DRebel flash problems/questions

[jrm]

I was shooting some family pics this weekend. Location was a nightclub - very dark. Only flash I had was the built-in.

I usually don't shoot flash. Still, I have read up on how the Rebel handles flash and though I understood. Apparently, I didn't. :)

Max aperture on the lens is 2.8. (Canon 70-200 f/2.8L IS). When on "P" mode, the camera selected 1/60sec, f/2.8. This makes sense - exposing for background (dark) and letting flash illuminate subject with 1/60sec sync speed.

However, I wanted to get greater depth of field by using a smaller f-stop. So I went to Av mode and dialed in f/5.6. Problem is that this made the shutter speed drop _way_ down... (forget exact number, but it was unacceptably long). I couldn't be bothered trying to figure out what was going on, so I just left it in "P" mode and snapped away.

Fortunately, no pics really suffered from depth of field problems, and most came out very well (considering I only used the built-in flash). Which leads to questions:

1. What happened to the bokeh at 70mm shooting f/2.8?
2. Why did the shutter speed drop below 1/60 when in Av mode?
3. What settings should I have used to get proper exposure at a smaller f-stop
(i.e. f/5.6)?
4. Am I correct in thinking that the DRebel meters the entire scene for exposure when using flash? (As opposed to just the single focusing point selected).

and related questions...

- why is autofocus so difficult in low light? Sometimes it would not work at all.
- is there any way to turn off the "pre-flash" machine gun effect?
- why did the "pre-flash" only go off sometimes?

I am looking into getting a "real" flash unit, but that is a question for another topic. :) For now, I am just trying to gain a better understanding of how it all works.

--Joe

[skimmilk68]

the pre flash is i believe red eye eliminating option...turn that off in your menu and it shouldnt do that.. and for your other questions, im wondering the same thing :D

nice lens btw, you should post some of the pics

[tim]

When I use an external flash I always use manual mode - F8 1/60 of a second, but I adjust it according to how it looks. Not sure how it'd work with internal flash, but it'd be worth a go.

[jrm]

I have the red-eye reduction turned off. That's what I thought the pre-flash was, so I checked.

Then I figured the pre-flash might have had something to do with auto-focus. Based on the difficulty the AF system had, I figured that guess was wrong too.

Hmmm.

[Bodog]

Joe, I'm certainly no expert on Canon's flash system, I think i can answer some of your questions.
You were OK in the "P" mode because your shutter speed would be limited to a "hand holdable" 1/60. However when you switched to AV, there is no such restriction and the camera tried to expose the background at the correct (low) shutter speed for the available light. Tim's suggestion of shooting in manual is a good way to get around this. Also, though it might not have helped in your situation, the Rebel firmware hack will allow you to force a 1/200 shutter speed with flash in AV mode.
As to the autofocus problem, I'm guessing your subject was too far away for the assist light to be effective. I don't recall the exact effective distance, but per the manual it is fairly limited. I'm also guessing that the darker the room, the shorter the distance...
Hope this helps,

[cmM]

in P mode the camera defaults to 1/60 or something like that, never really used P mode. In AV mode the camera will meter for ambient light, that's why you had really low shutter speeds.

Use M mode if you want to use f/5.6 or any other apertures.

As far as your Autofocus being slow or not working at all, that's because there is no light. The AF sensor needs to see *something* to focus on.

[jrm]

Thanks everyone for the info. I guess it is becoming clearer now. I was so pressured to just "take the pictures" that I didn't have time to think it through.

The P mode defaulting to 1/60 is the key that put it all together. I didn't realize that it "helped" you this way (I rarely shoot in P and am hardly ever in situations with such little light.). This info, along with remembering that Av will meter for ambient light only explains why the shutter speed would go so long.

I should have went with my gut feeling and just dialed M at 1/50 f/5.6 or so. If I did this, am I correct in assuming that the flash would have automatically adjusted itself for the subject/foreground exposure? In other words, would I have ended up with basically the same exposure that P gave me, only the "background" would have been more under exposed?

