Digital SLR Camera Crop Factor

Crop factor is a term that can cause a lot of confusion when you're trying to pick a digital SLR camera.

It doesn't help that it's referred to in several different ways. For example, all of the following mean the exact same thing:

Camera Z has a 1.5 times crop factor
Camera Z has a focal length multiplier of 1.5 times
Camera Z has a 1.5 times lens focal length conversion factor
A 50mm lens frames like a 75mm lens on Camera Z

The key thing to focus on is the multiplier: the three most common ones are 1.5, 1.6 and 2.0.

In a moment, I'll explain just what sort of multiplication is going on.

Digital Sensors vs. Film

It's easier to understand crop factor when you understand the difference in size between a digital SLR camera sensor and a standard 35mm film negative.

Key point #1: digital sensors are smaller than film.

Imagine that this circular image of a mountain is how the lens on your camera sees the world. In reality, the image captured by the lens is upside-down, but we'll ignore that for now.

The large box is the size of a 35mm film negative.

The smaller box is the size of a digital SLR sensor.

First you take the photo with a standard roll of film.

The example to the right shows how much of the scene is captured on the 35mm film.

The entire mountain is captured in this image.

Here's how our digital SLR sensor sees the exact same scene, because the sensor is smaller than the 35mm film.

Notice what happened?

Large parts of the top and side of the photo are cut off, creating an artificial zoom effect.

Key point #2: if you stood in the exact same spot and took the same image with the exact same lens on a film and digital SLR camera, the SLR camera would capture less of the scene.

The digital SLR sensor has "cropped" the view - hence the term crop factor.

In order to capture the same view with your digital SLR, you would have to step backwards, away from the scene you are photographing.

Crop Factor Multipliers

Time for a little bit of math. Not much, I promise.

Let's go back to those crop factor multipliers that I was talking about at the top of the page.

Make sure you understand this point before going on: even though the smaller size sensor is cropping the image, the perceived effect is that you have zoomed closer to your subject.

This is why a lens that would otherwise be wide angle on a film camera becomes less so when attached to a digital SLR.

Example 1 - 100mm Lens

Let's say that you have a lens with a focal length of 100mm.

When you attach this lens to a Nikon D50 with a 1.5x crop factor, this 100mm lens captures images more like a 150mm lens. [100mm x 1.5 = 150mm]

Let's work through this example for several more crop factors:

Focal Length	Crop Factor	"Adjusted" Focal Length
100mm	1.6	160mm
100mm	2.0	200mm

Example 2 - 28mm to 75mm Zoom Lens

When you have a zoom lens, you just have to apply the multiplier to both ends of the zoom range:

Focal Length	Crop Factor	"Adjusted" Focal Length
28mm	1.5	42mm
75mm	1.5	113mm
28mm	1.6	45mm
75mm	1.6	120mm
28mm	2.0	56mm
75mm	2.0	150mm

Is a High Crop Factor Bad?

Not necessarily. It depends on what you want to photograph with your digital SLR camera.

If you enjoy taking portraits, then a high crop factor is not going to affect you very much.
If you take landscape photos - or a lot of photos indoors - then crop factor will have an impact.

Why does crop factor have an impact on landscape photographers?

Beautiful landscape photography makes good use of very wide-angle lenses, to capture all of the scenery in front of the camera.

A lens with a focal length of 28mm is considered wide angle for a film SLR camera.

But it's not as wide angle when attached to a digital SLR (42mm remember?).

This means that landscape enthusiasts have to use really wide angle lenses to capture those scenic vistas. 18mm and even 10mm lenses are not uncommon for dedicated digital landscape photographers.

If you've been reading up on digital SLRs, you'll notice that many kit lenses (a package lens sold with the camera) are 18-55mm.

Now you know why this is such a common choice when it comes to digital SLR kit lenses.

Crop factor is the culprit.