Lens speed

Submitted by WhiteWall Team

The importance of lens speed

The speed of a lens is a crucial factor in photography that has a significant impact on the quality and character of your images. In this blog post, you'll learn everything you need to know about lens speed. We'll talk about the advantages of fast lenses, the relationship between speed and depth of field, bokeh and why fixed focal lengths are generally faster than zoom lenses. We also explain why telephoto lenses with long focal lengths are often less fast and what problems can occur with the imaging performance of very fast lenses.

What is lens speed?

The speed of a lens is described by the largest possible aperture and indicates how much light falls on the sensor of your camera. As you already learned in chapter 3 about the exposure triangle, a small f-number, e.g. f/1.4, means a large aperture, while a large f-number, e.g. f/16, closes the aperture further. This means that more light reaches the sensor with a small f-number than with a large f-number. Lenses with a maximum aperture of f/1.8, f/1.4 or even f/1.2 are therefore considered to be much faster than lenses with a wider aperture of f/4 or f/5.6. For photographers, speed is an important indicator of the quality of a lens, as a larger amount of light captured by the lens has many advantages. Lenses whose aperture can be opened to f/2.8 or more are generally considered to be fast.

What are the advantages of very fast lenses?

Photographing in low light

A major advantage of fast lenses is their excellent performance in low light conditions. A lens with a large maximum aperture, such as f/1.4, allows more light to fall on the sensor, making shorter exposure times possible. This is particularly useful for indoor shots, photos taken at dusk or at concerts - in other words, generally for shots with little ambient light.

Let's say you want to photograph a wedding in a dimly lit church without flash so as not to disturb the bride and groom or the guests. As you have already learned in chapter 3 about the exposure triangle, the aperture is directly related to the shutter speed and ISO sensitivity. With a fast lens, you can set the shutter speed fast enough to avoid motion blur while keeping the ISO sensitivity low to minimize image noise.

An example: At ISO 800 and aperture f/4, your camera will show a fairly slow shutter speed of 1/15 second in the dimly lit church. In this case, you must expect motion blur when taking hand-held photos. If you don't want to increase the ISO sensitivity any further to avoid image noise, you can open the aperture wide if your lens is fast enough to allow this. Each full aperture stop doubles the amount of light that falls on your camera's sensor. So if you open the aperture from f/4 to f/2.8 at ISO 800, the shutter speed is reduced to 1/30 second. If you open the aperture by a further f-stop from f/2.8 to f/2, you can take photos with an exposure time of 1/60 second. At f/1.8, the exposure time is reduced to 1/125 second and at f/1.4, the shutter speed of 1/250 second is so fast that you no longer have to worry about blurring, even with a light telephoto lens such as 85 mm. At the same time, you can maintain good image quality because you can keep the ISO sensitivity at ISO 800 by opening the aperture, thus avoiding the risk of image noise.

Creative control over the depth of field

Fast lenses also offer greater creative control over the depth of field. The larger the aperture, the smaller the area in focus in the image. With a large aperture such as f/1.8 or f/1.4, you can reduce the depth of field so that only your main subject is in focus and the foreground and background remain blurred. This is particularly useful for portraits or product shots where you want to draw the viewer's eye to the essentials. It is also referred to as cropping the subject from the foreground and background when the aperture is open.

As a rule, fast lenses produce a softer and more pleasant bokeh than slower lenses, as the large aperture creates a softer background blur. In particular, extremely fast lenses with a maximum aperture of f/1.2 are often regarded as true light and bokeh artists.

Why are fixed focal lengths often faster than zoom lenses?

Fixed focal lengths are, as the name suggests, optimized for a single fixed focal length and can therefore be built with a simpler optical construction than zoom lenses, which work with a variable focal length range.

As zoom lenses cover a wide range of different focal lengths and should offer the most consistent image quality possible at every focal length, manufacturers use more complex optical systems with often more lens elements than fixed focal lengths. For example, to build a 24-70 mm standard zoom with the same speed as a 50 mm f/1.4, a very complex design with quite large lens diameters would have to be developed, which would not only make the zoom lens very expensive, but probably also very heavy. This is particularly true for full-frame lenses, which have to expose a larger image circle than lenses for smaller sensors. Full-frame lenses therefore work with larger lenses, which are correspondingly heavy for fast lenses. Very large apertures of f/1.8 to f/1.2 are therefore generally only found in fixed focal lengths.

