(Originally posted by my buddy Rick over at Club3G) You have 3 basic input factors that when put together give you an output or image: Shutter speed, Aperture and ISO or Sensitivity. Together, these factorrs are known as the "exposure" of the image. We will look at these one at a time, and then put them together and see the relationship one has to the other and how changes to any one of them will affect your image. First a word from the editor on the term 'stop' Rick will refer to this term quite often in the following write-up. He'll talk about increasing one stop, or decreasing two, etc. A stop is simply the doubling, or halving of light allowed to strike the sensor or film for a given shot. All three input factors use the concept of doubling and halving so that they work well together. A bright but overcast day, for example, is only about one half as bright as a sunny day. This means that in order to shoot the same subject that you might have shot yesterday in full sun, you would need to increase your EXPOSURE by one STOP. You can arrive at that stop with any combination of the three input factors, as Rick describes below. Shutter speed: Shutter speeds can usually be adjusted in 1/3 stops. 1/10th sec, 1/13, 1/15, 1/20, 1/25, 1/30, 1/40, 1/50, 1/60, 1/80, 1/100, 1/125, 1/160, 1/200, 1/250, 1/320, 1/400, 1/500, 1/640, 1/800, 1/1000, 1/1250, 1/1600, 1/2000, 1/2500, 1/3200, 1/4000, 1/5000, 1/6400, 18000. The important thing to remember here is that anytime you double the exposure time you are increasing it by 1 stop (going from 1/100 to 1/50) and anytime you halve the time you are decreasing it by 1 stop (going from 1/125 to 1/250). The most obvious and probably the most important factor in determining the speed of your shutter is the the movement of the subject you are photographing. Is it a race car going 100MPH, an animal slowly walking by, or a bowl of fruit sitting on the table? The next most important factor is; What do you want this image to look like? And finally, which is just as important as the others, is the focal lenth of the lens you are using. A telephoto lens will not only magnify the image but it will also magnify any movement, whether it's the subject moving or the movement of the camera itself. If the camera is on a tripod or other stable platform, then this following rule does not apply. But for those that handhold their camera keep this in mind: You should try to keep the shutter speed equal to the focal lenth of the lens you are using (or wherever your zoom lens is set to). So for example, if you are using a 400mm lens, your minimum shutter speed should be 1/400. A 200mm lens should be 1/200 and a 135mm lens should be 1/160. Obviously everyone is different and you might be able to hold a 400mm lens to a shutter speed as slow as 1/250 or you might need a shutter speed as high as 1/640 and keep the subject sharp. Some lenses come with an image stablizing feature that can help you hand hold the camera for as much as 2 more stops. So for example, the 28-135IS lens at 28mm, which you should handhold at no less than 1/30, might get you as slow as 1/10s shutter speed with no visible blur. So what happens when you change that speed to something different? Lets go back to that race car going 100mph. Shooting that car at a high shutter speed will freeze the action and give you a super crisp image. However it tends to look a little unrealistic with the car just sitting there frozen in time. 1/1250 Slowing the shutter speed down a little will give you some motion blur in the wheels and make it look like the car was in fact moving when you took the shot. 1/640 (one full stop slower) But it's still not enough to really show what it was like to be there in person. Another trick you can use is panning. Panning is when you slow the shutter down even more and follow the movement of the subject with the camera. It takes practice, but once you get it down it will give you a crisp image of your subject and a motion blurred background giving you the appearance of motion. 1/200 Using this technique and a very slow shutter you can even make a subject moving slowly appear to be moving super fast. Another fun effect you can get with the shutter is to use a tripod or other stable platform and leave the shutter open for several seconds, minutes or even hours. This is the way you'd shoot fireworks or star trails. 2.5 seconds Leaving the shutter open like this allows the source of the light to "paint" a picture on the film or image sensor. You can also use a long shutter speed and a flashlight to paint light into a dark scene or write words in mid air. 10 seconds Aperture: This setting tells the lens how far to open its iris (like your pupil) when the shutter is released. Shown in f stops, lenses have apertures that fall in the range of the following sizes: 1.0, 1.1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.5, 2.8, 3.2, 3.5, 4, 4.5, 5.0, 5.6, 6.3, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, etc. Full stops are shown in bold while all others are 1/3 stops. The most difficult thing for beginners to grasp is that the smaller the "f" number, the larger the opening will be on the lens. The two most important things the aperture does for us is to help control the amount of light coming into the lens and controlling the depth of field (DOF) in the image. DOF is the area in your image from closest to farthest that is in focus. There are several factors that determine DOF and we will discuss those later. For now we want to understand how the Aperture setting affects DOF. f4.5 shows a shallow DOF. The camera is focused on the "2" button and you can not clearly see the screen of the phone or the ear hole. In the foreground the accessory plug is barely in focus. At f11 the entire phone is clearly in focus including the # key in the foreground and the earpiece farther away. As you can see, using a smaller aperture (larger number remember) gives you a greater DOF. for more info on what the f numbers really mean see "What is an f stop" in the stickies. ISO or sensitivity: ISO is a number that tells you how sensitive the film is. In a digital camera it's known as the ISO equivalent because the image sensor does not