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The OM-30 has focusing system limitations. Most of the time, you can use the LED in the viewfinder to help you focus an image. If the lens you attach has a maximum aperture of F4 or smaller, (F5.6, etc) you set this switch to F4, then focus the lens manually using the prism gridlines visible in the viewfinder.
In short, just set the F4/F2 switch to whichever is the closest to the maximum aperture of the lens you are using. With an F1.8 lens, for example, you would use the F2 setting. For a lens with a maximum aperture of F3.5, you would use the F4 switch setting.
What lens do you have on the camera? Many zoom lenses have a variable maximum aperture. This means that the maximum aperture varies depending on the focal length. If you set the lens to its maximum aperture and then zoom, the aperture may change.
f/5.6 is the maximum aperture for the 18-55mm lens zoomed in to 55mm. If you zoom out to 18mm you'll be able to open the aperture to f/3.5.
The maximum aperture is a matter of physics. The larger the aperture (smaller f/numbers) the larger the lens has to be. In theory you could make a 55mm lens with a maximum aperture of f/0.5, but it would have to be more than four inches across on the inside. Lens designers have to balance cost and size with performance.
If you really need a faster lens you can get something like this, but it'll cost you a bundle, about six times as much as your 18-55mm.
The maximum aperture is determined by the lens. At 55mm, the 18-55mm lens will not open up beyond f/5.6. This has nothing to do with the ISO or the camera's exposure mode. If you zoom out to 18mm, you can open up to f/3.5.
The varying of the aperture with the focal length is part of the design. While it is possible to build a lens with the same maximum aperture throughout its entire zoom range, that would result in a larger and more expensive lens. For example, Canon has a 17-40mm f/4 lens. That lens weight two-and-a-half times as much and costs more than four times as much as the 18-55mm.
You do have total control, within the constraints of the hardware. The 18-55mm lens has a maximum aperture range of f/3.5 to f/5.6. This means that at 55mm the maximum aperture is f/5.6. You can go the other way, all the way to f/38. At shorter focal lengths you can open up as far as f/3.5.
If you want a larger aperture, you'll have to get a faster lens. For example, Nikon has a 17-55mm f/2.8 lens which has a maximum aperture of f/2.8 at all focal lengths. It also costs $1300 new.
You will only be able to use the f3.5 aperture setting when the lens is zoomed out to the 18mm setting. As you zoom in, the maximum aperture available will decrease until you reach the 55mm setting where the maximum aperture will be f5.6.
Check to see if you can obtain f3.5 with the lens zoomed fully out. If not, it sounds as if the aperture blades could be sticking, in which case it will need to be professionally repaired.
There is no problem. The lens is f/3.5-6.3. That means that at 200mm the maximum aperture is f/6.3. At long focal lengths you will not be able to open up beyond f/6 or so. At the short end, at 18mm the maximum aperture is f/3.5.