Stereo Realist, I have decided to do a bit of an intro to those who want to use older cameras effectively. There are other sites, this is my favorite, out there to help you get a bit more in depth as far as exposure guesstimation, so if you have the want or need to figure out how to do this like a pro, then by all means do the research and practice a bunch. But if you want to cheat a little bit, just follow these handy tips.
First, the camera. I'm assuming you have an all-manual camera on which you can set A) shutter speed, and B) aperture. If you have an old film camera, like an SLR or rangefinder, you're all set. If you have a dSLR, you're all set there too, but you won't have to wait for lab prints to find out how your exposure guesses went. To do this, set your camera to M mode (manual!), and adjust aperture and shutter speeds accordingly.
Next, film speed. A good all-around film speed, or ISO, to start with is 400. Why? Why not - it will give good results in cloudy to sunny light. If you're using film, use a color negative film, as it has lots of room for error, or latitude if you will. If you're using a dSLR, change the ISO accordingly.
This is an easy rule to remember, and it works well. If you go outside, and it's bright and sunny, you should set your aperture to f/16, and film speed to the inverse of the ISO. So if you took my advice and loaded up color 400 film, that would make a good sunny day exposure f/16 at about 1/400th of a second. Simple!
It gets complicated when you have to change shutter speed or aperture to get different effects, and if you want to know how to do that, I'd suggest looking into the other sites I mentioned earlier. As it is, when you start out stick with f/16 on sunny days and you'll be a-ok.
I also like to use the rule "cloudy 5.6" to mention you can do this on cloudy days too. If it's really super cloudy out, you need to open aperture up to about f/5.6 at the inverse of ISO, or in our case 1/400. If it's somewhere in between, you can set the aperture at f/8 or f/11, depending on how bright it is. Use your instincts, and practice!
What does this mean when using a Holga? Well, the Holga is all-manual in a way - its settings are the same all the time, and the only thing you can change is the film speed. Suffice to say, the Holga shoots well in cloudy light with 400 speed film, and really sunny light can do well with 100 speed film. But don't overthink it. Matter of fact, I've shot with primarily 200-speed film in my Holga, and haven't had any real exposure problems.
Sunset & cloud shadows over West Bay
10 hours ago