Program versus Aperture Priority versus Shutter Priority versus Manual Exposure
Which is better, program mode or manual exposure mode? Which will yield better images, aperture priority or shutter priority? Is manual exposure the only truly accurate way to meter? Why do camera makers include so many choices for metering mode anyway? And why does the guy writing this article write nothing but questions?
OK, so I got carried away with all the questions.
But the truth is, this is indeed a frequent topic of questions, especially for new photographers. I'm going to do my best this week to put all these questions to rest so you can feel comfortable with how you are metering.
The first thing to understand is that exposure is determined by aperture and shutter speed (assuming ISO is kept constant that is), and no matter how you come up with a given value for each of these variables, you will get the same exact exposure. And because you are using the same aperture and shutter speed, your depth of field will also be the same as will the rendering (or freezing) of motion in the image. For example, an aperture of f/8 at 1/30 of a second will give you the same results no matter whether you came up with them on your own or whether the camera helped you do it. It does not matter. Going further, if you roll some dice to determine exposure and get lucky enough to pick the same aperture and shutter speed, you will still have the same results. In this sense, all four exposure modes are equal when used to their full advantage.
So, if each can yield exactly the same results, why have all four? While each has its own strengths and weaknesses, cameras generally include all four exposure modes since all four are useful in the right circumstances. To see why, let's look at each mode in turn.
In Program Mode, the camera makes all the decisions so you can relax and concentrate on composition or simply enjoying your surroundings. Program Mode on modern cameras can in fact be quite good at picking a reasonable aperture and shutter speed to give you both an acceptable exposure and a good balance of depth of field versus sharpness from freezing motion. For this very reason, it is often the mode most beginners start with and many long term shooters stick with. But Program Mode can only be as smart as what the camera maker can cram into its programming, and there is no way it can do the best job possible in every situation. This is no fault of the camera; it's simply that only you are there where you are taking any given picture, not the engineers from the camera maker. If you find yourself confronted by the unexpected, it is reasonable to assume that at least sometimes they might not have expected that circumstance either. The camera will try to fit it to what it has been taught, but it may not always do as good of job as you could do yourself if you know how.
Many cameras also include a "flexible" Program mode that allows you to override the camera's choice of aperture and shutter speed before shooting. This will have an effect on the depth of field or motion sharpness, but the camera remains in control of exposure. If you push the shutter speed up one stop faster, it will automatically compensate by opening up the aperture one stop to keep the exposure the same.
Sometimes there seems to be a stigma associated with Program Mode since its use gives so much control to the camera. But the truth is, Program is often uncannily good at dealing with the necessary decisions. And it deserves consideration as to whether it might in fact be better at these decisions than some photographers who refuse to use it. There really is nothing wrong with Program Mode. While it doesn't provide as much control as other exposure modes, that control is only useful if you know how to use it and have the time to do so. The light can change quickly and some subjects move even quicker, so a mode that doesn't slow you down while shooting can sometimes be just what you need.
If you're game for learning how to take more control of exposure yourself, Shutter Priority and Aperture Priority are good choices. In each, you can pick one variable and the camera will pick a value for the other to yield what the camera considers to be an acceptable exposure. Thus, in Aperture Priority, you can choose any aperture you want in order to yield the depth of field you want, and the camera will pick the shutter speed. In Shutter Priority, the reverse is true. You pick the shutter speed to either freeze motion or intentionally blur it, and the camera will select an aperture to make the exposure come out what it feels is correct.
When the camera picks the variable you have chosen not to, it does so so as to render a "medium toned" result. On average, many subjects have a medium gray brightness, and this is what camera meters are calibrated to. If you know your subject is not medium toned, you can use Exposure Compensation to push the exposure brighter (plus compensation) or darker (negative compensation). You can use exposure compensation in Program Mode too of course, but I generally don't recommend it unless you have a lot of experience doing so. In Program Mode, the camera doesn't always aim for a medium toned exposure if it makes the educated guess that your subject differs from medium. Camera makers have gotten more and more sophisticated at recognizing night shots as being subjects that should be darker than medium, or snow shots as benefiting from being exposed brighter than medium, and so on. So in general, as a starting point at least, I recommend using exposure compensation in both Shutter and Aperture Priority, but leaving it off in Program Mode.
