Photography Tutorial:


Focal Length, Aperture, F/Stop, Shutter Speed, ISO Setting and Exposure

Inside that lens assembly are a bunch of components. There are numerous pieces of glass (lenses), some sort of focusing mechanism, special coatings on the glass to cut down on flare and "chromatic abheration" and a mechanical component that changes the amount of light coming through the lens assembly during the time that the shutter is open. The "Focal Length" of a lens is the distance from the camera sensor to the optical center of the lens. With a zoom lens like the Nikon 70-300mm lens shown below, as you rotate the zoom ring of the lens you physically change the position of the lenses inside the camera lens, thereby changing the optical center position.

In the figure above, the light gray rectangle represents an APS-C sized sensor (DX in Nikon lingo). The dark gray rectangle represents a "full-sized" sensor (FX in Nikon lingo). The green circle represents the image circle cast by a DX lens and the red circle represents the image circle cast by an FX lens. Obviously, if you put a DX lens on a camera with an FX sized sensor, you'll have very dark corners because the DX lens image circle doesn't completely cover the FX sized sensor. You CAN put an FX lens on a camera with a DX sized sensor because the FX lens image circle more than covers the smaller DX sized sensor. You're actually using the best part of the glass of the FX lens in this case. This is also called the "sweet spot" of the lens.

Inside the lens are some blades that can change the size of the opening through which light can pass. These are the aperture blades. The diameter of the hole these blades make (in millimeters, mm) is the actual aperture value. The "F-Stop" number is a pure number (no units like millimeters) that you get when you divide the focal length of a lens (mm) by the aperture value (mm). Here's an old 50mm lens shown at three different aperture settings:

In the first case, the aperture blades are pulled completely out of the way, allowing the maximum amount of light to pass through the lens. The aperture value measures 27.8mm. If we divide the focal length of the lens (50mm) by the aperture value (27.8mm) we get an F-Stop value of F/1.8. In the second case, we've closed off the opening quite a bit, restricting the amount of light passing through the lens. Now the aperture value is only 8.9mm. Again, let's divide the focal length of the lens (50mm) by the new aperture value (8.9mm). This gives us an F-Stop value of F/5.6. In the last case, we've REALLY closed down those aperture blades so the opening (aperture) is only 2.27mm wide. Dividing the focal length of the lens (50mm) by the new aperture value (2.27mm) gives us a final F-Stop value of F/22. Clearly, the smaller the F-Stop number the larger the opening through which light can pass. The larger the F-Stop number, the smaller the opening through which light can pass.

Inside the camera (not the lens) is something called the shutter assembly. This usually consists of two sets of narrow, horizontal blades. When you're looking through your camera and focusing and composing your shot, these two sets of blades are up and out of the way. When you push the shutter release button, the first set of shutter blades moves into place to block all the light coming through the lens. The mirror inside your camera then flips up out of the way (so it doesn't block any of the light coming through the lens). The first set of narrow blades then collapse down, allowing the light coming through the lens (and through the lens aperture) to arrive at the camera sensor. After a certain "length of time", the second set of narrow blades then start moving across the open area exposed when the first set of blades moved out of the way. This progressivly covers up the open area of the shutter assembly, cutting off the light coming through and striking the sensor. The amount of time that your sensor receives light through the open shutter assembly is the Shutter Speed. If there's a (relatively) long time between the first set of shutter blades and the second set of shutter blades moving across the assembly, you might have a shutter speed of 1/30 of a second. As the amount of time between the two sets of shutter blades moving becomes smaller and smaller, you're cutting down on the amount of light that comes through the shutter assembly to reach the sensor. You'll see values like 1/60 of a second, 1/125 of a second, 1/250 of a second, 1/500 of a second, etc. Each of these values cuts the amount of light reaching the sensor in half so long as you make no change to the aperture in the lens. The shutter speed is simply the length of time that the shutter assembly allows the light coming through the lens to reach the sensor.

