So Aaron and I were talking the other day, and we didn't have a definitive answer to this potentially silly question:

• When you're shooting in manual, what exactly does the camera do in conjunction with your focus point and metering mode?

Am I even asking that correctly, Aaron?
For starters, I really don't see a big difference between (and therefore have very limited understanding of) the different exposure modes. I usually keep it on Evaluative on my 20D. So the camera normally uses - to some degree - the image going through your lens to determine exposure - correct?. When the camera is set to manual, does it simply change the amount of flash output? Does the flash need to be in ETTL for that?

Obviously I have much to learn on the topic so any insight is appreciated.
Thanks,
-Eddie

when shooting on manual, you can still see what the camera is metering in the viewfinder of the camera via the exposure diagram -2-1(0)+1+2

usually this serves to show me how far off the in camera metering is after I've taken an incident reading.

It might depend on the camera. For most cameras, the metering and focusing aren't linked at all. If you have it on manual (no metering) and auto focus, it'll still focus by itself.

If you're using a TTL flash and your camera is set to manual, the flash will put out as much light as it thinks it needs. So, in that way, you're still in an auto-metering mode. Lots of people photograph receptions this way. Set the aperture to something like 5.6, the shutter speed to 1/30, and let the flash determine the exposure.

If your flash is set to manual (with the camera still on manual), it'll put out as much light as you set it to, regardless of whether or not it's enough or too much light. For people who photograph receptions this way, they know at what distance the exposure will be correct for each aperture. So if they're further away, they'll open up the apterture a little. If they're close to the subject, they'll stop down as much as needed.

It's all about control. Do you want your camera or flash (or both) making exposure decisions, or do you want to be in control of that? There really isn't a "right" answer. Just whatever you're comfortable with.

Thanks guys. I'm really interested in the science behind what it's doing so that I can properly use my camera. If anyone else is confused by this whole flash business, feel free to jump in with questions too. It might help produce even more answers for all of us.
-Eddie

A 20d has three metering modes, evaluative, partial and center weighted. A 30d has the extra spot metering mode.

Eddie and I were a little confused as to how the different modes effect flash output. I am inclined to say they do effect the flash output because you will get different meter readings depending on the metering mode you are in.

Even though you have a set aperture and shutter speed in manual mode does the flash (set to ETTL) determine its output depending on the meter reading?

So imagine a scenerio where you are in one mode saying you are 1 stop underexposed and another mode saying you are two stops underexposed. Would your flash put out different outputs to get correct exposure?

Hey guys,

The answer is determined by your flash setting. So yes, if your "flash" is set to 1 stop underexposed it will underexpose by 1 stop under for the reading it took from your main subject regardless of the exposure setting for the camera.

Hopefully this answers your question, if not, let me know and I'll try again.