Art of Composition Part Four

In the last installment of this series on the Art of Composition, I discussed focal length and its impact on depth of field — and thus bokeh in an image. In this article, I’m going to continue this discussion on depth of field with a topic that seems to create a lot of disagreement in the photography world: the role that sensor size plays in all of this.

Before we do, however, I want to take a moment to reiterate that composition is all about how we arrange things within the picture space. It’s what we chose to include and what we choose to exclude. It’s where subjects sit in juxtaposition with other “things” in that picture space. It’s recognizing that both light and shadows are hard compositional elements. And it’s all about line and texture and patterns. But before we move into discussing concepts like the elements of design, we have to understand depth of field first and foremost as this is one of the technical ways in which we can choose to include or exclude details in our composition. While we cannot always choose all the stuff in our compositions given that wildlife photography is really all about organized chaos, we can make decisions on how that stuff will be rendered in our compositions, how much detail there will be, how hard or how soft those things will appear. So, while it may seem counterintuitive to spend so much time discussing depth of field in a treatise on composition, there is a method to my madness here.

Now, regarding whether or not a camera’s sensor size as any impact on depth of field, do a simple Google search for “sensor size + depth of field” and you will find just as many yes answers as no.

And the reason for this, as you will see in a moment, is because the answer is both yes AND no.

As we have already discussed, depth of field (DOF), and thus bokeh, is a function of many things such as distance, focal length, and aperture. And all three of these ultimately come into play when answering the question of how sensor size affects DOF.

Let’s start with aperture.

You probably already know that changing your aperture changes the amount of light that makes it through the lens to hit your image sensor. A larger aperture lets in more light. A smaller aperture lets in less light. And the depth of field in our image changes with this increase or decrease of that light.

An APS-C sensor, or what most people simply refer to as a crop sensor, is smaller than a “full-frame sensor.” Meanwhile, a micro four-thirds sensor is even smaller than an APS-C sensor.

Because a crop sensor is smaller than a full-frame sensor, it is not able to take advantage of the full amount of light that comes in from a lens at a given aperture.

Let’s look at it this way:

It’s safe to assume that everyone reading this is using digital cameras that are, in some fashion, based on the equivalent of old 35mm film. A 600mm focal length is such only when the image is rendered on film, or a digital sensor, that is 35mm in size. When considering cameras systems such as Olympus, which has a micro 4/3rds sensor, we say there is a “doubling effect” to the focal length of whatever lens we use because the sensor is half the size of a “full frame” sensor. So, a 300mm lens produces the effect we would expect a 600mm lens to produce when it comes to focal length. This is also why we call these smaller sensors a “crop sensor,” as it’s the effect is similar to cropping a photograph to get a desired closeness to our subject.

Still with me?

Good.

A full-frame sensor is bathed in the maximum number of photons our lens is designed to let in.

However, a camera with a 1.5 crop factor, such as a Nikon D500, is only going to be able to take advantage of about 75 percent of the light coming into camera because the sensor is smaller than a full frame.

Another pause.

Common sense here would say that since the sensor is not blocked by part of the lens, then regardless of sensor size, it is getting the same amount of light no matter what. The problem with common sense is that it’s often an oversimplification of a problem. And in this instance, it doesn’t consider the actual physical properties of light.

Picture, for a moment, that you have a 5-gallon bucket sitting next to a pint glass in your backyard when it starts to rain. Technically speaking, both the bucket and the pint glass are exposed to the same storm, the same clouds, the same amount of rain. However, over the course of a given amount of time, the bucket is going to collect far more water than the pint glass for obvious reasons.

Light and image sensors are, more or less, the same way. While the example is an imperfect one, it should give you the ability to at least visualize the concept here. Substitute photons for rain drops and a full frame sensor vs a crop or micro 4/3rds for the bucket and pint glass. The larger the surface area, the more photons, and thus light, the sensor will collect.

So, if the amount of light coming in through an aperture impacts DOF, then a crop sensor at that same aperture is going to be hit with LESS light than it would if it were a full-frame sensor. This lesser amount of light effectively means MORE depth of field.

