How do lenses determine the focus distance?

I did a quick search but couldn't find a straight answer.

How do lenses determine the actual distance of the object it is focusing on? like on the screen of the lens.

It uses magic


Seriously though, this may help you. While not an answer to your exact question it is a start in the right direction.

no it is magic that link is a lie.

there are little invisible gnomes that run out from inside your lens with a tape measure and calculate the plane of critical focus and then radio it back to your camera and then your auto focus takes over and BOOM your ready to go

Not sure what you mean by "on the screen of the lens". Are you asking about the distance reported in exif data, or the focus index on the lens itself?


If the data reported in exif:
I'm guessing that there is some sort of communication between camera and lens that relates raw data about the focus element position, which then gets converted to text and embedded in the exif data at time of record. For example: say actual focus element has 10 positions, the first would be the minimum focus distance of 10', second position at 20', etc., the last being infinity. The lens knows the position of the focussing element so it reports it back to the camera to be embedded in the exif. The camera and lens don't know the subject distance, but it knows generally where it's focussing at.

If you're talking about the focus index on the lens itself:
When they engineer the lenses, they know the focus distance based on calculations of the glass curvature, refraction, etc. Based on that, they know where critical sharp focus will occur (distance-wise) given a specific position of the focussing element in the lens. They can then put distances on the focus index of the lens based on where the focussing elements position is given a certain focus ring position.

If Mike's link didn't confuse you enough, try this:

http://www.zen20934.ze​n.co.uk …y/Canon%20AF%20​System.htm

Its a trick question!!! The lens does not determine anything. The part that makes the decision is in the camera body.

Yeah, that's true... It's the camera that focuses the lens in most modern cameras...

http://en.wikipedia.or​g/wiki/Autofocus has lots of info on how it works and discusses the different types of focus.

Canon DSLRs use Phase Detect for most focusing. Models with Live View switch to Contrast Detect when in that mode... it's slower but can be more accurate.

Mark1 wrote in post #13170863
Its a trick question!!! The lens does not determine anything. The part that makes the decision is in the camera body.

I don't think that's exactly true; if you don't have a lens that relays distance info to the camera, then the data isn't recorded in EXIF...so, most (anyway) of the consumer grade lens/body combos will not include that info.

The question was not how to record the info. The question was how the lens determines the distance. It simply does not. The AF system in the body tells the lens to adjust till it sees the maximum amount of contrast it can obtain. The lens makes no decisions itself. However, The lens will report its position and that can me calculated into a distance.

joebob23 wrote in post #13170335
Not sure what you mean by "on the screen of the lens". Are you asking about the distance reported in exif data, or the focus index on the lens itself?


If the data reported in exif:
I'm guessing that there is some sort of communication between camera and lens that relates raw data about the focus element position, which then gets converted to text and embedded in the exif data at time of record. For example: say actual focus element has 10 positions, the first would be the minimum focus distance of 10', second position at 20', etc., the last being infinity. The lens knows the position of the focussing element so it reports it back to the camera to be embedded in the exif. The camera and lens don't know the subject distance, but it knows generally where it's focussing at.

If you're talking about the focus index on the lens itself:
When they engineer the lenses, they know the focus distance based on calculations of the glass curvature, refraction, etc. Based on that, they know where critical sharp focus will occur (distance-wise) given a specific position of the focussing element in the lens. They can then put distances on the focus index of the lens based on where the focussing elements position is given a certain focus ring position.

I meant the focusing screen on the lens (ex: 1.5m to infinity)

I kinda thought it was some sort of light that is relayed through the lens, hits the object in focus, brings it back to lens or body.

Reading those links at the moment...

Mark1 wrote in post #13171645
The question was not how to record the info. The question was how the lens determines the distance. It simply does not. The AF system in the body tells the lens to adjust till it sees the maximum amount of contrast it can obtain. The lens makes no decisions itself. However, The lens will report its position and that can me calculated into a distance.

Gotcha; that makes more sense. Thanks!

amfoto1 wrote in post #13171293
Canon DSLRs use Phase Detect for most focusing. Models with Live View switch to Contrast Detect when in that mode... it's slower but can be more accurate.

Oddly enough, the 40D does not. When in live view, using autofocus lowers the mirror, turns off the LCD and focuses the lens with phase detect. Then it lifts the mirror and turns the screen back on.

Found that out the other day, I was surprised, as I was used to contrast detect from the XS.

Just something I thought was neat/odd/unusual.

b_ron wrote in post #13171670
I meant the focusing screen on the lens (ex: 1.5m to infinity)

I kinda thought it was some sort of light that is relayed through the lens, hits the object in focus, brings it back to lens or body.

Reading those links at the moment...

So, that's related to the info in Mark's responses. The manufacturer knows that at "position x" the lens is focused at APPROXIMATELY "y distance"; so they put those markers on there. That scale is just a rough approximation of how far out the lens is focused and not anywhere close to exact measurements.

Yeah. Mark's link explains the contrast differences. tomj's link basically says the same thing in different words? phase difference in light (contrast?), using that to calculate the distance.