Just stop already with the “This lens on a crop sensor is really Xmm” nonsense.

When choosing a lens, one of the primary data points to consider, depending on the photographer’s need or purpose, will be the focal length of the lens.

For example, wide angle lenses have smaller numbers (10mm, 16mm, 18mm) while telephoto lenses have larger numbers (85mm, 135mm, 200mm, 400mm).

Confusion can occur when comparing a lens against various sensor (or film) sizes. The two most common sizes are referenced as either “full frame” or the slightly smaller “crop” or “APS-C” sensor.

Sensor Sizes: A Brief History

Cameras considered “full frame” have a sensor roughly the size of a 35mm film frame; however, many cameras use a slightly smaller sensor, known as “APS-C.”

APS-C sensors, like “full frame” sensors, are also based on film frame size: prior to the advent of digital photography, some camera manufacturers explored film enclosed in a cartridge which was easily loaded and unloaded without winding; much of these used a smaller film type, which was dubbed APS-C. These were found primarily in point-and-shoot cameras.

(The ratio of a “full frame” sensor vs an APS-C sensor varies by manufacturer; typically it’s 1.5:1 but could be 1.6 or even 1.7 and still be considered “APS-C.” Any smaller and the sensor size ventures into “micro 4/3’s” consideration or smaller.)

Marketing Jargon

Unfortunately, during the infancy of digital photography as a technology, the 35mm film frame size quickly became a specific standard size to strive for. At the beginning of the digital photography age, sensor sizes were limited due to technical and financial constraints; a camera with a sensor size comparable to the most well-known consumer film size became a common comparison. Sensor size became a simple marketing value to use when separating “consumer” digital cameras from “prosumer” and full “professional” equipment.

Meanwhile, the APS-C sensor size quickly became popular as a balance between image quality and equipment cost. As more cameras were built using the APS-C reference, improvements were made in overall image quality captured by the sensor. While early camera systems would use the same lens mount type between full and crop sensors, manufacturers began to build lenses specifically for crop sensor bodies. Canon’s EF mount (full frame or crop) and EF-S mount (crop only) are an example.

I say “unfortunately” because before the digital age, professional photographers would actually use medium or large format film for commercial work; 35mm was considered “consumer” film by many professional photographers.

Selecting 35mm as a standard for “professional” digital equipment was, and still is, arbitrary — and primarily serves to give photographers G.A.S. (Gear Acquisition Syndrome). It’s still common today for enthusiast & semi-professional photographers to believe that full frame gear is a reflection of how “serious” one is about their photography.

Why would anyone use a number different than the one provided by the lens manufacturer to discuss focal length?

Primarily for convenience; far too often, Focal Length is confused with Field of View and even Depth of Field.

What is Focal Length, anyway?

The focal length of a lens is a measurement of how far away from the sensor (or film) the first glass element in the lens sits. The farther the glass is from the sensor, the further an image can be captured due to how the light refracts through the lens.

For Prime lenses, this number is fixed; “zoom” lenses have a dynamic focal length — this is provided by allowing the user to adjust the distance of the first glass element from the lens.

Focal length affects how the subject of an image is perceived: wide angle lenses will de-emphasize the middle of the frame, while telephoto lenses will increase it. If you’ve ever seen an image with a full moon and wonder why the moon is so large compared to our standard view of it, chances are (if the image wasn’t digitally altered) the photographer used a telephoto lens to capture the moon with more detail than our eyes’ focal length allows.

For reference, a 50mm lens most closely translates to the view from our eyes. If you look through the viewfinder of a camera with a 50mm lens attached while leaving your other eye open, the distortion between the two views should be nearly nonexistent. Wide angle lenses will appear to capture more information in the frame than the naked eye alone, while a telephoto lens will capture greater detail of a distant object than the naked eye can perceive.

Field of View (a.k.a. “Angle of View”)

As it relates to photography equipment, “field of view” is a measurement of how much scenery can be captured in a two-dimensional image as compared against the natural view from our eyes.

When capturing an image, we are effectively cropping out a slice of a subject/scene and choosing to frame a very specific amount of information in our image.

