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Introductory camera guide and beginners guide on film camera and digital camera for still photography

STsite's Guide to Still Photography Camera





F-Stop and Aperture

Leica 1 with 50mm Leitz Anastigmat f3.5, 1925 We often see on a camera brochure that says "Lens: 35-105mm f-3.5-f8". The "f-3.5-f8" is called the F-stop or the maximum aperture of the lens. This figure is derived from dividing the focal length of the lens by the aperture opening of the lens.
The Aperture of a lens
illustration of aperture
Aperture opening
The aperture is the opening formed by a system of metal leaves in the lens that open up and close down to control the volume of light passing through the lens. It is the lens's equivalent of the iris of our eye.

The photography term we often see in photo magazine "opening up 1 F-stop," means making the aperture size larger to allow more light through the lens and "stopping down" means making the aperture size or F-stop smaller to allow less light through the lens. A larger aperture size is represented with smaller number hence f2.8 is larger f-stop than f5.6. The standardised F-stop number runs as follows : f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, f45, f64. Each number admits half the light to the previous F-stop. So in our example above the f2.8 allows about 8 times more light through than f8.

illustration of F-stop opening
illustration of F-16, F-8, F-4 and F-2

Lenses with a wide aperture such as f1.4 are more expensive than one that has an aperture size of f4. Manufacturers can often enhance the quality of a mediocre quality lens by limiting the aperture size of the lens in effect limiting the use of the centre portion of the lens. This is because the centre of a lens has less distortion to the light passing through it than at the fringe of a lens. So limiting the aperture to say f5.6 will improve a cheap lens considerably. Inexpensive 35mm cameras usually have maximum aperture size of f5.6 or f8!

Now you have learnt that the larger the aperture the more light can pass through thus enabling you to take pictures indoor or in poor lighting condition, so if you use your camera indoor most of the time you should look for cameras with an aperture of at least f2. However, if you are an outdoor person you can save some money by buying a camera with a f5.6 lens. It is a compromise and most medium price range cameras will have aperture somewhere between f2.8 to f4 and often the in between number of f3.5.

A wide aperture protrait lens create a out of focus blackground - f2.5 90mm lens
A 90mm Protrait lens with a wide f2.5 aperture creates a soft out of focus blackground in this photo
To make things a little more interesting or more confusing is that most cameras now come fitted with a zoom lens. As we mentioned earlier the F-stop is derived from dividing the focal length of a lens with the aperture opening. We know that the zoom lens is a lens with variable focal lengths, so if the aperture size of the lens is constant as the focal length is changed, it means that the F-stop will also change! For instance our example above "Lens: 35mm-105mm f3.5-f8 " when the lens is at 35mm zoom range the aperture is f3.5 when you zoom the lens to l05mm zoom range meaning a 3X zoom the aperture size will be 3x smaller or f7, or f8 the standardised F-stop number.

When choosing a zoom camera we can't do much on this aperture figure so choose as wide an aperture you can find for the wide-angle range and use the telephoto for outdoor scene. This variation of aperture can cause problem to flash light admitting incorrectly exposed pictures at certain zoom range which is sometimes experienced with inexpensive zoom compact cameras. For the better designed zoom compact camera manufacturers have overcome this problem by limiting the maximum aperture to a narrower range using special mechanical zoom linkage to the aperture and programming the electronic of the built-in flash light to include this short coming. Camera with Through The Lens(TTL) flash sensor is the best solution to this flash exposure problem with zoom lenses.

The problem we mentioned above is easily solved with digital cameras due to the small size of the image sensor or CCD, thus making a large aperture lens a lesser problem with digital cameras as compared to film cameras. You can find a digital camera with specifications that read
Olympus ED1 SLR digital camera
The first Four Thirds System Digital SLR camera - the Olympus ED-1
"Lens: 35mm equivalent 35-105mm f2.8-f4.4" or something close to that value to be quite common. On a better digital camera you may find something like "Lens: 35mm equivalent 35-105mm f2-f2.5". You probably never see this f2-f2.5 figure with 35mm-zoom compact or with some luck you may or may not find them with high end 35mm SLR.

<<- Digital Zoom vs Optical Zoom TOP Shutter and exposure ->>



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