Alan321
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 p.1 #6 · Difference in AF on 1-series bodies with f/2.8 vs. f/4? | |
Up to the 1D4 the 1-series cameras have 45 vertical AF sensors that detect horizontal contrast. They each respond at any aperture down to f/5.6, except that the centre one responds down to f/8. They work at all of the larger apertures too. They all work at standard precision which is within the defined depth of focus, which in turn is related to to depth of field but is not the same thing. It varies with lens maximum aperture and focal length)
At some of those 45 sensors there is also a horizontal linear sensor that detects vertical contrast. It operates at a higher precision of 1/3 of the depth of focus but in so doing it is less sensitive can only operate down to apertures of f/2.8 or perhaps f/4. The more central sensors are the ones that respond best at f/4.
Whenever and wherever the horizontal AF sensors are operating the vertical sensors are also operating and in effect you have a cross-type (+) sensor that responds to both vertical and horizontal contrast. A vertical linear sensor has great difficulty focusing on a vertical flag pole unless you rotate the camera, but a cross-type will always work.
In practice the extra focus precision may not matter (e.g. if you are shooting at f/8 the focus precision at f/2.8 or f/1.4 is not so critical because you have lots more DOF at f/8 to mask any focus errors) but the closer you shoot to maximum aperture (at which all AF is done) the more benefit you can expect to see when you have more precision.
The cross-type vs linear may or may not matter depending on what you are shooting. So long as there are clear lines of contrast crossing the linear AF sensor then it will focus ok. If there aren't any then it won't focus so well (if at all) and a cross-type would probably be better because it is sensitive to contrast in more directions at the same time. As a work-around you can rotate the camera to achieve focus and then put it back to where it belongs for the shot.
All focal lengths are affected. What matters are the size of the AF sensor relative to the image area and also relative to the subject. Canon do not publish how fine a detail the camera can focus on, and so if the subject is too small relative to the sensor then there may be insufficient contrast to focus on. This is most likely at short focal lengths or longer shooting distances. However, there is usually something to focus on even if it is only the outline of the subject rather than the surface detail of the subject.
Chances are that cross-type focusing will operate faster because it has two lots of contrast detection at work at each focus sensor position, and more accurately because of the extra vertical precision compared with the horizontal precision, or simply because a linear sensor may be blind to the subject detail.
With the 1D4 there are some lens/TC combinations that work with cross-type capability and precision at f/4 even at what are normally f/2.8 sensors. e.g. the 300 f/2.8 L IS + 1.4x TC, or the 17-40 f/4 lens by itself. This is great but not all f/4 lens/TC combinations can do this.
The new 1Dx changes things somewhat by using some x-shaped cross-type sensors, having cross-type at any aperture down f/5.6, and discarding f/8 AF completely. I haven't studied it in enough detail to comment on its AF precision at f.5.6.
Some non-1-series cameras offer cross-type AF all the way down to f/5.6 at some AF sensors. i.e. They do not revert to linear sensors at f/4, which is often a good thing. However, they do not offer the higher AF precision that you can get from the 1-series cross-type sensors. Pros and cons. On the 1-series cameras "cross-type" inherently meant "high precision" and the terms were sometimes used interchangeably but that does not apply to the non-1-series cameras.
Some cameras, whether 1-series or not, have AF sensors that can work with a greater level of initial defocus. This is quite a separate issue from precision or cross-type but is often very handy because it can help avoid time-wasting lens focus searches. It's mostly on the more modern cameras and is something to look for. e.g. it is not on a 1Ds2 but is on a 1D4 or I think even the 5D.
- Alan
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