In the great swirling cauldron that is the internet, one of the leading contributors to the turbulence has to be photo gear-related commentary (I’m aware of the irony. Shut up.), particularly on the various and sundry discussion forums. Yet when it comes to lenses, there seems to be a rare level of consensus on a particular point: fast is always better than slow. Even Your Humble Filmosaur succumbed to that notion at one time, having swallowed the Kool-aid that bigger maximum apertures make your breath fresher, your clothes cleaner, and most importantly, your pictures better.
I now know that this idea is utter nonsense. Of course, fast lenses have their uses – nighttime available light photography and such – but beyond those specific realms, it’s just not true. For day-to-day photography, a very strong case can be made for slow lenses, yet the myth of the superiority of speed persists. Like every other gear-related fallacy about the direct and binding link between equipment and quality, this one is based purely upon a false assumption. We can debunk this notion on two separate fronts: aesthetic and technical.
First the technical. Put simply, the faster the lens, the more tricks the optical engineers have to use to get the light to bend the way they want it to. In order to do this, they need more elements. Back in the good old days, a fast (f/2 or faster) lens would be likely to have six or seven elements; now, with the advent of more advanced coatings and computer-aided design, element counts have more than doubled in many modern lenses. The simple fact is that light prefers to travel in straight lines, and the more you have to twist it off that path, the more likelihood there is for distortion. A slower lens, on the other hand, usually requires fewer elements and often has lower distortion because it allows the light to move in something closer to a direct linear path from front to back. True, modern technology has helped to lower distortion to very low levels, but a reduction of technical flaws alone does not equate to superiority.
Then there’s fact that very few people shoot at or near wide-open all the time. Once you get into the mid-apertures (say f/5.6 to f/11 or so) you’d be hard-pressed to find many lenses that can’t produce low-distortion photos with at least adequate contrast. If you’re shooting in or around these apertures the vast majority of the time, and you’re not under contract to produce available light photos in all conditions for someone else in order to feed yourself, exactly what advantage is a faster lens giving you?
This brings us to aesthetics. If high resolution and sharpness are important to you (and I’m not suggesting that they have to be, but reading the aforementioned forums, one could easily be led to believe that slightly unsharp lenses are a direct cause of bad posture, pestilence, and certain persistent social diseases), a slower lens (especially if you prefer vintage glass) will often be sharper across its range than a faster lens. Why do you think the humble four-element Tessar design has been around for so long and been so successful? Good sharpness, resolution, and contrast, with low distortion.
What if you don’t care about sharpness? Well, there are plenty of other things that define the optical qualities of a lens, but few if any of them have anything to do with lens speed. True, a fast lens can provide very shallow depth-of-field, but unless you’re shooting portraits a lot, this is a party trick, not a major functional selling point, and even if you are doing portraits, we’re talking about a relatively narrow range of focal lengths that are longer than what most people consider general-purpose.
If all things were equal, then sure, a faster lens would offer additional capabilities at no cost. But things are most assuredly not equal. Fast lenses are generally expensive compared with their slower brethren in a given focal length and range of quality. Even if you don’t mind paying that premium, there’s another price to pay, and it doesn’t go away: size. Fast lenses are bigger and heavier than their slower cousins, and you’ll be reminded of this every time you decide to carry one out. Even if you obviate the weight penalty by shifting to modern plastic lenses, you’re still saddled with the size and weight of the necessarily bigger glass. Why pay for and carry around a 50/1.4 Summilux when a smaller, lighter, simpler, and cheaper 50/3.5 Elmar will give you similar results 98% of the time?
Ultimately, anyone who cares about this enough to still be reading should be asking themselves just what kind of speed they need in a lens, then trying to match those requirements with appropriate equipment. For many, most of their photographic needs will be easily filled by relatively slow lenses. While I have a few relatively fast lenses, they get considerably less use than the slower ones. It’s just that it’s much easier to carry a camera with a lightweight compact lens on a regular basis (you’ve all heard the old saying that the best camera is the one you have with you), and it’s truly the rare instance in day-to-day photography that I find the need for apertures larger than f/5.6 or so, so why bother with the big, heavy fast lenses when smaller and lighter slow lenses will do just as well?