For e.g. an Apple external Firewire iSight does not have hardware autofocus. So how does it focus? Related questions:
- Does it focus to infinity?
- Does that mean it needs a "longer" minimum distance from lens in order for objects to be in focus?
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Sign up to join this communityFor e.g. an Apple external Firewire iSight does not have hardware autofocus. So how does it focus? Related questions:
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.
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.
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.
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.