actually change in sensitivity, however the processor in the camera will boost the signals it receives from the sensor and what it "sees". the greater the ISO number, the more sensitive the film is to light or the higher the boost from the digital sensor and thus the less light that is needed to make the image exposure correct. ISO values, like shutter speeds, double in value to indicate a full stop's increase in sensitivity. You can also find film and set some digital cameras to 1/3rd stop values. The range you can usually find is 25, 50, 64, 80, 100, 200, 400, 800, 1600, 3200, 6400. Lets say it's now getting dark at that race track, and your images are starting to come out a little dark (under exposed) using that 1/200th shutter speed. You're using ISO 200, so you can double that to ISO400 which lightens your image by 1 stop. So why not just always keep the ISO up high you ask? Nothing in life is free, and certainly not high ISOs. Higher ISO film is "grainier" and your images will suffer because of it (sometimes you may want that look). With digital format, the processor boosts the signals coming from the sensor, however it doesn't know actual image signals from noise signals. So it boosts both and you wind up with noisy images that look much like the grainy film images. Now if you're only printing out 4x6 inch prints this probably won't make a difference and you'll never notice. However if you print at 8x10, 16x20 or even 24x36 the problem will become increasingly obvious. Certain cameras deal with higher ISOs better than others. For the most part, the larger the sensor (DSLRs), the better they'll do. Point and Shoot cameras rarely perform well for even 4x6 prints at higher than ISO 400... with a few exceptions (Fuji F20/30). Putting it all together Exposure = Shutter speed * Aperature * ISO. Keeping the shutter speed at 1/125 you can increase the exposure in 2 ways. You can either increase the ISO (going from ISO 100 to ISO 200 ) or increase the aperture size (going from f8 to f5.6). Say you have the correct exposure using 1/125, f11 and ISO 200, but the shutter speed is too slow and you're getting blurred shots from camera shake. So you increase the shutter speed to 1/250, which is 1 stop less light (half), making your image 1 stop darker. Now you have to compensate for this in some way to bring your exposure back up. You can move the aperture from f11 to f8 (losing some DOF), or you can move the ISO from 200 to 400 (gaining noise). Or, if you have a camera that allows partial stops, you can adjust your aperature to f10 and your ISO to 320. This is 1/3 stop increase on the aperture and a 2/3 stop increase on the ISO, totalling 1 full stop. The same would hold true for aperture. Say your proper exposure is 1/80, f4 and ISO 400. You need more DOF in your image so you adjust your aperture to f8; 2 full stops, quartering the total light. Now again you need to compensate for this decrease in light entering the lens, and in this case you can increase the ISO to 800, and lengthen the shutter speed to 1/40th. Each of these is 1 full stop brighter and together gives you 2 full stops on your exposure. You could have just as easily increased the ISO to 1600 (if your camera allows), or lengthed your shutter speed to 1/20 alone. The important thing to remember is that a change in any one of these 3 factors will change not only your exposure, but also how the image looks and if you do change one after you've found the correct exposure, you have to compensate by adjusting another. DOF As I said before, the main effects the aperture has on your image are the amount of light entering your camera and the DOF in your image. But the aperture setting is not the only thing that can effect the DOF in your image. The focal length of the lens you are using and the distance from your camera to the subject are also both large factors that come into play. Using f4, I shot the two images below. With the camera in the same location I used a focal lenth of 28mm and then one of 200mm. @28mm note you can see the entire tray of candles is in focus and you can even see the bike far in the background. @200mm you can only see a small area at the back end of the candle tray that is in clear focus and you'd never know what that blurry thing in the background was. Here again I show two images. One was from about 5 ft away using a 100mm focal length. The other is from about 1.5ft away again using the same 100mm focal length. At 5ft. most of the tray is in focus and you've probably got 1 - 2 ft of clear DOF. At 1.5ft the stones in the back and the candle in the front are out of focus. There is maybe 2-4 inches of DOF in this shot. These factors are great for scenic photographers because they typically use wide angle lenses to capture as much of the scene as possible which allows them to use much larger apertures and thus faster shutter speeds. It also helps when shooting sunrises or sunsets. Using a 20mm lens and an aperture of f4 or f5.6 can still get everything from the meadow in the foreground to the mountains in the background in focus. Keep in mind, however, when shooting landscapes, that your focal point is just about in the middle of your total depth of field. About half of what is in focus will be in front of that point, and the other half (actually a little more), will be behind it. This means that if you are shooting a scene where you want everything from foreground to background in focus, try to avoid focusing at 'infinity'. The camera will most likely want to do this. Try focusing on an object 50 or 100 feet away. This will ensure the whole scene falls within the DOF. For the macro shooter however, this can be hell. Macro shooters use both longer lenses (100-180mm typically) and shoot very close. Sometimes less than 1 ft from the subject. To squeeze the most DOF out of this they need to shoot at f11 or even f16 which then means long shutter speeds, high ISO or the need for LOTS of light via the sun or flash. Standard flashes don't cut it here, because they normally don't illuminate the area that close to the front of the lens. Macro shooters use specialized ring flashes that mount around the end of the lens to illuminate their subjects.