DCR Workshop: Digital Crop Factor Demystified

by Reads (1,017)

Photography is one of those hobbies that can be as simple or as complex as you want it to be. This diversity is good for most people – you don’t have to be a professional level photographer to use a camera, or spend all your money to have a nice photo on your wall. Nor do you have to understand all of the intricacies of digital technology to get a good shot.

But sometimes a little technological knowledge can help clear up confusion, and one of the most commonly discussed – and commonly misunderstood – topics in digital photography these days involves digital crop factor. If you have heard of this concept but don’t know why you should care about it (or if you really should, that is), or if you have no idea what this mystical crop factor thing is, read on!

Back to the beginning
At its most basic, “digital crop factor” is a fancy way of referring to sensor size – more specifically the difference in sensor size from the our standard 35mm reference point, and how this in turn effects how lenses perform. Most people are familiar with (or have at least heard of) a 35mm camera: “35mm” refers to the most common size of still camera film, technically known as 135, which has a frame size measuring 36x24mm.

(As an aside, you may be wondering why it’s called “35mm” if the frame is actually 36x24mm? Answer: the 35mm dimension refers to the width of the film strip itself, including the perforated edges used to advance the film.)

Very few digital cameras – all of them high-end DSLRs – have a sensor size equal to a frame of 35mm film (these are the so-called “full-frame” cameras). Everything else, professional medium-format cameras aside, has a sensor smaller than 36x24mm. In order to describe the difference in sensor size between the conventional 35mm frame and the various sensor sizes found in most digital camera (and describe resulting differences in the way attached lenses perform on cameras with different-sized sensors) we use the crop factor concept, which helps relate differences in field of view in a picture between different size sensors.

Let’s start by exploring the more about how digital crop factor works. Then we’ll look at some practical applications of the idea. Finally, we’ll explore what practical impact crop factor on you as a photographer, based on the type of camera you use.

What does it all mean?
As we already explained, crop factor is a way of describing the practical difference in size between a digital camera’s sensor and the standard 35mm film frame – primarily in the context of DSLRs. Crop factor is commonly expressed as a “times magnification” value: some commonly seen crop factors include 1.5x (for Nikon‘s DX and related sensor sizes), 1.3x (Canon), 1.6x (Canon again), and 2x (Olympus and Panasonic).

DCR Workshop: Digital Crop Factor Demystified

The image illustrates a comparison of size of some common sensor sizes. Note that in this image 35mm is the largest (black box), but there are still larger formats out there for professional cameras. Note, also, that the smallest box is a typical sensor size in a Point and Shoot camera.

(Note that while crop factor primarily matters to DSLR shooters, point-and-shoot cameras certainly have a crop factor as well – their sensors are even smaller than those in DSLRs, and thus always smaller than the “full-frame” 35mm size. But we rarely see these crop factors expressed because we are less concerned with these numbers for a non-interchangeable lens camera.)

But what does this 1.5x or 1.6x or 2x number mean?

As mentioned previously, crop factor describes in part the relationship between a DSLR’s lens and its sensor size. Hence, to understand what a 1.3x crop factor means, for instance, we first have to understand a little bit about lenses.

One basic thing to know about a camera lens has to do with its “focal length” – basically, the distance between the lens itself and the sensor or film frame, almost always expressed in millimeters. This number tells us something important about how wide a lens’s field of view is, or how great its magnification is: a wide-angle lens would have a focal length somewhere around 28mm lens, while a telephoto lens might be a 200mm lens. A wide-range zoom lens (a lens that can be moved in or out to cover more than one focal length), then, might have a focal length of 28-200mm. More common kit lenses for DSLRs often cover the 18-55mm “wide to normal” range.

You’ll often see lens range and focal length referred to as a magnification value (3x, for instance), especially on point-and-shoot cameras. This is just a ratio of how the widest focal length to longest focal length: our 18-55mm lens above is almost exactly a 3x lens, since the ratio of 18 to 55 is roughly 1 to 3. Even though they’re expressed in similar ways, don’t confuse this magnification number with crop factor.

