lune Pro SBC that does both reflected and incident readings but I'd have to find it and dust it off. We have out of town company this weekend and then my wife is off work and we're planning on travelling next week so time is a bit tight. I'll see what I can do.

If I'm understanding your indoor test results correctly and you were having problems at those fairly bright indoor illumination levels then there must be something wrong with either the 7D's AF system or your AF test target. Just out of curiosity what does your AF test target look like ... could you post a resized image of the scene you are shooting and maybe a crop of the actual target the camera is trying to acquire, thanks.

Yes but remember the AF MA tests were done with Halogen not natural light and I don't know if that could affect the results. The test target is the standard USAF 1951 resolving power chart from Edmunds Optics which can be found here;

http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1665

The target is 24x36" and at 50x FL it fills the frame perfectly. I use a mirror for alignment and light the chart with two 500 watt Halogen lamps. At 50x FL the center target on the chart is slightly larger than the 7D's AF box in the viewfinder. I've found 50x FL to be a good compromise for AF MA that gives good close and infinity focus performance once dialed in.

Oh, one more thing just to make sure my understanding is correct ... the object distance you are shooting at during your AF testing is about fifty times the focal length; is that right?

Yes.

Bob
--
http://www.pbase.com/rwbaron

 

[J A K]

An EV of "11" is very bright; that's almost like an overcast day. I normally perform my AF testing at an EV of "3" or "4". Let me make sure I'm getting this straight ... you had some early units that had problems acquiring focus at an EV of "11"?

EV 11 is a reflected camera meter reading not an incident meter reading. Yes, EV 11 is bright but the AF problem with both of those bodies were only with my 70~200f4LIS which AF's fine with the latest body. The problem was discovered trying to MA the bodies to the lens not test the 7D's AF.

EV "9" is also pretty bright for indoor natural light ... the max I get is about EV "6.6" under the skylight. I should mention that we live in Arizona and to help keep the house cool we use shadow screens that block about 80 percent of the light.

Again, this was not an incident meter reading but reflected referenced from the camera in evaluative. Our house has a lot of windows that are unobstructed so it is quite bright during the daytime if full sun. I just tried this again with the shades in the room drawn and at ISO 100 the reflected reading is f4 at 1/4 sec or EV 6. The camera has some difficulty with the 24~105 finding focus (sometimes hunts and won't lock) at 24mm using center point AF but improves using center point spot. The AF target is a seam in a medium brown lamp shade so it's very low contrast. This is interesting because I just tried the 24~105 on my 50D and it's also not locking but it did with no problem the last time I tried this (in January) when I had 2 7D bodies to test and both exhibited the same behavior. This is not a very scientific test, only one of interest because of the difference in performance I saw between 2 7D's and my 40 and 50D when first performed last January.

What tests are you looking for?

Various targets shot at an EV values around "0" and "1". Note, according to the 7D specification the AF metering range is from -0.5 to 18 in EV units. Just out of curiosity do you have a light meter to measure incident illumination?

I have an old Gossen Lune Pro SBC that does both reflected and incident readings but I'd have to find it and dust it off. We have out of town company this weekend and then my wife is off work and we're planning on travelling next week so time is a bit tight. I'll see what I can do.

Hey, I've got one of those ... love that meter (also have the flash meter and 1 degree attachments). Please don't worry about running tests for now. Check out my comments below regarding what I think your AF issue might be. I hope you and the wife have a great vacation!

If I'm understanding your indoor test results correctly and you were having problems at those fairly bright indoor illumination levels then there must be something wrong with either the 7D's AF system or your AF test target. Just out of curiosity what does your AF test target look like ... could you post a resized image of the scene you are shooting and maybe a crop of the actual target the camera is trying to acquire, thanks.

Yes but remember the AF MA tests were done with Halogen not natural light and I don't know if that could affect the results. The test target is the standard USAF 1951 resolving power chart from Edmunds Optics which can be found here;

http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1665

The target is 24x36" and at 50x FL it fills the frame perfectly. I use a mirror for alignment and light the chart with two 500 watt Halogen lamps. At 50x FL the center target on the chart is slightly larger than the 7D's AF box in the viewfinder. I've found 50x FL to be a good compromise for AF MA that gives good close and infinity focus performance once dialed in.

IMO that target has insufficient low frequency spatial content for the AF to reliably acquire, YMMV.

