Friday, December 21, 2007

Technique (Exposure) - Aperature and Shutter Speed

A photographer must understand aperature and shutter speed before he can learn how to properly control exposure. Sure, you can put the camera on programmable or one of the other automatic modes and it will generally expose the picture properly. However, it will control the exposure of the subject for you and, to some extent, how the subject is displayed. It does not know you want a lot of depth of field (how far in front and back of the subject you want in focus) unless you tell it. One must understand aperature and shutter speed to really get good at controlling the light and deciding how to display the subject instead of allowing the camera to do this for him.

Aperature is the size of the opening in the lense that allows light to pass through and expose the film or hit the digital camera's sensor and create the image. One thing that is counter-intuitive (you would think the opposite was the case) is that the smaller the aperature number the wider the opening in the lense. The lense has a much smaller opening at an aperature setting of f22 (aperatures are in f-stops and are denoted by placing an f in front of a number - usually this is shortened to "stop" instead of "f-stop" when photographers speak or write) than at f4. However, at f22 the depth of field is greater than at f4. One must ingrain this into their brain until they essentially automatically understand this without thinking about it if they are going to take pictures quickly and properly expose them. However, don't worry if it seems to confuse you at first. It takes a while and only through practice will it eventually become ingrained.

Shutter speed is how long the opening in the lense stays open when you press the shutter release. A shutter speed of 1/250 means the opening will only be there for 1/250th of a second.

Let's spend a little more time understanding aperature and shutter speed and how they relate to one another. We'll begin with the aperature.

Full aperature settings (you can put the aperature fractions of the way between the full openings on many types of cameras) are f1.4, f2.0, f2.8, f4.0, f5.6, f8.0, f11.0, f16, f22, and f32 (if you see something written as an aperature of 1.4 the author is talking about f1.4). Again, a lense set at f1.4 has an opening much larger than the same lense set at f32. An aperature of f1.4 lets in twice as much light as an aperature of f2.0 (going the opposite direction, an aperature of 2.0 lets in half as much light as an aperature of f1.4). Likewise, an aperature of f2.0 lets in twice as much light as f3.5. Thus, the next larger f-stop lets in twice as much light. If you open up (this means changing the f-stop so the aperature is wider or more open) 1 stop (for example, go from f32 to f22) then twice as much light enters the lense. Open up 2 stops (from f32 to f16) and 4 times as much light enters, open 3 stops (f32 to f11) and 8 times as much light enters, and so on. Don't panic if you are thinking you need to remember the number of times the light that enters the lense increases when you open the lense more than one stop. I'll explain why later.

Shutter speeds, in seconds, are typically 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, etc. These can be as fast as 1/1,000 and faster on some cameras. Some cameras allow you to set a shutter speed slower (the shutter is open longer) than 1 second. There is also a setting typically called "bulb" that allows you to determine how long you want to leave the shutter open. It is usually designated by a "B" on the aperature ring or other way you set aperature on cameras. Of course, a shutter open for 1/125 of a second lets in much less light that one open for 1/2 of a second. Again, shutter speeds work like aperatures - the next slower speed lets in twice as much light. For instance, a shutter speed of 1/2 second lets in twice as much light as 1/4 second, and 1 second lets in twice as much light as 1/2 second. So, 1 second lets in 4 times as much light as 1/4 second. A shutter speed of 1 second lets in 16 times as much light as 1/15 second. Again, don't panic if you think you need to remember how many times as much light is being let into the lense when you change the shutter speed more than one setting.

So, why do you care about changing f-stops and shutter speeds? These affect your subject in a number of ways. As noted above, a smaller opening (larger f-stop number) gives more depth of field (an aperature of f16 will give you more depth of field than f11). Want to keep the subject in focus but make the background out of focus? Use a large opening, such as 1.4. This will make the background less distracting. Fast shutter speeds will let you stop a fast moving object so it is not blurred. For example, you may have to use a shutter speed of 1/500 second or more to stop the wings of a large bird in flight so the wings look sharp. Conversely, if you want to show motion by blurring the wings you will want to use a slower shutter speed. Ever see pictures of a biker or car that show enough detail that you can recognize it but the subject looks blurred to indicate motion? The photographer used a shutter speed that was too slow to totally stop the subject and make it sharp. This may be done intentionally. You can begin to see how these two factors affect the subject.

Now, let's try to understand how f-stop and shutter speed combine to give you exposures. Let's say you took a picture of a flower at f16 and 1/60 second with a digital camera and the picture looks properly exposed. However, you notice that the background is distracting. How do you try to fix this problem using the same camera and lense (there are ways to affect depth of field by changing equipment - a subject we will deal with later)? Well, you know that if the lens is open more you will get less depth of field which will put the background out of focus. However, opening the lense will let in more light. So, once you open the lense you have to change the shutter speed to let in less light to get the same exposure. How do you do this? Well, if you change the f-stop from f16 to f11 you have let in twice as much light. Now you have to choose a shutter speed that will let in half as much light to balance the amount of light entering the lense and get the same exposure. Changing the shutter speed from 1/60 to 1/125 of a second will let in half as much light. What you just learned is f16 at 1/60 (this is the short way of saying an f-stop of 16 at 1/60 of a second) is the same exposure as f11 at 1/125. You let in twice as much light by opening the lense (changing f16 to f11) but, at the same time, let in half as much light by using a faster shutter speed (changed 1/60 second to 1/125 second). So, the amount of light you let in by changing f16 at 1/60 to f11 at 1/125 is the same. You get the same exposure! However, you still notice the background is too distracting. You wish to try to open two f-stops from f16 to get even less depth of field. What shutter speed do you have to use to get the same exposure? Well, you want to go from an f-stop of 16 to 11 and then to 8 (this is a change in 2 stops) and f8 lets in 4 times as much light as f16. So, you have to change your shutter speed to let in 4 times less light. So you change from 1/60 to 1/125 and then to 1/250 second to get the same exposure. Notice a pattern? To get the same exposure, you change the aperature and shutter speed the same number of times in opposite directions (i.e., if you open the lense which lets in more light then you use a faster shutter speed to let in less light and vice-versa). f16 at 1/60 is the same as the following: f11 at 1/125, f8 at 1/250, f5.6 at 1/500, f4 at 1/1000, and so on. Going in the opposite direction (closing the lense), it is also the same as f22 at 1/30 and f32 at 1/15.

