[quote name='joze' timestamp='1294920275' post='5474']
wim: thanks for your input, this helps a lot.
Does 4) mean, that a F/2.8 24-70 L Zoom focuses more accurate and/or faster? (I've a 60D body).
Does the focus capability scale further with the speed of the lens, i.e. does a F/1.4 lens autofocus even more accurate and faster that a F2.8 lens?
[/quote]
In principle, an F/2.8 lens does indeed focus more accurately as a result, provided you use an F/2.8+ AF point, and in principle an F/1.4 lens also, or even more accurately, but that is much of a muchness with regard to accuracy, or rather with precision and repeatability (an out-of calibration fast lens will focus faster, but will still give you the same unsharp image repeatedly with more precision than any other lens which is in calibration <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />). You need more accuracy wide open with an F/1.4 lens than with an F/2.8 lens anyway.
However, at F/1.4 a slight body movement between AF and actual shutter button press generally is of much more influence than the accuracy of the AF system.
BTW, focusing accuracy also varies with exisiting light, as the faster AF points don't work down to the lower light limits where the slower AF points still do, in theory anyway. In practice I have never really noticed a big difference, and I generally do a lot of my shooting in low light.
With regard to faster focusing, that really depends on the implementation of the AF system. Generally speaking, ring USM lenses focus faster than non-ring USM lenses. However, if a ring USM has to move a lot of glass for AF, it will likely become slower again (f.e., 85L and 85L II). However, because of the way a ring USM lens works in relation to its power of moving teh focusing elements and groups in a lens forwards and backward for focusing, it is much more accurate, generally speaking, than a more traditional approach, because for the simpler AF stepper motors in relation to the force and power used the latter generally can't be as precise.
However, take two lenses of the same design and concept, one with ring USM an done with a more traditional AF motor, and the ring USM equipped lens will focus faster and more accurately.
Regarding your 60D: you'd have to use the centre AF point. That is the F/2.8+ AF point.
HTH, kind regards, Wim
wim: thanks for your input, this helps a lot.
Does 4) mean, that a F/2.8 24-70 L Zoom focuses more accurate and/or faster? (I've a 60D body).
Does the focus capability scale further with the speed of the lens, i.e. does a F/1.4 lens autofocus even more accurate and faster that a F2.8 lens?
[/quote]
In principle, an F/2.8 lens does indeed focus more accurately as a result, provided you use an F/2.8+ AF point, and in principle an F/1.4 lens also, or even more accurately, but that is much of a muchness with regard to accuracy, or rather with precision and repeatability (an out-of calibration fast lens will focus faster, but will still give you the same unsharp image repeatedly with more precision than any other lens which is in calibration <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />). You need more accuracy wide open with an F/1.4 lens than with an F/2.8 lens anyway.However, at F/1.4 a slight body movement between AF and actual shutter button press generally is of much more influence than the accuracy of the AF system.
BTW, focusing accuracy also varies with exisiting light, as the faster AF points don't work down to the lower light limits where the slower AF points still do, in theory anyway. In practice I have never really noticed a big difference, and I generally do a lot of my shooting in low light.
With regard to faster focusing, that really depends on the implementation of the AF system. Generally speaking, ring USM lenses focus faster than non-ring USM lenses. However, if a ring USM has to move a lot of glass for AF, it will likely become slower again (f.e., 85L and 85L II). However, because of the way a ring USM lens works in relation to its power of moving teh focusing elements and groups in a lens forwards and backward for focusing, it is much more accurate, generally speaking, than a more traditional approach, because for the simpler AF stepper motors in relation to the force and power used the latter generally can't be as precise.
However, take two lenses of the same design and concept, one with ring USM an done with a more traditional AF motor, and the ring USM equipped lens will focus faster and more accurately.
Regarding your 60D: you'd have to use the centre AF point. That is the F/2.8+ AF point.
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 ....