When browsing the Tamron lens catalogue, you will come across one term again and again: luminosity. It is generally regarded as a sign of quality in photographic lenses. The higher the light intensity, the higher the quality of the optical system. Anyone who takes photographs at a certain standard will appreciate the advantages of fast zoom lenses and fixed focal lengths. But what exactly do we mean when we talk about speed?
On this page, you will find answers to the five most important questions about the speed of photographic lenses.
1. What does luminous intensity actually mean?
The term luminous intensity actually comes from photometry, where it is used to measure the brightness of different light sources. In relation to lenses, however, luminous intensity means the largest possible aperture (open aperture). A large aperture (e.g. F/2.8 or F/1.8) is therefore synonymous with a high luminous intensity, although there is no fixed definition. For example, an aperture of F/5.6 is considered fast for some lenses.
2. Why is a large aperture important?
Photography is not possible without light. Only when enough light rays fall on the sensor can the camera produce an image. Fast lenses – as the term "fast" suggests – offer a decisive advantage: their large aperture lets in a lot of light. This means that photographs can be taken in twilight and indoors with a faster shutter speed.
Photo: Sally Hateswing. SP 35mm · F/1.4 · 1/800 s · ISO 100
3. What are the advantages of low light?
When more light falls on the sensor, faster shutter speeds are possible. The images become sharper. For example, aperture F/2.8 allows a shutter speed four times faster than aperture F/5.6 at the same ISO value, 1/250 s instead of 1/60 s, for example. Firstly, blurred pictures are prevented, and secondly, fast movements can be frozen razor-sharp.
Photo: Luke Stackpoole. 70-200mm at 200 mm · F/2.8 · 1/1000 s · ISO 500
4. How does a large aperture help me with the composition of the picture?
In almost all photos, a certain area of the picture should attract attention. Therefore, the focus is usually on this area, while the rest of the picture is blurred to a greater or lesser extent. A large aperture allows the plane of focus to be limited to a very small area, for example, in a portrait focusing on the face or the eyes. The background then blurs attractively out of focus. This is exactly what is meant by the bokeh effect.
5. Does a high aperture have disadvantages?
A wide aperture inevitably means a corresponding wide lens diameter. Particularly fast telephoto lenses are usually somewhat heavier and larger. However, advanced optical technologies and special glass make for increasingly compact and lighter lenses in general. For example, the 70-180mm F/2.8 Di III VXD weighs only 810 g – the SP 70-200mm F/2.8 Di VC USD G2, which is a few years older, weighs 1,485 g.
Photo: Oliver Güth. 24-70mm at 35 mm · F/4.5 · 1/160 s · ISO 250