Autofocus Accuracy

[bendixso]

Hey everyone,

I was reading the Wikipedia article on autofocus, and there was a line about the average accuracy of most autofocus systems. I read it a few times, and I wasn't quite sure what it meant. Is there anyone here who can help me demystify it?

The Wikipedia quote in question:
Autofocus accuracy within 1/3 of the depth of field (DOF) at the widest aperture of the lens is not uncommon in professional AF SLR cameras

Thanks for your help,
-Ted Bendixson

 

[imagemaker46]

Focus..........it's in or it's out........why do you need to know or understand anything more than that?

 

[analog.universe]

Depth of field is the physical dimension of the part of the scene that is in focus for the given aperture and focal length. Here is a table of various focal lengths, apertures, and their corresponding depths of field. Depth of Field Table

So the accuracy of 1/3, would be 1/3 of the difference between near and far in that table, for whatever lens is in question.

 

[ghache]

Focus..........it's in or it's out........why do you need to know or understand anything more than that?

having a bad day?

 

[KmH]

It really depends on the lens focal length.

For short and standard focal lengths, it is much closer to 1/2 than 1/3.

1/3 works for the longer focal lengths.

Sharp focus is a single plane in space that is parallel to the camera's image sensor (film or electronic), and focus sharpness deteriorates as a function of distance in front of and behind that plane of focus.

However, there is a range of distance in front of and behind that plane that the reduces focus sharpness is still acceptable. That range of distance is called the depth-of-field (DOF).

Back to auto focus accuracy, and I will be talking in terms of phase-detection auto focus, not contrast-detection or any of the other ways auto focus can be done.

There are 2 kinds of auto focus sensors in most DSLR cameras:

• single axis focus points (regular focus points)
• 2 axis focus points (cross-type focus points)

Most DSLR cameras have more of the single axis points than 2 axis points.

Cross-type points are more accurate than single axis (regular) focus points. Most entry-level DSLR cameras only have one, if any, cross-type focus points.

 

[Josh66]

I think what they're talking about in that article (can you post a link to it?) is that usually the point of sharpest focus will be 1/3 'into' the DOF that is 'acceptably' in focus. Say the DOF is 15 feet, and the subject is 30 feet away...

Focusing on the subject, 30 feet away, everything from 25 feet to 40 feet away would be in 'acceptable' focus, with the sharpest point being 30 feet away. Outside of that 25-40 foot range, the focus would be 'unacceptable', and thus - 'out of focus'.

 

[uriljack]

great post thanks very mmuch

 

[Garbz]

Focus..........it's in or it's out........why do you need to know or understand anything more than that?

He doesn't. you do. Focus is not in or out beyond any fluffy and completely meaningless definitions. Focus is judged by accuracy to the focal point, and resulting sharpness in the picture.

You are talking about sharpness of the target. This has little more to do with focus accuracy than the focus being the starting point. What's the depth of field? What's the printing mechanism. A picture can look quite out of focus on the computer screen yet perfectly tac sharp on a wallet sized print.

He's asking a legitimate question seeking to expand his technical knowledge and you poopoo him with some idle fluff. I suppose you shoot in fully automatic too because who cares what an aperture is, the photo is either good or bad right?

 

[imagemaker46]

Focus..........it's in or it's out........why do you need to know or understand anything more than that?

having a bad day?

Nope it's been good days. It's really only the camera geeks that always seem to find some reason to come up with the formula behind why cameras do what they do, instead of just going out and doing it.

 

[analog.universe]

Focus..........it's in or it's out........why do you need to know or understand anything more than that?

having a bad day?

Nope it's been good days. It's really only the camera geeks that always seem to find some reason to come up with the formula behind why cameras do what they do, instead of just going out and doing it.

Instead of? I really doubt that's the case...

You're not going to convince me that people are better off not understanding their equipment.

Instead of learning about exposure, why don't you just throw it in auto and forget about it? isn't all that theory getting in the way of your shooting?

 

[imagemaker46]

Well, I'm the first to admit that I am the least technical photographer I know. I don't worry about how my cameras work, I don't really have to many concerns about it. I know is how to use light. I understand what makes a good image without worring about the rules and regulations that people seem to think are the most important part of being a photographer.

I realize that the majority of people with cameras could use more understanding of light and composition. But do they all really need to know why a camera works? How autofocus works?

Learning how light works is the base, if you can see it, you can shoot it. Understanding light is the key to becoming a better photographer. I am a frim believer that people should turn off the auto functions on their cameras from time to time and really learn how to use a manual camera. Shooting auto is what people learn, it's easier to just point and shoot, and that's all the average camera owner wants.

 

[Derrel]

I think a LOT of people have a very vague, fuzzy concept of how their camera's autofocus system is engineered. SOme of the better modern AF systems are pretty amazing, but they have become rather com plicated compared with what we had available just five years ago; more modes of focusing, more custom function setting options, and even more buttons on some of the consumer and mid-range cameras; Canon even copied Nikon and decided it's mid-level cameras needed a dedicated back button focus control button, rather than the poorly positioned "*" button Canon users could program to function as a back button focus control. I think MOST of today's amateur photographers have never read their camera manual and its explanations of how the AF system is set up; I constantly see people woindering why they cannot get their AF assist light to come on...and then find that they are shooting in Autofocus-Continuous mode....duh....RTFM...reas the fine manual.....ahem...why drop $5,200 on a pair of cameras and then not know how the fricking AF system 'really' works???

 

[imagemaker46]

As much as I did say I wasn't a very technical person, I do know how my cameras work, I've always carried the manual with me everywhere I go. Yes there are functions that I still play around with, but I do know what to use and how to set things up for the events that I'm shooting. I think the big thing is just trying to not over complicate anything. Derrel you're quite right about everything you said, gear has become much more complicated than it used to be. It really is overwhelming when you first open up the box and the manual is 400 pages long, especially if you are switching systems. I went from Canon to Nikon a week before a big shoot, reading through the manual on the plane and playing with settings. I miss the old days when the manual was 10 pages long, and the camera had only a handful of options.

 

[Josh66]

I miss the old days when the manual was 10 pages long, and the camera had only a handful of options.

I don't...

Those kinds of cameras don't need manuals... I like having custom functions and auto-focus...

 

[SNBniko]

The people who make those fancy equations for why a camera does what it does are also the people responsible for improving it. Not just saying 'oh, it would be cool if my camera would do X and Y,' actually designing it. You can't design everything with a trial and error method, especially when most optic problems are easily solved with an equation, eliminating all expenses incurred with 'I wonder if...'

Do go poo pooing the engineers behind the camera. Even the camera with the 10 page manual had an engineer behind it somewhere, with the same set of equations for how the lenses work etc.