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1.1. Aperture mechanism

The size of an SLR camera's aperture is controlled by the movement of several aperture blades that behave much like the human eye, changing the size of the circumference. This type of aperture is known as an "iris aperture."

For artistic blur, the aperture should be as circular as possible. To achieve this, there should be an odd number of shutter blades in the shape of a polygon.

When you look through the viewfinder of a modern SLR camera, the lens aperture is always fully opened so the image is bright and easy to put into focus. When the shutter release button is pressed, the aperture blade is adjusted to the aperture value that has been set. At the same time, the reflex mirror in the camera body flips up and the shutter opens. When the shutter closes, the mirror moves back to its original position and the aperture returns to a fully opened configuration. Despite the speed of this operation, it is quite complicated. Sometimes it appears that the image in the viewfinder is different than the actual picture. Aperture setting is one reason for this. Because the image seen in the finder is observed with the aperture fully opened and the actual photo is taken after the aperture has been adjusted, there is a disparity.

Some SLR cameras have a stop-down (preview) mechanism that enables you to see what the actual picture will look like. Though this image may appear a bit dark, it is an accurate image representation and a good way to check depth of field. (The finder image might appear a bit more focused than the resulting photo, depending on the quality of the finder screen's matte surface.)

The lens of an SLR camera has an aperture that opens and closes in order to adjust the amount of light the film is exposed to. Also, when the aperture changes, the portion of the image that appears in focus, known as depth of field, is altered. By making the aperture smaller, the image in focus becomes longer as it gets farther from the camera ; by opening the aperture, the range gets shorter.

1.2. What is an f-number ?

Aperture size is indicated by an "f-number." Many cameras note f-number as ...1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90...... Though the arrangement of these numbers may seem strange, notice that every other number is doubled. Each step is a multiple of the square root of two. These numbers indicate the lens' focal length (f) divided by the diameter of the aperture (d). Expressed as a formula, that is "F = f / d". The amount of light that penetrates the lens, the image brightness, is proportional to the size of the aperture. Therefore, it is inversely proportional to the square of the f-number. The amount of light is halved with each increasing f-number increment.

Let's explore this relationship a bit further. We have just said that image brightness is inversely proportional to the square of the f-number. However, this is the case only when the subject is on the optical axis and is relatively far from the camera, or in other words, when the shooting magnification is relatively small. If "m" equals shooting magnification, then image brightness is inversely proportional to the square of F (1+m). So, when taking close-ups using an extension ring or bellows, a positive ("+") compensation is necessary. This adjusted f-number is called the effective f-number. (NOTE : If your camera has internal TTL metering, it is not necessary to make this calculation.)

Also, with some inexpensive wideangle lenses that are used with compact cameras and some expensive wideangle interchangeable lenses for rangefinder cameras, the decrease in light amount around the frame of the picture (= the four corners of the picture appear dark) becomes obvious.

When the aperture is fully opened, the f-number is referred to as maximum aperture or maximum f-number. These f-numbers are usually shown on the camera (1:1.4, F1.4, f/1.4, etc.). Conversely, the smallest aperture is called the minimum aperture.

By using a lens with a small maximum f-number, you will be able to take brighter pictures. Of course, this is advantageous when shooting in dark places or when you want to blur the background. In order to reduce the f-number, the diameter of the lens must be large, because it must be proportional to the focal length. This poses several practical problems, namely that the lens becomes bigger, heavier and more unwieldy. And more expensive. Lenses of this type are often called bright lenses or high speed, though these names don't actually denote high transmittance.

In theory, the maximum brightness of a lens is f / 0.5. Though it seems logical that a lens with a larger diameter would make an image brighter, this just isn't the case.

2. What changes with aperture

We now know that brightness is affected by aperture size, as is depth of field. This information always appears in an SLR's manual, so it should be clear to the user.

Though it's easy to change the aperture size by adjusting the lens, it is often difficult to get the desired results, depending on the purpose and subject of the photograph. The first step to familiarize yourself with aperture is to learn how to set it yourself using manual mode or aperture-priority AE mode ("A" mode). Another good idea is to make large changes in aperture size: One or two steps of aperture change might be enough to realize noticeable differences in your photos.

As standard procedure, try taking three pictures with the following settings, and compare the results :
1.) maximum aperture,
2.) minimum aperture and
3.) f-number set somewhere in between.

2.1. Aperture and brightness

( Photos 2.a.) to ( 2.c.) were taken with manual exposure with the same shutter speed. Only the aperture was changed.

( Photo 2.b.) is at normal exposure, f5.6. Photo (2.a.), at f2, is three(3) steps overexposed. ( Photo 2.c.), at f16, is three(3) steps underexposed.
The difference is apparent when the aperture is set at +/- 3 EV, for a total margin of 6 EV.

This does not mean the image will not be visible; if these photos are regarded as portraits, then ( Photo 2.a.), three steps overexposed, might be considered the best.

This helps highlight the value of taking a series of photos with different aperture values, then comparing them.

2.2. Depth of field and blur

( Photos 3.a.) through ( 3.c.) was taking using the same aperture values as ( Photos 2.a.) through ( 2.c.), only the values have been set at aperture-priority AE mode and different shutter speeds to achieve the appropriate exposure.

Background blur in each photo differs greatly. With an open aperture (( 3.a.)), the background is significantly blurred, but with a close aperture (( 3.c.)) there is only slight blurring. Adjusting the aperture allows you to make a difference in the distance-focus relationship.

The amount of blur is generally proportional to the diameter (d) of the aperture, and is inversely proportional to the f-number (F). Blur is proportional to the lens' focal length (f). Also, the wider the focusing plane and the wider the portion of the image that appears in focus, the more blur there will be.

Depth of field is the area in which the image appears to be in focusThis means that the smaller the aperture is, the deeper the depth of field will become.

So, depth of field changes with aperture, the lens' focal length and shooting distance. The general rules are shown below. But be cautious when shooting with telephoto lenses (long focal length) or when shooting close-ups (short focusing distance), because depth of field becomes quite shallow.