The Art of Composition part 4

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 go any further, however, I need to issue a warning about this article. What follows is a bit heady as I discuss this stuff. For some of you, this might be totally pointless. For others, it may be a game changer. Far from attempting to be overly pedantic, I think this is still an important part of the conversation as it impacts how all of us approach our wildlife photography – both full frame camera shooters and crop sensor folks alike. 

Adding to this, I want to acknowledge that this discussion might come across as being critical of camera system such as Olympus due to their use of micro four third sensors. Nothing could be further from my intentions. I have colleagues that use Olympus systems whose work I highly respect. This conversation is only about depth of field. 

​When it comes to sensor size and its impact on depth of field, there is a never-ending deluge of competing opinions. A simple Google search for “sensor size + depth of field” will produce just as many yes answers as no. And the reason for this, as you will see in a moment, is because of the complicated nature of this topic.

As we have already discussed, depth of field, 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. 

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 which translates to a shallower the depth of field. Alternatively, a smaller aperture lets in less light which equates to a larger depth of field. This is why us wildlife photographers love our f/2.8 and f/4 lenses while landscape photographers talk about f/22; the depth of field in our image changes with this increase or decrease of light.

An APS-C sensor is smaller than a “full-frame sensor.” Meanwhile, a micro four thirds sensor is even smaller than an APS-C sensor. And because both of these “crop sensors,” are smaller, they are not capable of taking the same advantage of light that comes in from a lens at a given aperture as a full frame sensor. 

Let’s look at it this way: 

A 600mm lens is a 600mm lens regardless of full-frame, APS-C, or four third sensors. We measure the focal length of a lens by the distance between the optical center of the lens to the image plane in the camera (your image sensor). Therefore, a 600mm lens is one with a 600mm distance between these two points. This is why a 300mm lens on an Olympus camera, for instance, is still called a 300mm lens despite have the equivalent field of view as a 600. 

Just like the focal length of a lens, the size of the aperture is universal as well. This doesn’t change based upon other components of a lens. An aperture of f/4 is the same across the board. And therefore, a 600mm lens, set to f/4, is always going to allow the same amount of light into the camera no matter what happens to be going on inside of the camera itself. 

But what is going on inside of the camera matters greatly when it comes to end results. 

What we call a full frame sensor today is based on the measurement of 35mm film – which is technically 36mm on the long end. This holds true across all camera manufacturers. 

What we call APS-C sensors (Advance Photo Systems-Common Format) varies with camera manufactures. APS-C sensors made by Canon are smaller than those made by Nikon, for instance. But in general, we can say these are roughly 25mm on the long end. 

And micro four third sensors, the smallest of the lineup, are 18mm in length. 

All this matters because of the impact that sensor size has on what your camera can do with the light passing through the lens. 

Lenses project a circular image into the camera. A 35mm sensor is going to read a large portion of this. A 25mm sensor will read a smaller portion. And an 18mm sized sensor will read an even smaller part of this projected image. 

This portion size is why we refer to these smaller sensors as “crop sensors.” A smaller sensor, because of the smaller portion of the image it records, is like taking a crop out of the middle of the larger image. 

So, on a micro four thirds sensor, which is half the size of a full frame sensor, the resulting image is a 50% crop out of the image being projected into the camera. For this reason, a 300mm lens on an Olympus camera produces the results we would expect to see with a 600mm lens. But just because the magnification of the subject appears the same, all similarities stop here.

Since the image projected by a lens is circular, even a full frame sensor cannot take advantage of everything there. But it’s the most light we can get with the technology available. As the sensor size shrinks, so too does the amount of light that it’s recording. 

A camera with a 1.5 crop factor, such as a Nikon D500, has a sensor that is 75% the size of a full frame sensor. For this reason, the APS-C sensor of the D500 is only going to be able to take advantage of about 75 percent of the light that a full frame sensor can. 

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, however, is that it’s often an oversimplification of a problem. And in this instance, it doesn’t consider the actual physical properties of those sensors.

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. 

This is similar to the way image sensors record light. 

