If you're new to digital photography or just looking at the wide selection of DSLR cameras available today, you've probably seen the term "crop factor" more than once. It's an important consideration in choosing cameras and lenses. Unfortunately, if you've tried to figure out what it means, you've probably run into a wall of formulas and terminology that only made the answers more confusing. Let's try and clear up that confusion with a few simple explanations, shall we?
It's all about standards.
Photography has evolved as technology has advanced. During that evolution, one of the earliest standards that made the medium more accessible to the general public was the advent of the 35mm film format in consumer cameras. (Don't worry, I'm not going to get into a long history lesson.) The point is, the 35mm film camera set a standard for the camera industry.
Like some other film camera standards, that format has been adopted in the world of digital photography as the norm. A "full frame" DSLR camera, which is the standard for many pros, uses a sensor that matches the 24mm x 36mm size of a 35mm film negative as closely as possible. In most cases, this sensor size is the reference point for determining the crop factor of a digital camera.
Why do we need it?
In short, the crop factor is used to determine how a camera's sensor will perform with different lens focal lengths, based on that 35mm, full frame standard. As you probably know, there are a surprising number of sensor sizes available in digital cameras. Each of these sizes will provide a different field of view for a given lens and focal length.
The difference is fairly easy to understand if you consider that a lens transmits a circle of light of a fixed size on a rectangular sensor. The size of the rectangle determines the camera's field of view. If we take the same circle of light and project it over a smaller rectangle, we're going to get less of the total picture within that rectangle, i.e. a smaller field of view. The opposite applies, too, of course, and it's important to note that some digital sensors are actually larger than the full frame format.
Now, if you think about the way that works, you'll realize that by making the field of view smaller, the size of the objects in the frame will appear to be larger, just as if you zoomed in. Because of that effect, the crop factor is sometimes referred to as the "magnification factor." What that means to you is that the effective focal length of a lens will be higher on a camera with a smaller sensor.
So, a smaller sensor is better?
Not necessarily. A smaller sensor also means that a photo will have to be enlarged more to fit in the same area as a full-frame sensor photo. That means that any noise present will be more noticeable and lower pixel resolution can become a problem. The best reasons for choosing a crop-sensor camera have very little to do with higher magnification.
How do I use the crop factor?
Here's the simplest part to understand! To use the crop factor, simply multiply the focal length of the lens by the crop factor of a camera. For example, an APS-C camera sensor has a crop factor of 1.6. That means that a 50mm lens on this camera will have an effective focal length of:
50 x 1.6 = 80mm
That's quite a difference. Now, let's look at the same lens on a full frame camera, which has a crop factor of 1.0:
50 x 1.0 = 50mm
See how that works? Finally, let's put the same lens on a 645 camera with a crop factor of 0.79:
50 x 0.79 = 39.5mm
This decimal crop factor and negative magnification mean that the field of view of the camera is going to be larger than the circle of light coming through the lens and you probably won't like the result.
Is that all it's good for?
Nope. In fact, if you're using a crop-sensor camera, one of the best ways to use the crop factor is in reverse. Let's say I want to know what lens will give me the same field of view as a "nifty fifty" on a Canon APS-C camera. In this case, We'll DIVIDE the focal length by the crop factor:
50 / 1.6 = 31.25mm
How about on a Nikon crop-sensor camera?
50 / 1.5 = 33.33mm
That means that the closest prime lens focal length for either camera will be 35mm. Now you know why that focal length is often considered the "normal" lens for crop-sensor cameras.
Is that it?
No. There's plenty more to know about crop factors, sensor sizes and types and lens types. I promised a simple explanation in this article and you now have enough information to understand how the crop factor works, how simple it is and why it's important.
If you'd like to know more about the relationship between lens and camera, how sensor sizes and types affect your photos or anything else about being a photographer, check out our unique photography community here. You'll be glad you did.