Exposure - The Confusion Factor

I hope we are not moving to fast here but we should all have a pretty good grasp on how exposure is controlled. Another quick overview, we have 3 things on the camera to adjust to effect exposure aperture (f-stop), shutter speed and ISO (light sensitivity). When taking a photo we can let the camera's auto modes choose all of these settings, we can set all or some and the camera will set the others. The camera uses the built in light meter to determine the settings it will use.

I just realized I'm wrong when saying that there are 3 things on the camera used to effect the exposure. Indirectly there a more ways to alter the exposure but on camera a fourth way to effect exposure would be the camera's metering mode. The various metering mode's interpret the light in the scene differently. The use and effects of the different modes will be covered in a future article. The default matrix metering mode attempts to be intelligent and compares current readings with what Nikon has programmed into the camera and the camera will set the aperature and shutter based on this. Most of the time this 'intelligence' works pretty good but the problem I have is that I really never know just how that intelligence is going to set the camera for my next picture. While on this topic I might also add that we can also indirectly effect exposure outside the camera by adding filters or controlling light that is in the scene.

Confusion Factor
In the last article I introduced the standard set of f-stops 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. The f-stop is our aperture measurement and remember a bigger aperture, opening in the lens, lets in more light. Now for the confusion factor, the actual size of the opening in the lens (aperture) is inversely proportional to f-stop numbers. So, f/1.4 is a bigger aperture opening than f/2. There is 'scientific' reasons that they are backwards but we won't go into that. You just gotta remember if you want more light to get in the camera, you need a bigger aperture opening which means a smaller f-stop number. It takes time but will sink in. Also remember that opening the aperture, smaller f-number, will decrease the depth of field.

Stop Down
If you read much about photography you will frequently come across the term stop down. When you stop down a lens, you are going to a larger number/smaller aperture and therefore less light. Changing an f-stop from f/1.4 to f/2 is referred to as 'stopping down'. The opposite is opening up, going from f/2 to f/1.4 is moving towards the smaller number/larger aperture and therefore more light.

So now hopefully we all know, or are at least getting a grasp on, the basic mechanics that cameras use to control exposure. In order to take your photography to the next level you must understand these basics. If you just leave the camera in auto mode all of the time you will get good pictures a lot but not all of the time. As you get more experience you will start to see some particular lighting scenarios where the camera goofed up, this is where your knowledge of exposure will start to come into play.

While writing this article I came across a dated but very easy to read site that goes into slightly more detail than I have here. If you want to dig a little deeper or read from a different view point here is the site.

Whats Next
Now that we know what all goes into exposure the next step is to learn when we need to step in and change the exposure for our current scene. In low light we need to make sure the shutter speed is fast enough to not blur movement, in an action shot we must make sure the shutter speed is fast enough to stop the action, if so desired. In a landscape scene we would probably want a large depth of field which means a smaller aperture, bigger f-stop number. While photographing a single person in a crowd we will probably want a shallow depth of field so we will 'open up' by larger aperture, smaller f-stop number.

RTFM - Read The Freakin Manual
So for now pull out the manual, look at how to change the ISO, set the shutter speed and aperture. Get familiar with how to get to the various settings I have talked about so far. Check out the different metering modes by taking some photos of the same thing in various modes, a back lit or scene with a lot of contrast will be particularly reveling in comparing metering modes. Take the camera out near dusk and change the ISO settings note how this effects the shutter and aperture speeds chosen by the camera. Take a picture looking down a fence row or for city dwellers a line of parked cars, change the aperture and note the effect.

I think the next article will be on white balance, check that out while you have the manual out. Luckily, the D40 does a pretty darn good job at white balance in auto mode but there are times when you will want to change it. Til next time get the camera out and play with it, all it costs is time.