AF question

[woollybear]

Is AF more accurate/faster when using a better lens? The lesser lens are often described as not as sharp, or good lens as soft wide open.

When using a good lens, does stopped down result in better AF performance?

Does using a lesser quality lens result in degraded AF performance?

 

[Mt Spokane Photography]

Is AF more accurate/faster when using a better lens? The lesser lens are often described as not as sharp, or good lens as soft wide open.

When using a good lens, does stopped down result in better AF performance?

Does using a lesser quality lens result in degraded AF performance?

Generally, the low end lenses are not as fast or accurate, but it really depends on the lens. The Expensive 85mm f/1.2, for example is relatively slow, it moves a lot of glass. It is accurate though.

The telephoto lenses intended for sporting events tend to be fast. and accurate. A wide angle lens does not have to move far, and has a very deep depth of field, so accurate AF is usually not a problem.

Stopping down a lens, does not, to my knowledge affect AF performance. The lens is first focused wide open and only stopped down just as the shutter opens.

Don't be too worried about AF performance, they all do the job they were intended for pretty well. Its only if you try to use a 85mm f/1.2 portrait lens to photograph sports or racing that you might have a issue.

If you have a specific usage in mind, there are some really knoowledgeable people who will give you a specific.

A generalization can mislead you.

 

[woollybear]

Thanks for the help. Right now I am just trying to learn the mechanics of cameras and lenses, and their various interactions, from a strictly technical standpoint (as opposed to artistic).

Another question, I know AF points are specified for various f-stops. 1 at 2.8, 4 at 5.6, etc. Why is that? and if the lens is focused at the maximum f-stop, why do they do that?

 

[MikeHunt]

Is AF more accurate/faster when using a better lens? The lesser lens are often described as not as sharp, or good lens as soft wide open.

When using a good lens, does stopped down result in better AF performance?

Does using a lesser quality lens result in degraded AF performance?

Been using a 50 f/1.4 USM for almost a year and it hunts quite a bit for AF. Just got an EF 24-70mm f/2.8L two months ago and the AF is 100 x faster. It seems like the L lens takes just nanoseconds to AF lock ;D

Get yourself some good 'L' glass and you'll never look back.

 

[chito]

Stopping down shouldn't affect af unless you're talking about the infamous 50L and its focus shift issues

 

[marekjoz]

Thanks for the help. Right now I am just trying to learn the mechanics of cameras and lenses, and their various interactions, from a strictly technical standpoint (as opposed to artistic).

Another question, I know AF points are specified for various f-stops. 1 at 2.8, 4 at 5.6, etc. Why is that? and if the lens is focused at the maximum f-stop, why do they do that?

"Another question, I know AF points are specified for various f-stops. " - Not exactly. AF points are specified for various MAXIMUM lens apperture, the lens has. You can have ie 24-105 f4 and when you will focus with this lens, then the camera will use points intented for f4. If you put 24-70 f2.8, then camera uses autofocus points intented for 2.8. If you use 50 1.4 - then also points for 2.8.
Before you shoot, when you try to focus your subject loooking through viewfinder and pressing shutter to half, then lens is open to maximum it's apperture (so in lenses I gave you for example - 4, 2.8, 1.4). Camera uses max apperture of given lens and according focus points to set a focus. When you finally make a picture after having set your focus distance, lenses are closed to apperture you set before, for you optimal exposure parameters.
Why is that? It's easier to focus at wider apperture because there is more light on a focus sensor When you focus, mirror before sensor directs light to viewfinder and focus sensor. When you shoot, mirror goes up and light goes through the lens directly to the camera main sensor.

 

[jwong]

Thanks for the help. Right now I am just trying to learn the mechanics of cameras and lenses, and their various interactions, from a strictly technical standpoint (as opposed to artistic).

Another question, I know AF points are specified for various f-stops. 1 at 2.8, 4 at 5.6, etc. Why is that? and if the lens is focused at the maximum f-stop, why do they do that?

They refer to the baselines for the autofocus sensors. Only lenses at or below a given AF f-stop can use it. The wider the baseline (lower f-stop), the more accurate the phase detect system works. See explanation below (Neuro):

http://www.the-digital-picture.com/Photography-Tips/Canon-EOS-DSLR-Autofocus-Explained.aspx

 

[Mt Spokane Photography]

Thanks for the help. Right now I am just trying to learn the mechanics of cameras and lenses, and their various interactions, from a strictly technical standpoint (as opposed to artistic).

Another question, I know AF points are specified for various f-stops. 1 at 2.8, 4 at 5.6, etc. Why is that? and if the lens is focused at the maximum f-stop, why do they do that?

To understand this, you first need to understand how phase detect autofocus works. Its basically a rangefinder, and larger lens openings can have a longer baseline for more accuracy. Thus, there are two rangefinders, one for smaller apertures and one for wider apertures. Since you need better focus accuracy with wide aperture lenses, this makes a difference.

Try searching for phase detect autofocus and get up to speed on that.

 

[LetTheRightLensIn]

Thanks for the help. Right now I am just trying to learn the mechanics of cameras and lenses, and their various interactions, from a strictly technical standpoint (as opposed to artistic).

Another question, I know AF points are specified for various f-stops. 1 at 2.8, 4 at 5.6, etc. Why is that? and if the lens is focused at the maximum f-stop, why do they do that?

because the max fstop of a lens may be f/2.8 or 4 or 5.6 etc. ;D