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For e.g. an Apple external Firewire iSight does not have hardware autofocus. So how does it focus? Related questions:

  1. Does it focus to infinity?
  2. Does that mean it needs a "longer" minimum distance from lens in order for objects to be in focus?
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+1 for mentioning an interesting piece of "vintage" equipment. I think I still have one of those in a box somewhere... – Paul Cezanne Feb 12 at 19:21
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Hmm... The external iSight camera had autofocus (50mm to infinity), it was the built-in version that did not. – John Cavan♦ Feb 13 at 0:38
Note that "iSight" is just a generic Apple-fied term for a camera on a computer. It used to mean webcam. Now, they use "Facetime" to refer to webcams. Now it just means the cameras on the backs of iPhones and iPads (which cannot be used as webcams). Note that the original external iSight webcam was autofocus. All subsequent iSight cameras have been fixed focus. – thomasrutter Feb 19 at 2:32
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Basically, there is no such thing as software autofocus, because all autofocus mechanisms require some motorised mechanism to adjust the lens. – thomasrutter Feb 19 at 5:44
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@Globalnomad - yes, there has to be a hardware actuator for there to be autofocus. What isn't neccessary is a dedicated AF sensor. The imaging sensor can be used and software can then control the motor for a cheap and slow AF, where as putting in dedicated AF sensors will result in a far superior and faster autofocus. – AJ Henderson Feb 19 at 14:14
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4 Answers

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Firstly, the Apple external Firewire iSight was auto focus. So it focused by way of a tiny motor inside that adjusted the lens much like other auto focus lenses. Therefore it probably was a bad example to choose.

When you refer to lenses that don't have auto focus, I believe you are talking about fixed focus cameras.

To illustrate how both auto focus fixed focus works, first here's a re-cap of the different types of focusing.

  • Auto focus

    In an auto-focus lens, electronics in the body and/or sensor detect when the lens is in focus and adjust it in order to bring it into focus.

    There are a few different types of auto-focusing systems, but they all attempt to change the focus automatically by detecting when the lens is in focus or the distance to the subjects, then physically moving the lens by way of a small motor or servo.

  • Manual focus

    In a manual-focus lens, the operator has to mechanically adjust the focus of the lens so that the subjects come into focus.

  • Fixed focus

    In a fixed focus lens, the lens has no focusing mechanism at all, and is permanently fixed upon a certain focal distance.

    The focal distance is usually chosen in order to get as much as possible in acceptable focus. So, if you have a webcam that is fixed-focus, its focal distance may be about 1.5 feet in front of the lens, where someone's face is likely to be. In a general-purpose fixed-focus lens, the fixed focal distance is often the hyperfocal distance. This is the shortest possible focal distance that would still keep objects at infinite distance in acceptable focus.

    Fixed focus lenses usually have a fairly small aperture, since this increases the depth of field, ensuring that more subjects in the picture will be in focus. Fixed focus lenses with a small aperture and with their focal distance permanently fixed to the hyperfocal distance can have a surprisingly large depth of field, with acceptable focus stretching from a small distance (a couple of feet) out to infinity. This varies according to how small the aperture really is, and the definition of acceptable focus that was used to calculate the hyperfocal distance (in other words, how much blur the manufacturer thought would still be acceptable).

    Of course, having such a small aperture also means you're not letting much light in, so this is a compromise in low-light gathering ability. The fixed focus also means that even though your subjects will almost always fall within acceptable focus (by some definition of acceptable) the focus will never be tack sharp unless the subject happens to be exactly at the lens' fixed focal distance.

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According to Wikipedia, the firewire, external iSight camera is not fixed focus, but has an autofocus. – AJ Henderson Feb 19 at 4:20
You're right, I've added a bit more to my answer. – thomasrutter Feb 19 at 5:50
up vote 10 down vote

Something that cannot focus simply does not focus. This how most disposable cameras are made. They have a fixed-focus lens where the focus plane is at a certain distance and the aperture is small enough to give a large depth-of-field.

With small sensors like in webcams and other tiny cameras, the effect is multiplied because depth-of-field is huge. In other words almost everything is acceptably sharp despite not being in focus. The best way is to focus at the hyperfocal distance which means everything to infinity is sharp enough, starting at some distance before the focus-distance. Some ultra-compact cameras can even fix their focus at the hyperfocal distance.

