Why did some of my indoor event shots come out under- and overexposed?

Available light can vary dramatically with the presence of light sources, shadows, and the colours and reflectivity of subjects and background.

Flash photography is probably the only kind (short of a controlled studio setup) where exposure will be similar keeping the same aperture/shutter/ISO settings. Both indoor and outdoor photography will need the exposure to be considered for each different subject - which probably means each individual shot.

Switching to an automatic mode (even if just for ISO) is one way of doing this, but many cameras have an exposure meter in the viewfinder (or on the screen) which will tell you whether each shot in manual mode is likely to be over or underexposed, and adjustments to the three settings (shutter, aperture, ISO) can be made.

Flash photography can be a challenge, mostly because it is the intersection of your camera mode and the flash modes. This will be in Canon-speak, but most camera systems are similar, using different names and acronyms.

On Canon cameras, you have Manual (M), Program (P), Aperture Priority (Av) and Shutter Priority (Tv). The Canon flash system communicates with the camera, so the flash knows the camera mode and settings. The flash also has its own modes: TTL and Manual. TTL uses info from the camera, and a 'test flash' to determine subject distance and exposure and automatically determines the amount of flash to provide. In Manual mode, the user sets the amount of flash.

It is likely you have your flash in TTL mode since you do not mention otherwise.

In Manual mode on Canon, using TTL, you set the aperture and exposure, and the flash responds with the appropriate amount of flash. When you push the shutter button, the flash sends out a pre-flash, which it uses to measure the scene, then adjusts the power of the 'real' flash. This happens so fast you don't notice it.

So, if you stand closer or further away, the flash will respond by adjusting the power to get the exposure correct based on the pre-flash. If you put the flash in Manual mode, you can set the flash to constant power, rather than this variable power.

If you put the Camera in Program mode, the camera will always try to maintain high enough shutter speed to keep a photo sharp. But when you attach a flash, the camera has to make assumptions about how you want to use the flash: it the scene is fairly bright, it will assume you want to fill the foreground but not at the expense of the background exposure, so it will adjust the flash power to do fill only. If the scene is dark, it will assume you want to light the foreground, and will adjust the power to light the foreground at the expense of the background.

If you put the Camera in Av or Tv, the flash always works in fill mode, because the camera is holding either aperture or shutter speed, respectively. It will also work to ensure that you do not exceed the flash sync speed, and flash in the viewfinder to tell you when you get above the sync speed (depending on your camera and flash models).

So, the modes are tricky, and Manual is not always 'best'. My advice it to choose the camera mode that best fits your scene and subject.

Manual mode works well under stable conditions and for stable subjects. With practice and experience (I.e. making many mistakes) it is also possible to learn how to adjust manual mode settings to produce successful pictures in changing conditions.

However in event photography, often the most important thing is just getting pictures of reasonable. quality. Automatic modes make that easier and fully automatic modes make the easiest way to produce reasonable pictures in variable conditions.

Indoors, available light will often vary significantly within a few feet. When people are few feet away from a light source the inverse square law means small changes in distance result in one to three stops of available light.

For some events, it might be possible to plan by metering the light before hand (for example a wedding in a church). Also knowing where important pictures must be made (as for a wedding) helps planning.

But mostly it is a matter of learning from mistakes and sticking with it.

I'm assuming that you weren't using flash at this event. I think your problem might have been the lighting. Fluorescent and LED lights can vary in intensity 120 times a second (or more depending on the ballast) in the US, and depending on your shutter speed you can catch the light at a high point or low point in the cycle. A fast shutter speed (~1/125 or faster) is the most problematic while a longer exposure (1/60 or slower) will catch the full cycle and start to even out the lighting.

Here is a discussion on a Canon site that explains the issues quite well. Depending on your camera you may have an 'anti-flicker' setting that times a fast shutter release for the peak of the light output.