Fast Prime Lenses by J. Dennis Thomas
There’s been resurgence in popularity of the fast prime lenses. Perhaps it’s due to the recent release of ultra high-resolution cameras such as the Nikon D800 and the Canon 1Ds MkIII that have the ability to out resolve many of the current crop of zoom lenses, so photographers are looking for the sharpest tool in the box, and when people think of sharp lenses thoughts inevitably turn to primes.
First things first, when I use the term fast prime I’m referring to lenses faster than ƒ/2.8. While ƒ/2.8 is fast in a zoom lens, I personally don’t consider ƒ/2.8 fast when it comes to prime lenses. There are plenty of reasons why people may buy and use ƒ/2.8 prime lenses, such as their compact size and good image quality for low prices (although Canon’s 24 & 28mm ƒ/2.8 IS lenses are quite pricey), but in my opinion a good ƒ/2.8 pro zoom beats out an ƒ/2.8 prime lens with versatility any day.
Please note that this isn’t a buying guide or a lens shootout comparing which lens is sharper and has better bokeh than the other, or a discussion on which brand has the best fast primes…
Now that we got that out of the way, what I really wanted to talk about is the numbers; what they mean, what you gain from lenses with larger apertures, the benefits of lenses with smaller apertures and whether it’s worth the extra money to get the fastest lens that you can.
For the uninitiated let’s take a look at what an aperture is; the aperture, simply put, is the opening of the lens through which light travels. A term that is used interchangeably with aperture is ƒ/stop and the size of the lens opening or aperture is referred to as the ƒ/number. The ƒ/numbers are derived from an elementary mathematical equation, N = ƒ / D, wherein N is the ƒ/number, ƒ is the focal length of the lens and D is the diameter of the aperture opening. Broken down into simpler terms, the focal length of the lens (ƒ), divided by the diameter of the aperture opening (D) gives you the ƒ/number (N). For example if the focal length of the lens is 50mm and the lens opening is 25mm the ƒ/number of the lens is ƒ/2. Since the f/number is directly proportional to the focal length divided by diameter, the f/number lets in a constant amount of light no matter the focal length.
The aperture numbering system seems a bit backwards to some people when first exploring the concept, the larger the aperture the smaller the number and vice-versa, but when you stop to take a look at the equation you see that the f/number is really a fraction; ½ is greater than ¼ therefore f/2 is larger than f/4. Pretty simple actually.
There are a pretty impressive number of fast lenses available for photographers today and it seems that the manufacturers are taking notice of the new popularity in primes because the both of the leading manufacturers (Canon and Nikon) as well as a very ambitious third party company (Sigma) have been revamping their fast prime lens lineup with some excellent offerings.
Traditionally fast lenses were offered back in the early film days for news and reportage photographers so that they could have the ability to capture more light with their lenses in order to get faster shutter speeds to freeze movement and action while covering breaking news. Remember that before digital cameras you had one single ISO setting per roll of film and film faster than ISO 400 wasn’t readily available before 1954 and ISO 800 film wasn’t available until 1960. So to get a usable photo you needed a fast shutter speed, which required a lens with a wide aperture, hence the term fast lens was coined.
The fastest current production lens is the Leica Noctilux ƒ/0.95. This is a specialty lens made for Leica M-mount cameras and runs approximately $11-15K so, we’ll just skip over that one (the fastest lens ever made was a Zeiss 50mm ƒ/0.7commisioned by NASA to shoot the dark side of the moon and later used by Stanley Kubrick to film a scene for Barry Lyndon using only candlelight).
The largest aperture lens available today that is reasonably priced (read sub $10K) is the Canon 50mm f/1. These were discontinued over a decade ago and are about as rare as hen’s teeth these days, but you can sometimes find one on eBay for $4-5K. For the more frugal fast aperture photographers fortunately we have no shortage of options in production today. Starting on the faster side we have the ƒ/1.2 and on the slower side we have ƒ/2. The options break down to (with no regards to focal length) from fast to slow; ƒ/1.2, ƒ/1.4, ƒ/1.8, and ƒ/2. So how do we benefit from a fast prime in low light situations over a fast ƒ/2.8 zoom? A ƒ/2 lens is one stop faster than ƒ/2.8, ƒ/1.8 gains you a stop and a third, ƒ/1.4 gives you two stops, ƒ/1.2 delivers two and one third stops, and a lens with an aperture ƒ/1 gives three full stops of light over your standard ƒ/2.8 zoom lens. Although on paper the numbers may not look like much, in practicality there’s a pretty big difference between ƒ/2.8 and ƒ/1.2. A lens with an aperture of f/1.2 captures an impressive 6X the amount of light than a lens does at f/2.8!
Now that we’ve established a frame of reference to how much light is captured according to the ƒ/numbering system what does it mean to us as photographers in practical terms? Photographers use fast primes these days for three distinct reasons; to gain a faster shutter speed without increasing the ISO sensitivity, to shoot at a lower ISO sensitivity setting in low-light situations to decrease the amount of inherent noise in the images, and finally to gain the ability to achieve an extremely shallow Depth of Field for artistic purposes.
