Exposure - Getting Technical Now

Ok, so now hopefully after the first article on Exposure you have the concept of how the camera controls the amount of light exposed to the sensor. A quick refresher, a bigger hole in the aperture means more light and less shutter open time for same exposure as a smaller aperture opening and longer shutter open time.

Camera Meter
The cameras metering system is responsible for figuring out how much light it thinks is needed to properly expose a photo. In all modes but manual (M) the camera will always set at least one of the 3 things it can set to make what it thinks is the proper exposure for the current scene. There are various metering modes and they all effect how the camera actually figures what it thinks is a correct exposure for the current circumstances. I have a future article in mind for this topic alone but for now we will just stay in the default matrix metering mode.

Astute readers may be wondering what the 3rd thing is that the camera can change, that would be ISO and it is basically a light sensitivity setting. The higher the ISO setting the less light it takes to make a proper exposure. It doesn't come for free however, more sensitivity brings more noise into the photo. So we now have another choice when it comes to getting a proper exposure. This choice is typically along the lines of, I don't have enough light under the current circumstances to make the photo so I must up the ISO. Sometimes there will be enough light for a properly exposed photo but we need a faster shutter speed because we have things moving in the picture and we don't want them to be blurred. In this case again we can up the ISO, this will allow us to use a faster shutter speed because we now don't need as much light for a proper exposure.


Camera Modes
The camera's various full auto modes will set both aperture, fstop and sometimes ISO as it desires. In P, S and A modes you can effect either the aperture or fstop used and the camera will automatically select the other setting to make the exposure as the camera thinks it should for the current ISO setting. Actually, P mode you effect both shutter and f-stop, more on that later. In full manual mode, M, you set both shutter and aperture as desired, in this mode you can, and should in most cases, use the exposure meter for reference. The camera's exposure meter is still measuring the scenes light and the metering bar tells you if it thinks you will over or underexpose the photo and by how much for the current fstop, aperture and ISO settings you have chosen.


F-stop's and Shutter Speeds
Finally I we must introduce the actual f-stop numbers and shutter speeds. I'm not going to get anymore technical than need be here, you can find much more detail if needed via Google. The camera industry long ago derived a standard set of f-stops as follows 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 that go along with a standard set of shutter speeds 1, 1/2, 1/4, 1/15, 1/30, 1/60, 1/125, 1/500, 1/1000, 1/2000, 1/4000, 1/8000, 1/4000. These f-stops and shutter speeds are in what the industry calls 'one stop' increments. Most all cameras now days have these number in 1/3 stop increments so there is probably 2 numbers between each of these standard stops on your camera. i.e. going from f-stop 2.8 to 4 is one stop but in 1/3 increments we have 2.8, 3.3 and 3.5 before we get to 4.

The beautiful thing about these 'standard' f-stop and shutter speeds are their relationship. You can derive the same exposure by changing either the aperture or f-stop by 1 (or more) stop(s) each. Lets say for a photo we want to take, the camera metering system tells us or auto mode has chosen, f/8 @ 1/250 as the exposure for the current scene. We can alter the look of the photo by choosing a new f-stop and aperture, all we have to do is change each by the same 'stop' amount. An exposure of f/4 @ 1/1000 will provide the same exposure as f/8 @ 1/250, the f-stop went down 2 stops and the shutter speed went up 2 stops.


Whew....
I hope we didn't get to technical here, I will end this article before we get brain overload. In quick summary, exposure is effected by 3 things f-stop, aperture and ISO. A higher ISO means it takes less light for an exposure at the expense of more noise in the photo. F-stops and shutter speeds have a corresponding relationship, the same exposure can be had by various combinations. Knowing the effects of the various f-stop and shutter speed settings is what will let us alter the effects/looks of our photos.

Please post comments, questions or mistakes in my writings to this in the D40 Plus groups thread on this article.