Zoom Factors: Why Your Feet Cannot Be Replaced By A Zoom Lens

When it comes to the focal length of your lens, too often we think about how far or how close a lens will reach.  The relationship between focal length and the distance between our subject and the camera should not be thought of as corollary.  In truth, the focal length affects far more than how close you can “zoom in” on a subject.  The one detail that many photographers forget – the detail that affects the spatial aspects of our photos the most – is also perhaps the most forgotten:  Angle of view.  Angle of view affects what fits into the frame which, in turn, affects how we perceive space in a photograph.  It distorts perspectives and bends reality far more than you think.  As photographers, we must take that into perspective, and we need to adjust accordingly.  The only way to do that?  Use your feet.

Compression Science

Corridor at 70mm.

If you are in the camp with those who believe that a photograph never lies, you should either change your view or give up the craft all together.  I believe that a photograph rarely tells a full truth, and angle of view and its side effects are the best evidence to support that believe.  In short, longer focal lengths have a narrower angle of view and in turn results in a more compressed image.  The perceived  distances between objects at different depths within the photo seems much tighter at these narrower angles of view.  Ergo, the longer the focal length, the more compressed the depth of your photo will appear.  The phenomenon is referred to as Perspective Distortion.  I could speak at length about the cause behind it, complete with all the boring details.  But as this is a photography discussion – and not a physics discussion – I will take a simpler approach.

Corridor at 18mm. To use the shorter focal length, I had to move about 15 feet closer to the doorway at left.

There are essentially two physical constants in the camera:  The size of the sensor, and the distance between the back of the lens and the sensor.  Bear with me for a minute and imagine this camera is a pinhole camera and we have control over the angle of view.  In such a simple camera, any change in the angle of view would change what gets dispersed across the sensor.  A more normal, and significantly more complex, camera lens is designed to keep the light spread across the sensor at a constant (I’ll refer to this as “magic”).  So the angle of view at the back of the lens is also constant, regardless of the angle of view at the front of the lens.

Focal length comes into play here simply because of how large the lens is, typically.  As a general rule, the longer the focal length, the narrower the angle of view.  No matter how narrow that angle of view may be, it still needs to fill the same angle when it gets spread across the sensor (thanks to the magic).  Think about the entire depth of your photograph as a cone of view emanating from the camera.  Somewhere in the middle of that cone, there’s a point at which nothing really distorts which we’ll call the distortion free zone (DFZ).    If we were to hypothetically take slices through other points of the cone, a slice closer to the camera than the DFZ would be smaller and one beyond the DFZ would be larger.  If we were to stretch the pre-DFZ slice and shrink the post-DFZ slice so that they were the same size as the DFZ slice, you can imagine the distortion that would result.  Objects in the post-DFZ slices would appear relatively smaller, objects in the pre-DFZ slices would appear relatively larger than their real-life (undistorted) counterparts.  This is essentially what the components within your lens are doing when it does it’s magic.  And that’s how the resulting image will look distorted.  But bear in mind, except in extreme cases, these distortions are rarely enough to be unnerving, and thus we can use Perspective Distortion to our advantage.

Perspective Distortion And Our Photography:  Hint, You’ll Need Your Feet

I am not looking to start a prime lens vs. zoom lens debate, but I want to highlight one argument in the pro-prime camp that I feel doesn’t hold water.  The argument is that a zoom lens makes one lazy as they should use their feet to frame the shot.  And while the argument touches lightly on the topic of discussion today, I feel the argument doesn’t hold water because it also highlights a disadvantage of prime lenses:  You can’t change your angle of view.  And while I do see the merit in prime lenses – and I do love all of mine – I will recommend that beginners purchase zoom lenses.  But that comes with a caveat:  You still need to use your feet.

For demonstration purposes, I shot these fruit with a horribly cluttered background. At 18mm (left), the clutter is very much a distraction. At 135mm, I can’t fully eliminate all of the clutter, but I can reduce some of the distractions.

When setting up a shot, I want you to consider the angle of view and the perspective distortion when you pick a focal length.  You’re picking a focal length for these reasons, not simply because you can’t reach a subject from where you stand.  If you’re not used to visualizing, move your zoom in relation to your footsteps.  Walk farther away, but zoom in to frame your subject similarly (or vice versa) and pay close attention to the background and foreground.  The focal length can dramatically change the mood of the shot.  The trick can also be used to obscure unwanted backgrounds or foregrounds as you can see in my examples shown here.  As for your feet…there’s no avoiding it:  Zoom or not, they’ll be doing some walking if you really want to compose your shot well.

Final Thoughts

So now I’m going to throw it out on the table.  From this point forward, I want to challenge you to stop thinking about zoom factors as a convenience.  I want you to think of the focal length as a tool, a means to bend reality to your needs – to compress or stretch space at will.  The use of focal length in this most appropriate manner will make you a better, more creative photographer.  So let your feet do their job and move yourself back and forth at the same time you’re rotating that zoom ring.