The ISO Setting adjusts how your cameras circuitry processes the information provided by the sensor. Every sensor has a "base" ISO value. For my Dad's Nikon D80, this is ISO 100 and for his older Fuji S1 Pro this is ISO 320. For my Nikon D300, this is ISO 200. This is the starting point for your circuitry processing the information coming from the sensor as it reacts to the light striking it. As you change the ISO setting, you're telling the circuitry that processes the information from your sensor to alter how it processes that information. As you increase the ISO value (from 100 to 200, from 200 to 400, from 400 to 800, etc) you're telling the circuitry that you want your sensor to act like it is more and more sensitive to the light striking it. Each of these ISO changes doubles the processing sensitivity of the information from your sensor. Because you're deviating away from the base ISO setting of the sensor, as you increase the sensitivity your circuitry amplifies the signal coming from the sensor. This amplifies both the real picture information AND the noise generated by the sensor. It's kind of like listening to the radio and you're tuning in a really distant station. As you increase the volume, the radio program becomes louder but the static or hissing in the background also gets louder. The ISO setting is simply how much you want the camera to amplify the information coming from the sensor. It acts like you are making the sensor more sensitive to the light that strikes it.

Exposure is the combination of the F/Stop setting (which controls the aperture size), the shutter speed, ISO setting and any other extra stuff that your camera does to process a given picture. In general, we only look at the big three (F/Stop, Shutter Speed, ISO Setting) when discussing the exposure of a given photo.

So how do these three settings interact to give you your photograph? How much light reaches the sensor for a given picture is determined by the aperture value controlled by your F/Stop setting and the length of time that the hole in the first shutter blade allows the light to go through it as determined by your shutter speed. How your camera processes (amplifies) the information coming from the sensor depends on your ISO setting.

Let's use the old tried and true analogy of the water hose, faucet and bucket to explain the interaction. For any given photograph, if you fill the bucket exactly to the top, it's a properly exposed picture. If you don't fill up the bucket, your picture will be dark and is said to be "under-exposed". If you overfill the bucket, your picture will be way too light and is said to be "over-exposed". In an under-exposed photo, you lose detail in the shadows and dark areas of the picture. In an over-exposed photo, you lose detail in the highlights. Instead of seeing detail in that white cloud, the whole cloud is pure white and the details are said to be "blown" or "blown out".

Ok, so a perfectly filled bucket is a properly exposed photo. You can have the valve mostly closed, just dribbling out the water. To fill the bucket you would have to let the water flow for a long time. You could open the valve a little more, letting the water flow faster and then you'd let the water flow for a shorter amount of time to exactly fill the same bucket. The opening of the valve is your aperture (as determined by the F/Stop setting of the lens) and the length of time that you leave the valve open to exactly fill the bucket is your shutter speed. If you double the area of the opening in the valve, you'll let twice as much water flow through the valve so to exactly fill the bucket you'd have to cut the length of time that the valve is open in half. Maybe you open the valve wide open. Now you have the maximum amount of water flowing through the valve, so you'd really have to cut down on the length of time that you leave the valve open or you'd overflow the bucket. If you have two lenses, one with a maximum F/stop setting of F/5.6 and the second with a maximum F/stop setting of F/2.8, the second lens can let through four times as much light as the first lens. This would be the same as replacing the first faucet with one much, much larger.

If I have a 12 liter bucket and have the valve set to allow 3 liters per second to flow through it, I'd need to leave the valve open for 4 seconds to exactly fill the bucket. If I change the valve setting so that 6 liters flow through it each second, I'd have to leave the valve open for only 2 seconds to completely fill the 12 liter bucket. The same thing is true in your photograph. For example, you set the F/Stop to F/8 and it takes a shutter speed of 1/250 second to get a perfectly exposed photo. If you change the F/Stop to F/5.6 you're letting twice as much light through the lens so you'd have to change the shutter speed to 1/500 of a second to again have a perfectly exposed photo. If I change the F/Stop to F/4 you're again letting twice as much light through the lens so you'd have to cut the shutter speed down to 1/1000 of a second to again get that perfectly exposed photo.

