Lens speed and aperture – What is a fast lens?
Using a fast lens at open aperture can be used to effectively highlight a subject. But what is ‘fast’ and ‘slow’ referring to in terms of photographic lenses? This article explains all about f-numbers, what is meant by ‘stopping-down’ and how shutter speed and aperture interact to create different depth-of-field effects. It’s a useful guide for beginners to understand what is meant by a fast lens and why they are so prized by keen photographers.
Using a fast lens at open aperture can be used to effectively highlight a subject. But what is ‘fast’ and ‘slow’ referring to in terms of photographic lenses? The concept can be confusing for beginners but doesn’t have to be.
Some photographers might confuse lens speed with autofocus speed or shutter speed. Actually, lens speed has nothing to do with the concept of speed as relating to motion. Lens speed refers to the amount of light that travels through the lens while the shutter is open. This is controlled by the size of the hole or aperture in the lens’ diaphragm. The faster a lens, the larger the maximum aperture in the diaphragm and the more light will hit the sensor at the same shutter speed.
Photographers love using a fast lens because of the creative possibilities it allows. They love having a shallow depth-of-field and focusing on the essentials. This adds an emotional layer to a photo – whether it’s a portrait, a detail in the depths of a pedestrian subway, or an appetizing plate of fruit.
How do f-numbers relate to lens speed?
A lens’ speed is usually referred to by the size of the maximum aperture of the lens. This diameter is expressed as an f-number, such as f/2.8 or f/5.6. Note the smaller the f-number the larger the size of the opening and the more light is let through when the shutter is open. High f-numbers refer to the smallest apertures. Modern digital cameras allow a large range of apertures between the minimum and maximum determined by the lens. In traditional analogue cameras, there were fewer, set apertures that could be selected. The standard choices were the f/numbers that halved or doubled the amount of light allowed on the sensor, known as “f-stops”. These are shown in the diagram below. Hence, going from say f/4 to f/5.6 (or ‘stopping down one stop’) means that only half as much light will be used to expose the images. To get the same exposure with an aperture of f/5. 6 you need to use a slower shutter speed than you would at f/4 to let in the same amount of light. So an f/4 lens is “faster” than an f/5.6 lens.
How do f-numbers relate to depth of field?
Smaller apertures (large f-numbers) produce a greater depth of field, useful when you want front to back sharpness, for example in landscape work. Wider apertures, the lowest f-numbers, have the shallowest depth of field and can be used for creative effect.
Are “fast” lenses better?
Working with fast lenses offers two main advantages. Thanks to their shallow depth-of-field, photographers have the possibility to isolate the subject. The idea, especially for portraits, is to make the subject stand out as sharply as possible against a blurred background. This gives an impression of spatial depth and focuses the viewer’s attention on the key part of the image.
A fast lens is also helpful in available light photography. This type of photography is all about creating an image with the light you have.
Gear and settings are choices that must be carefully made. A fast lens allows for more exposure options in low light. Due to their very high optical performance, our high speed Otus lenses, for example, are ideally suited to be used wide open for outstanding results with a very shallow depth of field.