why cant this thing focus?!

[marieD]

I have a 20D and have been having really focus issues. I notice that it is in low light. Would getting a better lens ( ihave the kit on there now) solve this problem?

[PacAce]

Yes, getting a faster lens (f/2.8 and wider) will definitely help but low light focusing is always going to be a challenge even with fast lenses. If you have an external flash, you can use the focus assist beam that the flash puts out for focusing. And if you don't want the flash to go off when you take the picture, you can use custom functions to turn off the flash so that it is only used for focusing.

[marieD]

Thanks PacAce! Interesting idea using the flash assist beam to focus. I will have to try that.

[ronosmena]

what metering mode are you using?

[Dare]

Got a laser pointer, I've been using a laser pointer. The camera will focus on a dot as long as you get it in one of the boxes. Then you can either set the shutter on delay or switch the lense to MF after it focuses in.

[SkipD]

Got a laser pointer, I've been using a laser pointer. The camera will focus on a dot as long as you get it in one of the boxes. Then you can either set the shutter on delay or switch the lense to MF after it focuses in.Even better yet, with a 20D (and others) set Custom Function 4 to the "1" setting. Then, autofocus only works when you touch the * button with your right thumb. Use the center focus point only. Aim the center of the finder at the laser dot, press the * button, then worry only about exposure control and final framing of the shot.

[Andy_T]

Crank up the ISO to 1600, too. That usually helps.

How would that work? Haven't tried it, but don't understand how it will work.
You will have a faster shutter speed and avoid motion blur, but the sensitivity of the AF sensor would IMO be the same.

Opening the internal flash might help as it emits a little pre-flash ... don't know if that helps focusing, but I assume it.

For low light, you might look at the 75$ 50/1.8 lens...

Best regards,
Andy

[Cadwell]

Adjusting the ISO increases / decreases the imaging sensor’s sensitivity to light. As the imaging sensor plays no part in the AF function (it’s safely tucked away behind the shutter whilst AF is going on), the ISO setting isn’t going to affect focusing ability.

Using a wider maximum aperture lens will help as the more light you get onto the AF sensors the better it works

[PacAce]

How would that work? Haven't tried it, but don't understand how it will work.
You will have a faster shutter speed and avoid motion blur, but the sensitivity of the AF sensor would IMO be the same.

Opening the internal flash might help as it emits a little pre-flash ... don't know if that helps focusing, but I assume it.

For low light, you might look at the 75$ 50/1.8 lens...

Best regards,
Andy
When the internal flash is popped-up, the flash will emit a burst of pulsed flashes (separate from the preflash) for about half a second to a second to use for focus assist. But, although it does help with the focusing, it can get very annoying very fast to your subjects (and understandably so with those bursts of high frequency flash pulses going off in their faces. :lol: )

[Curtis N]

Crank up the ISO to 1600, too. That usually helps.There is no way that changing the ISO setting will help with low light focusing. The ISO setting only changes how the image processor amplifies voltage coming from the sensor. During autofocus, the sensor is behind the mirror and a closed shutter. The camera's AF system operates independently of the image sensing/processing system.

[claudermilk]

Besides a faster lens, the single most useful thing has not been mentioned: single point AF. I always manually select my AF point (normally the center one) and focus off that. THere's no guesswork as to where the camera will try and focus. The only time it has trouble then is if you center a blank space in the frame--then you just recompose to get a contrasty edge to focus on. I've done thousands of low-light shots with my 20D-50/1.8 and no AF assist light successfully. The only time I start having trouble is REALLY low light and when I grab a spot with no edge to help AF out.

[Curtis N]

Mine hunt like crazy in darker environments but when I kick up the ISO to 1600, they focus quickly and accurately.I'm not attacking you, we're all friends here. ;)

But I'm not buying your statement. From all the official (and unofficial) documentation I have read, the ISO adjustment of Canon digital cameras operates completely independently from the autofocus system. There is no way that changing the ISO setting can affect how the AF operates, and therefore no way that it can prevent the camera from hunting in dark or low-contrast environments.

If you're still convinced that cranking your ISO helps the camera focus, I wish you would provide some explanation on how it works.

[Exit]

Jeez! Before anyone else attacks me, let me clarify that I thought the poster was referring to a lens hunting in low light. Mine hunt like crazy in darker environments but when I kick up the ISO to 1600, they focus quickly and accurately. Sorry if my post confused or upset anyone else.

Your statment was understood correctly and it is wrong! ISO has nothing to do with AF performance. As the others have said the sensor is behind the shutter...and that is closed during AF ;)