Apature vs. Speed?, on Manual setting.

I'm confused about something and thought maybe someone can help me figure this out? I use basic flach photography in my studio. (Small in house studio.) I love taking portrait pictures and was wondering... I set my camera on Manual, speed at 125 and apature usually changes, depending on subject. Sometimes I use apature setting of 4.5 for teens and adults, and for children or babies as high as 9.5 or 11. I never change the speed, on my manual setting, and always have my ISO set at 100. Should I be changing my speed when I change the apature? (again, when using flash photography) or is my speed safe at 125.

Should I use the AV setting which lets me change the apature depending on the effect I'm looking for? if I do this? will it adjust the setting for the speed based on the flash units automatically? I'm not sure at all what to do? I have a cannon 10D

Depends on your flash units. If you are using Canon flashes w/ TTL metering, then yes, when you stop down your lens the flashes will bump out their output accordingly. If you using flashes in manual mode, like studio lights on pc synch cords or w/ radio triggers, as you stop down, you will change the exposure of your subject.

With flash photography, the shutter speed has little to nothing to do with the part of your image that is exposed by the strobes. Lowering the shutter speed allows ambient light to bleed in to your exposure, which may be something you want, or it may not. If you want to kill the ambient light, raise your shutter speed to the max synch speed of the camera (usually 1/200th ot 1/250th, but some lucky ones get to go all the way to 1/500th). The aperture, iso, and flash output settings are all that matter to the part of the image lit by the flash. So, what I do when usign off camera lights in manual mode, is get the strobes where I want, with the ratio where I want it, with the aperture set close to the range I may want to work in, and leave it. Then I dial the aperture for the dof I want, and fine tune the exposure w/ the iso. Lots of newer cameras have iso adjustments in 1/3 stop incriments which aids in dialing it in. The flash output settins are pretty coarse full stop increments, and the only other way to adjust how much light falls on your subject is to move the lights either closer or further away, but this also has compromises. That, and it's just a lot easier to adjust the iso a bit while shooting than to get up, and start messing w/ your flash settings.

BM

BM- Thank you for your replay, you gave some helpful hints and now I think I need to set up my studio and try some stuff out. Thanks again for your time.
Monica