A wide angle lens will capture a greater field of view, while a telephoto lens captures a much more narrow field of view.

Depth of field is commonly associated with bokeh, which is the blurring of the image outside of the depth of field.

Depth of Field

Depth of field is a measurement of how much space exists for a subject to be in focus. For example, if the subject’s “fixation point” is 6 feet away from the camera, a shallow depth of field may be 3-6 inches from the subject. This space can be both in front of, and behind, the fixation point.

Depth of field / bokeh are determined by three primary factors:

• Distance from the subject;

• Focal length of the lens;

• Aperture setting on the lens.

Focal Length and Sensor Sizes

A full-frame sensor and lens will capture the entirety of the lens’ diameter onto the sensor; however, using a full frame lens on a crop sensor will only use the more central region of the lens.

Using our Focal Length diagram, we can see that the full size sensor (blue) captures the edges of the lens, while the crop sensor (green) will not be able to capture that data.

This relationship is exactly why APS-C sensor sizes are called “crop” sensors: the total potential data is “cropped” by the sensor size, leaving out some information that would otherwise have been captured.

The problem with continuing this idea of an APS-C sensor being “crop” is that modern digital cameras have lenses built specifically for their sensor size, meaning the entirety of the lens diameter is captured by the sensor.

So What’s the Actual Problem?

(There is a point, and we’re getting close to it — I promise!)

As previously mentioned, focal length is commonly interchanged with field of view (FOV) when discussing the expectations a photographer should have for a lens. An APS-C sensor’s FOV is narrower than a full frame sensor’s FOV; a full frame sensor’s FOV is narrower than a medium format sensor’s; et cetera.

Using a full frame sensor and lens as a baseline, it’s become common to discuss the FOV a lens provides by simply referencing the focal length. If a 50mm lens provides a FOV of 47° FOV (measured diagonally) on a full frame sensor, an APS-C sensor with a 50mm lens may provide closer to a 32° FOV.

Why It Matters

Alright so, who cares? It’s certainly convenient to refer to the focal length when discussing relative FOV’s, even if the actual focal length isn’t different between cameras with different sensor sizes.

Recall that the subject of an image is impacted by the focal length of the lens. An extremely wide angle lens, for example, will create a “fish-eye effect” on subjects close to the lens, while subjects in the distance will be much smaller and appear farther.

Think of the passenger side mirror on a vehicle and the warning that “Objects in mirror may be closer than they appear” — this is because the mirror is slightly convex so as to provide more of a view from the perspective of the driver, which will cause objects to appear smaller (farther) than they may actually be.

While a smaller sensor size may change the field of view, it does not change any distortion effects caused by the lens. The fixation point does not vary at all based on sensor size. Using our “human eye comparison” test, looking through an APS-C camera’s viewfinder and 50mm lens will still align almost perfectly to the human eye as doing the same exercise with a full frame sensor; the latter can simply “see” more along the edge of the frame.

Advertising a 50mm lens as being a “75mm equivalent” is therefore grossly inadequate and misleading. A true 75mm lens provides increased telephoto capability over a 50mm lens; not only does this impact the field-of-view, but also the size of the subject in the frame and any distortion of the image.

Crop vs Telephoto

This image was taken using a 23mm lens, which is considered a relatively wide angle lens. A 23mm lens on an APS-C camera is often referred to as “35mm equivalent.”

Notice the snow-capped mountains in the distance?

Had a telephoto lens been used to capture those peaks, they would appear much larger in relation to the surrounding scenery.

(While any distortion or fixation point difference between 23mm and 35mm is relatively marginal, those differences become more pronounced as the focal length increases.)

So does simply cropping the image actually provide a “telephoto” effect?

Obviously, the answer is no.

Taking the same image above, I’ve cropped in on the snow-capped peaks; their proportion to the surrounding scenery does not change.

When discussing a lens in relation to the sensor size of the camera body, there is therefore no justifiable reason to use the focal length as a catch-all reference when the only deviation between them is the field of view.

But Wait! What About Depth of Field?

This is typically where the counter-argument mistakenly assumes that an increased sensor size provides improved bokeh.

