> I'm not familiar with the OM-10 but have used the OM-1, -2, and -4. What follows is from memory--check what I say against the manual: >
> - On the top, where you can set 'Auto' or 'manual adaptor', what is > 'B', the third option? > Possibly "Bulb," a term from the days when shutter releases were pneumatic (L-o-o-ng ago!). If that's what it is, it means the shutter will stay open as long as you hold down the release. (Digital cameras often set upper limits on shutter-open time; film cameras do not, or at least didn't use to.) > - Also on this dial, what are the numbers (ASA and 1, 2, . in red and > blue, and how are they changed? > ASA usually refers to film sensitivity, or "speed," -- same as ISO in a digital camera. But with film, ISO/ASA is a function of the film you load, not the camera settings: you change ISO by switching to a different film type, and then set the camera's dial to provide proper exposure of that film. Some cameras now read codes on the film cartridge and set themselves automatically to that film's ISO, but can be overriden (see next paragraph).
You can vary a film's effective ISO, to some extent, by processing it differently, but you're probably not developing your own film, so that doesn't apply. You can also reset the ISO to cause deliberate under- or overexposure: With ISO 100 film, setting the dial to 125 causes a 0.3-EV underexposure; setting it to 80 overexposes by 0.3 EV. But modern film cameras, probably including yours, usually have a separate exposure-compensation control like that on a digital camera.
On older cameras, which have no built-in exposure meters, ASA dials were strictly to remind the user what kind of film was loaded -- they did not affect camera operation at all. > - On the lense, closest to the camera, what are the numbers, and red > dot, and how is it changed? OM lenses have a footage scale (ending with an infinity symbol) for focusing -- or, more often, for reading the distance once you've focused by trning the lens until the viewfinder imae was sharpest. No autofocus.
Then they have an aperture, or f-stop, scale, with numbers that vary by ~1.4 (square root of 2): f/2, 3.5, 4, 5.6, etc. Just as with the aperture adjustment on digital cameras, each number sets the lens to let in half as much light as the next smaller number, twice as much as the next larger one. For ex&le, if proper exposure is f/5.6 with a shutter speed of 1/200 second, it will also be proper at f/4 and 1/400 or f/8 and 1/100. [It becomes easier to understand if you realize that f/4 means the aperture size is 1/4 the lens's focal length, f/8 means 1/8, etc. Lens transmission is proportional to the _area_ of the aperture tha tlets the light in, and area is figured from the square of the diameter; so doubling the diameter multiplies the aperture size by 4, which is the square of two.]
Using a smaller (higher-numbered) aperture increases the depth of field; if you want everything from nearby to the distant hills in focus, use a small aperture like f/11; if you want to emphasize a nearby subject by blurring the background, use f/2. On OM cameras, the meter sets the shutter speed according to the f-stop you select (aperture priority).
On other OMs, and maybe yours, the scale closest to the camera body is for manual setting of shutter speeds, with numbers like 125 (1/125 sec), 250, 500, etc. These only work in manual expoosure mode-in auto-exposure mode, the camera sets the speed regardless of setting.
The red dot is a guide when you're changing lenses. To remove a lens, find the lock button on the lens (check the manual), press it, and turn the lens about 1/4 turn counterclockwise (as seen from in front of the lens); to mount a lens, line its red dot up with the red dot on the camera lens mount, insert the lens, and turn 1/4 turn clockwise until it clicks. When the lens is in place, it covers the red dot on the mount, so you waon't see that. >