Do We Still Need Fast Lenses?

The combination of better sensors and more ubiquitous VR has a lot of folk wondering about that question. The answer, as usual, is much more nuanced than an outright yes or no. 

Lenses involve what I call primary decisions. By that, I mean decisions that directly impact what your photograph will look like. In particular, the lens controls two primary decisions: focal length (angle of view) and aperture (depth of field). "Fast" doesn't play into that first decision, but it might in the second.

Image sensors tend to be a bit more of a secondary decision. This isn't clearcut, but if you're a landscape shooter, the difference between a D810 and its 36mp sensor and the D850 and its 45mp sensor is likely not significant. You'd have to be printing pretty darned big to make the D850 a clear choice over the D810. 

Buried within that sensor decision, though, may be something that impacts your primary decision (aperture, in this case). Again, using the D810 and D850 as examples, the D850 sensor tolerates high ISO pushes better than the D810. It's not just dynamic range I'm talking about here, but other aspects, such as a magenta cast that the D810 will produce at a lower ISO value than the D850. 

So let's delve into why people ask the question in the headline: (1) they believe sensors are good enough that they can boost ISO to higher values; and (2) they believe that VR (IS) is good enough that they can use slower shutter speeds. Both of those things apply to low light.

Thus, the question really needs to be framed "do we still need fast lenses for low light?" 

The answer is based upon your tolerances for #1 (high ISO use) and your subject motion for #2 (shutter speed).

Higher ISO values have lower dynamic range, period. The question is how much lower. The general rule of thumb is about a stop for every stop. In other words, if you push ISO by a stop, you lose a stop of dynamic range. We all have a tolerance point at which the loss of dynamic range is too much, that the blocking up of tonal values becomes a problem (as does noise). Generally, I find that if I don't have 7 stops of dynamic range in my capture, I start to get very unhappy and spend a lot more time trying to "save" the image. That's very near ISO 3200 on the D850, it's a but less than ISO 2500 on a D810.  Or, about a third of a stop. Which happens to be the difference between an f/1.8 and f/2 lens, if I'm reduced to shooting wide open.

What most people don't figure, however, is #2: subject motion. My rule of thumb with relatively static subjects (e.g. people standing) is that I want to be at 1/60 or faster. At 1/30 and slower, any motion by the subject starts to be problematic. That includes blinking, sudden smiling, head nods, hand movement, and much more. VR (IS) does not stop subject motion. 

So the way I look at low light and what lens I need is this: 

  • Use my maximum tolerable ISO (3200 on a D850)
  • Use my minimum tolerable shutter speed (1/60 for people)
  • What aperture can I use for the light available?

That last bullet will answer your question. Curiously, today in my unlit office (ambient light through windows and from computer only), that turns out to be ISO 3200 at 1/60 and f/4. Do I need an f/2.8 zoom? Nope. Not unless I need the depth of field isolation it provides. 

And that's where a lot of people are finding themselves these days. The tolerable high ISO values coupled with VR (IS) for more static subjects doesn't have them requiring an f/1.4 lens. Thus, they start the dialog that asks "do we still need fast lenses?"

As with everything photographic, the answer is contextual. What are you trying to do, and can what you've chosen do that? 

I have found in recent years that I've gravitated to a few slower lenses in some of my work, as I just don't need the extra aperture. For example, I'm not at all concerned that the 19mm PC-E I've been using for landscapes lately is an f/4 lens. I don't at all mind the 300mm f/4E PF for shooting daylight wildlife. For outdoor day sports, I'm often using f/4 or even f/5.6 lenses just to stay light and nimble. 

Of course, I can point out plenty of situations where I do need a faster lens to stay within my tolerance levels (for ISO and motion, as outlined above). 

For more on this subject, check out my f/2.8 or f/4 article. 

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