Stupid question about flash...

Ok, I'm still pretty new to photography, and even newer to the 400D!

When using the flash, the shutter speed automatically reduces itself dramatically (Av mode) resulting of course, in major shake.
I can use a pretty fast shutter speed on Tv, and get a nice shot.

Can anyone explain why this is happening?
It didn't happen on my ex Nikon D40!!

Av mode when using a flash is very different than shooting without the flash. Basically it tries to set the exposure correctly **without considering the flash** and then uses the flash forr fill in. This is a great mode to use outdoors when someone's back is to the sun and their face is in shadow.

Try this - set the camera on manual, set the ISO400, set the lens wide open, set the SS at 1/160th and shoot with the flash up -

when shooting with an automatic speedlight, you can shoot on manual. If it's too dark, you have to drop the SS or raise the iso.

I made the same 'error' you did when I first got a canon dSLR.

In Av mode your camera will always set SS to the correct exposure without flash. In a dark scene this means the shutter speed may be very slow indeed. Av is a nice mode to use for fill flash in daylight. I'm not so keen on Av when flash is the main illumination or when dragging the shutter.

In Tv mode, the shutter speed is set by you and in a dim scene the camera will open the lens up as far as possible. Then when you enable flash the flash will fire and light the scene to the correct final exposure using E-TTL. This is an OK way to shoot flash, except the lens will always be wide open and you have little control over the DOF.

The best way to shoot flash is in manual mode. Pick the aperture you want for DOF, pick a shutter speed you can live with and let E-TTL meter the flash for proper exposure. Bounce flash can give excellent results this way.

Also in M, you may find you can run a slowish shutter (like 1/30) - especially with an IS lens since you can count on the flash to freeze motion to some degree.

I have no idea how the D40 works, although I would be really surprised to find the camera does not meter to ambient in Av mode.

Av mode on a Canon works similair to your D40 with slow sync enabled...

Thanks for the help guys - appreciate it.

Interesting stuff, and I'm sure I'll get used to it eventually!!

Thanks again

For more information on flash from the Digital Rebel FAQ, click here.

There is a custom function, at least on the 30D there is, that allows you to specify that 1/250 s always be used in Av mode with flash. I use this where I want to use flash to isolate the subject by underexposing the background. The converse, which is the default, is as the others have described, and allows fill-in flash with correct exposure of the background.

andy_fozzy wrote in post #4081746
Thanks for the help guys - appreciate it.

Interesting stuff, and I'm sure I'll get used to it eventually!!

Thanks again

don't get intimidated....just keep on trying and learning from the changes you make instantly (isn't digital great?!)

you'll wrap your head around it, and eventually wonder how you were ever confused by it.

keep on pluggin away!

xarqi wrote in post #4083778
There is a custom function, at least on the 30D there is, that allows you to specify that 1/250 s always be used in Av mode with flash. I use this where I want to use flash to isolate the subject by underexposing the background. The converse, which is the default, is as the others have described, and allows fill-in flash with correct exposure of the background.

Ah, I've seen that settings in the menu.
I'll give it a try

Thanks allot.

Cathpah wrote in post #4084099
don't get intimidated....just keep on trying and learning from the changes you make instantly (isn't digital great?!)

you'll wrap your head around it, and eventually wonder how you were ever confused by it.

keep on pluggin away!

Thanks Cathpah.
You're right, that's the beauty of digital!
Pretty much the reason I never got into film photog.

Cheers

Huge THANKS to everyone for the replies.

Lots of good help and advice, and I'll post my results!

Thanks again, Andy

xarqi wrote in post #4083778
There is a custom function, at least on the 30D there is, that allows you to specify that 1/250 s always be used in Av mode with flash..

1/200s with the 400D

burge wrote in post #4088865
1/200s with the 400D

Tried this option yesterday, and is working well for me.

JeffreyG wrote in post #4081651
Also in M, you may find you can run a slowish shutter (like 1/30) - especially with an IS lens since you can count on the flash to freeze motion to some degree.

Question about this: do you find you get better results using first curtain sync or with second curtain sync? I'd think that second curtain sync would be preferable in general when using a relatively slow shutter speed but that's not based on any real experience...

Andy,

It's because under Canon's system, Av resorts to 'fill flash' mode. Often people just assume that flash means 'fast shutter speed', but that's not always the case. With Av mode, the flash by default will try to just 'fill in' some extra light to make sure your foreground subjects are properly exposed. It will still however expose to ensure the background is also decently exposed. For this reason, you'll notice that the shutter speed doesn't actually differ compared to Av mode without using flash.

So just to recap, because Av mode's default flash paradigm is 'fill-flash', that means it cannot just raise the shutter speed like you want. Because if it did, the background may become poorly exposed, hence defeating the purpose of the mode being fill flash only. If you didn't care for fill flash, and just wanted the foreground subjects to be decently exposed (with fast shutter speeds in poor lighting), then use Tv like you have been or use Manual. Or, like already mentioned, there's a C.Fn available to force the shutter speed to over 1/200th second under Av mode. In other words, effectively disable it's fill-flash predisposition.