LightMeter App is a simple and easy to use light meter application for iPhone that will help you set your exposure right, especially if you're using manual camera.
Lightmeter App allows you to get shutter speed value, aperture and ISO (sensitivity) with ability to lock each. For example if you're using ISO 200 film in your camera you can set ISO to 200 and lock it. You can lock the shutter speed knowing what you are about to shoot and find what is the best aperture for your selected shutter value.
App also allows you to see the live preview of the photo as well as balance it with over and under exposure.