While the example is an imperfect one, it should give you the ability to at least visualize the concept here. If we look at full frame and crop sensor cameras with similar megapixel counts, the manufacturer must engineer those photosites to be smaller for the crop sensor so they can fit the same number into smaller real estate. Like the bucket versus pint glass analogy, the smaller photosites are not capable of collecting the same number of photons hitting them as the larger photosites on a full frame sensor. The larger the surface area of the photosite, the more photons, and therefore the more light the sensor will collect. This is why Canon has chosen to keep their flagship mirrorless cameras, the R1 and R3, at a lower pixel count as it allows for larger photosites and better lowlight capabilities. 

So, if the amount of light coming in through an aperture impacts DOF ( remember larger apertures equate to shallower depths of field), 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. And less light passing through the lens means MORE depth of field. 

In the video world, many camera sensors are smaller than 35mm. 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 by taking all of that lost light and bouncing it back to the image sensor. This, in turn, allows lenses to function like they would on a full-frame sensor. A 300mm is a 300mm. A 600mm is a 600mm. And therefore, an aperture of f/2.8 produces the same depth of field it would on a full frame sensor. 

Speed Boosters, like those produced by Metabones, were designed specifically for handling the problem of aperture and its associated depth of field when working with smaller than 35mm sensors. While the wild world of hobby photography endlessly debates such things, the professional cinematography world, from Nat Geo to BBC to Paramount to MGM, have entire industries built around solving this very basic problem. 

But there is another side of the coin, sort of. 

As mentioned above, APS-C sensors also pack together more pixels into a given area. 

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

This argument makes little sense, in my opinion. I admit that depth of field is all about what one perceives to be in focus versus out of focus. However, increasing resolution, if it has any impact on this what-so-ever, would in fact mean increasing the perception of what is in focus thereby making the depth of field even larger. 

This is far more ink than I intended to spill over this topic. So, it’s best to move on. 

The smaller sensors used in some cameras has given rise to the misconception that a crop sensor is better for wildlife photography because it gives “greater reach.” But there is a problem with this. By using a crop sensor, you don’t actually turn your 300mm lens into a 600mm lens. 

As mentioned above, a crop sensor does nothing more than take a crop out of the middle of an image projected by the lens. The image compression doesn’t change like it does with increased focal length. The look and feel don’t change because the physics of the lens didn’t change. The resulting image is no different than if you had simply cropped your photo in Lightroom or Photoshop. 

If you remember from the last article about focal length and depth of field, a 600mm f/4 has a shallower depth of field than a 400mm f/2.8. Likewise, an 800mm f/5.6 produces an even shallower depth of field than the 600 does. 

Now, let’s say we are both standing side by side in the field. I’m wielding a full frame camera with a 600mm lens. You on the other hand are carrying a micro four thirds camera with a 300mm lens. Your subject will be the same size as mine in your viewfinder, leading everyone to say you are working with the equivalent of a 600mm lens. 

If how big the subject appears in your viewfinder is the only thing that matters to you, then we are equally matched here. It will appear that your subject is just as close as mine. However, this is where the similarities abruptly end. 

Despite how big the subject may appear in your viewfinder, you are still working with a 300mm lens and all the physical properties that are associated with that – but at the same distance I am working with a 600. 

If we are both photographing a brown bear that’s 50 yards away, my 600mm f/4 will produce a depth of field of about 4.5 feet. Meanwhile, a 300mm will produce a depth of field of roughly 18 feet. 

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 focal length and aperture, the shallower the depth of field will be. The farther away I am from a subject, the larger the depth of field will be. 

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 extreme low light capabilities, I prefer to work with full frame cameras. 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, maybe that’s all part of the story telling for you. Maybe the size and weight savings of a micro four thirds system makes all the difference. Maybe your artistic vision doesn’t place the emphasis mine does on ethereal portraits made at extremely shallow depths of field. Maybe you love macro photography, and you are always starved for more depth of field. There are many reasons that one might chose a micro four thirds system or a crop sensor over full frame. It’s just important to understand how such things work and how the mechanical properties of that technology may impact your photography.