I'm having trouble getting nice even lighting when using my speedlight on my camera. I photographed a formal recently in a hall with high yellowish ceilings. I pointed the speedlight directly upwards at the ceiling to diffuse the light. However in many of my photos, especially those near walls, the light would be very bright near the top and dim near the bottom. What could I do to avoid this?
You need to think about the direction of the light when you bounce.
Bouncing isn't just about pointing the flash up, or up and forward. It's about picking the direction of your light. If you're bouncing, you have to stop thinking of the flash as your light source, and think of the reflecting surface your flash is pointed at as your light source. In other words, point the head of the flash at where you'd put a softbox in a studio. In this specific case, you put the light in the ceiling, straight over your head, more or less pointing downwards.
If you want the light to come from the front more, you need to bounce behind you. But there may not be a reflective surface (wall) there. So, choosing where in the room you do this, and how the room is shaped / how far away the walls/ceiling are all key. The higher the ceiling, the less liable you'll be able to bounce, because light, like all forms of electromagnetic energy, falls off according to inverse square. (1/x2). Double the distance, you quarter the light; triple it, and you've down to one-ninth. There are reasons to still use a high ISO setting, even if you use a flash.
You may need to set up a reflector or reflective surface, or rearrange where the group is standing, so you can use a bounce surface. Or. Consider taking that flash off camera, and remotely triggering it somehow (most commonly, with a radio transmitter/receiver).
See also: Neil van Nierkerk's Tangents website. He's got a ton of information on bouncing an on-camera flash.
In addition to the light falloff across the frame, there are some light-dark bands, possibly caused by flickering artificial lights?
Options to consider:
Reduce shutter speed or use sync settings on camera to eliminate banding.
Use diffuser in addition to bouce.
The problem should be fairly easy to fix with post processing.
There are more complicated methods, but the basic idea of using layer blending modes is the same.
In addition to what xiota suggests, you need to consider how the inverse square law applies here (What is the inverse-square law, and how does it apply to lighting for photography?).
If you were forced to only bounce the light off the ceiling, then to even out the exposure from top to bottom, you'd need to increase the total distance the light has to travel so that the difference between light path to top and light path to bottom are ratios that are closer together. This is often hard to control, as you can't adjust the ceiling height and often have little working room for which to move back - and the solution of decoupling the flash from the camera to move it further behind you, for example, actually opens the door to better lighting methods.
If you can get the flash off the camera - this would be the most ideal as you have more control over where the strobe sits relative to the subject and any sort of modifiers you want to use to soften the light.
If you cannot decouple the flash from the camera, then your next best option is to direct some of the light forward so that it can fill in where the main light path has started to fall-off. This is done by extending the bounce card up (if your flash has one), adding a home-made bounce card, or using a light modifier that throws the light in all directions (something like a Magmod of Fonger)
I pointed the speedlight directly upwards
Nop. The example image you are posting has different problems than the one described. Let me explore.
You have a harsh shadow clearly showing behind the guys' heads (red circles)
You have a clear gradient, this means you have a clear object producing an obstacle from your light to your scene. (Purple lines)
One interesting thing is that you still have a harsh shadow on the bottom part of the photo (green ovals)
Let's forget for now about the red and green circles and focus on that gradient.
It is clear to me that you are not really pointing the flash upwards, but only partially.
So, the main flash is still hitting the scene, (red zone) and the rest is illuminated by the bounced part.
This idea is reinforced now by the shadows on the red circles.
There is a chance you pulled the small bounce card some flashes have, this would explain the shadows on the purple circles.
If you are going to bounce the light, please avoid hitting your subjects with direct light coming from the flash.
Yes, you can pull now the small bounce flag of your flash. This will probably have a similar effect, depending on how wide is your lens, but will be a lot less noticeable because the power of the light coming from that little bounced zone is very small.