FWIW below is the target I use to test AF error at 12.1X the focal length; of course this is too small for you if you are at 50X the focal length. It would be easy for you to make a similar random pattern by shooting by shooting a bunch of gravel and then, "if" you want, it's easy to embed some of the inner groups from one of your chart's USAF patterns right in the middle of your random target as a measure of the quality of focus.

Oh, one more thing just to make sure my understanding is correct ... the object distance you are shooting at during your AF testing is about fifty times the focal length; is that right?

Yes.

Bob
--
http://www.pbase.com/rwbaron

--
Regards,

Joe Kurkjian

Galleries: http://www.pbase.com/jkurkjia



SEARCHING FOR A BETTER SELF PORTRAIT

 

[40d_dane]

Darn it, now you have me worried! I just bought a new lens and it is without camera. So anyway, I was thinking about a 7D because a couple of shooters I know are doing very well with that camera shooting BIF; however, to my knowledge they have never used the 7D at low levels of illumination. Do you recall what the shutter speed, aperture (I know you said widest), and ISO settings were for your tests?

I am currently doing some testing on my 7D to sort things out for my self. My 7D does AF OK at about 1s, f5.6 and ISO200. Don't think you could do much BIF'ing at that shutter speed LOL. I haven't pushed further. I've found that the 7D AF shifts slightly with color and light source color temp. Red-ish illumination of a black/red target makes the 7D front focus compared to focus on black/white target in natural light. My 40D doesn't seem to have the same effect. The shift, however is small and most likely within Canons limits of acceptable focus. I've redone my first round of tests on a new and better target with the same result, but I am not yet satisfied with the quality of the result so I am not ready to post new results yet. I will build a new and better target next week and hopefully have new results to post by next weekend. My first results have been uploaded to my gallery.

 

[photo nuts]

I've found that the 7D AF shifts slightly with color and light source color temp. Red-ish illumination of a black/red target makes the 7D front focus compared to focus on black/white target in natural light. My 40D doesn't seem to have the same effect. The shift, however is small and most likely within Canons limits of acceptable focus. I've redone my first round of tests on a new and better target with the same result, but I am not yet satisfied with the quality of the result so I am not ready to post new results yet. I will build a new and better target next week and hopefully have new results to post by next weekend. My first results have been uploaded to my gallery.

That agrees with my random testing at home. Interested to see more detailed results from you next week.

 

[rwbaron]

An EV of "11" is very bright; that's almost like an overcast day. I normally perform my AF testing at an EV of "3" or "4". Let me make sure I'm getting this straight ... you had some early units that had problems acquiring focus at an EV of "11"?

EV 11 is a reflected camera meter reading not an incident meter reading. Yes, EV 11 is bright but the AF problem with both of those bodies were only with my 70~200f4LIS which AF's fine with the latest body. The problem was discovered trying to MA the bodies to the lens not test the 7D's AF.

EV "9" is also pretty bright for indoor natural light ... the max I get is about EV "6.6" under the skylight. I should mention that we live in Arizona and to help keep the house cool we use shadow screens that block about 80 percent of the light.

Again, this was not an incident meter reading but reflected referenced from the camera in evaluative. Our house has a lot of windows that are unobstructed so it is quite bright during the daytime if full sun. I just tried this again with the shades in the room drawn and at ISO 100 the reflected reading is f4 at 1/4 sec or EV 6. The camera has some difficulty with the 24~105 finding focus (sometimes hunts and won't lock) at 24mm using center point AF but improves using center point spot. The AF target is a seam in a medium brown lamp shade so it's very low contrast. This is interesting because I just tried the 24~105 on my 50D and it's also not locking but it did with no problem the last time I tried this (in January) when I had 2 7D bodies to test and both exhibited the same behavior. This is not a very scientific test, only one of interest because of the difference in performance I saw between 2 7D's and my 40 and 50D when first performed last January.

What tests are you looking for?

Various targets shot at an EV values around "0" and "1". Note, according to the 7D specification the AF metering range is from -0.5 to 18 in EV units. Just out of curiosity do you have a light meter to measure incident illumination?

I have an old Gossen Lune Pro SBC that does both reflected and incident readings but I'd have to find it and dust it off. We have out of town company this weekend and then my wife is off work and we're planning on travelling next week so time is a bit tight. I'll see what I can do.

Hey, I've got one of those ... love that meter (also have the flash meter and 1 degree attachments). Please don't worry about running tests for now. Check out my comments below regarding what I think your AF issue might be. I hope you and the wife have a great vacation!