Each time you change the F/Stop setting a "full stop" you are doubling the area of the hole (aperture) thereby letting twice as much light through the lens. We said above that the F/Stop is simply the focal length of the lens divided by the aperture measurement. So, with a 100mm lens you'll have the following relationships:

F/stop = (Focal length) / (Aperture diameter)
F/16 = 100mm divided by 6.25mm
F/11 = 100mm divided by 9.1mm (twice the hole area than with 6.25mm diameter)
F/8 = 100mm divided by 12.5mm (twice the hole area than with 9.1mm diameter)
F/5.6 = 100mm divided by 17.6mm (twice the hole area than with 12.5mm diameter)
F/4 = 100mm divided by 25mm (twice the hole area than with 17.6mm diameter)
F/2.8 = 100mm divided by 35.7mm (twice the hole area than with 25mm diameter)
F/2 = 100mm divided by 50mm (twice the hole area than with 35.7mm diameter)
F/1.4 = 100mm divided by 71.4mm (twice the hole area than with 50mm diameter)
F/1 = 100mm divided by 100mm (twice the hole area than with 71.4mm diameter)

(See this explanation to find out where these numbers actually come from - Detailed F/stop explanation)

In each step above, you should notice that as the F/stop value decreases, the diameter of the opening (aperture) increases, making a bigger hole. We start out with an F/stop of F/16 which indicates an aperture diameter of only 6.25mm with a 100mm lens. As we change the F/stop to F/11, the aperture diameter increases to 9.1mm. This doubles the area of the hole. If the aperture diameter equals the focal length of the lens, you end up with an F/Stop of 1.

So, if I have a perfectly exposed photo with a shutter speed of 1/250 second and F/Stop at 8, I could change the F/Stop to 5.6 (doubling the area of the hole) and cut the shutter speed in half to 1/500 second. I could then change the F/Stop to 4 and cut the shutter speed in half again to 1/1000 second to get another properly exposed photo.

Now that you have this information, where do you start?

Sunny 16 rule and equivalent exposures

Although called a "rule", the Sunny16 Rule is more of a beginning guideline to setting exposure. In the days before automatic cameras, photographers had to adjust all the settings of the camera and lens themselves. The photographer would have to adjust the F/stop setting on the lens which affects the aperture or size of the opening through which light will travel through the lens. He'd have to decide how fast to set the shutter speed of the camera to determine how long the light traveling through the lens would have to "expose" the film inside. Before doing either of these things however, he'd have to select which film he was going to use. The type of film selected determined if he'd end up with slides or negatives, would determine the tone of the final exposure and would determine how sensitive the film was to the light striking it. So-called slower films required more light to get a proper exposure than faster films. This was indicated by the ASA rating of each film type. A film with an ASA rating of 100 required twice as much light to strike it as a film with an ASA rating of 200. For a long time, ASA 200 was about as fast as you could get for color film. Eventually, the manufacturers came up with ASA 400 and even 800 film. The ASA rating has changed names to ISO, but the interpretation remains the same. As you raise the ISO rating, you raise the "sensitivity".

So, when people were stepping into the world of photography it was nice to have some starting point to help them set up the camera to get decent pictures and the Sunny16 rule was born. Essentially, it said on a super bright sunny day, set the F/stop to "16" and the shutter speed to the reciprocal of whatever the ASA rating was for your selected film. If you were using ASA 100 film, set the shutter speed to 1/100 second. If using ASA 400 film, set the shutter speed to 1/400 second. Pretty easy to remember, isn't it?

Today, with most people using digital cameras, we're not limited to a single ASA (now ISO) setting for an entire roll of film. You can select ISO 100 for one shot and ISO 1600 for the next. What you're here to find out is how to know what F/stop setting to use, with what shutter speed with what ISO setting to get the shot you want.

