Lenses and Optics

Fast Lenses: A Primer

Published August 15, 2009

by Stephen Michael Garey

What is a Fast Lens?

A fast lens is any lens of any type and focal length that has an aperture design capable of opening to f/2.8 or wider. There are fast lenses currently available that open as wide as f/1.2, and there are any number of popular lenses that open to f/1.4 and f/1.8. One of the fastest lenses ever produced opened to an amazing f/0.7. It was custom-manufactured by Zeiss and used by director Stanley Kubrick in his filming of the very dark, candlelight scenes in his period-piece “Barry Lyndon.” Other lenses approaching this speed and quality have been produced through the years by Canon, Schneider, Nikon, Rodenstock and others. Until recently, Canon made a 50mm f/1.0, which they replaced with the current 50mm f/1.2.

Ok, now that we’ve gotten the bottom-line definition out of the way, let’s move on to the things that make so-called “fast lenses” so costly, so sought after, so misunderstood, and so both over-estimated and under-estimated in terms of what they actually bring to the photographic process.

Fast Shutter Speeds

This is the reason they’re called “fast” lenses. Actually, there’s nothing particularly fast about the mechanical parts of the lens itself. No, what a fast lens does is deliver more light to the film or sensor plane, therefore enabling faster shutter speeds given a particular camera’s (or film’s) ASA/ISO setting; hence the term fast lens.

Increasing shutter speed, of course, isn’t a desirable thing all by itself. There’s really no good reason to increase shutter speed just for the sake of increasing shutter speed. Shooting with a 50mm lens, 1/100 will do just about the same thing as 1/125 or even 1/250 if we’re talking about hand-held photography and if the subject and background are relatively stationery for the fraction of a second needed to grab the shot. Of course, we also have to factor in the aperture setting and make sure it’s appropriate to the shutter speed required (or desired) and to the scene itself.

For arguments sake and to simplify things, let’s say that the aperture we want is f/8 with a shutter speed of 1/160. But let’s also say that, at f/8 and 1/160, the shot will be quite underexposed. The first thing you might want to do is slow down the shutter speed to let in more light. But you can only slow it down so far before camera shake and/or subject blur (or both) become problematic. So you make an executive decision to keep the shutter speed at a minimum of 1/80 and, at the same time, you open your lens to its maximum of f/4. But even at f/4, you find you’re still underexposed by a couple of stops. So now you decide to crank up your DSLR’s ISO to the sensitivity that enables proper exposure at 1/80 and your selected aperture of f/4.

All is well now because, after all, that’s what high ISOs are for, aren’t they? But, as we all know, you’re now running the risk of increasing the noise in the image to perhaps obtrusive levels, depending on your level of tolerance for digital camera noise and depending on the camera itself. (If it’s the likes of a Nikon D700, Nikon D300, Canon 5DMkII or Canon 1DsMkIII, the risk is considerably less than with the greater majority of cameras out there. Most, as good as they are, can introduce shadow noise as low as ISO 400.)

Here’s where fast lenses come in.

As numbers, the difference between f/2.8 and f/4 might not seem like much, but it’s enough to make all the difference in the world to your photographic outcome. f/2.8 lets in twice as much light as f/4, allowing a lower ISO setting (reducing the risk of noise) and providing you with the shutter speed you need (or desire) to keep things from blurring.

The trade-off is depth of field; the wider the aperture, the more there will be background and foreground softness. The closer you are to your subject, the more severe the background and foreground softness will be. In some cases, and with the fastest lenses, this can mean that a mere fraction of an inch out of the entire scene is actually in focus. For some photographers, and with some types of photography, this shallow depth of field is one of the added bonuses of fast lenses, i.e. it can be very useful in achieving a certain photographic effect. In fact, Depth of Field (called DOF for short) is topic unto itself, regardless of lens used. You’ll find it discussed in-depth (no pun intended) at our article Controlling Depth of Field

For now, let’s just say that—all things being equal—a sharp, clear, bright, in-focus shot of the main subject is better than a not-so-sharp, dark, noisy, not-so-clear shot of the main subject and the background and the foreground.

All of the above is, of course, an extremely simplistic view of what fast lenses are primarily about. There’s more to fast lenses than this dumbed-down scenario … and more to know about fast lenses in relationship to focal length, a lens’ “sweet spot” and other fast lens topics … not to mention the contradictions that always seem to creep in whenever someone is talking about things photographic.

When f/5.6 is considered fast.

So let’s start with a good old contradiction. I started at the outset with defining a fast lens as any lens of any type and focal length that’s designed for an aperture of f/2.8 or wider. But, there are always exceptions and, in certain photographic circles, f/5/6 and f/4 are plenty fast enough for the job that needs to be done … and I’m speaking of wildlife here and/or other kinds of telephoto photography, including sports photography, which are most often shot in decent to very good to excellent light and at great distances from the subject, requiring a lens with a focal length of at least 400mm (usually longer) and sometimes as long as 800mm or more.

