I have a Canon rebel and bought a Focus external flash for it. When I have it set to ETTL, the pictures come out really dark. It's like it's thinking that since there is a flash, the camera is adjusting to compensate and the exposure isn't enough. Doesn't matter if I make the exposure length really slow. The other problem is that if I put my flash in manual mode, it won't even fire even though it's still attached to the camera.
There are two different issues coming into play here:
- Your Canon Rebel T100/4000D does not have a standard hot shoe with an ISO compliant contact in the center. Canon started doing this with the lowest priced models a few years ago. You can't trigger standard manual flashes directly with this hot shoe. There are a few workarounds that require hardware made specifically to work with the Canon cameras that have this partially disabled shoe. I call it a "semi-lukewarm" shoe.
We've had a few previous questions about this issue here at Photo SE:
How can I make use of a speedlight that does not work with the non-standard hotshoe on lower-end Canon cameras?
Is a Yongnuo YN560-TX compatible with Canon SL3 or T100?
What speed lights are compatible with canon 2000d? I can’t find any
Canon EOS 4000D Hot Shoe for Sync (with Studio Flashes)
- Many third party flashes don't work very well in E-TTL mode if the Canon camera's flash metering is set to Evaluative. Try setting the flash metering mode to Center Weighted Averaging, or partial. Please note that Flash metering mode is a different setting than regular Metering mode.
We've also had a few previous questions regarding this issue here at Photo SE:
Flash intensity levels aren't the same across consecutive photographs
Yongnuo YN-685 underexposure issue in bounce mode at longer focal length
Why does my pictures becomes darker while using external flash 😓?
I have a Canon rebel [T100] and bought a Focus [IFTTL] external flash for it. When I have it set to ETTL, the pictures come out really dark. It's like it's thinking that since there is a flash, the camera is adjusting to compensate and the exposure isn't enough.
If you use aperture priority (Av) mode, by default, the camera always tries to use the flash for fill, rather than as the main source of illumination. The assumed usage is firing a pop-up flash to illuminate shadowed faces when people are backlit by the sun. Your iso, aperture, and shutter speeds are kept relatively near where they would have been without using any flash.
See also: The EOS 'Bible': EOS Flash Confusion
Doesn't matter if I make the exposure length really slow.
Shutter speed doesn't have any effect on the flash portion of an exposure because a flash burst (at or below sync speed) will be faster than any shutter speed you can use. Leaving the shutter open for longer doesn't gather any more light from the flash. But I suspect your flash isn't actually firing in sync because...
The other problem is that if I put my flash in manual mode, it won't even fire even though it's still attached to the camera.
You have a T100. It is, unfortunately, one of the three camera models (the SL3/250D, T7/2000D, and T100/4000D) where Canon removed the central sync contact on the flash hotshoe. This is the contact that sends the main "fire" signal to the flash.
Only third-party flashes with eTTL II capability can deal with this missing sync contact, and only if they're firmware upgradeable so that they can deal with the communication protocol changes the missing contact needs to fire and control the flash. The main 3rd-party brand that does this is Godox. I would look to see if your Focus flash has a USB port, and whether there are any firmware updates you can apply to your flash for communication with the T100. But it's likely that your flash is simply incompatible with a T100, while it would work fine on a non-T100/T7/SL3 Canon dSLR body.