The last part is actually the easiest. Always shoot at the base ISO setting for your camera/sensor as long as your selected shutter speed and F/stop setting is attainable with your equipment. We'll cover the ISO setting in a little bit. For now, let's take a look at just the F/stop and shutter speed setting using the Sunny16 rule.

Let's say it's a painfully bright, sunny day and you've got a camera with a base ISO rating of 100 for its sensor. The following little chart shows the settings you could choose to get a well-exposed picture in these conditions.

Mouse over each F/stop setting to see the available combinations of F/stop and shutter speeds that will give a well-exposed shot.

F/16 F/11 F/8 F/5.6 F/4 F/2.8 F/2

Each of these individual combination of settings will give a picture with the same exposure, or relative brightness. So what is different between these shots? Remember from the depth of field page that smaller aperture openings (larger F/stop values) put more and more of the picture in focus. Larger apertures (smaller F/stop values) narrow that depth of focus and isolate objects. With moving objects, they can be blurry in your pictures if you use too slow of a shutter speed.

F/1.8 F/2.8   F/5.6   F/11 F/16

The settings given for the Sunny16 rule are the starting point for a specific set of circumstances: extremely bright sunny days. So what if it's only "hazy bright" or "cloudy"? All you have to do is change your starting point. On a cloudy day, you might start with F/11 or F/8. Remember, when changing from F/16 to F/11 you are doubling the area of the opening inside the lens, letting twice as much light through the lens. Changing from F/11 to F/8 doubles the area and the amount of light again so long as you keep the ISO and shutter speeds the same. So on a cloudy day, you might start with F/8, ISO 100 and a shutter speed of 1/100 second.

F/8 F/5.6 F/4 F/2.8 F/2    

What this chart and the first chart show you is that once you have a starting point, changing the F/stop means you have to make a corresponding change to the shutter speed to keep the same exposure.

So what if you're at the ballpark to watch your son or daughter playing baseball? You've got your trusty 70-300mm zoom lens and it's a cloudy day. You put your camera in "P" mode because you just don't understand exposure and want to let your camera do everything for you. Looking through the viewfinder, you see that the camera has selected F/5.6 with ISO 100 and a shutter speed of 1/200 second. You're all set! Your daughter is up to bat and you frame the shot, anticipating the swing. She takes a mighty swing, you time the shot perfectly and she hits her first home run ever! Woo hoo!! She's going to be so excit .. Hey! Wait just a dang minute. As you review your shot you see that the bat, the ball and her arms are all blurry! What happened? It just so happens that 1/200 second shutter speed is just not fast enough to "freeze" the motion of these things. Your 70-300mm zoom lens has a maximum F/stop of F/5.6 at the long end so you can't even try a larger aperture like F/4, which would then require a shutter speed of 1/400 second to keep the same exposure. So what do you do?

If only your "film" were more sensitive you could take a shot at twice the previous shutter speed and get the same exposure! This stupid camera. I guess you'll just not be able to get any good pictures today, right? Wrong! Now we can talk about that ISO setting. Each time you raise the ISO setting, you're telling the electronics inside the camera to amplify the signal coming from your sensor. This effectively makes the sensor more "sensitive", needing less light to get the same exposure.

Look at the following scale to see the effects of changing the ISO from 100 to 200 to 400, even up to 800 in the given situation at the ball park while keeping the F/stop set at F/5.6.

ISO 100 ISO 200 ISO 400 ISO 800      

Quickly adjusting your ISO setting from 100 up to 400 you're just in time to get a series of dead sharp pictures of your son hitting into the seasons only triple play!

So now you see! For a given set of circumstances (cloudy day) keeping the F/stop the same but raising the ISO setting will necessitate a corresponding change in the shutter speed to keep the same exposure. The same would be true if you kept the shutter speed the same and raised the ISO. This would necessitate a corresponding change to the F/stop setting.

You need to consider all three elements for getting the exposure right. To keep the same exposure between shots, if you change one of the three elements (F/stop, ISO or shutter speed) you need to change at least ONE of the others to compensate. In this situation, 1/800 second should be fast enough to freeze anything you see happening at the ball park, and raising the ISO from 100 to 400 for just about any digital camera available today should still give you a virtually noise-free picture.

