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I think I've more or less understood how AF works now. However, I'm not sure how the lens influences the whole process. In my recent research for my next lens I found reviews stating the one lens' AF is "dead-on" while the other one's "often fails". Where exactly does it fail? Is the AF motor actually imprecise? Or does it relate to the optics?

I know there are cameras have the AF built-in while other don't (AF motor has to be in the lens). It would be nice if you could explain if this makes any difference.

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3 Answers 3

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Because a "lens" is actually a complex collection of simple lenses which move in groups to perform functions of focusing distance and effective focal length, there are mechanical tolerances that affect performance. Choice of gear ratios, and the mechanics the the gearing become important to remove slack (play). With better quality machining, and design, the mechanical play can be minimised. Some still argue that because prime lenses do not have to worry about changes in effective focal length, the internal movements of the lens groups are simpler to control, with much tighter tolerances. These mechanical factors can affect both the accuracy of the focus, and the speed of the focus.

Other and above the physical factors, the sheer amount of light that can get through helps with auto-focus sensors, as there will be a greater contrast (very light/dark vs. dim/dark) to get a confident focus hit -- this can affect the speed of finding the foccal point, as the camera searches for the highest contrast in the scene. The camera will always attempt to focus with the aperture wide open, and obviously, an aperture of f/1.8 is going to be able to get more light to the sensor than an f/5.6 lens.

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I think this (mechanics) is the actual answer to the original question. As for the prime vs zoom, when I received my EOS 7D, I tried MFA on the 50/1.4 lens. I reached the max +10 settings to get it barely accurate, while my other, zoom lenses seem to focus much more accurately. –  ysap Jan 28 '11 at 13:59
    
Another advantage of a prime lens in this regard: for zoom lenses, the error is often different depending on focal length (front-focus at short lengths, back-focus at long lengths, for example.) Camera bodies which allow adjustment on a per-lens basis don't (at least as far as I know as of 2011) allow for multiple settings per lens, so with a zoom you have to pick a compromise, whereas with a prime you can get it just right. (At least, if unlike @ysap's lens it's within the range of the adjustment!) –  mattdm Jan 28 '11 at 14:19

Actually, the lens influences the focus in two ways:

1) The speed with which it focuses (speed of its motor)
2) The amount of light it gathers (linked to f stop)

Obviously, the faster the focusing motor and the more light a lens gathers, the faster it will focus. But the focus confirmation and seek is also dependent on the camera you use, which can bring inconsistencies between various camera+lens combinations.

You can fit a 24-70 f2.8 nikkor lens (damn fast lens) on a D40 which has about 3 autofocus points and a relatively simple focusing system, and it will still focus pretty fast; HOWEVER, it probably won't get the focus right every time, because of the simplistic nature of the camera's autofocus system.

If you stick the same lens on a D300s or above, which has probably more than 50 autofocus points and a much better processor/system to make use of all of them, you will get much less badly focused ones.

Answering your question: AF lenses (Nikon) are lenses without autofocus motor, they focus by turning a screw on the lens mount - the camera does all this, and only the semi-pro or above models do it (d90 or better). AF-S lenses have a motor built in, they autofocus on all modern cameras, even cheap ones.

I don't know the precise differences between the performance of these two kinds of lenses, but as newer, and more expensive lenses all have dedicated AF-S motors, i assume that a dedicated focus motor improves performance.

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Obviously, this is assuming you know what each autofocus mode of your camera does, and are using it correctly. The camera manual is your best friend! –  JoséNunoFerreira Jan 28 '11 at 12:00
    
Well, one thing that is impossible to accomplish with an in-camera motor is the ring-type ultrasonic motor used in recent better lenses which is silent and very fast. It's hard to blame anyone here. When Nikon thought up its lens mount that wasn't foreseeable and it definitely made the lenses cheaper since they didn't have to include as much electronics and the motor. –  Joey Jan 28 '11 at 14:50
    
It is only Canon and Nikon who think beginners with cheap DSLR don't deserve an in-body AF-motor. Sony is different. Even the cheapest entry level Sony SLT cameras have autofocus motor in body. –  Esa Paulasto Oct 18 '13 at 23:22

Example:

If you have a lens that with aperture 2.8 or less, more light will be available for the focusing system on your camera. The camera has a important role in this.

Now with the ultrasonic motors in many lenses, the focus is more fast.

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2  
Actually, in my particular case the reviews say the 1.8 lens focuses much better than the 1.4.. The aperture may be part of it, but obviously not the only/main factor. –  eWolf Jan 28 '11 at 11:16
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Also, ultrasonic does not necessarily mean faster. On Pentax, the "SDM" lenses are much slower to focus than the small screw-drive-focus Limited lenses. –  mattdm Jan 28 '11 at 14:20
    
Try a Canon 85mm f/1.2L, mark I. You will never confuse 'USM' with 'fast' again, ever. It is positively glacial. It is still a magical lens, of course :) –  Staale S Jan 29 '11 at 0:42

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