However, this does not mean that there are no fast zoom lenses. All major camera manufacturers offer zoom lenses with a constant maximum aperture of f/2.8. These usually include a wide-angle zoom with, for example, 16-35 mm or 14-24 mm with a constant aperture of f/2.8, a fast standard zoom with 24-70 mm f/2.8 and a telephoto zoom with 70-200 mm f/2.8. These three lenses are generally aimed primarily at professional photographers and are often referred to as "trinity" lenses. With these lenses, the aperture can be opened up to f/2.8 at any time, regardless of the focal length being used. To return to the example of wedding photography in a dimly lit church: With a 70-200 mm f/2.8 telephoto zoom, you can comfortably shoot from a greater distance while opening the aperture wide enough to keep the ISO sensitivity low for better image quality.

In addition to these fast professional zoom lenses, there are many cheaper and more compact zoom lenses with a correspondingly lower speed. But what does it actually mean when a zoom lens, e.g. a 15-30 mm, is labeled with an aperture range of f/4.5-6.3 in the product name? The aperture range refers to the specified focal length. At the shortest focal length, in our example 15 mm, the aperture can be opened up to a maximum of f/4.5. If you zoom to 30 mm, only a maximum aperture of f/6.3 is available at the longest focal length, which is therefore more closed in comparison. The aperture range in between is not usually specified by the manufacturers. In practice, the lens does not suddenly change from f/4.5 to f/6.3. Instead, the maximum possible aperture is closed in small steps as the focal length increases. In this way, slower zoom lenses can be made more compact and offered at a lower price for photo enthusiasts on a smaller budget.

Why are telephoto lenses with long focal lengths often less fast?

Telephoto lenses for wildlife and sports photography have very long focal lengths so that photographers can cover large distances to their subjects. This requires complex designs. The longer the focal length, the larger the lens usually is. And if it also needs to be fast, it also becomes heavy and expensive.

An example: The very fast telephoto full-frame fixed focal length NIKKOR Z 400 mm 1:2.8 TC VR S, built for professionals, offers a maximum aperture of f/2.8, but weighs around 2.9 kilograms and costs around 14,500 euros. The comparatively slower NIKKOR Z 28-400 mm f/4-8 VR telephoto zoom covers the same focal length of up to 400 mm - however, the aperture can only be opened up to a maximum of f/4 at 28 mm and up to a maximum of f/8 at 400 mm. Compared to the professional telephoto lens, significantly less light falls on the camera sensor, which is why you either have to rely on a lot of ambient light or significantly increase the ISO sensitivity when using the zoom. On the other hand, the NIKKOR Z 28-400 mm weighs only 725 grams and is already available for around 1,550 euros. The speed therefore has a major influence on the quality, size, weight and price of a lens.

What problems can occur with the imaging performance of very fast lenses?

Although fast lenses offer many advantages, they can also present some optical challenges. The most common problems include chromatic aberrations (color fringing), vignetting and reduced sharpness in the corners of the image at maximum aperture.

Chromatic aberrations

Chromatic aberrations, also known as color aberrations, occur when a lens refracts different colors (wavelengths) of light to different degrees. This can lead to color fringing or blurred color edges at high-contrast edges in the image. Fast lenses with a large aperture (e.g. f/1.4) are more prone to chromatic aberrations, especially when the aperture is open.

Vignetting

Vignetting is the reduction in brightness towards the edges of the image. At maximum aperture, vignetting can be more pronounced with very fast lenses. As a rule, the drop in brightness is visibly reduced when the aperture is stopped down by two stops. However, vignetting can now also be easily corrected in standard image editing programs.

Loss of sharpness in the corners of the image

Many fast lenses only achieve their best sharpness when stopped down by one to two f-stops. With an open aperture, the sharpness in the corners and at the edge of the image can be less than in the center of the image. The better a fast lens can resolve the corners at large apertures, the better the optical construction and the more expensive the lens.

Conclusion

The speed is an important quality feature of lenses and influences many aspects of photography. For example, very fast lenses make it easier to take hand-held photos in low light conditions and offer creative photographers greater control over the depth of field for appealing cut-outs with soft bokeh. However, they are often associated with greater weight, a higher price and potential optical problems.

When choosing a lens, you should consider your own needs and intended use. If you often take photos in low light or indoors, a fast lens is recommended. If, on the other hand, you are looking for a compact and lightweight lens and often only take photos during the day in good light, a less fast lens at a lower price may be the better choice.