Now you should have a good understanding of what is meant by "Focal Length", "Aperture", "F/Stop", "Shutter Speed", "ISO Setting" and "Exposure" and how they all work together to give you your properly exposed photograph.

So if all these different settings give the same "exposure", what is different in the resulting photos taken with all these combinations? Why would you want to use one combination of settings instead of the others?

The first point we've already told you. Increasing the ISO setting increases the noise in the resulting photo. Most DSLR's do a great job internally handling the increased noise up to about ISO 640 or 800. More expensive DSLR's and DSLR's with bigger sensors (in general) handle noise better than cheaper cameras or smaller sensors. You can think of each photosite on your sensor as one of those buckets we talked about above. The bigger the "bucket", the greater its ability to actually gather the light striking the sensor.

Just remember, as you increase the ISO value you are telling your sensor to magnify its reaction to the light hitting it. This also magnify's the noise generated by your sensor. Some sensors are better at higher ISO values than others, so how high you can go with your ISO setting and still get acceptible results will vary from camera model to camera model. You'll always get your best results (lowest noise) with lower ISO values.

The second and probably more important point is that when you change the size of the aperture, you change the depth of field in the resulting photograph. What is the depth of field? This is simply how much range of the stuff in the photograph (from near to you to far from you) is acceptably "sharp". The larger the aperture (smaller F/stop number) the smaller the depth of field. The smaller the aperture, the greater the depth of field. If you're shooting a landscape picture, you want a huge depth of field so that not only the foreground elements are in focus, but so are the mountains in the distance. Shooting a portrait or your daughter out there on the soccer field? Now you want to isolate the subject and have the stuff between you and her and the stuff behind her blurred out. This really makes the subject of the photo stand out from all the rest of the photo.

This was shot at 24mm and F/10. Both the wide angle focal length (24mm) and the very small aperture at F/10 result in a huge depth of field. Yes, the fat guy on the right is me. Note that I have enough, um, insulation that I don't need a jacket like the skinny guy to my right.

This was shot at 300mm and F/5.6. Both the telephoto focal length (300mm) and the much larger aperture at F/5.6 result in subject isolation. Notice how the girl in front of the baton twirler and the band members behind her are blurred, helping to draw the eye to the main subject of the photograph.

Higher ISO values are useful when:
There is not much available light
You need higher shutter speeds to stop fast-moving action
You need smaller apertures for greater depth of field.
You're shooting in a dark place where you aren't permitted to use a flash (museum, church).

Changing the F/stop number (aperture size) you control the depth of field. Small F/stop numbers (large apertures) isolate your subject from the surroundings. Large F/stop numbers (small apertures) give you greater depth of field, making more and more of the photograph sharp and in focus. Larger apertures require faster shutter speeds. Smaller apertures require longer shutter speeds.

Faster shutter speeds allow you to "stop" motion. How much do the arms of a running child move in 1/4000 second? Not very far, I'd guess. How far do they move at 1/100 second? Enough to give you a blurry picture. Faster shutter speeds require larger apertures. Longer shutter speeds require smaller apertures.

Don't be afraid to step away from that "P" camera setting. Select "S" in situations where you need to control the shutter speed to "freeze" fast-moving objects. You set the shutter speed you need and the camera will adjust the Aperture (F/stop). Select "A" for situations where you want to control the depth of focus. You set the Aperture (F/stop) and the camera will adjust the shutter speed. You might even find yourself selecting that intimidating "M" setting (gulp!) where you'll select both the Aperture (F/stop) as well as the shutter speed for ultimate control. Simply start with your camera in "P". The camera will give you the starting point numbers for the F/stop and shutter speed. If possible, keep the ISO setting at the base value unless your equipment won't get you the F/stop and/or shutter speed you need for your shot. Only then raise the ISO value.

Now get out there and experiment with your camera's exposure settings!!

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