The real facts about low-light focusing

[defordphoto]

Awesome article...

http://www.cps.canon-europe.com/kb/detail.jsp?faqId=1003

[scottbergerphoto]

Good reference Jim. I'll just add two things that the article omits. Low light AF is slowed by:
1. Using an AF assist beam as on an EX Series Flash- there is a slight delay for the AF beam to emit.
and
2. CFn.17-1, expanding the number of AF points around the manually selected one, also slows low light AF in AI Servo.
Regards,
Scott

[defordphoto]

2. CFn.17-1, expanding the number of AF points around the manually selected one, also slows low light AF in AI Servo.


AHA! That one I did not know! Thanks.

[tim]

Good info there. In low light I always have the 550EX on, even if flashes aren't allowed, the AF assist beam really does help. A custom function tells the flash not to fire.

[Mills]

Thanks Jim. Informative read!

[allanc]

I use my Digital Rebel for cave photography. AF is totally useless under these VERY low light conditions. I only use manual focus. I use a subject in the distance for the point of focus. If the person shines a light directly at me, I can focus the light to a pinpoint.

[ron chappel]

That site has some very usefull info!
I was planning on re-reading my instruction manual sometime soon so this is good timing :)

I'm most impressed that everything is written in plain english !!!

[NYC2BGI]

Thanks for the link. It was very informative and just what I needed.

[Pekka]

What the article does not really say but what actually is very clear when you read the conclusions paragraph is that in low light the focus is more accurate if camera is steady and focusing target is steady. If the camera shakes during "information accumulation period" the information gathered will be unreliable.

it takes X amount of time multiplied by Y amount of light multiplied by Z amount of contrast for the AF sensor to accumulate enough information to function.

[cmM]

Interesting read... thanks Jim

Another reason to start focusing with *

[slin100]

It's interesting to learn that color temperature does not affect focus accuracy. This should dispel the notion that focus accuracy is not affected by, for example, the use of tungsten lighting.

[scottbergerphoto]

Interesting read... thanks Jim

Another reason to start focusing with *
Using * to focus via CFN. 4-1 while my preferred way of using my camera, doesn't change the issues with low light. I'm not sure what you mean.

[cmM]

Using * to focus via CFN. 4-1 while my preferred way of using my camera, doesn't change the issues with low light. I'm not sure what you mean.
Technically it doesn't, but...

Custom Function 4-1 or 4-3 can also be very useful for low-light AF, especially with an EF lens that supports Full-Time Manual (FTM) focusing. With these settings you can manually focus the lens at will if the AF system isn't able to lock on. You can also stop the lens from searching by simply lifting your thumb off the AE lock button.

[scottbergerphoto]

Technically it doesn't, but...
True that.

[Jon, The Elder]

Nice read guys - Thanks much, confirms some impressions I had. This is shaping into one of the better forums, through your efforts.