Aperture in photography is a hidden an obscure feature that most amateur photographers ignore. All the cameras have aperture but can’t be controlled by all them. Mobile phones and tablets can’t control the aperture of their cameras. The same is true with cheap compact cameras.
Aperture affects mainly the depth of field and the shutter speed. Aperture is not the diameter of the lens diaphragm as it is the common misconception. Apperture is the ratio of the focal lenght to the diameter of the diaphragm.
That sounds scientific and is confusing at the start.
The human eye is actually a lens. Its sensitivity and accuracy in relation to its weight and size is beyond any image sensor of the present or the near future. Its diaphragm we called iris. It expands and shrinks acording to amount of light. The same happens with the diaphragm of photographic lenses. Like the the human optical nerves, the image sensor in order to bring good results has limits on the duration and the amount of light.
Focal length is not the distance of the image sensor to the furthest element of the lens as it is a common belief. It is the distance from the image sensor to the optical center of the lens. That’s why lenses with the same focal lenght have different physical lengths.
The physical equivalent of the aperture is a tunnel. If the tunnel has short distance (focal length) and has a big diameter (diaphragm) , it is bright inside. If it is very length and has small diameter, it is shadowy inside. That’s the reason why aperture has great importance in low light photography.
Aperture is written with the symbol f/. The f represents the result of the fraction between the focal lenght and the diameter of the diaphragm. The f is called f-stop. Most cameras uses standarised f-stop scale like f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16. The smaller the f-stop the more light the image sensor receives. The difference between each stop corresponds to the double amount of light is received. For example a lens with f/2.8 aperture receives the double light from the f/4 aperture.
The aperture is controlled in three ways.
- Regulating only the diameter of the diaphragm. It is typically used in prime lenses. Prime lenses have a fixed focal length and can’t zoom.
- Regulating only the focal length. It is used on cheap compact compact cameras with zoom and fixed diaphragm.
- Regulating both the diaphragm and focal lenght of the lens. This is used in lenses with zoom and diaphragm blades .
In practice the aperture usually creates this effects on photographs:
- Lower f-stops creates vignetting. Vignietting is the darkening of the photo’s corners. Sometimes is artistic, sometimes is unpleasant.
- Higher f-stops produce sharper photos. The group f/64 was a group of photographers like Willard Van Dyke and Ansel Adams. The high f-stop f/64 on large format cameras produced sharpness evenly with great depth of field.
- Lower f-stops accept the maximum light and permit the minimum time of exposure. Despite creating less sharp photos, it is the ideal setting in low light handheld photography. Human eye has approximately f/2.1 in darker places and f/8.3 in brighter places.
- Lower f-stops create shallower depth of field. When lower f-stop is combined with larger focal length we have the bokeh phenomenon. Bokeh is the artistic blur of the out of focus backround and foreground. The object in focus is more evident. It is artistic and very usefull in portraits.
- Higher f-stops create more lens flare. lens flare is the artifacts or haze created by a very bright source of light, like sun or a naked lamp. That can be artistic or unpleasant.
- Lower f-stops conceal the presence of dust on the sensor or the lens.
Since the lower f-stops results in the beneficial bokeh but also to softer photos, the construction of sharp lenses with very low f-stop is very demanding.
That kind of lenses are called portrait lenses because bokeh helps the eye to focus on persons. They are also called fast lenses because they send more light to the sensor and require minimum shutter speed. Normal and wide angle sharp lenses below f/1.4 and telephoto sharp ones below f/4 cost thousands of dollars each. They make the difference between amateurs and professionals.
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