Is there something I can substitute for an off-camera flash? I'm broke, and an amateur.
First off, the tell tale of cat portraits are their ears and whiskers. That will define a good shot from a snapshot, but more importantly it will define their character. Be mindfull of that.
You'll need better lighting and control of said lighting. Of the images you posted, that control is not made IMO. #2 is the only one that I can discern that flash was used. Unfortunately, it also looks like the built-in flash was used giving the catch lights dead center on the eyes, lose of detail (to blown highlights) in its fur around the face and the harsh shadow against the wall. (I won't mention the clipped ear or the whiskers being lost). Place a white business card or folded piece of aluminum foil in front of your flash to bounce it off the ceiling or nearby wall. Direct built-in flash kills an image IMO.
HERE are a few tutorials about the use of reflectors.
I'm not sure what's going on in #1, but it appears to be harsh ambient daylight bleeding through that is blowing out your background. You could possibly bring down the exposure in PP a bit and recrop so that the RoT is more adhered to. This may help. <More to follow below.>
The last two are backlit. There is no way to get detail from a backlit black cat other than using some additional light. An external flash is by far the easiest and most useful remedy, but careful use of reflectors can also add another dimension to your repertoire. They allow you to use the available light but direct it to YOUR needs.
Right, so here's the more to follow about external flash. A little testing I've done recently to correct backgound ambient light. Mind you, all of these shots are SOOC.... no post processing.
This is a shot of Winston as the camera wanted to meter at zero.
The exposure on Winston is aw'right, but I didn't like the ambient, background exposure. So I dialed it down to get the background to my liking.
I liked the background, but Winston is underexposed quite a bit..... needs a bath. With that background exposure, I added a flash. This was a hotshoe mounted flash.... nothing fancy, and I added enough fill to illuminate him, but keeping the background.
I probably should've backed the flash power off a tad more, but afterwards I could also do some PP to bring it in line with what I was looking for.
All that being said, it is worth saving up for an external flash. It will undoubtedly improve your keeper rate IMO.
As always...... just my 2¢.