I‘ve been using cameras since before they had any kind of automatic modes. I had to learn about exposure and aperture back in those days of film because it was so expensive to take photographs and any blown-out or underexposed shots were downright painful to receive back from the chemists.
The first digital cameras had automatic modes, but they weren’t very good and it took progressive years of work by Canon, Nikon and the other large camera manufacturers to arrive at a solution that would meet the demands of photographers. The compromise they arrived at was the priority mode, of which there are two main versions – aperture and exposure.
So how do they work? Well, in Aperture Priority (AP) mode, you determine the aperture (the width of the iris during a shot) and the camera determines the best exposure to reach good light levels. Thus if you are shooting in low light levels and you set a relatively small aperture of f/12 or f/16 then the camera will realise that means not much light will reach the sensor and it therefore needs to employ a long exposure. If you are shooting in low light levels and set a large aperture of f/3.5 or f/5 then the camera will choose a much shorter exposure time to compensate. Exposure priority mode is simply the other way round – you choose an exposure and the camera selects an aperture that will expose the sensor to the correct amount of light.
Landscape photographers are usually more concerned with depth of field than exposure so it makes sense to use AP mode when shooting outdoor scenery. If I want a sharp image from front to back then I dial in f/16 on my camera, pre-focus a third of the way into the scene (to set the hyperfocal distance for my lens) and then let the camera determine exposure time. If I want to nudge the exposure in one direction (to get some blur in water or clouds for instance) then I usually juggle aperture and ISO.
Typically speaking I leave my camera in AP mode for 90% of the shots I take. You’ll always get people telling you that real photographers only shoot in manual mode, but I think that’s a load of crap. Aperture priority mode enables me to react far more quickly than someone juggling manual and it means I can get shots that would otherwise have been missed. For instance if a bird suddenly decides to fly through my lovely sunset and I decide I want that bird in the shot then I quickly dial down the aperture to increase exposure, capture the scene and then dial my original aperture back in. Speed is of the essence and having to set aperture, exposure and ISO means you might miss something.
So when don’t I use aperture priority mode? Typically speaking I don’t use it for panoramic shots because I need to have identical exposures and apertures to ensure a seamless blend later on. Sometimes I disagree with the light meter in the camera and manually set aperture, exposure and ISO to get the effect I’m after. I also never use AP mode for night shots, because I need complete control over all of the image’s elements.
Photography should be enjoyable and I find that some photographers get obsessive about the in-camera settings at the expense of their photos. So relax, get a tripod, choose AP mode and concentrate on composition, not histograms.