In a scene lit by a single standard 75W tungsten light bulb a typical exposure at ƒ/2.8 is about ISO 3200 for 1/30. Excluding the artistic principles of using a wide aperture and just focusing on the practical settings (no pun intended), using a fast zoom at this exposure would be OK for a static subject using a relatively wide setting as long as you didn’t mind an image with a little noise. Using a fast lens and opening up the aperture yields new possibilities either for freezing motion or reducing noise and capturing more fine detail.
The following table shows the dramatic increase in shutter speed when using a fast lens. Most current cameras deliver relatively clean images up to ISO 3200. While at ƒ/2.8 you would need to be shooting a subject that is pretty much static, but at ƒ/1.4 you have a shutter speed fast enough to freeze the motion of most typical subjects outside of sports.
|ƒ/number||Shutter Speed||ISO Sensitivity|
If you’re shooting a relatively static subject and your concern is more on capturing images with little noise and more fine detail, a fast aperture lens has great practical use as well. Assuming you are planning on hand-holding your camera and you are using a marginally wide to standard focal length of 35 mm I chose a base shutter speed of 1/30. Using an ISO of 800 at ƒ/1.4 will dramatically increase the image quality over the same image shot using ISO 3200 at ƒ/2.8.
|ƒ/number||Shutter Speed||ISO sensitivity|
*Note that currently there are no DSLR lenses in the 35mm focal length at ƒ/1.2
For the artistic purposes of shallow Depth of Field (DoF), the plane of focus becomes increasingly thin at wider apertures. Since that background blur is the main criteria we are going to assume a relatively close focus of about 3 feet. The DoF also changes with format so distances are listed full-frame/APS-C. At further focus distances the DoF is increased. To help you calculate the DoF with just about any lens at any focus distance using any aperture check out http://www.dofmaster.com/
|ƒ/number||Near limit||Far limit||Total DoF|
|ƒ/2.8||2.91/2.94 ft.||3.09/3.06 ft.||0.18/0.12 ft. (2.16/1.44 in.)|
|ƒ/2||2.94/2.96 ft.||3.06/3.04 ft.||0.12/0.08 ft. (1.44/0.96 in.)|
|ƒ/1.8||2.95/2.96 ft.||3.06/3.04 ft.||0.11/0.07 ft. (1.32/0.84 in.)|
|ƒ/1.4||2.96/2.97 ft.||3.04/3.03 ft.||0.09/0.06 ft. (1.08/0.72 in.)|
|ƒ/1.2||2.96/2.98 ft.||3.04/3.02 ft.||0.07/0.05 ft. (0.84/0.6 in.)|
Truth be told, once you start fooling with shallow DoF the tendency is to push it to the extremes, which often leads to images that have focus problems. Often people want to shoot portraits wide open and end up with tack sharp eyes, but with a fuzzy nose and ears. Sometimes the plane focus is too thin causing the subject to be partially out of focus in the foreground causing the shallow DoF to detract from rather than add to the image. If you’re buying a lens exclusively to be used for shallow depth perhaps you may want think about buying a lens with a smaller maximum aperture to avoid the trap of having not enough depth in your photos (and saving a few bucks in the process).
So what is the bottom line here? Excluding the topic of bokeh and sharpness, which are too subjective for one person to judge, and assuming the point in buying a fast lens is to shoot wide open, when I crunch the numbers, I draw the following conclusions. The ƒ/1.2 aperture is simply not enough of a step up from ƒ/1.4 to be considered a necessity. 1/3 of a stop isn’t going to make or break your image. Stepping up from a ƒ/2.8 lens to a ƒ/1.8 or ƒ/1.4 lens is definitely going to have a major impact on your images. Faster shutter speeds, lower ISO’s, and much shallower DoF are all good things.
When it comes down to making a decision between choosing between ƒ/1.8 and ƒ/1.4 the lines get drawn pretty quickly. The difference in shutter speed and ISO is a definite improvement, but might not be a complete deal-breaker. The DoF factor is noticeable, but less dramatic. The biggest differences between the ƒ/1.8 and ƒ/1.4 lenses aren’t about photography at all. The main differences between these two types of lenses are that ƒ/1.8 lenses are built for the consumer while ƒ/1.4 lenses are built for the pro. The ƒ/1.8 lenses are smaller, lighter, and much more inexpensive than their faster ƒ/1.4 counterparts. In my opinion if you want more bang for the buck go with the ƒ/1.8 lenses, the difference in speed and DoF field is negligible in comparison to the price differences. That being said, all of the fast primes I use for assignments are of the ƒ/1.4 variety. Why? It’s simple; I need the build quality. The pro lenses are built to take the rigors of everyday shooting whereas the ƒ/1.8 lenses should be treated a little more gently.
In the end of course the choice is yours, but going into the market knowing more about the science and practical applications of a fast lens will help you make a more informed decision.