Before you run out and start your black and white photography adventure you need to prepare your camera for use. One of the first things you should do on your camera is match the film speed dial of the camera to the film speed of the black and white film you selected. The location of the film speed dial varies for different camera models but will always be on the top side of the camera. In order to change the dial you may need to press a button close to the dial – this button is designed to lock the film speed so that it doesn’t accidentally move while photographing. If using a film speed that is not specifically listed on the dial (like ISO 125) then simply place turn the dial so that it is approximately on the speed. The purpose of setting your film speed is to help “calibrate” the light meter inside the SLR camera – a faster film needs less light while a slower film needs more light and the film speed dial will tell your camera this.
Note: If you accidentally shoot a roll of ISO 100 film and the camera was set to ISO 400 film it can still be developed through a process known as push processing. (Pull processing would be the opposite, shooting ISO 400 film when the camera was set to ISO 100). When processing the film you will develop it as the speed your camera was set to and not the actual speed of the film. It is also very important to note that you cannot change the ISO speed halfway through a roll of film or else you will likely over/under develop half the roll. So if you notice you forgot to change the film dial halfway through shooting then leave it as is and make a note to develop the roll of film according to the film speed the camera was set to.