I was trying to take a photo of some fireworks, but it turned out like "pegs". I’m not sure what happened - this was my first time using manual on my Canon Rebel t6 and I was hoping they turned out better. What should I have done differently?
There are two things happening here.
Firstly, they're out of focus. That's why the lines are so wide. It's asking a lot of a camera to auto-focus on fireworks. If you want the fireworks to be sharp, either use manual focus and a bit of trial and error; or auto-focus on a static well-lit object at the right distance and then switch to manual focus. Of course, you might well make the artistic decision not to have them sharp: focus blur sometimes works quite nicely with fireworks if your lens has good bokeh.
Secondly, I understand your reference to "pegs" to refer mainly to the length of the lines. This is down to exposure time. If you want something looking more like points than lines then you need to drop the exposure time by three or four stops, and compensate with aperture and ISO. If you want longer lines, you need to increase the exposure time by a few stops, again compensating with aperture and ISO.
You did well in framing and focused fairly well for your first time attempting fireworks.
You did well in terms of selecting an aperture and ISO value that exposed the glow from the burning powder nicely bright without losing the color by overexposing.
You didn't leave the shutter open long enough.
Shooting fireworks against a black sky is a lot like using flash in a dark room. The length of time the shutter is open does not affect how bright the image is. The amount of power put out by the flash over a much shorter period of time does that.
With fireworks, the bright parts of the picture are constantly moving. Leaving the shutter open longer doesn't usually make the image that much brighter, it just shows the glowing fireworks along a longer portion of their travel. Now obviously, if you left the shutter open for several minutes the background light and the number of fireworks crossing the same spot of your sensor over and over would eventually overexpose your image.
I normally shoot fireworks using a wired remote cable release and the camera set to 'Bulb' mode. With your Rebel T6/1300D you access 'Bulb' mode by using Manual exposure mode and setting the shutter speed (Tv) past the setting for 30 seconds to "B" for 'Bulb'. The shutter opens when either the camera's shutter button or the button on the remote is pressed and closes when the button you used is released. I use a tripod to shoot fireworks. This allows exposure times in the range of about one to five seconds.
You pick a spot of sky to point at, focus manually at infinity (or just a tad shy of it), and you wait until the show starts to see if you're pointed in the right direction. Adjust your aim as needed. Don't forget to refocus if you zoom in or out. I find manual focus works best for me. Once I've got the camera pointed in the right direction and focused where I want it, I don't even bother with the viewfinder (or LV). I just watch the show with my own eyes and use the wired remote to open and close the shutter, occasionally glancing at the image previews on the LCD after a shot to be sure exposure, focus, etc. is still where I want it to be.
When using 'Bulb' mode you can anticipate a shell before it bursts (by watching it climb - the trail of the climb usually goes dark a half second or so before the shell breaks) and you can already have the shutter open when it "breaks." You can then hold down the button until the glowing powder has spread out or fallen.
Canon EOS 5D Mark II + EF 24-105mm f/4 L IS @24mm, ISO 100, f/10, 1 second (Bulb - EXIF only reports in whole seconds, so it could have been anything from just shy of 2 seconds or shorter to less than 1 second).
Some folks prefer to include the shell's climb in the image. I prefer to leave it out and open the shutter just as the shell breaks or just after it breaks.
Sometimes, if there are multiple burts going off at once, like in the "big finish' of a professional display, you'll go ahead and close the shutter before every burst spreads completely out to prevent areas with multiple breaks going off very quickly from washing out those parts of the frame.
If you are shooting wider to catch the surroundings as well as the bursts, you have to pay more attention to length of exposure combined with aperture and ISO. Keep in mind that everything else is a constant brightness, while the fireworks is very bright but constantly moving so that it only lights up a particular part in the frame for a short instant as it takes several seconds to move across parts of the frame.
I'd like to have been able to hold the shutter open a bit longer for this one to catch more of the foreground, but I did not have a tripod at the time. I wasn't even planning on shooting fireworks at the end of an airshow I'd been shooting all day. While walking back to my car a long distance from the main crowd area, I saw the reflections of the fireworks on the antique planes parked on the tarmac. I feverishly dug my cameras back out of my backpack, swapped lenses, moved a QC2 plate from the 7D to the 5D Mark II, mounted the camera on my monopod (tripod was in the car 3/4 mile away) and started shooting about 3 minutes before the end of the display. That limited my shutter time to about 1/5 second with a non-stabilized lens at 35mm on a monopod. The higher ISO definitely had an impact on washing out the colors and blew out the center of the shell breaks. ISO 1600 is 4 stops from ISO 100, which reduces the full well capacity of the sensor by a factor of 16 and reduces the dynamic range. But I got the shot I wanted.
After trying it a few times, you'll find getting the wide, generic full burst shots will become old hat. For something different, try shooting tighter and seeing what kinds of abstract shapes and colors you get. You're basically shooting "blind", even if using Live View, because you won't be able to see the entire image captured over several seconds until after you've closed the shutter and the camera shows you a preview image.
Catching the very end of one burst and the very beginning of another can be fun. I called this one 'Hyperdrive tractor beam.'
Pulling focus during a burst takes a lot of practice, but the results can be unique. I generally focus at a very close distance and pull focus to infinity as the the shell breaks. Live View can be helpful here, and a tripod is an absolute necessity. Even with the tripod, the camera movement caused by zooming the lens is evidenced by the 'squiggles' as the focus was moved.
Stopping down a little for the big finale allows you to leave the shutter open a bit longer and fill the frame with various types of shells as the break.