Canon EOS-1D Mark II Digial Conseils et techniques - Page 23

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II. LENS ISSUES

AF Speed According to

Just as EOS cameras have improved over time in terms of AF calculation speed, EF

EF Lens Vintage

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