Do most camera's af systems struggle with fast lenses?

stevebrot · 04-23-2014, 08:27 AM

Originally posted by monochrome

Learn something every day. I have always been mystified by autofocus query/complaint threads since I rarely seem to have problems. Now I suppose I know why.

There is a very good article on the Web that describes the hows and whys in very understandable detail. Unfortunately, I always forget to bookmark it, but found a link on the Web this morning (thank you RiceHigh...oops! Did I say that?). It also addresses the similar limitations to focus screen microprism and split-image focus aides. It is enough to say that the inclusion of the three f/2.8 center point sensors on the K-3 is a big deal for people using faster AF lenses.

http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf

It might be asked why adaptation of the f/2.8 sensors has been slow. Good question. The linked article is almost a decade old and the last paragraph indicates that Canon had a minimal implementation on the 20D at that time. I can just imagine that there are technical issues.

Steve

SteveB · 04-26-2014, 10:59 AM

Lenses don't always focus the same at all apertures due to aberrations, and yet AF is always done wide open, so there's another problem to add to f5.6 AF sensors inaccuracy with bright lenses.

awaldram · 04-26-2014, 12:14 PM

Originally posted by stevebrot

The short answer is yes.

The long answer is complicated and has to do with how phase-detect AF (PDAF) systems work. Most of the AF sensor sites for most dSLR cameras have a working aperture of f/5.6. I say most because some cameras (the K-3 being the sole Pentax example) have at least some sensors optimized for f/2.8. The result is relatively poor precision when used with faster lenses at wider apertures.

Contrast-detect (CDAF) systems such as used in live view do not have this limitation and generally provide both greater precision and greater accuracy.

Steve

The k5ii series has a single f2.8 basined sensor the the K3 has 3 ... at least the canon 1ds has multiple f2.8 sensors.

As to contrast detect it a double edged sword.... because it works on contrast not phase the camera has no idea which way to focus so has to guess this means performance is so so and in moving targets generally no-no unless the lens is designed for contrast detect (no Pentax lens are).

Also as light drop contrast falls off faster than phase so under low light level CD will fail.

Both PD and CD and be fooled into false locks as they find a peak that is no 'in focus'

As they are both using differential between two points both can only be as accurate as the baseline in use. I have no idea what the base line is for CD off sensor but have not noticed any improved accuracy with Fast lens over PD once PD is calibrated.

jkomp316 · 04-30-2014, 12:27 AM

Collimation and divergence are a good read. Factors also change depending upon the temperature of the light your capturing. Manufacturing tolerances affect things as well. Crisp shots at F/1.4 with point and shoot operation is quite a feat. Its much easier when the lens is permanently attached to the camera...

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04-23-2014, 08:27 AM	#16
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by monochrome Learn something every day. I have always been mystified by autofocus query/complaint threads since I rarely seem to have problems. Now I suppose I know why. There is a very good article on the Web that describes the hows and whys in very understandable detail. Unfortunately, I always forget to bookmark it, but found a link on the Web this morning (thank you RiceHigh...oops! Did I say that?). It also addresses the similar limitations to focus screen microprism and split-image focus aides. It is enough to say that the inclusion of the three f/2.8 center point sensors on the K-3 is a big deal for people using faster AF lenses. http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf It might be asked why adaptation of the f/2.8 sensors has been slow. Good question. The linked article is almost a decade old and the last paragraph indicates that Canon had a minimal implementation on the 20D at that time. I can just imagine that there are technical issues. Steve

04-26-2014, 10:59 AM	#17
SteveB Veteran Member Join Date: Mar 2007 Location: U.K. Posts: 685	Lenses don't always focus the same at all apertures due to aberrations, and yet AF is always done wide open, so there's another problem to add to f5.6 AF sensors inaccuracy with bright lenses.

04-26-2014, 12:14 PM	#18
awaldram Veteran Member Join Date: May 2008 Location: Hampshire Photos: Gallery Posts: 732	Originally posted by stevebrot The short answer is yes. The long answer is complicated and has to do with how phase-detect AF (PDAF) systems work. Most of the AF sensor sites for most dSLR cameras have a working aperture of f/5.6. I say most because some cameras (the K-3 being the sole Pentax example) have at least some sensors optimized for f/2.8. The result is relatively poor precision when used with faster lenses at wider apertures. Contrast-detect (CDAF) systems such as used in live view do not have this limitation and generally provide both greater precision and greater accuracy. Steve The k5ii series has a single f2.8 basined sensor the the K3 has 3 ... at least the canon 1ds has multiple f2.8 sensors. As to contrast detect it a double edged sword.... because it works on contrast not phase the camera has no idea which way to focus so has to guess this means performance is so so and in moving targets generally no-no unless the lens is designed for contrast detect (no Pentax lens are). Also as light drop contrast falls off faster than phase so under low light level CD will fail. Both PD and CD and be fooled into false locks as they find a peak that is no 'in focus' As they are both using differential between two points both can only be as accurate as the baseline in use. I have no idea what the base line is for CD off sensor but have not noticed any improved accuracy with Fast lens over PD once PD is calibrated.

04-30-2014, 12:27 AM	#19
jkomp316 Senior Member Join Date: Feb 2013 Photos: Gallery \| Albums Posts: 227	Collimation and divergence are a good read. Factors also change depending upon the temperature of the light your capturing. Manufacturing tolerances affect things as well. Crisp shots at F/1.4 with point and shoot operation is quite a feat. Its much easier when the lens is permanently attached to the camera...