Hey guys... I put this together for some friends and another board that I'm on. I know that there are tons of great photographers here, but I figured that I'd post it here to help out any newcomers... This tutorial assumes you have a DSLR and starts you in Manual mode. I shoot Canon and wrote it for Canon users, but the buttons should be pretty universal. This is by no means they only way to shoot or think about photography, but I think it represents a clear explanation of the three major factors that determine exposure. Please excuse the diagrams... I had few resources when I put it together and didn't even have those files on my computer. I'll upload the original diagrams on Monday from my work machine.
If you are reading this, chances are you just got a new camera, but I bet you didnít realize that you already owned the most advanced and sophisticated camera and lens combination available, the human brain and eyes. As you look around the room or even at this written text, your eyes (the human version of the lens) are constantly adjusting the zoom, focus and aperture to read and see. The brain then takes that information, processes it and applies adjustments so that everything is understood and colors are correct (our own version photoshop). Have you ever looked at a sunset and marveled at how perfect the colors where and then tried to take a picture and the result didnít look as good as what your eye saw? The reason for that is that your eyes and brain are constantly making adjustments so that every color looks perfect. When you look at the orange sky, your pupil dilates to let the perfect amount of light in to recognize the best orange. As your gaze moves to the ocean, it readjusts so that you get the best blue color. When taking a picture, you can only dial in one set of adjustments to capture the entire scene. You might dial in the perfect settings to capture the blue, but then everything else might be lighter than you like, or if you dial in settings to capture the orange perfectly, then the blue might end up being too dark, or almost even black. You have to find a best fit, compromise to get all of the colors to match as closely as possible. The wider the range from dark colors to light colors, the more compromise you have to make.
In its simplest terms, Photography is the capture of light on a light sensitive paper or sensor. In order to properly expose the scene that you are trying to photograph, there are three variables that can be adjusted: ISO, Aperture, and Shutter Speed. While making changes to each of these affects the amount of light captured, each one has its own characteristics, advantages and disadvantages that come along with those adjustments. In order to properly expose the scene, you must balance how much light each one is allowing to reach the film or sensor. Iíll outline each one along with its positives and negatives.
Shutter Speed is the measurement of the amount of time used to expose the picture. It is probably the easiest aspect to grasp, so weíll cover it first. When you press the shutter button on your camera, the mirror that is directing the light through the lens and up to your eye in the viewfinder, flips up and a metal curtain lifts up (or sideways or rotates, the direction doesnít really matter) and out of the way allowing what youíve seen in the viewfinder to reach the film or sensor and create a picture. The curtain then comes down and the capture is finished. The amount of time that the curtain is up, allowing light to hit the film, is the shutter speed. It can range from 30 seconds to 1/8000th of a second (in some professional model cameras). The longer the shutter is open, the more light gets in, so in a dark situation, youíll need a longer exposure. Letís imagine that we want to take a picture of something moving fast, like a bird in flight, and it is a sunny day. We could use a very fast shutter speed, 1/1000th of a second for example, to freeze the bird in mid-air and perhaps even in the midst of it flapping its wings. Now, letís imagine that we want to take a picture of a potted plant on the window sill, but its dark in the room. We can either turn a light on or we can use a slow shutter speed (think longer amount of time) to capture the picture. This might seem like a viable option, but youíll quickly realize that as hard as you try to be steady, if you use a slow shutter speed, your hands will shake and it will produce a blurry image. This is a situation where you might need a tri-podÖ or you can adjust your Aperture or ISO, allowing you to use a faster shutter speed? Using a tripod mounted camera with a slow shutter speed works for stationary objects, but if you are going to shoot people, even people sitting still, youíll need to use a faster shutter speed.
**Note- As a general rule of thumb, the slowest shutter speed that I can hand hold is, ď1/focal length of the lensĒ, so if Iím using a 50mm lens, I can hold the camera steady 1/50th of a second. If Iím using a 70-200mm zoom lens and I have the lens zoomed to 135mm, I can hand hold to 1/135th of a second. This is different for everyone. If you have really good technique and you can be steady like a sniper, you might be able to go much slower, so test it out for yourself**
Letís go back to the example of our potted plant. We have all of the lights turned on and we just canít make it any brighter. Additionally, we donít have a tripod, so we canít steady the camera and the picture is blurry because we have to use a slow shutter speed in order to get a good exposure. For the sake of our example, letís say we are using a 85mm lens and our shutter speed is 1/50th. If we try to make it any faster to reduce the blurriness, it is too dark. This is a perfect case for ISO adjustment. ISO is a measurement of the film or sensorís sensitivity to light. When referring to film, some people call it film speed or ASA. Most of the digital cameras reference it as ISO. The most common numbers associated with ISO are 100, 200, 400, 800 and 1600. These were the common sensitivities that film could be purchased in, and as digital cameras have come into existence, theyíve kept the same number and scale referring to the sensitivity adjustments of the sensor. Youíll notice that the numbers double with each step up. This is to show that ISO 400 is twice as sensitive to light as ISO 200 and ISO 100 is half the sensitivity of ISO 200. The higher the number, the more sensitive to light the film or sensor is going to be, so in a dark or dimly lit situation you want a higher number. There is less available light, so you want the sensor to be more sensitive to capture as much available light as you can. Recently, camera companies have begun adding intermediate 1/3 stop increments of the standard ISO settings, as well as allowing the capability to expand ISO settings with custom functions for extremely low-light situations. In these cases, the ISO values are as follows, 50, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, 1250, 1600, 2000, 2500, 3200, 6400. This allows for greater flexibility with your settings and allows you to only go up just as much as you need to limit any negative effects. Now that you understand ISO, letís apply it to our example of the plant. We were using an 85mm lens and we see now that with our ISO setting of 400, we needed a shutter speed of 1/50th to get the exposure, but that was too slow to hand hold the camera without any shake or blurriness. If we adjust our ISO up to 800 (doubling the sensitivity), we can now use a shutter that is twice as fast, 1/100th, to get the same exposure and now weíve eliminated the shake. You might think to yourself, wow this ISO thing is great, Iím just going to set it high and leave it there, however, there are drawbacks to using higher ISO settings. On film, the higher ISO produces a grainier picture. On digital cameras, higher ISO settings produce noise. This shows up as green, red and blue speckles and is caused by the individual pixels on the sensor beginning to heat up and incorrectly capturing the colors. The general rule of thumb is that you want to try and keep your ISO setting as low as possible in order to achieve the shot you want.