I agree completely with null. I tried to learn flash with an Olympus OM-10, and I actually learned flash with a Canon 50D. It's a lot easier with digital instant feedback and EXIF substantially lowers your burden of note-taking so that you can remember what you did when you see the results (you'll still need to track light placement and power settings). It's not impossible to learn with film, but it is a much higher and steeper mountain to climb without the instant feedback.
However, there are a few alternatives to running through a lotta rolls and development to learn flash since you can't actually see what you're doing. Because the OM-10 does not have a built-in flash, you have to add an external light source. But it doesn't have to be a simple manual-only flash.
- You could use a constant light source, such as an LED panel or video light. This may not provide as much light as a flash, but has the advantage of letting you see what you're doing while you're doing it.
- You could use a manual flash with an autothyristor/external sensor mode.
Autothyristors (or External Sensors; Auto Mode)
Today, most camera/flash combinations can automate the power required to make a good exposure by using something called TTL (through-the-lens metering). The camera tells the flash to send out a small burst of light (the preflash), meters it, and then adjusts the flash's output power. But this requires that the camera and flash both know this communication protocol.
The OM-10 is pre-TTL, so the only way to get automated flash power is to use a flash that has an external sensor (autothyristor) on it. This does not require any camera/flash communication, so can be used both on and off camera, and does not require a same-brand/system flash.
The sensor, after it's received a certain amount of reflected light, tells the flash to cut the power, in effect, automatically setting the power out of the flash based on the shooting situation. You do have to set the iso and aperture you're using on the flash for this to be effective, so if you change your aperture setting as you go, you will have to still fiddle with the flash. And you do need to take care to keep the sensor unobstructed and clean. But this is what folks did in pre-TTL days to not waste rolls and rolls of film on bad flash settings (see The Strobist's "Q&A: How to Shoot Events Without TTL Flash").
The problem is finding a flash with this capability in these TTL-days, as TTL has supplanted Auto modes on most flashes. Canon and Nikon's current high-end flashes can do this (on Canon, it's called "external sensor" on Nikon it's the A-mode). The Olympus FL-600R/Panasonic FL-360L calls it "Auto". Older film-era manual flashes also have it; but you might have to check sync voltage. But I don't think there are any current cheap 3rd-party manual flashes that offer it, other than the Vivitar 285HV/Cactus KF36, but if you use that, you won't have swivel for bounce, or 1/8 power, and it has a weird proprietary sync connector, so not the greatest choice.
Balancing Flash and Ambient
I love shooting pictures of my friends indoors at night, so that the light sort of 'illuminates' them as the subject whilst the background remains relatively dark. ... How can this be done?
In flash photography, this is about tipping the flash/ambient balance more towards the flash, and making sure that the flash illuminates your subject and not the background. It can be done in any lighting conditions with a powerful enough light, but tends to happen more often if you're shooting in darker situations. You're basically underexposing the ambient, and then adding enough flash to bring your subject up to the right exposure level.
When you take a flash image, it's like combining two different exposures. Think of it like Photoshop layers combining to make an image--there's the ambient light (all the light that isn't the flash), and there's the flash. The amount of light you get from ambient is controlled by iso, aperture, and shutter speed. The light you get from flash is controlled by iso, aperture, flash power setting, and flash-to-subject distance (closer the flash, the more light you get).
So, if you set the ambient exposure to be one or two stops below "good" exposure, and then throw some flash on your subject to make up the difference, you'll get a darker background with a well-exposed subject.
See: "Dragging the Shutter" at neilvn.com, and the Strobist's Lighting 101: "Balancing Flash and Ambient".