In the video world, many camera sensors are smaller than 35mm, despite the fact that the lenses that are being used were designed for a 35mm-sized sensor. This fact gave rise to a series of adaptors called Speed Boosters. Speed Boosters work to increase the amount of light hitting the smaller sensors of these cameras and thus allow the lenses to function like they would on a full-frame sensor. A 300mm is a 300mm. A 600mm is a 600mm.

This is the basis for the argument that YES, sensor size impacts DOF. And it is one of the stated reasons why many macro photographers prefer to use APS-C sensors given that they INCREASE depth of field by roughly one full stop.

But there is another side of the coin.

APS-C sensors also pack together more pixels in a given area.

Depth of field is all about perception — what the human eye PERCEIVES to be in focus. Thus, with additional megapixels, with additional resolution in smaller areas, some argue that smaller sensors should in fact decrease depth of field — giving you more bokeh.

Functionally, however, all of this is subjective to some degree since depth of field is about illusions and perception. I have always noticed what appears to be a full stop of difference between the look and feel of an image with a full-frame sensor versus a crop sensor using the same lens and aperture. However, we need to take another facet of this into consideration, given all this talk about perception and subjectivity. And that is how using a crop sensor fundamentally changes how we approach photographing wildlife and thus changes the depth of field of our images.

A 600mm lens on a full-frame camera is, of course, 600mm. From one full-frame camera to the next, the magnification of this lens remains the same. But with a crop sensor, this changes. A 600mm on a sensor that provides a 1.5x crop gives us the magnification we would expect at 900mm.

But this isn’t really 900mm, though.

In the previous article, I talked about how focal length affects depth of field. By using a crop sensor, you don’t actually turn your lens into a 900mm lens. It’s nothing more than a crop out of the middle of an image from a 600mm lens. The image compression doesn’t change. The look and feel doesn’t change like it would by using an actual 800mm lens versus a 600mm lens. The actual impact on the photograph is no different than simply pulling the photo up in Lightroom or Photoshop and cropping in to make the subject larger. Sure, the subject is bigger, but the mechanics of it all doesn’t change because that’s all it is — a crop.

But it’s also different than cropping in on the computer, because in the field, you ARE seeing the subject so much larger in your viewfinder.

Because the subject now appears larger in the viewfinder, you then adjust the distance between yourself and the subject accordingly.

Let’s say we are both standing side by side behind our respective 600mm f/4 lens. I’m using a full frame camera. You are using a crop sensor camera. For this reason, the subject you see in your viewfinder appears 1.5 times larger than the one I see in my viewfinder. As a result, for us to create the same photograph, this means either you will need to back up or I will have to move in closer.

And here is where the devil lies within the details.

As I discussed in previous installments of this Art of Composition series, distance is everything when it comes to depth of field and bokeh. The closer I am to a subject at a given aperture, the shallower the depth of field will be. The farther away I am from a subject at a given aperture, the larger or deeper the depth of field will be.

And so, if you and I are both in the field together, using the exact same 600mm lens set to f/4, but are wielding different-sized sensors in our cameras, we are going to create very different looks and feels to our images.

If I use a full-frame camera, I will have to work my way closer to the subject than you will to fill the frame with my subject. And by getting closer, I decrease the depth of field and the amount of details you see in the environment around my subject, and thus increase the bokeh in the photograph.

Returning back to macro photographers here, this is the major reason photographers use APS-C sensor cameras to get greater depth of field. Using the same 100mm f/2.8 macro lens as you would on a full-frame camera, their WORKING DISTANCE to the subject is greater because of the crop factor. And this greater working distance INCREASES depth of field.

As you can see, there are big differences here in terms of the results we can expect from crop vs full frame sensors. This is not to say that one is better than the other. It’s just very important to recognize how the technology in your hands works so you can understand how it will all impact your photography.

For me, as a wildlife photographer that prioritizes shallow depths of field and bokeh in my photographs, I only work with full frame cameras. There are a few others reason, chief amongst them being the they handle high ISO situations better than crop sensors (think back to the bucket and pint glass example). But this is my style of photography. Maybe a little more depth of field is something you are always looking for in your wildlife photography. There is nothing wrong with that. It’s just important to understand how the tools in your hand are playing an ancillary role in all these things.

However, in wildlife photography, as you will see in the next installment of this series, you will learn one very big reason I believe shallow depths of field are better – and why I always try to shoot at f/2.8 or f/4.

To Be Continued. . .