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With focusing to hyperfocal distance not everything will be in focus. Subjects closest to the camera will still be out of focus - approximately from the camera till half of the hyperfocal distance. – Evaldas Dzimanavicius Feb 12 at 19:02
@EvaldasDzimanavicius Clarified. Thanks! – Itai Feb 12 at 19:56
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The focusing method used by NASA's Curiosity rover currently on Mars is to take several pictures at different focusing distances and compare the size of the resulting jpeg files. By iterating with on-board software until the largest file size is obtained, that image represents the sharpest focus, which in image compression software correlates to a correspondingly large file size needed to hold a given level of detail. While a focusing motor is needed, the determination is done by software in a flash drive--not by firmware. This system can focus from infinity to as close as the motors allow lens extension.

Inexpensive web cams can autofocus by using current from a bridge circuit to deflect a spring on which a focusing element is mounted, again using software tests to verify the position of highest contrast or key frame size. The only "motor" in this case is the back and forth deflection of the spring.

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The autofocus mechanism you describe in the MAHLI camera on the Mars Rover is effectively the same as that used in point and shoot and (most) mirrorless cameras, in that a sample area is read and the lens hunts back and forth until the best is found. It's interesting that they're re-using the jpeg engine for that purpose; I don't know if any earth-bound cameras do the same. They're also sampling across the whole frame, which is presumably because focus speed is not a concern. Anyway, as interesting as all this is, it doesn't really answer the question, because this is autofocus hardware.... – mattdm Feb 13 at 0:05
@mattdm I assume the reason for using JPEG is that the chips that do JPEG encoding have been around for a very long time, and presumably all bugs would have been discovered by now. This yields a more robust autofocus scheme requiring only a very short program to do the AF which is easier to check for correctness. – Matt Grum Feb 19 at 10:53
@mattdm - yes, it is autofocus hardware, but it is not a hardware based AF sensor. It is an imaging sensor which is being reused for AF purposes by analyzing the captured image. (As opposed to say a phase detect or cross type AF that utilizes a discrete HW AF sensor.) – AJ Henderson Feb 19 at 14:12
@mattdm - never mind, within the context of the clarified question, the asker was specifically talking about an AF motor. – AJ Henderson Feb 19 at 14:15
up vote 0 down vote

Not having a hardware autofocus could simply mean that it lacks autofocus sensors and instead focuses using contrast based methods in software. It also could simply be a fixed focal length. Not sure without looking at details of the specific device.

According to Wikipedia, the external firewire version of the iSight has a 50mm to infinity autofocus. If it lacks "hardware autofocus" then it most likely means that there are not dedicated AF sensors in the imaging sensor. This would require a "software focus" where the software would drive the focus in one direction and then the other, looking for the setting that resolves the sharpest image by looking for the highest contrast.

It is worth noting that the internal versions of the iSight are fixed focus as described in other answers.

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CDAF is still "hardware autofocus". I believe fixed focus is what he is referring to. – thomasrutter Feb 19 at 2:30
@Thomasrutter - From Wikipedia - "The external iSight's ¼-inch color CCD sensor has 640×480-pixel VGA resolution, with a custom-designed three-part F/2.8 lens with two aspherical elements. It features autoexposure, autofocusing from 50 mm to infinity, and video capture at 30 frames per second in 24-bit color with a variety of shutter speeds. However, the iSight has an image delay of approximately 120 ms." It does appear to have an autofocus, it just must work off of software processing of the image to resolve edges via contrast rather than having actual AF sensors. – AJ Henderson Feb 19 at 4:16
The internal version of the iSight however IS fixed focus. – AJ Henderson Feb 19 at 4:16
If the question was referring to the external version of the iSight, then it's a simple case of the person asking the question being mistaken, because it does have autofocus. There is no such distinction as "software autofocus" vs "hardware autofocus" as autofocus always means there is an automatic focusing mechanism that drives a little motor that focuses the lens - you can't move that lens without hardware. So just a case of confusion on the part of the person asking the question - hopefully he's less confused now! – thomasrutter Feb 19 at 5:42
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@Thomasrutter - but you can or can not have hardware to sense if the image is in focus. On many cameras there are dedicated sensors for detecting if the image is in focus. This is what I would interpret as "hardware autofocus". It is still possible to autofocus without these sensors, but it requires using contrast AF. – AJ Henderson Feb 19 at 14:04
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