Canon EOS 80D Instruction Manual page 229

Effective Range of Built-in Flash
ISO Speed
(p. )
148
ISO 100
ISO 400
ISO 1600
ISO 6400
* When a high ISO speed is set and focusing distance is long, appropriate
exposure may not be obtained depending on the subject conditions, etc.
Detach any lens hood when shooting with the built-in flash.

If a lens hood is attached or if the subject is too close, the built-in flash

will be obstructed and the bottom of the captured image may look dark.
Do not perform flash photography when the built-in flash is held down

with your finger or not fully raised for some other reason.
If you use a super telephoto lens or large-aperture lens and the bottom of
the picture looks dark, using an external Speedlite (sold separately, p.233) is
recommended.
EF-S18-135mm f/3.5-5.6 IS USM
Wide Angle
f/3.5
1 - 3.4 / 3.3 - 11.2
1 - 6.9 / 3.3 - 22.6
1.7 - 13.7 / 5.6 - 44.9
3.4 - 27.4 / 11.2 - 89.9
D Using the Built-in Flash
(Approx. in meters / feet)
Telephoto
f/5.6
1 - 2.1 / 3.3 - 6.9
1 - 4.3 / 3.3 - 14.1
1.1 - 8.6 / 3.6 - 28.2
2.1 - 17.1 / 6.9 - 56.1
229