T5I Issues with built-in flash

Question

If I take photos in a dark room or in low lighting, the camera's built-in flash does go off but the image comes out dark. I tried two lenses and it is the same thing (so I know the problem is not with the lens). This was fine just a few weeks ago. I did reset all the settings. 

Answer 1

The built-in pop-up flash is intended for relatively close distances... perhaps around 10'.   A vast assortment of externally mounted flashes (on or off camera) are available however -- and that's one of the advantages of a DSLR camera (most point & shoot cameras tend to have no ability to take advantage of supplemental lighting.)
The flash in your camera is rated with a "guide number" of 13 meters.  A "guide number" is the distance that the flash can adequately illuminate a scene EXCEPT it assumes a base ISO of 100 and also a base aperture of f/1.0.  Your lens will not be able to use f/1.0 -- but the reason the industry uses this as a standard is because it makes it very easy to determine the effective distance using any f-stop.
All you do is divide the guide number by the f-stop you are using.  So for example, if your lens is using f/5.6 then you'd divide 13 by 5.6.   13 ÷ 5.6 = 2.32.  That value is in meters (since the guide number was in meters... some older flashes were represented in feet.)   If you convert 2.32 meters into feet, you get 7.6 feet.
You can increase that distance by increasing the ISO.  Each time you increase the ISO by a stop you get to increase the distance by a factor of 1.4.   That means at ISO 200, your flash will cover distances out to just shy of 11' (at f/5.6).  At ISO 400, you can get out ot about 15'.  
Canon makes it easy to determine the speedlight guide number because if you omit the trailing "0" from the model number (e.g. on the Speedlite 430EX II you'd just take the "43" and omit the final "0") those values actually represent the guide number in meters.  BTW... most other companies do NOT do this... don't attempt to use this if you evaluate a 3rd party flash.  But for that means a 430EX II has a guide number of 43 meters (again... ISO 100 and f/1.0).   43 meters works out to about 141'.  At f/5.6 that's still about 25'... and that's still using the base ISO 100.  The 430EXII is considerably stronger than the built-in pop-up flash.  The 600EX-RT provides even more coverage (60 meters).
The other VERY nice advantage of these external units is that the head both tilts and swivels.  You can "bounce" the light off a ceiling (the ceiling needs to be white otherwise the color of the ceiling will tint the color of the flash and create a color cast on your subjects.)  But this creates a nice soft shower of light on your subject so they don't get harsh / flat lighting from using on-camera flash.
Tim Campbell
5D II, 5D III, 60Da