Pre-Photography 101: The Numbers

In my previous post (Pre-Photography 101), I talked about some basics. One of which was Exposure. So let’s look at Exposure a little closer. Today’s cameras are great for measuring the amount of light entering the lens. This is where things get started – measuring the amount of light. Before we get started, let me ask you to do a couple of things. First, reach into that junk drawer. You know the one that has all the pens, dead batteries, paper clips and expired coupons. In that drawer dig way to the back of the drawer and pull out the manual for your camera.

There are three sections I’d like to read about. First, find the instructions to change your camera’s ISO Setting. Your camera ISO is probably set for “Auto”; change the ISO setting to 100. Now don’t put the manual away. If you want to return that ISO setting to “Auto”, you may need the instructions again. You’re going to ask me why you should do that. And, Should I always shoot with the ISO set at 100? The answer is NO. Your next question will be why not? For right now, cause I Said So (LOL)! I’ll talk more about ISO and answer those questions in another post. Setting the ISO to 100 will simply keep all the light readings set for the same light scale reference. If I tell you any more, I’ll likely confuse things.

The second area to look up, is how to control the flash on your camera. There should be a flash mode selection feature on a setting dial or a menu tab. Please set the flash to “Off”. I’ll bet you didn’t know you could have so much control over your camera – did you? Don’t you feel powerful with all this control!

The third area in the manual to look at is the description of your display information. You need to know what information is shown in the display and where to look for it. The reason to look at your camera manual for this is because I don’t know what camera make/model you have. So I can’t tell you where the information is displayed. I’m simply going to explain the numbers and ask you to watch how they change under different lighting situations.

Now that you know where the information is positioned in the display, you should be able to find the numbers that indicates the Shutter Speed and Lens Opening. The shutter speed is simply that – the length of time the shutter is open to expose the sensor. Shutter speed is measured in time. You may read something like 5s (5 seconds), 1/30 (1/30th of a second), 1/125 (1/125th of a second), 1/250 (1/250th of a second)….. are you getting the picture here? Yeah bad pun, I know. This number indicates how long the shutter will be open. So if the display shows 1/125, the shutter will be open for only 1/125th of a second. That’s a real short period of time! So which is a longer exposure time, 1/30s or 1/250s? If you’re having a bit of a struggle here – the larger the bottom number in the fraction, the shorter the shutter speed. So 1/30s is a longer exposure than 1/250s.

The next number you will notice will look something like: 2.8, 4.5, 5.6, 8.0, and 11.0. These are referred to as “f-stops”. An f-stop is the opening size of the lens. So what does the number mean? You ask. If you really need to know, it’s not a measurement like inches or millimeters. It’s the ratio of the lens opening and the distance from the lens to the film or sensor plane. Aren’t you glad you asked?

The point to understand about the f-stop is the smaller the number, the larger the lens opening. The comment I expect to hear is if the lens opening is larger why isn’t the number larger? It goes back to the simplified definition given earlier. You just have to get used to remembering the larger the number the smaller the opening.

So what’s the relationship between the two you ask? As a photographer you can use shutter speed and lens opening as creative tools. I’ll explain more about them as tools in another post. But for right now, let’s do a little exploring. If you have a dark area in the house or if it’s night time, aim the camera at a dark scene. Look at the exposure numbers in the display. You should see a very slow shutter speed. Depending on the light available or not available, you may see a shutter speed as slow as ½s, 1s, 2s or whatever the time is. In fact, why not write the number down. In addition, look at the lens opening number also known as the “f-stop”. You may see a number like 2.8, 3.5, or 4.5. Going back to my lens opening discussion earlier, these numbers should be small numbers, meaning a large lens opening.

Now try the same exercise in a well lit area indoors during daylight. Indoors with a lot of light, you may get shutter speed numbers like 1/15, 1/30, 1/60 and an f-stop like 3.5, 4.5 or 5.6. Again based on the brightness of the scene, the numbers will vary and I can’t guess them from here. OK, if it’s daytime and you aim your camera outdoors in your yard, you may see shutter speeds like 1/125, 1/250, 1/500 and f-stops like 5.6, 8.0, 11 or 16. These are very fast shutter speeds and the lens opening is getting much smaller. Remember the bigger the lens f-stop number the smaller the lens opening.

So what is going on here? In dark situations the shutter speed is very long and the lens opening is large. In these dark light levels, the camera metering system is slowing the shutter and opening the lens up to let as much light enter the lens and hit the sensor to get an acceptable exposure. Going to the daylight scene, the metering system has set the camera with a very short shutter speed and set the lens opening to a very small one. Again the metering system is using settings to control the amount of light directed to the sensor.

If you’re beginning to wonder “how does the camera know how to set these lens settings in these light situations”, then you’re making headway. It’s all in the metering system measuring the amount of light in the scene and the algorithms written and programmed into the camera software for exposure control. For right now, don’t worry how the algorithms work. If you’ve begun to understand the change in shutter speed and f-stops, then you’re on your way. This is the very basic start to understanding exposure.

So congratulations, you’re on your way! Oh you didn’t think you’d know everything by now did you? Oh and don’t forget to go back and return your camera setting to where they were before I asked you to change them. That will give you more practice reading the manual and getting familiar with your camera settings/menus.

So now that we’ve talked about those numbers, pay more attention to them as you take pictures. Just begin to notice how they change as the amount of light changes. Come back in the next couple of weeks and we’ll talk more!