So what determines a “Correct” exposure? Well, for the sake of argument a correct exposure would record each color tone as it appears in real life. You will often hear people referring to a “Grey Card”. This is simply a card that reflects 18% of all light that hits it. If you point your camera at this card and fill the view finder with just the card, you would have a “Correct” exposure when the light meter was at 0 or centered.

It is really a pretty simple concept, your camera has a light meter in it. This measures the amount of light being reflected off of the subject. Based on the amount of light being reflected the meter determines what the object will look like if you take the photograph with your current settings and displays that tonal value to you on a chart. The chart (for most of you reading this) is on the bottom edge of your viewfinder and has a + sign at one side, a – sign at the other and a 0 in the middle.

• +||||||0||||||- Nikon
•  
• -||||||0||||||+ Cannon

So if the indicator is on the + side of 0 then the image will be brighter or closer to white and if the indicator is on the – side of 0 then the image will be darker or closer to black. This means, if you are photographing a white wall you want the indicator to be all the way to the + sign without going over and if you are photographing a black wall you want the indicator to be all the way to the – sign without going over.

The light meter we find in the viewfinder helps us to get the exposure right before we take the picture but what about after. The LCD display on the back of the camera is NOT an accurate rendition of the image that we have captured so how do we know if we got what we wanted? That is where the histogram comes in. That thing that looks like a photo of jagged mountains, that is important.

In the image above, the histogram is the image to the right. The histogram tells us how many pixels fall into each tonal value from 0 to 255. The higher the peak the more pixels in that tonal value. Every histogram is read from left (0) to right (255) or black to white respectively. Looking at the photo the scene is about 1 stop brighter than middle grey, so our light meter in the view finder should have looked like this:

• +||||||0||||||- Nikon
•  
• -||||||0||||||+ Cannon

And our histogram ends up looking like the one above, shifted 1 stop to the right. This particular photo is a composite of exposures combined in Photoshop to reach a good overall exposure. This is done most often in architecture and landscapes because the sensor is not capable of capturing the dynamic range or range of tonal values with one exposure. To the sensor black and white are only 256 integers apart while to the human eye black and white are closer to 2,560 integers apart. As a result the photo has areas that are over and under exposed. so we take multiple exposures and combine them to make one image.

For now, I only did this to show the varying degrees of exposure and the histograms to go with them. So here is the exposure for the brightest parts of the image.

Notice the histogram! It has shifted dramatically from 1 stop to the plus side all the way to the right hand edge.

Now look at the exposure for the dark parts of the image.

The the dramatic shift again. This time from one stop over to actually running off the right side of the histogram. The info that runs off the right edge of the histogram shows up as a big block of pure white, no information at all. This is called clipping.

So now you have seen an image that is under exposed, one that is over exposed and one that is just right. When you look at the histogram, clipping is bad 90% of the time. The big thing to remember, is that the light meter in your camera has a +(white) and a -(black), look at the scene you are photographing and determine if it is really dark, really bright or somewhere in between and then set the indicator accordingly with the camera settings. Double check with your histogram to make sure you haven’t gone to far over or under and you should be taking usable exposures quickly.

    Be advised, this series of lessons is based upon the assumption that you have and will be using an SLR (Single Lens Reflex) camera. The best way to learn the basics is to start with the basics so film is preferred for those of you that have a film body. If you do not own a film camera and have no desire to purchase one (they are pretty cheap) then I suggest placing gaffers tape over the LCD panel on the back of your digital camera. Seeing immediate results is an incredible advantage to digital shooting, unfortunately it can become your biggest crutch while you are still learning the basics. I promise it won’t be long before you can start playing with it but for now, just forget it’s there.

    To get started lets discuss basic camera functions. Every camera is based on the same basic principals. A piece of light sensitive material is placed behind a curtain that prevents light from touching it. When the curtain is opened, the amount of light that touches that light sensitive material is recorded by the material itself. Based on this principle, we have two variables within our control:

1. The sensitivity of the material behind the curtain and
2. The amount of light that we allow to pass the curtain and be recorded by the material.

Pretty simple right? We only have to control two variables.

    So how do we control the sensitivity of the material? We choose an ISO(International Standards Organization) rating. With film this number appears on the box and is typically between 100 and 3200. To start out just stick with the lowest ISO available to you, if you are using film start with 200 ISO(it is actually closer to 100 most of the time). The rating number indicates the sensitivity of the material. ISO 100 film requires twice as much light as ISO 200 film to achieve the same result and ISO 200 film requires twice as much light as ISO 400, etc.

    Try not to get caught up in the how and why, just keep in mind that there is a measuring system and that system is incredibly accurate. Now, if we place ISO 100 film in our camera behind the curtain then open the curtain for 1 second we will achieve a certain shade of gray. If we open the curtain for 2 seconds that shade will be lighter and if we open the curtain for half a second that shade will be darker. This is your shutter speed. The shutter controls how long light is allowed to fall onto the film or digital sensor.

    We have one more variable that allows us to control the amount of light passing the shutter. We place lenses on our camera to focus light from a particular scene onto the film or sensor, within the lens is a diaphragm. This diaphragm can expand and contract leaving a larger or smaller hole in its center for light to pass through, the hole is the aperture. The aperture controls how much of the available light is passing through the shutter while it is open. A larger hole allows more light to pass through itself and a smaller hole allows less.

    Think of shutter speed and aperture like a faucet and the film or sensor like a glass. An 8oz glass needs half as much water to fill as a 16oz glass would. This is your ISO. Then, if you choose the 8oz glass and place it under the faucet you have two options to fill it. You could turn the faucet just a little and leave it running for a long time with a slow stream of water or you could open it all the way up and turn it off quickly. Either way you get the same amount of water in the glass.

    That’s it folks, those are your camera functions. The only controls you have when you use a camera with available light. Now, how do you know how much light you need to allow in. This answer is a little complicated, so for now just use your light meter.

    When you look through the view finder on your camera, underneath the image there is a bar with a + sign at one side and a – sign at the other. in the middle there is a 0. When your camera indicates 0 the scene in the view finder will be given enough light to be recorded as a middle tonal value. If you are using black and white film, 0 would be middle gray. So if the object is white you want the meter to slide to the + side of zero and if it is black you want the meter to slide to the – side of zero.

    Now go try it. Don’t worry about Depth of Field and motion blur just try to get good exposures. if an object is light get a photo that the object looks light, if it is dark get a photo that looks dark and if its in the middle…

Good luck…