Shutter speed — Aperture or F-stop — ISO

Aper­ture size and depth of field

When you take a pic­ture, light enters your cam­era through a ‘hole’. This hole is referred to as the aper­ture. The size of the hole is referred to as the f-stop. The shut­ter that we talked about in the last sec­tion resides behind the hole or aper­ture. Like the shut­ter, the aper­ture size can (usu­ally) be adjusted. It can be made larger, let­ting in more light. It can be made smaller, let­ting in less light. The shut­ter speed and aper­ture size in com­bi­na­tion with the ISO (the sen­si­tiv­ity of the record­ing mate­r­ial or the speed of the film if you are using film) is what deter­mines the over­all expo­sure. These f-stop num­bers are occa­sion­ally writ­ten around the lens bar­rel, and can be seen in the camera’s viewfinder as well. What to do these num­bers around the lens mean? They refer to the size of the aper­ture, how much light is going through the lens, then through an open shut­ter, and then get­ting to the dig­i­tal sen­sor or film where the scene is recorded.

The aper­ture (AKA an F-stop) also helps deter­mine depth of field. Depth of what? It’s not as com­pli­cated as it sounds. I sug­gest get­ting your cam­era right now and check­ing the num­bers in the viewfinder. The num­bers always start out small and get larger i.e. F-1.8, F-2.8, F-4, F-5.6, F-8, F-11, F-16, F-22, F-32. Your lens may not have all of these num­bers but it will have some of them. Each num­ber rep­re­sents the size of the aper­ture or open­ing of the lens dur­ing the expo­sure. (Expo­sure once again, refers to the moment that the light passes through the aper­ture and the open shut­ter in order for the image to reg­is­ter on the dig­i­tal sen­sor or film). The smaller the num­ber i.e. 1.8 or 2.8, the LARGER the open­ing of the aper­ture. The larger the num­ber, the smaller the aper­ture open­ing. Although this may seem con­trary to logic, think of it this way. Pre­tend these num­bers are all frac­tions of an inch or a cen­time­ter.  In real­ity these num­bers are indeed frac­tions of the focal length of the lens. So F-22 is SMALLER than F-1.8 because it lets in a smaller frac­tion of light. A good thing to know is that each suc­ces­sive num­ber lets twice as much light hit the film as the pre­vi­ous one. NOTE:THIS IS THE SAME THEME THAT WAS MENTIONED IN RELATION TO SHUTTER SPEEDS. So F-2.8 lets in twice as much light as F-4. If we go in the other direc­tion F-11 lets HALF the light that F-8 does because F-11 is SMALLER. Ok now that the num­bers are out of the way let’s talk prac­ti­cal depth of field.

Depth of field-refers to the over­all fore­ground to back­ground sharp­ness of your image. This is most strongly deter­mined by the aper­ture size you choose and how far away you are from what­ever the cam­era is focused on.

Try this exer­cise: Go look out your win­dow at a scene 50 feet away. Then squint with one eye or make a small cir­cle the size of a pea with your fin­gers and look through the hole at the scene again. Keep mak­ing your fin­ger smaller until the hole is teeny, let’s say the size of a pea. You should notice that as the squint or fin­ger cir­cle gets smaller, the depth or how far you can see gets clearer or sharper. That’s depth of field. And the lens on a cam­era works the same way. The smaller the aper­ture F22, F16, the sharper the details in BOTH the fore­ground and back­ground. The larger the aper­ture F-1.8, F-2.8, only the ele­ment focused on will be sharp. The clas­sic exam­ple is a man in a for­est. If you focus on the man at 1.8, the man will be sharp but the trees behind him will be out of sharp focus. But at F-22 both the man and the trees behind him will be in focus. The mid­dle aper­tures will give more depth of field than the large aper­tures like F-2.8 but less depth of field than the smaller aper­tures like F-16


F-2.0 A shal­low depth of field. Note how the fore­ground is sharper than the background.


F-16 A large depth of field. Note how both the fore­ground and back­ground are sharp.

So why do we choose dif­fer­ent aper­tures and not always the small­est or sharpest one? Basi­cally for two rea­sons: Although the smaller aper­tures( F16,F22, F32) pro­duce sharper focus from back­ground to fore­ground, they require more light to achieve proper expo­sure. This is because the hole that the light passes through in order to hit the record­ing mate­r­ial  is small. If the hole were larger, you would need less light. This is not a prob­lem on a sunny day, but would be a prob­lem on a cloudy day or at night. The sec­ond rea­son for choos­ing a larger aper­ture boils down to intent. Even if there is enough light to choose what­ever aper­ture you desire, some­times you want to ISOLATE the fore­ground from the back­ground. This is espe­cially use­ful when tak­ing a por­trait. Let’s say we’re talk­ing about that man in the for­est again. If we want the viewer to focus on the man, we will choose a larger f-stop. When we look at the result­ing image, we will notice that the man is in sharp focus but the back­ground is some­what blurry. This will cause the viewer of the image to pay more atten­tion to the man and less atten­tion to the trees. If the man is a lum­ber­jack how­ever, and we want to show his envi­ron­ment, then we may wish to choose a smaller aper­ture (light per­mit­ting), and so more of the image from back­ground to fore­ground will be sharp.

