The guy who said there is no such thing as a dumb question, never read this!

What exactly are you focusing on? Choose something with lots of contrast. A desk maybe? Or a book on a coffee table or something. A wall or things of similar color won't work too well.

Woogie wrote in post #2313679
What exactly are you focusing on? Choose something with lots of contrast. A desk maybe? Or a book on a coffee table or something. A wall or things of similar color won't work too well.

I guess my question is::

Will the camera allow you to take a picture with AF on if it is too dark, whatever, for the auto-focus to lock onto the target?

No, not in One shot. If the camera can't focus or can't tell it is focused, it will not actuate.
I checked this on my 30D for you. Even with the safety shift disabled; No focus lock, no shutter actuation.

That said, if you manually focus the lens while the shutter is half-pressed, it is possible for the camera to recognize when it is in focus and light up the focus indicator in the viewfinder. However, without enough light for the AF sensor to operate it is highly unlikely that the camera will recognize focus, lock and allow you to take a photo.

Now for the crazy part. If you recall, earlier we told you that the AF-assist beam will not fire in AI servo. Well here is some insult to add to injury. The camera will take a picture without achieving focus lock in AI Servo.

Ken

canonphotog wrote in post #2313729
No, not in One shot. If the camera can't focus or can't tell it is focused, it will not actuate.
I checked this on my 30D for you. Even with the safety shift disabled; No focus lock, no shutter actuation.

That said, if you manually focus the lens while the shutter is half-pressed, it is possible for the camera to recognize when it is in focus and light up the focus indicator in the viewfinder. However, without enough light for the AF sensor to operate it is highly unlikely that the camera will recognize focus, lock and allow you to take a photo.

Now for the crazy part. If you recall, earlier we told you that the AF-assist beam will not fire in AI servo. Well here is some insult to add to injury. The camera will take a picture without achieving focus lock in AI Servo.

Ken

Thanks, and thanks to the rest of you. Am I on safe ground then when I say my equipment is working fine, within parameters, even though the nut that holds the lens (me) is not working to well?

Looks like everything is working as designed, but I have learned new things about a couple of custom functions I might want to use.

Were you simply trying to focus closer than the minimum focus distances of the lenses?

The 70- 200 has a longer close focus distance.

That means the closest you can focus with the 70-200 is 1.3M.

That's why the 17-40L could focus on your hand but not the 70-200L

cdi-ink.com wrote in post #2313846
Were you simply trying to focus closer than the minimum focus distances of the lenses?

With the 70-200 I was 25 feet away. It was with room light pointed at a black cat. So, it was the inability to focus due to light (ambient as well as a dark subject) that prevented it from auto focusing, not distance.) I was well aware of the reasons for not being able to auto focus, the issue was why wouldn't it let me shoot anyway, even if not in focus? If I am in a totally dark room, with a Speedlite attached, I figured I could still take a picture. Looks like the only way to snap off a picture is by turning off auto focus and then doing it manually. As I stated, my ONLY question now was" can you take a pic if the camera is not in focus?

joegolf68 wrote in post #2313877
With the 70-200 I was 25 feet away. It was with room light pointed at a black cat. So, it was the inability to focus due to light (ambient as well as a dark subject) that prevented it from auto focusing, not distance.) I was well aware of the reasons for not being able to auto focus, the issue was why wouldn't it let me shoot anyway, even if not in focus? If I am in a totally dark room, with a Speedlite attached, I figured I could still take a picture. Looks like the only way to snap off a picture is by turning off auto focus and then doing it manually. As I stated, my ONLY question now was" can you take a pic if the camera is not in focus?

Oh ok I missed that part (tired eyes). But yeah it's normal that the Canon system won't allow firing when focus isn't achieved (whether it's accurate or not is a different story) in One-Shot and AI Focus. In AI Servo you can fire away no matter how far out of focus you are.

As an aside, 25 feet might be the limit of the AF assist beam as well. I'm not sure of the specs on that.

joegolf68 wrote in post #2313877
As I stated, my ONLY question now was" can you take a pic if the camera is not in focus?

Not in 'One Shot' mode.
If the camera believes it is focused it will take the shot though. is anything else in the scene in focus?

If you switch it the manual focus, you can take a picture at any time, wether it's in focus or not. You can have a cf card full of out-of-focus shots if you want. Please post some, they'd make for a good laugh.

Custom function 4 set to 1 will allow you to take photos at any time, whether the camera is in focus or otherwise
It also moves focus from the shutter button to the * button which can be handy

LOL,This subject definitely doesn't come under the term 'dumb question' !
Unfortunately it's one of the many mproblems from having so many auto/semi auto functions on cameras

Rehuel wrote in post #2314428
If you switch it the manual focus, you can take a picture at any time, wether it's in focus or not. You can have a cf card full of out-of-focus shots if you want. Please post some, they'd make for a good laugh.

Obviously you have missed the entire point!

I had a 70-200 with a 580EX attached. If the light is too low to focus, I still have the 580EX attached, so I expect it to go off, and allow the auto focus to work, but if it won't allow for a pic due to low light and no focus, then how do I shoot in low light in this situation. A Catch-22 I guess. So, as I asked, is this the case? The answer is yes, I must use manual focus. My attempt was not to take out of focus shots, it was for the Speedlite to go off, allow for auto focus and then take the shot. But your comment leads me to believe many things. Thanks to all of you who HELPED with my question.

ron chappel wrote in post #2314640
LOL,This subject definitely doesn't come under the term 'dumb question' !
Unfortunately it's one of the many problems from having so many auto/semi auto functions on cameras

Thanks for the comment, and thanks even more for understanding what I was asking!

Col_M wrote in post #2314535
Custom function 4 set to 1 will allow you to take photos at any time, whether the camera is in focus or otherwise
It also moves focus from the shutter button to the * button which can be handy

Thanks. I have learned several good tips about the custom fn buttons, and will begin using several of them!

Joe