T5I Issues with built-in flash

[Hina]

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. 

[TCampbell]

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.

[Hina]

Thanks for your reply. I have been wanting to get an external flash.

The issue with the built in flash is that it does not matter the distance, even close distances images are dark. 

[TCampbell]

You may have inadvertently adjusted your 'Flash Exposure Compensation" setting (or FEC).  The icon on your camera looks like a white box with a lightning-bolt next to a tiny +/-.  

Each value of compensation either halves or doubles the power output of the flash when using ETTL flash (but of course you can't go beyond full power.)  

For example... suppose you are taking a photograph outside on a sunny day.  You can use the pop-up flash with FEC set to a value of about -1 and improve the shot.    The camera calculates the amount of power that would be required if the only source of light were the flash.  But then it adjusts that power level by the amount of compensation you've selected.  "-1" means you want it to reduce the power output by 1 "full" stop.  A "full" stop is the halving (or doubling if you were increasing) the amount of light.  The effect is that the brightness of the sun wont change but you've asked the flash to fire at 50% of the calculated amount of power.  This makes the sun the primary source of light, but uses only half as much flash (so the light ratio ends up being that the sun provides 2/3rds of the light and the flash provides 1/3rd of the light) and this improves the overall shot because the strong shadows that would have been present with no flash, will now appear as weak shadows.  You get the shadows to provide the dimensional look to the photo... but now they are just moderately dim shadows instead of extremely dark shadows.

See page 120 of your manual.  (Here's a link to the manual:  http://gdlp01.c-wss.com/gds/5/0300010905/02/eos-rebelt5i-700d-im2-en.pdf )

Set this value back to 0 and it should provide a normal amount of light on the subject as long as the subject is within range of what the flash can handle.