Let's look at one more example, using the same flower subject as above and the same equipment. You got the proper exposure at f16 and 1/60. What if you really want to put that background out of focus and open the lense 4 stops. You go from f16 to f11 to f8 to f5.6 to f4. That lense is really open now! You better let in a lot less light with the shutter speed or your picture is going to be too light (overexposed - you let in too much light and the picture is too light). So, you go from 1/60 to 1/125 to 1/250 to 1/500 to 1/1000. The two are the same exposure- f16 at 1/60 is the same exposure as f4 at 1/1000.

Wow, you say, "My head is spinning after all of that." Mine did too when I first learned all of this. What did I do to understand it? One way is to put your camera into manual mode (usually indicated by an "M" on the exposure ring or the dial that lets you decide which mode you will use-- other example modes are programmable usually indicated by a "P," aperature priority by an "A" or "Av" and shutter priority by a "S" or "Tv"). Now, as you increase or decrease the opening (change the f-stop) watch what your meter tells you the shutter speed should be for the same exposure (nearly all or all cameras made these days have a built in meter). Watch what happens if you change the f-stop two times in one direction, three times, more. Another way is to initially carry a chart with you of f-stops and shutter speeds like the following one (you can put it into a table to make it somewhat easier to follow):


Aperature 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32
Shutter Speed 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4


If you open up or close (or stop) down the aperature a certain number of stops from the original proper exposure, you move in the same direction to find the proper shutter speed. For instance, the original exposure is f4 at 1/60. However, you want more depth of field so you stop down to f8. You moved two f-stops to the right on the chart; f4 to f5.6 to f8. Now, you must move two shutter speeds to the right; 1/60 to 1/30 to 1/15. The exposure equivalent to f4 at 1/60 is f8 at 1/15. Note in the chart as the aperature is closed down (smaller opening) from right to left the shutter stays open longer. This makes sense since closing down the lense lets in less light so you have to leave the shutter open longer.

The chart above is a crutch. Don't become dependent on it! It initially helps you understand the relationship between aperature and shutter speed. As soon as you can, start doing this in your head so you don't have to rely on a chart. The sooner the better. You don't want to carry an 8.5"x11" paper around with all of the crutches for doing photography written down. Besides, if you have to make these changes relatively fast, like during a quickly setting sun, you don't want to be looking on a piece of paper for you answers. You can do it much faster in you head once you commit it to memory.

I know this post is a long one. However, understanding how aperature and shutter speed work and relate to one another is critical. You will not become a good photographer until you commit this information to memory.

Now, let's have a little fun. My next post will cover one aspect of composition - the rule of thirds.

12 comments:

angie said...

Bill Gates, ty ever so much for taking your time to share your wealth of information. Your P.M.S. is a superb post, hope you do not mind if i share it with my friends.

William R. Gates said...

Thank you Angie. I'd be thrilled if you share it with friends!

Anonymous said...

Thank you, thank you, thank you! This is the first time somebody explained this that made complete sense. I haven't put it into practice yet (too late) but I just wanted to thank you for sharing your knowledge. I'm an amateur photographer teaching myself as I go along and I appreciate it when professional photographers share their knowledge. I look forward to reading your other posts.

Simone

William R. Gates said...

Your welcome. This is the whole reason I write this blog - to help budding photographers take better images they can be proud to hang on their walls and, perhaps, other people's walls. Cheers, Bill Gates

Anonymous said...

Fabulous ...thank you so much.
Val

William R. Gates said...

Someone recently told me I'm misspelling a couple of photography terms on this site. "Lense" should be "lens" and "aperature" should be "aperture." Thank you and I will correct these in future posts and, perhaps, present posts if I can find the time.

art faker said...

this is a great posting!! understanding the relationship the way you have explained is new to me and way more practical. keep posting....

William R. Gates said...

Thanks Art, if that is your real name (I suspect this is a web or cyber name). Cheers.

peenkfrik said...

Thanks for this post. I was the photographer for the group tour today but most of the images were underexposed because it was noon time. I hope to apply these concepts next time.

Anonymous said...

Domo Arigato Gozaimashita....

Keep posting.

Marie (^_^)//

lhgkjmdfkg said...

8 - 1/2000 seconds, in simple terms, how long is this for shutter speed? im rather confused, how many seconds between clicking the button and it capturing the image!

William R. Gates said...

ihg...: There are at least two things going on when you press the shutter release on a camera. First,it takes the camera time to send a signal to the electronics which tell the shutter to open. Second, the shutter will then open for 1/2000 second and close. The first one is referred to as shutter lag or lapse. You can find this technical information in the cameras manual (how long the shutter lag is). However, the lag time is normally very short (much less than one second). A key point- if you wish to get a picture of something happening fast and don't have your camera set to shoot multiple frames, press the shutter release just before the action you wish to capture to account for the lag time. Lag time does not affect the exposure because the shotter will still open for only 1/2000 of a second if this is the shutter speed you have chosen.