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Understanding EXPOSURE and the Advantages of RAW Format
in Digital Photography

Alton Vance

What is Exposure?
How does exposure affect my picture?
Film, JPEG and Exposure.
RAW & Exposure with Digital Photography.
Getting an Extra Stop or Two with RAW
Illustration of Aperture settings

From the Beginning - What is Exposure?(menu)

I'll start at the beginning for those who may not already have a knowledge of what exposure really means. Simply put; EXPOSURE is adding light to film or a digital sensor in order to create an image.

PROPER EXPOSURE is adding the proper amount of light to film or digital sensor in order to create a good image.

There are two basic elements built into every camera I know of that affect PROPER EXPOSURE: 1.aperture settings (sometimes called f/stop) and 2.shutter speeds. Both of these settings must be balanced to give a PROPER EXPOSURE to any picture.

However there is one more element that affects PROPER EXPOSURE: the speed of the film you use called the ISO number (or for those of us old enough to remember the ASA number). Here is where digital photography really begins to shine. This third element, the ISO setting, is built into the camera. With film cameras the only way to change the ISO setting was to change film. Film comes with a certain sensitivity to light. The more sensitive it is to light the faster the film and the higher the ISO number. That means that it takes less light to expose a film that has an ISO rating of 400 (exposes faster) and more light to expose a film that has an ISO raing of 100 (exposes slower). But the only way to change that rating with a film camera is by changing film.

But, walla! With a digital camera there is no film and these ingenious engineers have built in a series of electronic circuits that allows us to change the ISO setting on our digital cameras at any time we want.

But none of that makes any difference if we don't know how to use it so that's the topic of the next section so don't stop here, read on!

How does exposure affect my picture? (menu)

Here is where experiment will mean a lot. Try it you'll like it.

In reality all of the three elements above, APERTURE, SHUTTER SPEED, and ISO will affect the exposure of your picture.

Under ideal conditions most automatic cameras today will do a desent job of exposing our pictures by setting the dial on auto and letting the camera decide what's best for our picture. But for those less than ideal conditions or for the more creative among the shutter bug society we need a little more knowledge of just how these things affect our picture and how to control it.

Let's start with the aperture setting.

Every camera lens has a piece of glass on the front that covers a hole in the lens that lets light into the body of the camera. In the barrel of the lens is a set of blades that make that hole larger or smaller. The size of that hole is called the aperture. The smaller the hole the less light comes through at once. The larger the hole the more light comes through. It's like turning on your water faucet at home. If you turn it just a little a small amount of water comes out at once. If you turn it all the way a lot of water comes out at once. This is the aperture setting.

Here's where it may seem confusing but the higher the aperture number the smaller the aperture of the lens, i.e. f/22 is a very small aperture setting - a very small hole in the lens barrel. f/5.7 is a fairly large aperture setting and creates a fairly large hole in the lens barrel. The smaller numbers let in more light. You might ask me why, but that is a mathmatical study in itself and it's fairly complicated to explain but there is a reasonable explanation and it all has to to with the proportions built into the lenses. But let's just accept the fact for now, that large aperture numbers means small holes in my lens - less light, and smaller numbers mean larger holes - more light. Here is an illustration to give you a visual picture of what I just said.

Apertrue Picture

There is a good reason for having the ability to change the aperture settings but I'll talk about that in another article. For now let's just say it controls how fast light comes into the camera.

Now let's talk about the shutter speed.

Shutter speed also affects the exposure of your picture. A shutter is a window or door in front of the film plane or image sensor that opens either vertically or horizontally to let in light and then closes to block out light. The longer that door is open the more light comes in. Shutter speed times are a little easier to understand. For normal everyday photography a shutter will stay open for about 1/250 th of a second. If the shutter stays open for 1/100 th of a second then more than twice as much light comes in. If the shutter stays open for 1/500 of a second only half as much light come in. Shutter speeds can be as fast as 1/8000 th of a second. The faster the shutter speed the more accurately you can freeze the action of a moment. Here is an example of two pictures taken at different shutter speeds.


Shutter Speed 1
Taken at 1/250 second

Shutter Speed 2
Taken at 1/40 second

There are creative times that you may want to take an action picture with a slower shutter speed to emphasize action by allowing the picture to blur as in the bottom picture above. There are also times you may want to stop the action and freeze the subject so that it is crystal clear.

But if slower shutter speeds let in more light it would seem logical that you might let in too much light if you continue to reduce the shutter speed? Yes that would be true but when you lengthen your shutter speed you should always reduce the size of the aperture to keep the proper amount of light in balance. It might be compared to filling a gallon bucket with water from a faucet. There are two ways to do it (with many variations in between). You can turn the water on very slowly and let out just a trickle of water but it will take longer to fill the gallon bucket. Or you can turn the water on full blast. This will take a shorter time to fill the bucket but it will still be the same about of water when the bucket is full. Two different speeds, two differnt amounts of flow, but still the same amount of water. The aperture is compared to how wide open you turn the faucet on. The shutter speed is compared to the amount of time it takes to fill the bucket. The bucket is the measure of the amount of light it takes to expose the picture correctly.

