Single Aperture vs. Variable Aperture Lenses


The topic of single aperture lenses (e.g., f/2.8) versus variable aperture lenses (e.g., f/4 to f/5.6) comes up often when I talk with people about photography as the conversation inevitably moves in the direction of gear.

Single aperture lenses tend to be a difficult concept for people to wrap their heads around because a Nikon 70-200mm f/2.8 telephoto lens can cost more than double the that of a camera body and as much as 6 times the cost of a variable aperture lens such as a Nikon 55-300mm f/4.5-5.6. Here’s why:

Lenses Do One Thing - Cameras Do A Lot

Lenses collect light... that’s it!

Digital cameras are small computers, and have many more functions than a lens. Capturing an image is only one aspect of a digital camera, albeit the most important aspect, but all too often people get caught up in all the other things cameras do. They tend to minimize the image capturing ability of a camera and focus more on the "bells and whistles". 

In fact, the more simple a camera the more likely you will capture good images. You won't be distracted by all the settings and functions in a complicated menu structure. And the most important aspect of photography, besides good glass, is learning the exposure triangle.

Camera Functions In Order Of Importance

  1. Light gathering ability [quality of the lens (a.k.a. glass)].  This is not a camera function, but it is the most important aspect of photography, so I made it number one.
  2. Image capturing ability - size and quality of the sensor. It's ISO capabilities and megapixels. 
  3. Accurate focus
  4. Accurate light metering
  5. Ability to shoot in RAW format
  6. Everything else

"Everything else" it seems is what the majority of people pay attention to. Beaming your photos to your computer through WiFi, or having the ability to create HDR images, apply noise reduction, and adjusting white balance can all be done in post with programs such as LightRoom, Apple Photos, DxO, etc., especially if you shoot in RAW format.

I think the primary reason people would rather spend money on a camera body instead of glass is the perceived price per function. It seems easier to justify spending $2000 on a camera when there are more than 200 different settings, functions, and other photo processing and editing features.  Whereas, spending that same amount on a lens is hard to wrap your head around. That is, if you don't understand the importance of light gathering ability.

Consider these two lenses.:

On its face they both appear to do the same thing, so the vast majority of people would think you are crazy for spending close to 6 times the money for the lens on the left when compared with the lens on the right.

Do single and variable aperture lenses do the same thing? Yes and no.

Yes, they gather light, but how much light?  The most important part of photography is the amount of light hitting the sensor.  

No, because they don't gather light in the same way. Here's where it gets technical. The f/number (a.k.a. f/stop) is the size of the opening that lets light into the camera. The lens' iris. 

I'll spare you the math, just know that the smaller the f/stop (f/1.4) the more light the lens lets in. The larger the f/stop (f/16) the less light is allowed in. 

Each f/stop larger lets in double the amount of light whereas, each f/stop smaller lets in half the amount of light. Modern cameras and lenses can make half and third of a stop adjustments.

f/stop and Shutter Speed

Here is why a single aperture lens is important. With variable aperture lenses you lose light when you zoom. That means the lens above at 55mm starts at f/4.5 and as you zoom the aperture stops down to f/5.6, so you lose light. 

1/500 of a second is a fast shutter speed and the faster the shutter the less time the sensor has to gather light. The variable aperture lens at 300mm on an APS-C Nikon has the effective focal length of 450mm and a minimum aperture of f/5.6 when compared with the 200mm single aperture lens that is f/2.8 at the effective focal length of 300mm. 

Therefore, the single aperture f/2.8 lens has 3 times the light gathering ability at its max focal length when compared with the variable aperture f/5.6 lens. When taking a photograph in the same lighting conditions you would need to slow the shutter speed by 3 stops from 1/500 to 1/125, which would increase the risk of motion blur. Or you can increase the ISO by 3 stops, but that would impart more noise (grain) onto your image.

That is why f/2.8, f/1.8, and f/1.4 lenses are known as fast lenses - Low f/number = fast shutter.

F-Stop, Focal Point, & Depth Of Field

One last thing to note when shooting with a low f/stop is that your focal plane will be razor thin.  Shooting at f/1.8 and focusing on a person’s eye will make everything in front of the nose and everything behind the ears blurry.  This is known as bokeh.  With that same lens shooting at f/22 everything from the camera to furthest background will be in focus.

Notice below how Uncle Fuggles’ eye is in focus with both images and how the background shot at f/1.8 is blown out while the shot of him taken at f/22 is in focus as far back as the trees. 

It's important to note that the camera settings were not the same. I needed to accommodate for a lot less light at f/22 and way too much light at f/1.8. The f/1.8 shot was taken with 100 ISO at 1/2000 shutter speed whereas the f/22 was taken with 500 ISO at 1/100 shutter speed, which allowed the sensor to be more sensitive and the shutter speed slow enough to give the sensor enough time to capture the necessary amount of light. Both shots were taken with a Nikon 35mm f/1.8 prime.

The Intangibles

Another benefit of a large aperture lens such as a f/1.4 or f/1.8 is the sharpness sweet spot. My f/1.8 lens is sharpest at f/4. The reason is due to where the light enters the glass. As you stop down the iris light enters the lens closer to the center. As the lens' iris opens wider light enters nearer the edges of the glass, which can cause "soft focus".

This begs the question, where is the sweet spot of a variable aperture lens? If you are zooming to 200mm at f/5.6 has that lens moved beyond the sweet spot? I couldn't tell you since I don't shoot with variable aperture lenses.

Purchasing A New Camera & Lens

Now that you understand the importance of good glass my recommendation when purchasing a new camera is to forgo the camera kit, which is usually a camera body and a variable aperture lens such as an 18-105mm f/3.5-f/5.6. 

Instead, I recommend purchasing a camera body that is well within your budget and putting the rest of your money into a good lens. If your budget is tight buying used is always a great option.

Name Brand vs. Off Brand

Lenses are no doubt expensive, but you don't have to spend a ton of cash to get good glass. Name brand lenses from your camera manufacture are the best choice, but companies like Sigma, Tamron, and Tokina make quality products for much less money. 

Take our example of the Nikon 70-200mm f/2.8 above. You can purchase the equivalent Sigma 70-200mm f/2.8 new for $1200, which is about half the price of the Nikon and way better than the variable aperture I compared it to.

One final thought

The variable aperture lens above has less glass, so it is much lighter, and it has a longer reach (i.e., 300mm vs. 200mm). With that said, variable aperture lenses are good for outdoor photography in bright conditions, but you will struggle to make a good photograph indoors without a flash.

A good lens will last for years even when your current camera becomes obsolete and worn out.  When good glass is taken care of it will survive numerous camera bodies, which can help justify the cost.