The Myth of Fast Lenses

Since I took up photography full-time 3 years ago, I’m much more informed about equipment and techniques.  There are some well-rehearsed lines in this industry:  photography is about shaping the light; remember to work the shot; don’t take pictures – make pictures.  And on and on.

Many of those tomes are also around gear – usually put out by manufacturers I think.  As an example, fast lenses (those with a maximum aperture of f/2.8 or larger) are always better than other lenses.  Better for low light capture, better for managing depth of field, better for autofocus performance.  Always buy the fastest lens you can afford.

Whenever I venture to purchase a new lens, I’m typically presented with the fastest lens first – the advanced option.  But the more I’ve shot, the more I’ve come to question this equipment mantra.  Most of the time, you DON’T need the fastest lens.  Here’s why.

I started with an entry level full frame camera, and quickly realized that image quality was decent but not good enough.  Too soft at times, too noisy at times and missing some features like back-button focus that I really really like.  By that time, I had purchased two additional lenses, pro-grade, but not top of the line.  I decided to rent a better body and try my lenses on it.  The difference was amazing.  I was immediately convinced that the body plays a much bigger role in image quality when body and lenses are matched, but are not necessarily all at the same end of the price range.

I upgraded my old body for a new one (wish that were true of the flesh and bone variety as well!) and haven’t looked back since.  Here’s where you might ask:  if the better body made such a difference, don’t you think the better lenses would as well?

Over the past couple of years, I have borrowed or rented top of the line lenses to try with my body.  Surprisingly, I didn’t see the same quantum difference in image quality.  I had more of an eye-opening experience trying different types of lenses rather than different speeds of lenses.

The differences between primes and zooms can sometimes be astonishing in terms of image sharpness and quality.  And primes certainly force a different approach to shooting, which often results in more creative angles and composition.

The differences between wide angle and telephoto are equally revealing, permitting multiple views and contexts for the same subject.  Emphasis can be placed where you want it.  The background is also very interesting to play with, with wide angle lenses creating more of a separation than telephoto lenses (which tend to compress the scene).

But the practical differences between an f/4 maximum aperture and an f/2.8 or larger maximum aperture have been far more difficult for me to discern.

I rarely shoot in darkness, and when I do, it’s usually for the purpose of a long exposure of a static scene, which works equally well at 1, 2, 4 or 8 seconds.  If I do try to capture motion in low light, it’s usually with the intention of creating a trail or blur (such as in light painting), so again, I can work with longer exposures.  So there goes the low light argument.

The one darkness exception I’ve found is maybe astrophotography.  Here, a fast lens can be better, since the sky is continuously moving relative to our position on Earth.  If the shot involves night sky objects that are relatively faint (like the Milky Way or individual constellations, particularly in high light pollution areas like Toronto), a faster lens will gather more of the available light more quickly.  Having said that though, the noise reduction capabilities of my good camera make it possible to shoot at higher ISO settings, which amplify the available light, essentially achieving the same result.  So fast lenses are not absolutely necessary even here.

I don’t shoot portraits as a rule, and when I do, I try to ensure I have the light I need (either natural or flash).  I also rarely completely blur the background (since I prefer shooting environmental portraits) but if I need to, I have three lenses that can take care of the problem – a 50 mm f/1.8 prime, a 100 mm f/2.8 macro (yes, you can use this lens for portraits) and a 70-200mm f/2.8 zoom.  But given my preference for showing the background as context, there goes the depth of field argument.

The other area where depth of field is the main consideration is macro photography.  As noted, I have a 100 mm f/2.8 macro.  Having a fast macro is actually of no help at all for my macro shots, even though few lenses are offered at slower speeds.  The argument here is that with the lens so close to the subject, a fast lens lets in the maximum amount of available light (and may help freeze those moving insects through a faster shutter speed).  But if depth of field is so compromised that you can’t ever use the lens wide open, what’s the point?  When needed. I add supplemental lighting to my macro shots by using a ring light that sits on the front of the lens.  I also use focus stacking to maximize the depth of field at all aperture settings.  So again, there goes the depth of field argument for a fast lens.

The biggest benefit for me in a fast lens has come when shooting events.  I admit here that lower ISO, the ability to isolate subject from background, and a faster shutter speed can indeed be beneficial.  So I do use the 3 lenses mentioned above for events.  But I’ve shot an equal number of events with an f/4 kit lens, and they’ve turned out just fine.

What about sports and wildlife?  I have never shot wide open to compensate for light in sports and wildlife.  Some isolating shots might benefit from a blurred background, but for me the surroundings and context are typically important.   A fast maximum aperture supposedly helps with freezing action, both in terms of autofocus and detail.  I agree it can be helpful, but with practice and good anticipation to prepare me to take the shot, and with a higher ISO to give me a faster shutter speed, I do just fine.  So, again, there goes the motion argument for a faster lens (also see autofocus below).

So the only question left is the question of autofocus speed and accuracy, important to all types of photography, but especially anything involving motion.

Lenses matched to their cameras, regardless of price point, should do a decent job of finding focus.  It may be a bit slower, it may struggle a bit in lower light, but for most shooting, autofocus should work just fine.  If it didn’t, camera manufacturers would quickly discover they couldn’t sell the camera or the lenses.   But fast, precise autofocus is usually what is advertised and what sells a larger maximum aperture lens.

My camera manual advertises “high prescision” autofocus at the viewfinder centre with f/2.8 or larger maximum aperture lenses.  But the autofocus sensor also includes “high precision” sensors for f/4 or larger maximum aperture lenses, as well as sensors for f/5.6 and f/8 or larger maximum aperture lenses.  Better camera, better coverage.

An f/2.8 maximum aperture lens will let in twice as much ambient light as an f/4 maximum aperture lens, supposedly improving autofocus speed and accuracy.  That sounds like a meaningful stat, but I’ve rarely had trouble getting focus with my “slow” lenses (because of the better camera), and when I do, I switch to manual focus.  It’s not that hard to focus manually in most conditions (except fast motion).  You might also be interested in knowing that fast lenses larger than f/2.8 can introduce so many challenges with depth of field that autofocus speed and accuracy are reduced, not improved (but no retailer or manufacturer will tell you that).

And there you have it.  There is one twist to this story:  one feature I won’t go without in any new lens I now buy.  That is image stabilization.  Unrelated to the speed of the lens, it is a feature of higher-end lenses that corrects for the microvibrations of hand-holding a camera.   This feature can add 3 to 5 effective stops to your camera, partially compensating for any slow shutter speed that might be in play because of your “slow” lenses.  It won’t, of course, compensate for a moving subject.  But it is a life-saver for me and the shaky hands of my advancing years.

So, to summarize, pick the lenses you need for the type of shooting you do and don’t be mislead into thinking you always need “fast” lenses.  In my view, that is just a myth aimed at those with the money to indulge it.  Always match the camera and the lenses, but budget appropriately for your shooting interests.

And always keep in mind that a good photograph isn’t about the gear – it’s about the photographer.  Last year’s winner of the Wildlife Photographer of the Year (a world-wide competition sponsored by the British Natural History Museum) was taken by Tim Laman with a Go-Pro (see image above).