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Aperture, Speed & Sensitivity Part 3

2013-04-30 16:08:00 GMT

By ACDSee pro photographer & guest blogger Alexandra Pottier

We already talked about Aperture and Speed, it is now time to talk about Sensitivity, and close the chapter on the basics of a good exposure.

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Sensitivity is the sensibility of the sensor to the light. It is the measurement of the sensitivity of the surfaces, a sensor in digital photography, a film in silver based photography.

Sensitivity is expressed in ISO, most of the times vary between 100 and 3200. Usual numbers are: 50, 100, 200, 400, 800, 1600, 3200.

A big number (ex: 3200), represents a high sensitivity where a smaller amount of light will be necessary to expose the picture correctly.

We can compare it to human skin. A high ISO (small number) can be compared to a light skin type (as of a blonde or redhead) which is very sensitive to the light and will burn if it is exposed to too much light. On the contrary, a low ISO (high number) corresponds to a dull or dark skin, which will take longer to burn or correctly expose in photography.

We can also compare the aperture to the clouds (the more clouds there are, the less light comes through) and the speed to the amount of time the skin is exposed to the sun.

As for the rest in photography, when you double the ISO number you will need half the amount of light to expose correctly. Therefore, you can double the speed or you can use a smaller aperture (everything is connected together!).

In traditional photography. a roll of film is set for one sensitivity only. On a digital camera you can change the ISO for each picture.

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The sensitivity has a big impact on the quality of the picture. When the ISO is set up high, the grain of the picture increases.

The grain in traditional photography is the bigger silver salts that appear on the prints. It is called noise in digital photography, where the pixels appear.

When you have very little amount of light, the first thing to do is to open up the aperture as much as possible or use the lowest speed before you increase the sensitivity. If you need speed, you’ll have to use a bigger aperture to compensate the loss of light.

Sometimes noise has nice effects. You can also add noise with your favorite software!

You’ll have to juggle between the three parameters speed, aperture, and sensitivity to expose your images correctly, compromise is what makes photography interesting.
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Aperture, Speed & Sensitivity (Part 2)

2013-04-10 17:20:00 GMT

By ACDSee Guest Blogger & Professional Photographer Alexandra Pottier

After aperture, one of the three facts that make a good exposition in a picture is the shutter speed.

It is the time while the sensor is exposed to the light while the curtain is open. If we think of it as a window, it is the time while the window is open.

Usually, we express the shutter’s speed in seconds or fractions of a second.

A long exposure time, 1 sec for instance, exposes the sensor for a longer period of time. That is useful when there isn’t much light in the scene.

On the contrary, a short exposure time, 1/1000 sec, exposes the sensor very shortly to the light when there is a lot of it.

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Usual shutter speeds are : 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, ¼, ½, 1 second, up to 30 seconds. Then you have the Bulb mode, where you can decide for yourself the length of the time of aperture.

As you can see the numbers are equally proportional. When you double the exposure time (from 1/250 to 1/125 ) you let twice as much light in.

Of course the speed has consequences on the final result of the picture. The use of a fast shutter speed (1/1000s sec) will freeze the action even if the subject is moving.

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The use of a slow speed (1 sec) will show an amount of panning.

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The shutter speed must be chosen according to four criteria which are :

The effect you want for your picture. Frozen action or fuzzy yarn.
The movement’s speed. The photographer’s stability is important, below 1/60s, it is better to use a tripod, because the human, even very still, suffers from micro-movements, and the result is a fuzzy picture.
The subject’s speed. The more the subject is moving, the more fuzz there will be. And vice versa.
The focal length. It is common to say that it is better to use a focal length ratio 1/focal length. For example, if you are using a 200mm lens, it is better not to go under 1/200 sec.

If you want to keep the same exposition while changing the shutter speed, you’ll have to change the aperture increasely.

To change the shutter speed, there are two options : use the manual mode or the S mode. This way, you get to choose the speed you want for your camera.
To practice, you can start with the focal length rule (1/focal length) then you can try on different moving subjects, a walking person, a cyclist, a jogger, etc…

Have fun!

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Aperture, Speed & Sensitivity (Part 1)

2013-03-01 21:07:00 GMT

By ACDSee Guest Blogger & Professional Photographer Alexandra Pottier

In photography, three things matter: Aperture, speed and sensitivity. These three facts, when combined, make a good exposure.

About aperture in photography:

The diaphragm opening in a camera is composed of thin metal curtains that open or close in increasing sizes.

The aperture of the diaphragm corresponds at the size of the surface that will allow the light to come in and hit the sensor during the exposure. You can compare the diaphragm to a window, the bigger the size, the more light comes in and the room is more bright.

The size is expressed in f/stops where large numbers (f/22) are letting very little light in, which is useful when there is a lot of light, like a bright sunny day.

Whereas smaller numbers (f/2.8) allow a lot of light to come in when there isn’t much light around.

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f/2.8                                                 f/8                                               f/22

The numbers are proportional, if you increase one f/stop (from f/11 to f/8 for example) you double the size of the hole from which the light comes through. It is the same proportion if you decrease from one f/stop, (from f/8 to f/11) you let half of the light to come through.

The most common f/stops are f/2.8, f/5.6, f/8, f/11, f/16 , f/22.

Usually, the biggest or the smallest aperture given by a lens depends on its focal length. The average lens with a focal length of 50mm, has a maximum aperture around f/2.8. A bigger zoom lens usually offers a smaller aperture, around f/5.6.

The more a lens opens wide, the more complex it is to build. The zoom lenses with a big focal length (200mm) and a wide aperture are usually very expensive.

The aperture also affects the depth-of-field (DOF), the sharpness of the different levels of the picture. In other words, the depth of the depth of field.

With a large aperture (f/2.8) the depth-of-field is small and, on the contrary, a small aperture (f/22) gives a large depth-of-field.

In Portrait photography, it is recommended to use a small depth-of-field, so everything that is behind the subject will have a nice artistic blur. Where in landscape photography, it is better to use a large DOF, and so a small aperture to have every level in the picture sharp.

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The image on the left has a big aperture (f/2,8) and has a small depth of field
The image on the right has a small aperture (f/22) and has a large depth of field.

You can change the aperture by using the manual mode of your camera, or you can use the A or Av mode. When you do so, your priority is to set the aperture first, the other settings will follow.

Some cameras have a depth of field measurement where it shows you what is invisible. When you change your aperture while using this tool, you can see the depth of field changing through the lens. Very handy to compose a nice picture.

Now get your camera and practice!

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