If I'm understanding your indoor test results correctly and you were having problems at those fairly bright indoor illumination levels then there must be something wrong with either the 7D's AF system or your AF test target. Just out of curiosity what does your AF test target look like ... could you post a resized image of the scene you are shooting and maybe a crop of the actual target the camera is trying to acquire, thanks.

Yes but remember the AF MA tests were done with Halogen not natural light and I don't know if that could affect the results. The test target is the standard USAF 1951 resolving power chart from Edmunds Optics which can be found here;

http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm?productID=1665

The target is 24x36" and at 50x FL it fills the frame perfectly. I use a mirror for alignment and light the chart with two 500 watt Halogen lamps. At 50x FL the center target on the chart is slightly larger than the 7D's AF box in the viewfinder. I've found 50x FL to be a good compromise for AF MA that gives good close and infinity focus performance once dialed in.

IMO that target has insufficient low frequency spatial content for the AF to reliably acquire, YMMV.

I've never had problems with using AF on that target with digital bodies and even back in the film days with an EOS Elan, EOS 1, A2, and Elan 7E. I've used it for all my AF MA on the 50D and 7D's. I only had problems with the two very early 7D bodies with my 70~200f4LIS but they were all over the place even on flat sides of buildings with contrasty elements in full sun. I doubt it's the target as the larger elements are quite legible in the viewfinder.

FWIW below is the target I use to test AF error at 12.1X the focal length; of course this is too small for you if you are at 50X the focal length. It would be easy for you to make a similar random pattern by shooting by shooting a bunch of gravel and then, "if" you want, it's easy to embed some of the inner groups from one of your chart's USAF patterns right in the middle of your random target as a measure of the quality of focus.

You nailed why I use the resolving power chart and that's because it makes it so easy to determine quality of focus. I've made other focus charts on the computer and printed them on both gloss and matt photo paper but have not seen any measuable difference in AF performance using them.

Bob
--
http://www.pbase.com/rwbaron

 

[J A K]

I've never had problems with using AF on that target with digital bodies and even back in the film days with an EOS Elan, EOS 1, A2, and Elan 7E. I've used it for all my AF MA on the 50D and 7D's. I only had problems with the two very early 7D bodies with my 70~200f4LIS but they were all over the place even on flat sides of buildings with contrasty elements in full sun. I doubt it's the target as the larger elements are quite legible in the viewfinder .

You might be in for a surprise.

I used to AF on a target with a VERY high contrast repetitive pattern (pics below) with an internal pattern that included part of the USAF pattern along with a vertical sliding resolution scale to determine at what point aliasing errors would occur.





For several years I was content to use that AF pattern ... no problems at all when using a 17-40, 50mm f/1.4, 28-135, or 100-400.

When my 70-200 f/4L IS arrived I noticed focus acquisition loop was oscillating at a low frequency while trying to acquire focus from infinity ... somewhere between infinity and the the real focus distance the the system was not happy ... you could hear the vibration and see the focus dial move. Note, the lens "appeared" to be fine from the standpoint of AF accuracy except for tests using the AF test pattern above. The lens was definitely fine if I adjusted focus off by just a little bit towards infinity ... it would always converge back to perfect focus. Okay, bottom line, the oscillation was somewhat unacceptable to me ... I figured this lens might be (I was never 100 percent positive) a lemon and B&H promptly replaced it.

My second copy didn't oscillate but it did acquire a false lock at several points between infinity and the true focus distance. So I thought maybe I had a camera problem ... I had nearly identical results on every xxD body in the bag. Oh, this is interesting ... FWIW, the 1DmkII waltzed right trough from infinity and locked perfectly (no false nulls).

I changed the AF target to two broad lines, one vertical and the other horizontal ... no false lock problems on any xxD body acquiring the new target. Eventually, to completely eliminate the possibility of a false lock I migrated to a random pattern.

I'm not sure why the false lock didn't show up with other lenses but I "think" it has something to do with the fact that the 17-200 f/4L IS is much sharper wide open than any of the other lenses I had tested and the AF system was locking on a harmonic.

Anyway, although my situation was not due to a spatial frequency limitation per se I passed along the story to provide an example of weird lens-dependent sh!t that can happen.

Regarding your specific situation at 50X, if you made a checkerboard (two white and two black) on an 8x10 sheet of paper (or a single broad horizontal and vertical line) and then embed a small section of very high resolution (for QC) I "think" the results will be different (in a positive way).

