Fast lenses, eh? Who doesn’t want one mounted on their camera? And with new models coming thick and fast, including the recently announced Nikon Z 50mm f/1.2 S and Z 14-24mm f/2.8 S, Fujifilm’s stunning XF50mmF1.0 R WR, Sigma’s 85mm F1.4 DG DN Art – and plenty more besides – the choice gets wider with every passing year. So, to help with your next purchase, we thought it was high time to look at what fast lenses are, the benefits of using them, and some of the drawbacks you’ll need to factor in, too.
What are fast lenses?
A lens’s speed – and whether it’s considered ‘fast’ or not – is dependent on the maximum aperture available on it. The term ‘fast’ defines lenses that have very large maximum apertures, and this literally means that faster shutter speeds can be used with these lenses, than with ‘slower’ models that have smaller maximum apertures. Of course, the term is relative and subjective, and the apertures considered ‘fast’ have changed over time. So, while both an f/1.4 and f/1.8 lens are ‘fast’, the f/1.4 model is faster than the f/1.8. And while once a 50mm f/2.8 lens was considered fast, modern lens designs and manufacturing processes have made it possible to create much faster optics.
How does a fast zoom compare to a fast prime lens?
It all depends on focal length. While you could say that today’s fast primes lenses are those of f/1.8 and wider, that doesn’t cover long telephoto prime models. So while f/2.8 might not sound fast in isolation, on a 300mm lens like the Canon EF 300mm f/2.8 L IS II USM, it most definitely is. Therein, an EF 300mm f/4L IS USM would be considered ‘slow’ by comparison, even though it’s still an amazingly useful lens.
What about zooms? Well for starters, the lens needs to have a constant maximum aperture to be considered ‘fast’. That means that the maximum available aperture won’t get smaller as the focal length is extended, for example falling from f/2.8 at the wide end of the zoom to f/3.5 at the long.
You can get ultra fast zoom models like Sigma’s 18-35mm f/1.8 DC HSM ART, but such wide apertures are rare, and the wider the maximum apertures, the bigger and heavier they get. f/2.8 is usually considered fast for a zoom, and that’s why you’ll find lenses like 14-24mm f/2.8, 24-70mm f/2.8 and 70-200mm f/2.8 to be in the bags of many working professionals and serious enthusiasts.
The benefits: More light means faster shutter speeds
The primary benefit of a fast lens is to let more light hit the sensor during an exposure, and more light means that faster shutter speeds can be used in a wider range of lighting conditions. So for example if you were shooting with a maximum f/5.6 at ISO 400 the shutter speed might be only 1/60sec, which depending on the focal length and other factors could be too slow to keep a subject sharp. But if you have a maximum aperture of f/2.8 at ISO 400 you’d get around 1/250sec, so this extra speed means you can capture moving subjects or shoot handheld with less motion blur.
The benefits: Creative low light photos
More light is also very useful for longer exposures. A great example of this is in low-light landscapes and astrophotography. Fast lenses mean you can expose the sensor to more light in a given period, so for instance if you want a starry sky as the backdrop to your landscape scene, but you need to keep the exposure under 20secs to stop the stars from blurring, a fast lens can help you do that. Fast wide-angle lenses like a 20mm f/1.8 or 14-24mm f/2.8 are good examples of models that help in these situations.
The benefits: Shallower depth-of-field
Many people think that a shallow depth-of-field is the defining characteristic of wide-aperture lenses, and in some ways it is. In reality though, depth-of-field is a byproduct of the lens aperture controlling the amount of light entering the camera. But there’s no doubting that with a fast lens you can use this to your advantage. The razor-thin depth-of-field created by, for example, an 85mm f/1.4 lens can keep portrait or nature subjects separated from cluttered backgrounds, while a 70-200mm f/2.8, 300mm f/2.8 or longer lens does the same for subjects further off like sports, action and wildlife.
The benefits: Faster focusing
In most camera and lens setups, focusing takes place with the lens at its maximum aperture setting, and the lens then stops down to your chosen aperture to make the exposure. Why? Simply because the greater the amount of light there is, the easier it is for the autofocus system to work. It’s like when you’re trying to read a book in dim lighting – it’s harder than in bright light. Therefore, pound-for-pound, faster lenses are more likely to lock on instead of hunting for the subject, and this makes them especially useful for sports, action and wildlife subjects.
This principle also means that fast lenses are more likely to work when fitted with a teleconverter. Teleconverters rob aperture stops – for example a 2x teleconverter will reduce an f/2.8 lens to an f/5.6 lens, so the wider the aperture you have at the outset, the better.
The drawbacks: The widest aperture isn’t always the best
Lenses with very wide maximum apertures can seem highly attractive, but be warned, you may not always get the sharpest results, or the best image quality at those settings. That’s not to say all lenses are created equal – of course they’re not. So some lenses will be excellent wide open, but others may be a compromise. However, pick a fast lens and what you’ll also find is that it’s still likely to outperform a slower lens at equivalent apertures. For instance, the f/1.8 setting on an f/1.2 lens is almost certainly going to be superior to f/1.8 when that’s the maximum aperture the lens allows.
The drawbacks: ND filters can be required
If you’re working in bright light, you may find that the widest apertures are unavailable to you, or will overexpose the subject. In these cases there’s simply too much light entering the camera for the shutter speed or ISO to compensate for. In these cases, fitting an ND filter will be needed, blocking out some of the light, and meaning that the widest apertures are once again available.
The drawbacks: Size, weight and price
Another indisputable factor is that faster lenses tend to be larger, heavier and more expensive than slower ones. That makes sense because of the greater level of engineering required to make them, and the larger glass elements required in their light path. And greater weight means you’ll get tired faster or not be able to take as much other kit out as you’d like. Fast telephoto lenses may need additional mounts or gimbals to help with this, especially if you’re using them for extended periods.
Fast lenses will also likely have larger front elements, meaning bigger filters are needed, so some used smaller slot-in filters at the rear instead. And some fast wide-angle lenses don’t allow screw-in filters to be used at all, so more complex and bespoke filter systems are needed.