There are many factors that go into every photograph, the most basic of which is Exposure. The one thing in every photograph that can be controlled is the exposure or the amount of light that is recorded by the light sensitive material. The light sensitive material can be a digital cameras sensor, a piece of film, or anything else that reacts when introduced to light. From here on however, I will be referring to the digital camera’s sensor.

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.

Leave a comment