Canon EOS-1 - Manual (Page 23)

II. LENS ISSUES 23
AF Speed According to
EF Lens Vintage
EF Extender Issues
Just as EOS cameras have improved over time in terms of AF calculation speed, EF
lenses have improved in terms of lens drive speed. Although most if not all EF lenses
regardless of vintage autofocus quickly enough to satisfy the needs of photographers
who specialize in stationary subjects, there is a significant improvement in lens drive
speed for fast-moving applications such as sports photography when the newer lenses
are used with newer cameras like the EOS-1 class digital SLRs.
Generally speaking, all USM lenses introduced since 1999, starting with the first 4
models in the IS (Image Stabilizer) super-telephoto series (300mm f/2.8L IS, 400mm
f/2.8L IS, 500mm f/4L IS and 600mm f/4L IS), autofocus noticeably faster than the
non-IS versions they replaced when used with an EOS-1 class digital SLR. Therefore, if
you want to experience the maximum performance of your EOS-1 class digital SLR in
terms of AF speed, you must use a recent lens.
The technical reason behind the improved performance lies in new programming on the
circuit board in the lens that supplies an initial burst of electrical power to the USM
motor that’s twice as high compared to the older lenses. The USM motor takes
advantage of this extra power to focus the lens faster.
All single focal length L-series lenses from 135mm to 1200mm plus several L-series
telephoto zooms are compatible with Canon’s 1.4x and 2x EF Extenders. These
accessories are understandably popular among professional and advanced amateur
photographers because of the extra value they offer in terms of increased focal length
without forcing the photographer to invest in longer prime lenses.
Of course, there are trade-offs for this convenience. The 1.4x and 2x Extenders cost 1 or 2
f/stops respectively. This means, for example, that a 300mm f/2.8 lens when used with a
2x extender becomes a 600mm f/5.6. Additionally, EF Extenders reduce lens drive speed.
As noted in the instruction sheets supplied with the Extenders, the EF 1.4x or 1.4x II
reduces lens drive speed by approximately 50~67% depending on the lens in use. The EF
2x or 2x II reduces lens drive speed by up to approximately 75%. This speed reduction
gives the AF system more time to detect focus. This is helpful since the depth of focus is
reduced with the longer effective focal length and the chance of defocus increases.
However, the reduced tracking speed and smaller maximum apertures (that sometimes
result in the loss of cross-type sensor focus detection as mentioned in the description of
the 45-point Area AF Sensor at the beginning of this document) caused by the use of
Extenders can be a disadvantage with fast moving subjects, particularly in low light.
II. LENS ISSUES