Auto focus vs Manual focus

[funky_snowhead]

Ok this isn't a tutorial - but it might be when we get some reponses.

I don't really understand how it works and that bothers me...
When I half press my shutter release I can hear the camera work and it will focus to make the object in the centre of the viewfinder be in focus. (Sometimes it can't do this - and warns me)
What is actually happening in the camera to bring the item into focus?

Currently I have a bridge - but when I move up to the 'adult' world of an SLR - theres a choice of lenses that either auto focus or manual focus. Which suggests to me that its something to do with the lens?

But then I know when talking about camera bodies - it talks about the number of focus points - and I have seen on my friends DSLR a lovely pattern of dots which all light up when those areas are in focus.

Can anyone help to explain this in simple language? Or point me to a thread or webpage?

I'm sure this has done before - and honestly I have searched - but if you can imagine the hits you get on both TP and google for auto focus and manual focus you may understand why I haven't progressed much in understanding this!

thanks

 

[RobertP]

Moved to talk equipment from tutorials. If you want to write up your findings into a tutorial another time you are welcome to


Oh... the noise you hear is a small motor moving glass elements inside the lens assembly so the light focuses on the sensor. More AF points are usually better at tracking moving objects in servo focus mode. In single focus mode where you want say someones eyes focussed but they are not central in the picture you can manually select one of the side points and use that. The centre one is more sensitive so its usually best to use that one only.

Focus points work on contrast so if you point the camera at a plain wall it can't focus. Point it at a newspaper headline and it will focus easily.

 

[dfrear]

Currently I have a bridge - but when I move up to the 'adult' world of an SLR - theres a choice of lenses that either auto focus or manual focus. Which suggests to me that its something to do with the lens?

It's in camera, the different between an MF lens and an AF lens is that the AF lens will focus the lens for you using a motor, rather than your hands. The lens then interfaces with the camera in order to determine where the lens should end up to be correctly in focus.

Most SLR's these days use contrast focussing (or passive autofocus) which uses the difference in contrast to bring the image into focus. The actual math and technology behind it isn't really worth explaining, but its basically the intensity difference between adjacent pixels.

If you want to read into a bit more you can read more about it on Wiki: http://en.wikipedia.org/wiki/Autofocus

 

[funky_snowhead]

Thanks - so the action of focusing is to move the lens so the light falls correctly for the bit you are focusing onto the sensor...
This can either be done manually by moving the lens yourself - manual focus or automatically by the camera - which uses complex bits of jiggery pokery to decide what it needs to do for the object to be 'sharp and in focus'.

I guess i get confused because I move the lens to adjust the zoom and how big an object is on the sensor. The camera then adjusts the lens again on my half click to focus on that object.

Is it two different kinds of movement that are happening? My manual zoom moves the lens backwards and forwards in the horizontal plane which controls the size of the object. What kind of movement is the focus doing? Is it tiny amounts also on the horizontal plane - or is it something more fancy including a bit of vertical adjustment?

I think its the second one but not sure as it seems v. complex but then again the more I learn about what cameras are doing the more I am amazed... (I should just stick to Terry Pratchetts explanation of imps and a paintbrush!)

So the difference between an auto focus lens and a manual focus lens is that the camera tells the lens how to move in the first scenario to focus and little motors in the lens make necessary adjustments - but in the second scenario one you have to do it manually till you see that the object is sharp.

 

[HoppyUK]

Thanks - so the action of focusing is to move the lens so the light falls correctly for the bit you are focusing onto the sensor...
This can either be done manually by moving the lens yourself - manual focus or automatically by the camera - which uses complex bits of jiggery pokery to decide what it needs to do for the object to be 'sharp and in focus'.

Yes. There are very few purely manual focus lens available new these days, although they all have manual override.

I guess i get confused because I move the lens to adjust the zoom and how big an object is on the sensor. The camera then adjusts the lens again on my half click to focus on that object.

Is it two different kinds of movement that are happening? My manual zoom moves the lens backwards and forwards in the horizontal plane which controls the size of the object. What kind of movement is the focus doing? Is it tiny amounts also on the horizontal plane - or is it something more fancy including a bit of vertical adjustment?

I think its the second one but not sure as it seems v. complex but then again the more I learn about what cameras are doing the more I am amazed... (I should just stick to Terry Pratchetts explanation of imps and a paintbrush!)

Zooming and focusing are two separate functions. Zooming (framing) is always manual and each time you adjust the zoom the image needs very slight refocusing. There is nothing 'vertical' going on.

So the difference between an auto focus lens and a manual focus lens is that the camera tells the lens how to move in the first scenario to focus and little motors in the lens make necessary adjustments - but in the second scenario one you have to do it manually till you see that the object is sharp.

Yes, although with some cameras the focus drive is also contained in the camera body, but it's the lens that moves.

All DSLRs use 'phase detect' AF, which uses contrast but with a dimensional element which gives it speed and sensitivity. Contrast-only systems are only used for live view and although this is getting better and has a few potential advantages, it is currently slow and doesn't like low light much. Your bridge camera has contrast-detect AF.

 

[funky_snowhead]

thanks! its starting to make sense!