Speed flash anomaly
 

I am relatively new to photography but I am learning fast. I have just bought a Nissin Di622 speedlite for my Canon 40D. I set it to TTL to try it out indoors and found that on Av mode it selected a 2 sec exposure, which I thought was rather long; When I switch to Auto it set an exposure of just 1/60 sec: both identical settings and shots, both came out ok, histograms similar. When in Av I was shooting in RAW, does this make such a big difference.
I would appreciate any input to this as I am quite confused.

Grahame
nbgemini

What aperture was selected in both Av and Auto?

in AV mode the camera will meter for the ambient light only, this means indoors you will get a long exposure with the whole room (depending on your metering) correctly exposed, the flash will then expose for a subject in that scene, say a person, as if it was using fill-in flash.

In auto mode your camera selects a shutter speed based on the flash sync so typically 1/60 or 1/125 & then uses the flash TTL to correctly expose the scene.

Try switching to manual & selecting 1/125 & then use your aperture to control how much ambient light you want in the shot.

You'll also find the flash exposure is different depending on what exposure mode you are on, so on matrix, area or whatever it's called, it thinks you want the whole frame correctly exposed, on spot or centre it thinks you only need the central subject exposed so the background will be much darker.

The thing to do is to try all the different settings to get to know how your flash meters the scene & what difference it makes to shutter speeds & apertures.

Speed flash anomaly
 

Many thanks guys, just for the record, the exposure on Av was set to F4.5 and evaluative metering, but I am now going to play a bit more using spot or centre weighted, I tried manual, but with F4.5 still set I had to dial in up to 2 sec before the meter centred. Anyway I'm going to try altering the metering mode. Again, many thanks; I'll let you know.

Grahame
nbgemini

you need to understand the difference between what the camera meters & what the flash lights. The meter in the camera won't know that the flash will light the shot so meters for an exposure which should get a decent exposure without flash, you then add extra light with the flash. If you are metering for the camera to expose you will always get a slow shutter speed & then additional light with the flash.

The difference is do you want the camera to light the scene or the flash or a combination. If you want the flash to light the scene don't pay attention to centering the meter. It will do it with the flashlight, not the ambient light, your meter only knows about ambient light.

Thanks again Cathus
I've been reading up in the manual and have a better understanding of slow speed flash, after some successful experimenting I can set the shutter to 1/250 fixed, or use TV at say 1/200; anyway I now understand that on Av the flash was lighting up my subject while the slow shutter speed caters for the background. I think this would be a good subject for Photoplus to write about; maybe I'll drop them an email.
Thanks again

Grahame