Regards,

Joe Kurkjian

Galleries: http://www.pbase.com/jkurkjia

--



SEARCHING FOR A BETTER SELF PORTRAIT

 

[photo nuts]

For several years I was content to use that AF pattern ... no problems at all when using a 17-40, 50mm f/1.4, 28-135, or 100-400...

When my 70-200 f/4L IS arrived I noticed focus acquisition loop was oscillating at a low frequency while trying to acquire focus from infinity ...

My second copy didn't oscillate but it did acquire a false lock at several points between infinity and the true focus distance. So I thought maybe I had a camera problem ... I had nearly identical results on every xxD body in the bag...

I changed the AF target to two broad lines, one vertical and the other horizontal ... no false lock problems on any xxD body acquiring the new target. Eventually, to completely eliminate the possibility of a false lock I migrated to a random pattern.

Do you think a target like this can be a problem"
http://www.lensalign.com/products.html
Lots of repetitive structures

 

[40d_dane]

You might be in for a surprise.

Actually, your target is listed in every EOS camera manual as a case where the AF system doesn't work !

When my 70-200 f/4L IS arrived I noticed focus acquisition loop was oscillating at a low frequency while trying to acquire focus from infinity ... somewhere between infinity and the the real focus distance the the system was not happy ... you could hear the vibration and see the focus dial move. Note, the lens "appeared" to be fine from the standpoint of AF accuracy except for tests using the AF test pattern above. The lens was definitely fine if I adjusted focus off by just a little bit towards infinity ... it would always converge back to perfect focus. Okay, bottom line, the oscillation was somewhat unacceptable to me ... I figured this lens might be (I was never 100 percent positive) a lemon and B&H promptly replaced it.

My second copy didn't oscillate but it did acquire a false lock at several points between infinity and the true focus distance. So I thought maybe I had a camera problem ... I had nearly identical results on every xxD body in the bag. Oh, this is interesting ... FWIW, the 1DmkII waltzed right trough from infinity and locked perfectly (no false nulls).

I can think of two possible explanations for why you didn't have this problem previously: This may have been the first time you have had your target perfectly parallel to the image sensor plane or this may have been the first time that no part of the AF sensor hit the edge of the target. Remember that the AF sensor is actually quite a bit larger than the square marking on the focus screen. On the 40D for example, the center AF sensor cover an area approximately the size of the metering cycle marking. There may be other and maybe even more likely explanations but I could think of these two right of the top of my head. The oscillation you've observed with the first 70-200 can probably be reproduced with the second if you give it exactly the same setup.

I changed the AF target to two broad lines, one vertical and the other horizontal ... no false lock problems on any xxD body acquiring the new target. Eventually, to completely eliminate the possibility of a false lock I migrated to a random pattern.

What you want to achieve with tests like this is to check AF calibration. For that the simplest target is also the best. Canon service centers uses one or two (I think it was two) fat black lines on a white background. I used to use a center cutout of an ISO res chart but when I had to fight a service technician to have my 40D calibrated to work with one of my lenses the guy only had a brief look at my test shot and then said that he wouldn't do anything based on a complex target like that. So I redid my test with a dead simple target - a piece of black tape on white paper - and got the exact same result but this time the technician couldn't dismiss my test shot.

 

[J A K]

For several years I was content to use that AF pattern ... no problems at all when using a 17-40, 50mm f/1.4, 28-135, or 100-400...

When my 70-200 f/4L IS arrived I noticed focus acquisition loop was oscillating at a low frequency while trying to acquire focus from infinity ...

My second copy didn't oscillate but it did acquire a false lock at several points between infinity and the true focus distance. So I thought maybe I had a camera problem ... I had nearly identical results on every xxD body in the bag...

I changed the AF target to two broad lines, one vertical and the other horizontal ... no false lock problems on any xxD body acquiring the new target. Eventually, to completely eliminate the possibility of a false lock I migrated to a random pattern.

Do you think a target like this can be a problem"
http://www.lensalign.com/products.html
Lots of repetitive structures

Yes, it's a possibility but not necessarily probable. Note, the LensAlign isn't as strongly repetitive (i.e. weak harmonics) as my original AF target and don't forget, the issue that I experienced didn't come up until the 70-200 f/4L IS came on the scene.

Regards,

Joe Kurkjian

Galleries: http://www.pbase.com/jkurkjia

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