Well, an f/2.8 lens at 800mm would be the behemoth of behemoths … a lens so large, wide and heavy, it certainly couldn’t be used hand-held … and it certainly would take a tripod of equally behemoth proportions to keep it stable. So, with an 800mm lens, f/5.6 is regarded as quite a fast lens. In the case of 600mm lenses, of which there are several, f/4 is fast. At 400mm, we’re back to several possibilities at f/2.8, the usual standard for the fast lens category.

So here’s a general guideline to consider: if a lens is shorter than 400mm, it shouldn’t be regarded as fast if its aperture doesn’t open to f/2.8. Which means that Canon’s 300mm f/4L IS, while certainly faster than an f/5.6, isn’t really a fast lens per se. But, hey, it does have Image Stabilization, which does make up for its “slower speed,” doesn’t it? Not really (more about this in a bit).

So where does my f/2.8-f/4 lens stand

That’s a very good question, and one that demands a good answer. An f/2.8 – f/4 is called a “variable aperture” lens, one of the most common and most successful of all lens designs. Variable aperture lenses are zoom lenses that can be found in a wide variety of focal length and aperture combinations such as 24-135mm at f/3.5-f/4.5, 70-200mm at f4-f/5.6, 28-300mm at f/3.5 to f/5.6, etc., etc., etc. The meaning of “variable” here is that the lens automatically changes its widest aperture setting (closes it down actually) as its focal length increases.

There are several lenses made today that start at f/2.8 at their wide end but end up as f/4 lenses (or smaller) at their long end. But because they have that magical f/2.8 as part of their specification, many regard these lenses as fast. Well, I guess they are if you stay at their widest angle and never go beyond. But surely that defeats the whole purpose of having that lens as a zoom. The problem is, as soon as you get only a few millimeters beyond wide, the lens is no longer fast. And quite often enough, the change comes rather abruptly, meaning you’re at f/3.5 or so just 2-to-4mm from the lens’ shortest focal length. So, as good as these lenses are … and many are very good indeed … I for one do not bring them into the fast lens category. Remember, in certain situation, f/2.8 gets you the shot while f/4 doesn’t.

Speed is one thing. Image quality is another.

There is a widespread belief that, in terms of image quality, a fast lens is almost always better than a slower lens, i.e. sharper, more accurate color rendition, higher contrast, less distortion, etc. One of the reasons for this belief is the cost of a fast lens, which is almost always more expensive than a slower counterpart. One of Canon’s best lenses, the venerable 70-200 f/2.8L IS, is currently over $1,800. Nikon’s 17-35 f/2.8 is also in that price range, and there are many others similarly priced or higher. One of the most expensive is Sigma’s 200-500 f/2.8: nearly $30,000.

What generally supports if not justifies these prices is:
1) It’s very costly to make glass of this type and size
2) These lenses have great build quality as well
3) They do deliver terrific optical performance, even wide open

But it’s really a mistake to assume that you can’t get similar or even better image quality in a slower, less expensive lens. Here’s where the mistake lies:

A fast lens is designed to deliver quality performance at a very wide aperture, i.e. f/2.8 or wider. Because it performs well at these wide apertures, it can do even better when stopped down one stop or two, say to f/4. In fact, some of the Zeiss manual focus primes really don’t get any better beyond f/4, meaning they reach their “sweet spot” rather quickly and remain at that performance level even at f/8 to f/11.

Slower lenses, on the other hand, aren’t designed for best performance wide open. If the lens is an f/3.5-f/5.6, it probably doesn’t deliver really good border-to-border image quality until at least f/5.6. It then gets progressively better as you stop down further, with its best performance at f/8 or so.

However, its performance at f/8 could very well match or even exceed the performance of a faster optic set at the same aperture! The main difference between these two lenses is that the fast lens reaches good-to-excellent quality sooner than the slow lens, and with the slow lens there’s no super-fast wide open aperture option at all.

However, at f/8 … and all else being equal … the two lenses could very well deliver similar quality. A good example is Nikon’s 18-55 VR kit lens, an f/3.5-f/5.6 lens. It costs around $185, but the price isn’t at all indicative of its image quality. Its performance at f/8 is rather astonishing and is nearly indistinguishable from several lenses costing $1,000 or more. The build of this lens leaves an awful lot to be desired, and the performance at f/3.5 isn’t something to write home about. But stopped down: wow, an amazing lens, which compares favorably to my Zeiss 25mm f/2.8 … except that my Zeiss delivers the goods at f/2.8! You get the point.

Enter Image Stabilization/Vibration Reduction/Vibration Control.

These are relatively new technological advancements in SLR lens and camera design. Canon was first with its Image Stabilized 28-135mm lens in the late 1990s.

Whether designed in-lens or in-camera, the idea behind stabilization is the same: to effectively counter hand shake and other types of physical movement of body and camera when shooting hand-held.

And effective it is!

