Why does my T3i provide a red box and not allow me to take a picture while in "A" automatic mode?

[Frisb]

In Automatic Mode (A) and if the lighting is low, the flash comes on automatically, a green box is provided when ready to expose and I then fully depress the shutter button.  Sometimes though, I get a red box and cannot take the picture.  It is a bother not knowing why or how to correct it.  I end up having to relocate and being denied the original view I selected.

[ScottyP]

That probably means that there isn't enough light for the camera to autofocus. The flash may be trying little flickers trying to help it focus. It is frustrating sometimes.

If you are shooting something that is not moving like a landscape at dusk and you are on a tripod you can manually focus on infinity and use Tv or Manual mode to set a long shutter exposure and get some interesting shots that way.

[Frisb]

Thanks for the reply Scott. "fer-tographer" - some questions just seem to have no answers. :)) lol

[Skirball]

If the flash is up won't it emit a focus assist flash to lock focus instead of just a red box?

[Frisb]

i believe the flash assist is trying to help the focus but i can't get a green box and it won't take a picture.

Thanks for the reply,
Steph

[TCampbell]

By default the camera is in "One Shot" mode -- but you can change it to "AI Servo" mode.  

In "One Shot" mode, the camera will focus (or at least attempt to focus) and ONLY after it is able to confirm focus will it take the shot.  This behavior is known as "focus priority".

In "AI Servo" mode, the camera will continuously re-evaluate focus.  This mode is intended for moving subjects where the focused distance is constantly changing.  However... one nuance of this mode is that if you DO fully press the shutter button, then the camera WILL take the shot -- whether focus was able to be locked on any subject or not.  This behavior is known as "release priority".  

On Canon's mid and pro level bodies you can over-ride this behavior (e.g. you could use, say, "One Shot" mode, but still set it for release priority.)  On Rebel bodies such as your T3i you cannot override the mode.

Unrelated to your question, the "red box" / "green box" tells us that you're using "live view" when shooting.  If you haven't done so, you may want to become familiar with using the viewfinder instead of live view.   There area  lot of advantages to using the viewfinder over live-view mode.  One of those advantages (there are many) is that the camera focuses much faster -- which would be important if you're shooting anything with action.

[Skirball]

I was wondering about that, but not too familiar with Live View.  It's possible if you're in Live View that it's not using focus assist? 

+1 on TCampbells suggestion to try using the view finder...  it's why you have a SLR  🙂