As noted, the focal length of the lens affects how much we can see – what’s called field of view. A wide-angle (shorter focal length) lens allows us to see a broader field of view – such as a large group of people. A telephoto (longer focal length) lens allows us to “zoom in” on things – so we can see things up close that we might not otherwise see so well, such as wildlife. As we change the focal length we change what we are able to see. But also, if we were to change the size of the sensor in the camera, we would change how much of the total frame focused by the lens we can capture on that sensor.

A simple experiment you can do to see for yourself how this works involves cutting a quarter-sized hole in a piece of paper. If you look through the hole at an object while moving the paper closer to yourself or farther away, you’ll notice that the field of view changes. It gets narrower the farther away the paper is from your eye, or wider the closer it comes. Though it ignores for the moment lens magnification, this is the essence of focal length: the longer the focal length (the farther the paper from the focal plane, which in this case is your eye), the narrower the field of view – just like in a telephoto lens.

Similarly, though, if you were to make the quarter-sized hole in your paper a dime-sized hole instead – that is, if you were to make the hole smaller – you would see a narrower field of view than you saw with the larger hole with the paper at the same distance from your eye. The logical extension of this experiment is that as we make the sensor in our camera either farther away from the lens (by zooming out to a longer focal length) or smaller (by using a smaller sensor), we see less of the image we are taking a picture of.

By the numbers
Here is where the numbers come into play. Despite seeing less of an image with a 1.3x crop factor camera sensor than we would have with a 35mm film frame, the image appears magnified. This is because we take less image, but print or view it at the same size. In reality, we just did more enlarging when we printed or viewed the images, but it appears that we had more magnification due to the camera.

Think back to our lenses – how else do we magnify an image? We use a longer focal length lens. So, the digital crop factor is a way of converting the focal length of a 35mm lens to the equivalent focal length on a digital camera with a different sized sensor.

DCR Workshop: Digital Crop Factor Demystified

The above image shows the relative photo that would be produced with cameras with different sensor sizes. This assumes that the camera was in the same place, and that a lens of the same focal was used. Note that you don’t achieve a zoom with these smaller sensors, but simply capture a smaller, tighter image.

Let’s look at an example: If you take a picture on a 35mm film camera with a 50mm lens, to recreate that picture with a digital camera with a 1.3x crop factor you would need a 38mm lens to cover the same field of view. Likewise, if you used the same 50mm lens on your 1.3x crop factor digital camera, it would appear to perform like a 65mm lens on a 35mm camera.

Why? It’s simple math, really, based on the relationship between focal length and crop factor. If you want to see what your 35mm lens would appear to be on any particular digital camera, multiply the focal length by the camera’s crop factor (50mm x 1.3x = 65mm, in our example above). In the same way, if you want to know what focal length you need to be equivalent to a 35mm lens, divide the focal length by the crop factor (50mm/1.3x = 38mm). See, it’s not rocket science!

Free teleconverter?
This is the part where I need to apply some cautions to you. Many people like to think that their digital camera has a built in magnifier – suddenly their 200mm lens is really a 300mm lens on their Nikon DSLR with its 1.5x crop factor.

This simply isn’t true. The lens doesn’t change one bit. What changes is the field of view you get from the lens. Remember, in actuality, with a digital camera with a crop factor greater than 1, you actually lose some of the image that the lens is able to focus on. The reason it appears that you have a built in magnifier is that you took the same photo, got less in your frame, but then printed/viewed the picture at the same size.

DCR Workshop: Digital Crop Factor Demystified

DCR Workshop: Digital Crop Factor Demystified

Wowit’s a magic zoom! No, this is not a magic digital camera zoom. Rather, these photos illustrate the difference between a 35mm frame (top) and a digital 1.5x crop factor frame (bottom). Note that the 1.5x frame appears to be zoomed in. This is misleading because it contains less image data, but is enlarged to be the same physical size. While it might not be noticeable at normal print sizes, there is a decrease in image quality due to the extra enlargement.

For 99 percent of photography, the way digital crop factor affects the camera will be irrelevant. Just don’t get sucked into thinking that the new 1.6x crop factor camera you bought is magic. While it might help to more tightly frame a telephoto shot, if you have full-frame lenses on a camera with less than a full-frame sensor, you’re simply cropping out part of the lens’s captured image from the get go.