These mechanical/electronic control additions allow one to shoot at shutter speeds as much as three and four stops below what would normally be required to prevent hand/body-movement-induced blurring of the scene or subject. So, for example, if you’re shooting at 100mm, then 1/100 is really the minimum shutter speed for preventing blurring due to physical shaking. An Image Stabilized (Canon’s term) lens allows the same capture … sharp and crisp … at a speed as low as 1/30.

“So … wait a minute,” you ask, “if that’s true [and it is], that means I can take sharp shots in low light, without a flash, at shutter speeds that would otherwise be too slow to deliver critical sharpness. So I don’t really need a fast lens to bring more light in, correct?”

Correct. But you still need f/2.8 or wider, so you can increase the shutter speed in order to stop action. Thanks to the slower shooting speed, shooting at 1/30 with a stabilized lens or camera will give you the proper exposure, but if someone in the scene moves, or the wind blows leaves, or the cat yawns, you’re going to see motion blur in your shot … not from hand-holding the camera but from something or someone in the scene that moved.

An f/2.8 lens, combined with stabilization, lets you increase the shutter speed to 1/60 or higher, significantly reducing the risk of motion blur.

One more thing.

Actually, three more things:

There are some very fast lenses out there that—while they allow the necessary light to reach the film or sensor plane and thereby let you increase the shutter speed—do not at all perform well at their widest settings. Speed doesn’t necessarily equal image quality. Some of these fast lenses actually don’t reach their potential until f/5.6 or smaller, completely negating their value as “fast lenses.” Don’t let yourself gravitate towards a fast lens just because it’s fast or because it’s fast and reasonably priced. Check the reviews, read the test conclusions, ask your dealer if the lens is returnable.

Second: fast is good, but great fast is really expensive. Decide how much speed you actually need before you plunk down $1,800 for a Canon 24mm f/1.4L Mark II or similar. Do you really need f/1.4? If you’re a wedding photographer you might. If you’re a landscape photographer, probably not.

Review your needs and your frequency of needing wider apertures. Maybe f/2.8 is all you need. Maybe f/4 is really the widest you need to go. Case in point: I sold my Canon 24-70 f/2.8L (great lens) in favor of a Canon 24-105L IS f/4. First, I wanted the longer reach of the 24-105. Second, I wanted Image Stabilization. Third, the vast majority of my work is at f/8 to f/11. I require fast apertures only occasionally, and for that specific need I carry around a Canon 35mm f/2 for my Canon cameras and a Zeiss 25mm f/2.8 for my Nikons.

Finally, two often unacknowledged advantages of fast lenses are:

1) When you’re looking through the viewfinder … particularly if your camera is a crop sensor camera with a small and somewhat dim viewfinder … fast lenses make your subject or scene brighter and therefore easier to see. A lens that starts at f/5.6 is considerably darker through the viewfinder than a lens that starts at f/2.8. A lens that opens up as wide as f/1.2 really shows you the scene you’re shooting in all its glory.

2) The auto-focus points in the major brands of digital cameras … particularly professional models such as the Canon 1-Series … are designed with different levels of sensitivity for fast lenses and slower lenses. With most DSLRs, an f/5.6 lens will utilize fewer AF sensors than an f/4 or f/2.8. An f/4 lens, while it may activate more sensors than an f/5.6, will still activate fewer sensors than an f/2.8 or wider lens. The fact is, those “cross-type” auto-focus points that increase AF speed and accuracy (and might make one camera more attractive to you than another) are generally only functional with the f/2.8 primes and zooms. Bottom line: if you’re not using a fast lens, the cross-type sensors perform absolutely no autofocus function at all.

So, carefully evaluate your needs, understand what fast lenses are designed to do and how they do it, and you’ll end up with the best possible fast lens at your side … ready whenever you need it and whatever you need it for.

—Stephen Michael Garey

About the writer: Stephen Michael Garey spent the majority of his creative career in the advertising and design industries, where he served some of the world’s best-known companies, including Disney, America Online, General Electric, CBS News, Apple Computer, IBM and Honda. Over the years, his creative work has earned numerous national and international awards for excellence. Stephen turned his attention to fulltime freelance photography in 2002. You can see some of his work here

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Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Posted in Lenses and Optics
  • Mickey Peters

    I have been teaching photography for 40 years. The term “fast lens” is so confusing to anyone trying to learn photography. It has a larger aperture, it allows a faster shutter speed, but the lens is NOT fast or slow. Yes, if you read through your long and laborious article it finally says that, so why start the confusion in the first place?

  • It?s really a nice and helpful piece of info. I?m satisfied that you shared this helpful info with us. Please keep us up to date like this. Thanks for sharing.

  • John Suzenski

    Dear Stephen,
    Any suggestions for outdoor night arena lighting situations? I took a few pictures of some rodeo action for a friend. During the day performance the shots were great…after the lights came on there was a lot of motion blur. I am using the Nikon D5000 and a Tamron 70 -200mm f4 lens. I tried a f 2.8 rental lens but still had a motion problem with some shots. Lighting conditions could best be described as fair to moderate from the stadium lighting….
    John S

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