Please help me understand...

It seems that my Canon XTi, by default, tends to underexpose.

It gets fairly well exposed pictures on a bright sunny day.

It seems that it tends to underexpose a lot in low-light conditions.

It seems that it tends to underexpose quite a lot with the new external 430EX I got.

I am trying to understand if it's this particular camera body's fault, the XTi specific issue, or if it's a normal problem associated with shoting in low-light situation (e.g. a bedroom with a simple 2-bulb light fixture).

I had a camera sent for service when I just bought it, complaining about front focusing and exposure. They sent it back saying they found & corrected a focusing calibration issue but didn't find anything wrong with exposure.

Here's a partial 100 crop of what I consider a "good" exposure:

If the EXIF is correct, your XTi is in P mode, with EC @ -1, FEC @ -1 2/3, then of couse everthing is underexposed. How about reset the EC and FEC to 0 and try again?

Lotto wrote in post #5586802
If the EXIF is correct, your XTi is in P mode, with EC @ -1, FEC @ -1 2/3, then of couse everthing is underexposed. How about reset the EC and FEC to 0 and try again?

There's a space between "-" and the digit, it was +1 and + 1 2/3. These are the values I used taking the picture which crop I posted - in my opinion this one was properly exposed.

If I remember correctly (correct me if I'm wrong) P mode doesn't let you go below what it deems is a hand holding speed (I think 1/60)

Also it's normal for a Speedlite to underexpose. I find my 430EX underexposes 1 stop relative to the On-board flash

I regularly use +1 with all my speedlites.

Get out of P and use M for indoors flash. There are also some useful FAQ about ETTL flash in Curtis's signature.

smorter wrote in post #5587195
If I remember correctly (correct me if I'm wrong) P mode doesn't let you go below what it deems is a hand holding speed (I think 1/60)

Also it's normal for a Speedlite to underexpose. I find my 430EX underexposes 1 stop relative to the On-board flash

I regularly use +1 with all my speedlites.

Get out of P and use M for indoors flash. There are also some useful FAQ about ETTL flash in Curtis's signature.

Thanks ! So setting a + 1 FEC is normal.

I still think XTi tends to underexpose in low light.

One thing that I did see right away is that "shiny skin" effect seems to be almost nil with speedlite even if I don't aim at ceiling. The built-in flash was very harsh in that respect, I have some otherwise beautiful picture of my wife with very shiny spots I couldn't get out in processing soft.

RE: Shiny spots, it tends to happen when your flash power is too high and your subject has oil or sweat on their face. The latter is not controllable by us but here's a method I use to get rid of it as best as possible in PP.

In PS, get your Brush tool and ALT+Click on an area of skin that isn't shiny but of the same colour of what the shiny skin should be. Then using as large a brush size as possible "paint" over using button clicks only (i.e. don't hold it down and spread it everywhere). I start with very low opacity (20% for me) settings and click only if necessary.

Sometimes the results will look unnatural depending on your brush settings and if the colour you select by ALT clicking does not match the skin colour of what the shiny area was supposed to look like, and sometimes if you don't select a big enough brush it can start getting blotchy, so you do have to experiment a lot to get good results.

You need to experiment with the hardness and opacity settings of the brush, I set mine to 20% for both, so often it requires a few dabs to clean up something.

Not sure if that made sense, but shiny skin can be mitigated definitely, as long as you haven't blown out too much of it. e.g. if the whole forehead is blown to white, then there's nothing that can really be done, but if it's just patches then it's definitely fixable.

Thanks for the tip. I was using Clone tool and the other one (forgot the name - the bandaid icon) with mixed results, however I hope with the new external flash I won't get nearly as much skin reflections, the built-in flash looks much harsher (does it have anything to do with it being so close to the lens ? It can't be nearly as bright as the 430EX).

The EC will do nothing in P mode in this case, since ambient was too low.
In P mode the shutterspeed won't go below 1/60s, and the aperture couldn't open up further. Dialing in +2 EC would still have given you 1/60s at f/2.8, and the same exposure for the BG.

If you'd have used Av, the shutterspeed would have gone lower, to expose the BG properly. If I'd have to guess, something like 1/8s or so would give proper exposure.
And motion blur quite likely

Use M to set the shutterspeed to what you want, and take control of the ambient lighting ratio that way.

If I'd have taken that shot, I would have been using ISO 800 or 1600, and bounce flash for fill. Probably bouncing backward, into the corner of the room gives good results.

The FEC of + 1 2/3 is rather much, but is probably explained by the lot of white sheets around the subject, and the fact that ETTL sucks .

By the way, your image is in AdobeRGB, which it shouldn't for web posting.

It's certainly not unheard of for the Xti to under expose by default - in fact I've read so many threads now discussing this very issue I think it's safe to say that the Xti (400D) does under expose in the majority of situations. It's also certainly not unheard off for the 430EX (and the 580EX for that matter) to need to be set at +1 exposure comp. I have two 580EX and as a rule of thumb both require setting at +1 regardless whether they're on my 400D's or my 5D. I'd stop worrying about what you need to dial in to get a correct exposure if I were you and just be content (and pleased) that you've got it right.