Why does an autofocus camera need to emit IR if objects already emit it?

[nightsparkling]

Hi everyone,

An auto-focus camera emits an infrared pulse and detects the pulse reflected from the target, then it calculates the distance of that target from the camera (i.e. it can focus the target). As far as I know, all objects will emit infrared radiation, so what is the use of the pulse emitted by the camera? Or if I put it differently, even if the camera emits an infrared pulse, but can it receive another pulse from the target ? or even receiving pulses from another object which is not the target?

 

[jedishrfu]

There are different wavelengths of infrared:

https://en.wikipedia.org/wiki/Infrared

I think the camera uses a shorter wavelength than what is emitted by people similar to what is used in night vision goggles.

 

[nightsparkling]

But sometimes, the target of the camera may not be a person, right? (forgive me if I ask a stupid question...)
So, will the camera detect other infrared radiation and focus wrongly?

 

[jedishrfu]

I imagine if you were taking a picture of something radiating the same frequency of light it could fool the camera.

Someone else here at PF more familiar with camera tech should be able to answer that.

Here's more info on auto-focus:

https://en.wikipedia.org/wiki/Autofocus

The camera may have other features that allow it to overcome various issues like this.