With digital cameras, I hedge this advice somewhat though since you can use the camera's histogram function to help fine tune your choice of exposure compensation in any of the exposure modes discussed so far. If you take a shot in Program Mode and anticipate from experience that the camera may over expose it based on how you feel it should be rendered, you can be more comfortable tweaking the exposure compensation, knowing that you can see whether you got it right after the shutter fires by reviewing the histogram. So long as your subject cooperates by staying still long enough, you can check the histogram, make any needed adjustments to your exposure and re-shoot.
Exposure Compensation doesn't even work in Manual Exposure mode; all it will do is mess up the meter reading, so leave it off if you decide to go with Manual.
Your choice between Aperture Priority and Shutter Priority should be based on which exposure variable is most important to you in a given situation. If you are shooting flying birds, you probably want to be sure you have a fast shutter speed, so pick one you think will work in Shutter Priority and let the camera deal with the aperture. If on the other hand you are shooting a vast landscape, you may want as much depth of field as you can muster, so pick an appropriate f/stop in Aperture Priority and let the camera figure out the shutter speed. Either way, these modes can give you more control than Program, yet allow you to work faster than pure Manual since you are still delegating some of the work to the camera. When coupled with the use of exposure compensation when needed, these two modes can work quite well.
There is one small drawback to using Aperture and Shutter Priority modes though: it's hard to switch between them. If you shoot similar subjects for an entire session, you can set your camera to whichever of these assisted modes is appropriate, but if you tend to shoot multiple subjects you might find that Aperture Priority may be best some shots, while Shutter Priority is better for others. To get from one mode to the other, you not only have to deal with the physical controls necessary to change modes, you also have to deal with the mental gymnastics necessary to think in terms of whichever you are in at any given time. After all, these two modes are essentially the inverse of each other. In one mode, you have to think about aperture but get the benefit of mostly ignoring shutter speed, while in the other you have full control of shutter speed while delegating most of the aperture decision making to the camera. Yes, switching back and forth can work, but it can also be hard to get used to, and there is an alternative that still gives you control when shooting a variety of subjects.
This alternative is Manual Exposure, and some may view it as being a bit of a heavy handed way to not have to mentally shift modes while shooting. I mean, after all, in Manual, you always have to set both aperture and shutter speed yourself. That's why they call it manual — the camera still meters for you, but it's completely up to you what you do with what the meter says. The camera won't in any way change either shutter speed or aperture for you. If it thinks you're about to over or underexpose a shot, it will tell you via the meter display in the viewfinder, but it won't step in to try to stop you from doing so.
But to me at least, having to control both variables myself is frequently an acceptable price to pay since it gives me a consistent way to think about exposure while I work. And since you generally have to keep exposure compensation in mind when working in either Aperture or Shutter Priority, you've pretty much got two things to think about no matter which mode you choose unless you go with fully automatic Program Mode. Manual Exposure forces you to contend with both aperture and shutter speed, Aperture Priority means you have to think about aperture and exposure compensation, and Shutter Priority means you will need to consider shutter speed and exposure compensation. Although not the same two things in each mode, there are two in each nonetheless.
Yes, Manual Exposure can seem somewhat like you are working without a net, but as mentioned, you still do have full use of the viewfinder's meter display. And once you get used to it, Manual can actually be easier than other Aperture or Shutter Priority since it's a consistent way to work. Manual is generally my preferred exposure mode.
But not always. If I'm working in fairly even lighting and want to be able to work quickly, I will sometimes switch to one of the Priority modes, I don't do a lot of wildlife shooting, but when I do I will often choose Aperture Priority to ensure a blurred background from a shallow depth of field, or Shutter Priority to be certain of fast enough shutter speeds. And if I'm doing more casual shooting and want to take it easy, I may well switch to Program Mode and let the camera take full control.
So when trying to decide which mode is best, consider what you will be shooting as well as how you prefer to work. When first starting out, don't let anyone make you feel bad for using Program Mode — although not all, it really is quite good in more situations than most people give it credit for. As you gain experience, try learning about all four modes so you can use them comfortably when the situation calls for each. After all, the camera manufacturer did include all four, so there must be a reason have all four available.