I can see clearly now!
One positive advantage of using cameras with tighter crop factors is the increase in lens clarity. Due to the smaller sensors used, the image used from the lens doesn’t require as much of the surface of the lens. As a result we get cleaner images.

The reason has to do with a basic tenet of optical design. The clearest part of a lens is the very center of the lens. As you progress farther and father out you lose some clarity. How much clarity is lost depends largely on the quality of the lens: a high-end professional lens is expected to be sharper at the edges than a consumer zoom, and lenses with fixed focal lengths (what are called “primes”) are generally sharper all around than zooms.

In any event, because the image from the very edges of a lens is cropped out from the beginning with a smaller-than-35mm sensor, we end up with cleaner images in many cases. The impact isn’t dramatic, and if you’re using a digital-only lens (a lens purpose-designed for smaller-sensor DSLRs) you won’t see this benefit. But in some cases you will get slightly sharper photos all around.

Likewise, it should be noted that crop factor has an impact on depth of field as well, in the same way that it affects focal length. Calculating depth of field is complex, and differences in field of view further complicate the matter: depending on how you analyze the situation, you either gain or lose depth of field with a smaller sensor, though it’s generally accepted that smaller sensors give you more depth of field for the same size aperture used when compared to full-frame ones. If you’re interested in learning more about depth of field and crop factor, we’re glad to continue the discussion in the forum.

Applications for DSLR users
As noted throughout, crop factor has applicability for fixed-lens cameras just as it does for DSLRs. But since you can’t change lenses on a point-and-shoot, there’s very little reason for compact camera users to spend much time worrying about crop factor.

Grasping the concepts involved in dealing with crop factor are slightly more important for DSLR users, however. There are several considerations for your choice of lens, as well as some increased concern in how you shoot with your camera. These concerns shouldn’t, however, be overstressed. For most people, they will have a minimal impact at best.

It’s important to know your DSLR’s specific crop factor, and to keep it in mind when you purchase lenses. This matters particularly with a wide-angle lens: if you need true wide angle, you need to buy a wider lens for your camera with a crop factor than you need for your 35mm camera. An 18mm lens with a 1.5x crop factor shows a field of view equivalent to a 27mm lens on a full-frame camera: wide, but not the ultra-wide field of view 18mm would suggest. If you need ultra-wide viewing on a small-sensor digital, you’ll want to look in the 10-12mm range (where, unfortunately, lenses get rather pricey). If you’re an Olympus or Panasonic shooter, the narrow 2x crop factor of the Four Thirds sensor used in your DSLRs makes finding a sufficiently wide lens even harder.

Just the opposite considerations apply to telephoto lenses. With wide-angle shots you lose parts of the shot: with telephoto lenses you usually prefer to get closer, so the crop factor doesn’t usually offend as much. Few people are bothered by having 300mm lenses that perform like 450mm and so on. Just don’t let the crop factor fool you into thinking you’re getting free magnification: you are still losing part of the frame.

Finally, consider that with “older” lenses, or newer lenses that are designed for use on full-frame cameras, you will gain a bit of clarity from the optics. Some caution is needed, since lenses designed for film cameras often aren’t coated to control digital-specific aberrations, but this concern aside, sometimes it pays to buy older equipment.

Digital crop factor does directly affect image capture, but with the way we treat digital photography in this day and age we are able to account for these effects without knowing it. You don’t need to lose sleep over this idea, or go sell your lenses to buy new ones because you found out that you aren’t getting the right depth of field or need a longer focal length.

This information can prove to be invaluable in some instances, and it’s something that DSLR shooters should have a basic grasp on, but it’s impact should be overstated – instead of chatting about crop factor, or worrying about it, get out there and shoot! The best pictures are the ones you take – not the ones you dream of taking.

Published at the beginning of each month, DCR Workshop provides tips, tools, and tricks for taking better pictures and using digital photo technology. If there’s a how-to topic you would like to see covered in a future DCR Workshop installment, send your suggestions to editor@digitalcamerareview.com.

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