01-13-2011, 07:24 AM
This is frontfocus, be it only slight, and AFAIAC, within spec.
A few things to take into consideration, however.
1) Canon lenses are calibrated for AF at 50 X FL. At close distances and longer distances the results may therefore vary. From my experience when focusing closer than 50X FL the tendency with well calibrated bodies and lenses, especially zoom lenses, is to get slight frontfocus, be it within spec, and with longer distances slight backfocus, although in the latter case DoF tends to make up for this.
2) AF zoom lenses are not parfocal, which means the camera needs to refocus after zooming every single time, and the tolerances required in the focusing mechanism in order to make AF possible may account for differences in focus across the zoom range for the same subject and subject distance. Often this is not linear either, from my experience it varies up and down across the zoom range, very likely coinciding with the variable movements of the groups and elements with changing distances and zooming.
3) The size of the actual AF sensors is three times the size of the red square in the view finder, linear, or 9 times for area. This means you need to choose a contrast transition which is large enough for the AF sensor selected to get accurate AF results. Furthermore, the AF point may not be exactly in the spot that the red square indicates, it may be offset slightly due to unavoidable tolerances. This is something to experiment with to find out where exactly it is most sensitive.
4) For lenses with maximum apertures smaller than F/2.8 (larger f number IOW), AF accuracy according to Canon specs is within 1 f-stop of aperture selected, and therefore the resutl you seem to be getting is within spec (AF is just within DoF), as the 0, which I assume is yoru focusing point, is still within the DoF zone. Note that with lenses with a maximum aperture of F/2.8 and faster, this is 1/3 of DoF, provided you use one of the fast AF points (with most non professional models, which includes even the 5D II, this is the center AF point). Of course, with the 24-105L this doesn't help you, as it will only work with the AF-points for slower lenses (effectively, a fast AF point will become a slow one with a slow lens, they are generally implemented as dual AF-points).
5) AF depends on lighting. The 1D series cameras (1Ds, and 1D) have much more advanced AF-systems, and generally work well in all kinds of light. All amateur cameras up to and including 500D, 40D, and 5D have AF-ssystems targeted for a single spectrum band of light, and may therefore have problems with AF especially in artifical lighting, such as fluorescent light. The newer cameras from 50D, 5D II onwards use two bands, and are more accurate in this regard. IOW, the 7D and 60D are also more accurate. I am not sure about the 550D, however, although from my experience it seems to be a bit better too
6) AF is about statistics, due to the tolerance of lenses and bodies. When a body-lens combo reaches focus by AF, every time you press the AF button focus will be slightly different, within a certain band, even for static objects and focusing from a tripod with a cable release. However, it should always reach focus conform Canon specs (1/3 of DoF with fast lenses and fast AF points, 1 stop of DoF for all other).
7) Optimal sharpness at pixel level is only required at very large magnifications (like 50% or 100% on screen). For anything else the standard in-DoF sharpness obtained from calibration in spec will suffice for normal shooting, especially if you do some post-processing, either in-camera or on RAW files.
8) AF is a result of the body and lens working together, which is why you really need to send in both when you want a lens calibrated, and ideally you should send in all of your lenses and bodies together for optimal results.
I suggest you try shooting a few normal, real life subjects at different distances, and see how the camera-lens combo fares for AF in those instances. Also test different FLs, not just 60 mm. Really go through the range, 24, 35, 50, 70, 85, 105 mm. You may find it is the same as the results you describe at 60 mm, but it may just as well be better at other FLs. You will liekly find that teh results are ok. There is a big difference between testing these things and actual real life shooting, and i am talking from experience here. You only need to take notice, if the results from real life shooting are not what you exppect. Even so, just like it was in the days we didn't have AF, a number fo shots will be out of focus. Also ote that the best AF system lacks the intelligence of the average person, when it comes to picking the correct contrast transition for focusing, even if, generally speaking modern cameras with AF generally perform way better than we ever did ourselves in the past with manual focus and split wedge or prism collar focusing screens. There is therefore no shortcut in this regard to exactly learn how your camera and lens combo or combos behave under different circumstances in order to get optimal results from them, just as there wasn't in the past with our MF systems.
If you are still unhappy with the AF focus point positioning of your lens-camera combo after the above test, and you don't have micro focus adjust on the body you are using, I suggest you have it calibrated by Canon. If you are not a CPS member, you have to take quite a delay into account, however. Over here in the Netherlands it takes anywhere from 3 weeks to more than 2 months, depending on how busy it is at the repair centre(s). I don't know what the situation in Germany is, however.