For those of us who began in photography when there was no truely automatic cameras we had to remember to consider all of this everytime we took a picture. We had to use hand held meters or use meters built into the camera to measure how much light was needed for each situation and then set each of these settings manually. Or you tried to make a lucky quess (which was more often wrong than right).

Todays world of automatic cameras allow us to avoid thinking about much of this mechanical stuff when we shoot a picture. The reason many more people take pictures today than they did in the 50's, 60's, 70's and even the 80's is because they don't have to bother with setting anything on the camera. But the more you know about APERATURE and SHUTTER SPEED and ISO settings the more creative you can become with your camera. The knowledge and use of these three things is what seperates the world of the snap shot shooter from the creative photographer. This is true whether you shoot with film or a digital camera.

Now let's consider some practical application of what I've just said.

Film, JPEG, and Exposure(menu)

Enter the world of digital. For the creative photographer it has changed the way we think about photography. Almost every digital camera on the market today will produce a standard type of image called JPEG. It is a standard format that the computer world understands. It doesn't matter what camera it came from, if it is in JPEG format a computer, both PC's and MAC's, can read it so you can display it on your screen or print it from your inkjet printer. There are advantages of having standards.

But there are also disadvantages. For all practical purposes in this article I'm going to liken a digital JPEG image to a FILM image. Once you have taken the picture, what you see is what you get. Yes you can manipulate that image in an editing program to some degree. But, if the image is overexposed and highlights like white clouds or feathers on a swan are blown out that detail is gone forever. You can't put back in what isn't there. If you shoot only in JPEG format your exposure better be right or your picture will be wrong, just as if you were using film. The safe solution to this short coming is to do lots of bracketing (using different settings to increase or decrease exposure). But if you don't know how to use any more than your automatic setting on your camera you won't be doing any bracketing. One step up from here is to find your bracket setting on your camera and remember to take three shots of every picture you take. Many digital cameras today have "automatic bracketing" settings. But I've discovered that even bracketing isn't always enough to save a favorite shot, especially when there is lots of sky involved in the picture.

That's where shooting in RAW format becomes so advantageous. Read the next section to find out how.

RAW and Exposure with digital photography(menu)

What is RAW format? RAW is not an acronym. It simply means raw format, just like a vegetable before you cook it is raw. That said, what does raw mean in the world of digital photography? Every camera captures its original image in RAW format and may be proprietary to the camera (i.e. only the type of camera that took the picture understands the information). All digital cameras have firmware (software) built into the memory electronics. This firmware interprets the RAW infomation and converts it to JPEG, a format that the rest of the digital world understands. That JPEG image can now be read by many different brands of software on your computer and without thinking about any of this you can display your beautiful image on your computer screen or have it printed on your inkjet printer or take it to a photo lab and have it printed on actual photographic paper.

But there is the option in many digital cameras to have your information written to your memory card in its origianl RAW format. The one disadvantage of this is that it takes up more room on your memory card. But there are several wonderful advantages of recording that raw information if you can afford the space.

First my article on this site "Understanding WHITE BALANCE" explains one wonderful advantage. Exposure adjustment is another very big advantage. You will have to have special software, which usually comes with the camera, to read and convert that RAW information. See may article on "Understanding RAW Format" for more on RAW conversion software.

This software will allow you to adjust your exposure settings plus or minus 2 to 4 stops. After you take the picture, if your picture is too dark or too bright, you can adjust the exposure until it is just right. One word of advice in this area - It is easier to adjust an under exposed picture than an over exposed one. If the highlights are blown out the information is gone. But sometimes there is information in the shadows that can be brought out with RAW conversion that would be gone in a JPEG image.

Here is an illustration of how you can adjust exposure with RAW conversion software.

RAW Exposure 1
This is the original exposure. It is underexposed.

RAW Exposure 2
Here I increased the exposure by 1.5 stops in PhotoshopCS2 RAW conversion software.

RAW Exposure 3
Here I adjusted the original shot to enhance the sky and combined the two images in Photoshop.
Combining the two shots is a subject for another article. (Later)

Read the next section to understand how to get one or two stops to help stop that action.


Getting an Extra Stop or Two with RAW(menu)

Another practical reason to shoot in RAW.

Remeber the times you were shooting in low light and thought "If only there was just a little more light." Now you have it, right inside that camera if you shoot in RAW. It is possible to under expose a picture by at least one full stop and sometimes even two and then add that full stop exposure when you develop in RAW format.

This method can be very helpful when trying to stop the action in a sports shot. You can set your camera on manual and shoot with a higher shutter speed. Or set it on shutter priority and let the camera chose the widest opening even if it is not high enough. You can add the extra stop or two in the RAW conversion software at a later time.