Fiddling with Depth of Field

[gnelson]

I'm a complete amateur Now that we have that out of the way, I hope this question doesn't sound too silly.

I've noticed that when I take pictures of two people (like my husband holding my daughter) that I'll focus on them and then one or the other will be out of focus. I figured this is a depth of field issue, so I increased the f-stop from 2.8 to be slightly higher. I'm mostly indoors when I do this.

That makes the picture blurry, at which point I blame the lighting. Everything I've read says that increasing the f-stop to 8 or higher will decrease the bokeh effect... but I don't know how to stop the fuzzy appearance in that situation when, for example, I'm indoors. Do I bump up the ISO?

Thanks in advance!

[OsmosisStudios]

The problem with stopping down from f/2.8 to f/4 or f/8 is that youre losing shutterspeed too.

What this means is that if youre (pushing it) at 1/30s at f/2.8, when you stop down to f/4, you lose a stop of shutter speed (1/15s). Stopping down to f/8 in this situation puts you another 2 stops down in shutter speed (1/8s, 1/4s). At such long shutterspeeds, youre bound to get motion blur from either your hands holding the camera or your husband/daughter moving.

Sounds like you need a flash. With the XSi, youve got several options. The 270ex, the 430EX (and EXII) and the 580EX (and EXII). If you just want the flash for on-camera work and bounce-flash work, you may only need the 270ex, but if you plan on doing any off-camera flash work, then a 580exii would be better, despite the insane cost.

[ericw]

It sounds like everything your assuming is correct. Depending on the distance to your subjects and how they are posed, shooting at f/2.8 on the 50mm could cause DOF issues - making one of the subjects out of focus. Closing down your aperture is the appropriate move but forces a slower shutter speed to obtain proper exposure. If your indoors and not using flash (dark) the shutter speed will probably be low enough to cause motion blur if you are handholding and not using a tripod. So, increase your ISO and you will be able to acheive a faster shutter speed at your smaller aperture, avoid motion blur, and get your sharp image. Or use a tripod and hope they stay still. Or use off-camera flash.

Regarding DOF and such, I reference this Depth Of Field Calculator when I'm thinking about these things. Basically you put the info in and hit calculate and where it says "Total" is your Depth of Field for the given circumstances

EDIT: I spent all that time typing and osmosis snuck one in on me... Maybe the calculator will be useful to you