HTH, kind regards, Wim
A few things to take into consideration, however.
1) Canon lenses are calibrated for AF at 50 X FL. At close distances and longer distances the results may therefore vary. From my experience when focusing closer than 50X FL the tendency with well calibrated bodies and lenses, especially zoom lenses, is to get slight frontfocus, be it within spec, and with longer distances slight backfocus, although in the latter case DoF tends to make up for this.
2) AF zoom lenses are not parfocal, which means the camera needs to refocus after zooming every single time, and the tolerances required in the focusing mechanism in order to make AF possible may account for differences in focus across the zoom range for the same subject and subject distance. Often this is not linear either, from my experience it varies up and down across the zoom range, very likely coinciding with the variable movements of the groups and elements with changing distances and zooming.
3) The size of the actual AF sensors is three times the size of the red square in the view finder, linear, or 9 times for area. This means you need to choose a contrast transition which is large enough for the AF sensor selected to get accurate AF results. Furthermore, the AF point may not be exactly in the spot that the red square indicates, it may be offset slightly due to unavoidable tolerances. This is something to experiment with to find out where exactly it is most sensitive.
4) For lenses with maximum apertures smaller than F/2.8 (larger f number IOW), AF accuracy according to Canon specs is within 1 f-stop of aperture selected, and therefore the resutl you seem to be getting is within spec (AF is just within DoF), as the 0, which I assume is yoru focusing point, is still within the DoF zone. Note that with lenses with a maximum aperture of F/2.8 and faster, this is 1/3 of DoF, provided you use one of the fast AF points (with most non professional models, which includes even the 5D II, this is the center AF point). Of course, with the 24-105L this doesn't help you, as it will only work with the AF-points for slower lenses (effectively, a fast AF point will become a slow one with a slow lens, they are generally implemented as dual AF-points).
5) AF depends on lighting. The 1D series cameras (1Ds, and 1D) have much more advanced AF-systems, and generally work well in all kinds of light. All amateur cameras up to and including 500D, 40D, and 5D have AF-ssystems targeted for a single spectrum band of light, and may therefore have problems with AF especially in artifical lighting, such as fluorescent light. The newer cameras from 50D, 5D II onwards use two bands, and are more accurate in this regard. IOW, the 7D and 60D are also more accurate. I am not sure about the 550D, however, although from my experience it seems to be a bit better too
6) AF is about statistics, due to the tolerance of lenses and bodies. When a body-lens combo reaches focus by AF, every time you press the AF button focus will be slightly different, within a certain band, even for static objects and focusing from a tripod with a cable release. However, it should always reach focus conform Canon specs (1/3 of DoF with fast lenses and fast AF points, 1 stop of DoF for all other).
7) Optimal sharpness at pixel level is only required at very large magnifications (like 50% or 100% on screen). For anything else the standard in-DoF sharpness obtained from calibration in spec will suffice for normal shooting, especially if you do some post-processing, either in-camera or on RAW files.
8) AF is a result of the body and lens working together, which is why you really need to send in both when you want a lens calibrated, and ideally you should send in all of your lenses and bodies together for optimal results.
I suggest you try shooting a few normal, real life subjects at different distances, and see how the camera-lens combo fares for AF in those instances. Also test different FLs, not just 60 mm. Really go through the range, 24, 35, 50, 70, 85, 105 mm. You may find it is the same as the results you describe at 60 mm, but it may just as well be better at other FLs. You will liekly find that teh results are ok. There is a big difference between testing these things and actual real life shooting, and i am talking from experience here. You only need to take notice, if the results from real life shooting are not what you exppect. Even so, just like it was in the days we didn't have AF, a number fo shots will be out of focus. Also ote that the best AF system lacks the intelligence of the average person, when it comes to picking the correct contrast transition for focusing, even if, generally speaking modern cameras with AF generally perform way better than we ever did ourselves in the past with manual focus and split wedge or prism collar focusing screens. There is therefore no shortcut in this regard to exactly learn how your camera and lens combo or combos behave under different circumstances in order to get optimal results from them, just as there wasn't in the past with our MF systems.
If you are still unhappy with the AF focus point positioning of your lens-camera combo after the above test, and you don't have micro focus adjust on the body you are using, I suggest you have it calibrated by Canon. If you are not a CPS member, you have to take quite a delay into account, however. Over here in the Netherlands it takes anywhere from 3 weeks to more than 2 months, depending on how busy it is at the repair centre(s). I don't know what the situation in Germany is, however.
HTH, kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