I know the AF needs to see something - just thought it had some low light/no light feature (Or am I thinking of the IR AF assist on the 550ex flash unit?)

Thanks again for the answers.

--Joe

[scottbergerphoto]

Your camera meter will always try to meter the ambient light. It doesn't know or care if you have a flash attached. In P mode, the camera doesn't default to 1/60, it just is programmed to not go below 1/60 sec. Go out into some daylight and watch it get faster. In Tv, Av, and M, there is no such limit to the shutter speed. The 10D allows you to lock the shutter speed at 1/200 in Av in the custom functions.
My own personal preference is to use the camera in Manual Mode and the flash in ETTL/II. I set my shutter speed based on how much ambient light I want to record and still have fast enough shutter speed to avoid camera shake. I set my aperture for the depth of field I want.
These pictures were taken with a 1D Mark II and 550EX. The camera was in Manual Mode 1/60, f/8 and the flash was in ETTLII bounced off the ceiling. The added advantage of the camera in Manual Mode is more consistent flash exposure as you eliminate the Auto Reduction of Fill Flash found in all the other modes.
http://www.pbase.com/scottbergerphoto/christmas_2004_srgb
There is more about this in the EOS Flash Sticky.
Scott

[PacAce]

I
- why is autofocus so difficult in low light? Sometimes it would not work at all.
- is there any way to turn off the "pre-flash" machine gun effect?
- why did the "pre-flash" only go off sometimes?


The "pre-flash" you are talking about is actually the focus assist light used to aid in focusing in very dark settings. It is used only when the lighting is so low that the camera can not auto focus without it. Since the DRebel does not have custom functions, I don't think there's a way to turn the focus assist light off without installing a firmware hack.

[tim]

Your camera meter will always try to meter the ambient light. It doesn't know or care if you have a flash attached.

Given how often external flashes are used, I wonder if there's scope for adding an "external flash" position on the dial? I don't know how much you could automate and how much would have to be manual, but it could be interesting. I don't have the energy to give it any more thought right now.

[jrm]

<snip> In P mode, the camera doesn't default to 1/60, it just is programmed to not go below 1/60 sec. <snip>

Yes, that is how I understood it and what I meant to say. "Default" was not the right term to use.

Thanks for the other information. I read up on all this a while back, and somewhere in the back of my head this all rings a bell. Thing is, it was probably last spring when I looked into the question and I haven't used the flash since then. When I found myself (by surprise) in a situation where I needed a flash, all that old information decided NOT to wake up and be useful.

Already checked out the EOS flash sticky, but am having trouble with some of the links. (Pretty sure that is where I picked up most of my info when I originally looked into all of this.)

I like your shots. The bounced light give such an improvement over the built-in flash. I am strongly considering the 580ex (successor to the 550). Just want to make sure I really understand flash before I take the plunge.

--Joe

[jrm]

The "pre-flash" you are talking about is actually the focus assist light used to aid in focusing in very dark settings. It is used only when the lighting is so low that the camera can not auto focus without it. Since the DRebel does not have custom functions, I don't think there's a way to turn the focus assist light off without installing a firmware hack.

If it is the focus assist, then I wonder why it had such a hard time focusing. With the way it was strobing, there was plenty of relatively close subjects to focus on. I had only the center focus point selected, and it was smack in the middle of someone's face.

Am I correct that units like the 550/580ex have their own IR focus assist? And if so, this would both do away with the need for the camera's flash to fire the focus assist and also be less intrusive?

Thanks,

--Joe

[tim]

My 550 has a focus assist lamp - a bit red thing.

[scottbergerphoto]

EX Series flashes have a focus assist lamp, the red plastic square in the front of the flash. It puts out a cross beam that aids the camera in focusing. When an external flash is mounted, the camera's internal focus assist is disabled. Some cameras allow you to disable the focus assist (internal/external) in the custom functions and some in the personal functions(1 series). You can also disable the focus assist by going into AI Servo.
Scott