Because a full frame sensor captures a wider field of view, the composition of the subject within the frame changes. After all, there’s more scenery surrounding the subject when the FOV increases. To compensate for the additional scenery, the full-frame photographer must move closer to the subject to match the composition. (Inversely, an APS-C photographer must move away from the subject to match composition.)

When the subject moves away from the camera, we see that the focus point shifts and the band between the lines narrow considerably.

In the second illustration, the house will appear much more in focus relative to the primary subject; any scenery in front of the subject will be blurred, but an observer may struggle to tell if the house is meant to be a part of the subject or not.

Aperture’s role in this process is similar to distance from the subject: a larger/”faster” aperture would be similar to standing closer to the subject, while a smaller aperture is akin to increasing your distance.

Recall that distance from the subject is one of three key variables when determining depth of field / bokeh.

Using the illustration, the lines represent the focus and depth of field in relation to the subject. Optimal focus is achieved where the lines cross.

Any region of the image where the lines are apart from one another will be blurry; how blurry is represented by how far apart the lines are.

By moving closer to the subject, the full frame photographer changes the fixation point which then impacts the degree of bokeh behind the subject.

Clearly, the sensor size itself does not factor into the perceived bokeh. Even a wide angle lens can create bokeh if the subject is close enough to the lens.

(Using “large” or “small” to describe aperture is meant to convey the diameter of the lens’ ability to capture light; the smaller the number, the wider (larger) the diameter.)

As we can see, increasing the aperture diameter impacts the depth of field similar to distance from subject.

It’s not uncommon to see aperture “equivalents” alongside focal length “equivalents” — this really only reflects the aperture setting necessary to compensate for the increased distance from the subject in order to achieve a similar depth of field.

This adjustment can be applied simply based on a change in distance between the subject and camera, regardless of the sensor size.

Where Misrepresentation Becomes Insidious

All too often, lens reviews will take these comparisons or adjustment values and use them to suggest inferiority. I’ve seen lens reviews for a lens with an aperture maximum of f/1.4 state that this is somehow actually “just” f/2.8 thanks to the sensor size.

This is a completely false narrative!

Modern digital cameras are exceptionally good. Even cell phone cameras, using tiny sensors and lenses, can create incredible art.

Any and all comparisons of gear to “full frame equivalents” are just that — comparisons. It’s an attempt to establish some sort of baseline with which to compare the gear in question.

So why does full frame continue to be the standard bearer?

• APS-C gear is far more available and popular by both manufacturers and consumers;

• Professional photographers are increasingly embracing APS-C equipment due to its size and quality of images;

• Medium format digital cameras are increasing in availability as well.

Full frame sensors continue to be the baseline… well, just because.

Be Sure Your Upgrade Has Purpose

If you’re considering upgrading your gear from APS-C or Micro 4/3’s to full frame because you believe your images will improve, please stop now. The money would be better spent on higher quality lenses or on improving your technique.

Larger sensors require larger lenses, which increases the size and weight of your equipment. An extremely common argument put forth by professional photographers for converting to APS-C is that doing so reduced the size and weight of their gear, which in turn increased the opportunities to take photos because the photographer was more likely to have their gear with them.

Larger sensors do capture more information. This can help with low light conditions or for when the final result needs to be reproduced extremely large (billboards, et cetera). Their purchase should be driven by these needs, not by marketing techniques designed to drive consumerism.

Upgrading to full frame simply to “prove” your commitment to photography or your value as a photographer, or with the expectation that doing so will vastly improve the quality of your work, will only result in extreme buyer’s remorse.

In Conclusion

A 50mm lens is a 50mm lens, whether or not it’s paired with a full frame sensor, an APS-C sensor, a Micro 4/3’s sensor, or a smartphone camera sensor.

While the field of view may change, the actual focal length does not; lost in the comparison is the practical difference a lens makes on your subject based on the focal length, which does not change simply because the sensor size differs.

Manufacturers, lens reviewers, and retailers should discontinue the “equivalency” verbiage when discussing lenses; it only functions to perpetuate consumerism and to establish artificial superiority of one technology over another.