ISO — The sen­si­tiv­ity of the record­ing mate­r­ial or Film Speed

The last ele­ment in this trin­ity is the sen­si­tiv­ity of the record­ing mate­r­ial known as the ISO. If we are still using film it is known as the film speed or the ASA. What is the dif­fer­ence between  ISO 200 and ISO 400 for your dig­i­tal cam­era or film cam­era?  You’re gonna love it. AGAIN ITS BACK TO THE SAME THEME AS IN THE PREVIOUS SECTIONS. ISO 200 is twice as sen­si­tive to light as ISO 100. Going in the other direc­tion, ISO 200 is HALF as sen­si­tive as ISO 400 or 400 speed film. The higher the ISO, the less light you need to take the pic­ture. This is why if you are tak­ing indoor shots or shots at night (where there is less light) use a higher ISO or a higher film speed.

In dig­i­tal pho­tog­ra­phy, higher end cam­eras can go to ISO 6400 and higher. Film speeds can go as high as 1600 or even 3200. These ISOs are extremely sen­si­tive to light and are there­fore often used when the light level is really low. So why not always choose a high ISO, what’s the catch? The catch is in the pixel struc­ture that makes up the dig­i­tal file or the grain struc­ture of the film that you may still be using. The lower the ISO the finer the pixel struc­ture or grain. The higher the ISO or film speed the more ‘pix­elly’ or ‘noisy’ or grainy the image will appear.  You might not see it so much in a small image, but if the image is enlarged to 8x10 inches, you can def­i­nitely see the dif­fer­ence. Pix­elly images or grain aren’t always bad. Many pho­tog­ra­phers use noise or grain for artis­tic effects. Most peo­ple how­ever, pre­fer less noise or grain­i­ness in their images.

ISO - Film Speed

In dig­i­tal pho­tog­ra­phy or film pho­tog­ra­phy, ISO, or ASA or film speed all refer to the same thing; the sen­si­tiv­ity of the record­ing material.


Shut­ter speed — aper­ture — depth of field — film speed — summary

Based on the ISO, the shut­ter speed and the aper­ture, the camera’s light meter will eval­u­ate the scene and pro­duce a proper expo­sure. This expo­sure will be an f-stop along with a shut­ter speed at a given ISO. NOW THAT YOU KNOW SOMETHING ABOUT EXPOSURE, YOU CAN CHOOSE DIFFERENT SHUTTER SPEED/APERTURE COMBINATIONS FOR CREATIVE EFFECTS AND STILL MAINTAIN PROPER EXPOSURE.

So here we go: First put your cam­era in man­ual mode. Let’s hope it’s day­time. Go to the win­dow and focus on some­thing. Some­where in the viewfinder you will see the expo­sure. Let’s say it’s F-5.6 at 1/250 This means that the size of the aper­ture the ‘scene’ passes through is F-5.6. It reaches the shut­ter which stays open for 1/250 of a sec­ond and is then recorded on the dig­i­tal sen­sor or on film. Now as we have learned, shut­ter speeds and aper­ture sizes work math­e­mat­i­cally . Let’s say that we decide we want to shoot not at F-5.6 but at F-4. Since F-4 is one f-stop larger, it will let in more light than we need. The final print will be too light or overexposed.

Pic­ture it this way; let’s say the aper­ture is a win­dow. If we make the win­dow larger, more light will enter the room. Let’s say that right behind the win­dow are shut­ters. If we open the shut­ters for a longer time, again more light enters the room. There­fore the solu­tion for let­ting in 1 unit extra light through the win­dow (aper­ture) is to decrease the time the shut­ter stays open. So if the cor­rect expo­sure was F-5.6 at 1/250 and we change to the larger aper­ture F-4, we must decrease the shut­ter speed by 1 unit to 1/500. Thus F-5.6 at 1/250=F-4 at 1/500. BOTH COMBINATIONS ALLOW EXACTLY THE SAME QUANTITY OF LIGHT TO BE RECORDED. In this case the main dif­fer­ence between the two shots will be depth of field. The F-5.6 shot will be sharper from fore­ground to back­ground than the F-4 shot.

ISO works in a sim­i­lar math­e­mat­i­cal fash­ion. Let us say that it is an hour after sun­set and the light is really low. We are using a 50mm lens at ISO 100 and tak­ing a por­trait of Bozo the clown on the beach. We look through the viewfinder and find that the expo­sure is F-2.8 at 1/30th of a sec­ond. We know that since Bozo is stand­ing he is mov­ing slightly, and we there­fore need to go to 1/60 to ensure that there is no move­ment which will blur the scene.

What do we do? Basi­cally we need 1 more unit of light to get the shut­ter speed to 1/60. If we raise the ISO to ISO 200 and keep the same aper­ture, then the shut­ter speed becomes 1/60.  We would have the iden­ti­cal over­all  expo­sure and our sub­ject would be sharp.  We could also raise the ISO more if we like. If we raise it to ISO 400 (adding 1 more unit of light) then we can keep the shut­ter speed at 1/60 and change the aper­ture to F-4. This would allow us to get more depth of field if we wanted it. Rais­ing the ISO to 800 would allow us to shoot at F-5.6 for even more depth of field, while keep­ing the shut­ter speed at 1/60.

Once expo­sure is fully under­stood, it’s easy to see how we can manip­u­late the vari­ables that make up expo­sure (ISO, shutter-speed, and aper­ture) to pro­duce very cre­ative images. Until expo­sure it is fully under­stood, prac­tice shoot­ing at dif­fer­ent aper­tures (while being mind­ful of the shut­ter speed). The depth of field pro­duced by dif­fer­ent aper­tures will be one of the most cre­ative deci­sions you make for every shot that you take.

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