Lens Speed (Relative Aperture)

Relative aperture (or lens speed) is ability of the lens to give more or less bright image on the film at same conditions. Larger relative aperture allows shooting at lower light levels. The aperture balue also affects depth of field.

There are two different relative apertures - geometrical and effective.

Geometrical relative aperture value is determined by maximum diameter of the focal aperture and focal length as

C = D/F


where D - maximum focal aperture diameter, F - focal length.

Focal aperture is an image of the aperture diaphragm visible through the lens front part. In most lenses focal aperture in fully opened state is  almost equal to diameter of the front glass element.  Excludes from this rule are super-wide lenses that have front element much larger that focal aperture.

Let's take some prime lens and check this. For example, front glass diameter of the Takumar 58mm lens is approximately 24.14mm. 24.14/58 = 0.416. This number says nothing to us since we used to operate reciprocal numbers for aperture values. 1 divided by 0.416 gives us much more clear value of 2.4 - exacly what we see on the lens! And now you understand why there is 1:2.4 marked on the lens, right? Some manufacturers may mark this a bit differently - f2.4, f:2.4, f/2.4, or even just 2.4. But it will ALWAYS be the reciprocal value of 1 divided by the actual geometrical aperture.

Geomertical aperture explained

When we calculated the geometric aperture value, we did not consider light loses due to reflection and absorption in the glass. Therefore, actual lens speed (effective relative aperture) is always smaller than geometrical one by value of all light loses in the lens. In complex lenses with many glass elements, such loses may be about 30-40%, and they should be considered for exposure calculation. That is why all modern movie lenses have aperture scale marked in values of the effective apertures. Value of the geometrical aperture is marked on the front ring of the lens housing unually. Some foreign lenses have aperture scales marked in both geometrical and effective aperture values. In this case, effective aperture values are marked with red paint, whereas values of the geometrical aperture are marked with white paint.

Here is the LOMO OKC11-35-1 lens, for example. It's geometrical aperture is 1:2 while its maximum effective aperture is 1:2.3. This means that it actually pass only 2/2.3 = 0.87 (87%) of the light that it would pass in ideal conditions. 100% - 87% = 13% of the light is lost in the glass.

Geometrical aperture vs effective aperture

To reduce light loses and to increase image contrast, all modern lenses have coated elements. This means that surfaces of the elements bordering with air are covered with thin transparent material that has average refraction ratio between glass and air. Such a thin film reduces amount of reflected light significantly, that causes more light to pass through the lens, and less light is being dispersed.

The best performance is possible when thickness of the coating layer is a multiple of 1/4 wavelength. This condition can be true for a single wave length only, so it is impossible to eliminate reflections completely. Coated lenses are marked by Russian letter П on Russian lenses.

Some lenses may have few coating layer purposed for different wavelengths. They are marked as MC usually (Multi-Coated).

Even if there is no any special mark on the lens body, you can easily tell that it is coated of the glass has some clearly visible color tint of reflected light - yellow or blue in most cases.

The plot below shows relation of the reflected light amount of a single glass/air border before and after coating. As you can see, reflection is eliminated completely for the single wavelength only (λ=560мμ in this case), but reduced significantly for other wavelengths as well.

Relation of the reflection ratio of different wave lengths for non-coated and coated glass surface

In accordance with USSR GOST, diaphragm scales of lenses are marked in the effective aperture values. Series of the marked values is set so the each following aperture mark corresponds to double or half light amount passed through the lens compared to preceding mark. Amount of the transmitted light is directly proportional to the area of the aperture opening, so suitable relative aperture values are 1:1, 1:1.4; 1:2; 1:2.8; 1:4; 1:5.6; 1:8; 1:11; 1:16; 1:22.

First mark of the aperture scale lens corresponds to the value of the full opened diaphragm, and it may differ from the series mentioned above. All other values should match the specified row. Second mark may be without inscription if it's value differs from the first mark less that 10%.

For user's convenience, aperture values are marked with their denominators only - 1, 1.4; 2; 2.8; 4; 5.6 etc.

Here is a simple diagram showing a diaphragm size depending on the relative aperture settings. Let's assume we have a lens with 1:1 maximum effective aperture (the lens does not lose any light when its diaphragm is fully opened). When you step it down to the next value of 1:1.4, you cut 1/2 of the ligh. When you step it down to 1:22, you get only 1/484 of the light.

Relative aperure value and the diaphragm size

If you need this for some practical reason, here is the amount of light you get at different effective aperture values for different values:

Relative aperture11.422.845.68111622
Light fraction11/21/41/81/161/311/641/1221/2561/484
Light percentage100%50%25%13%6%3%1.6%0.8%0.4%0.2%