People talk about the crop factor, but actually there are four of them. If you have heard the word before but are thoroughly confused, rest assured, it’s not your fault. The camera manufacturers’ marketing departments don’t really care, and their apathy ruins it for the rest of us.
This article looks at:
- What a crop factor is
- The four types of crop factors, and
- When to use each
By understanding all the four kinds you can confidently put your knowledge to good use when selecting lenses for your camera. Moreover, you will be immune from crappy marketing gimmicks forever.
What is a crop factor?
Start by reading the meaning of the 35mm equivalent, if you haven’t already. The basis of the crop factor stems from the need to use a particular lens on different camera bodies. Whereas in the 35mm equivalent, you are judging and comparing lenses for your sensor, the crop factor compares sensors directly.
The crop factor is simple: It is the ratio of (something) of one sensor to the same (something) of another sensor, so you know how they compare.
E.g., if sensor A is 36mm x 24mm (full frame 35mm), and sensor B is 24mm x 16mm, then the crop factor could be 36mm (something of A) / 24mm (same something of B) = 1.5.
It’s a simple number which you can then use to calculate the equivalent lens coverage, or angle of view. E.g., if a 50mm lens is used on sensor A, the same lens will have an equivalent focal length of 50mm x 1.5 = 75mm on sensor B. Now, if he had a third sensor, C, with a size of 44mm x 33mm, then the crop factor of C with respect to A is 36mm / 44mm = 0.82. The same 50mm lens on sensor C will be the equivalent of 50mm x 0.82 = 41mm.
Here’s the cool part: As long as you fix the ‘something’ across all sensors, you can use the crop factor for all lenses. If the crop factor between two sensors is 1.9, say, then no matter what lens you use, you can multiply it by 1.9 to find the equivalent focal length on the smaller sensor.
As you might have guessed, you can also use the other quantity, the height, of each sensor. This leads us to the next section. But before we move on, know this: Since I have adopted 36mm x 24mm (full frame 35mm) as the gold standard, I will be using this sensor size as the baseline.
The four types of crop factors
Let’s assume a sensor has a width W and height H. There are four somethings (dimensions) of a sensor that you can use:
- Width = W
- Height = H
- Diagonal = ?(W2+H2)
- Area = W x H
Width (W), or the Horizontal Crop Factor
If you don’t know much about lenses, I suggest you begin by reading What lens to get?
The most common ‘logic’ for selecting a focal length is to fit something horizontally, whether you have a mountain range or a car or even a human being in your frame. But people are taller than they are wider, you say. Well, yes, but you want to fit them horizontally in the frame first, don’t you? Now matter how you slice it, a video and cinema frame is horizontal.
Even our eyes defer to a greater horizontal angle of view, and that’s just the way we see things. Here’s how it works:
Like the example in the previous section, if you have two sensors, you compare their widths. What you get is the Horizontal Crop Factor. Once you have this relationship fixed, you can always select a lens and immediately know what it’ll look like on either sensor.
Height (H), or the Vertical Crop Factor
You guessed it, the Vertical Crop Factor compares sensor heights: If the width of an object is the most important attribute, its height is not far behind. After all, if you want a long shot of a person you are directly referring to the vertical size of the sensor, which must accommodate the person. If you want to get the full height of a tall building, you must either use an extremely wide lens or walk far enough to get it in your frame.
One scenario the vertical crop factor comes in handy is if you want to match the shot size (close-up, mid-shot, long-shot, etc.) between different sensors. Here’s an example:
As you can see, the width of the sensor just ‘hangs around’ as you pin your model to the desired height. When you change a sensor, you want to keep this height constant, so you will choose the vertical crop factor to find a lens that will give you a similar angle of view – in the vertical direction.
Even so, you’re always limited by the size of your set for long shots. Close ups are slightly easier to deal with.
Diagonal, or the Diagonal Crop Factor
Why would anyone need the Diagonal Crop Factor? They don’t, but there is a good reason for its existence: the image circle. The image circle of a lens is circular, and a sensor is always designed to fit within the image circle. If a sensor fits perfectly, the diameter of the image circle should correspond to the diagonal of the frame:
However, you will immediately discern that, if you change the size of the sensor (which is what this is all about) or its shape (the aspect ratio), this parameter becomes meaningless. Furthermore, a sensor usually falls within the image circle. Nobody designs the two to match precisely, or you risk serious vignetting.
Area, or the Area Crop Factor
Comparing the areas of two sensors is like playing “My daddy can bench press your daddy…”. No he can’t, but in marketing it’s allowed. The Area Crop Factor is as beneficial as a megapixel, or a pat on the back when you’re sinking in quicksand.
So, which crop factor should I use?
Use either the horizontal or the vertical crop factor. Both have good uses. Leave the diagonal and area crop factors for the kids and the trolls.
Between the horizontal and vertical crop factors, I prefer the horizontal crop factor as the defining standard for the crop factor. This is simply for the reasons stated above – our use of lenses are primarily governed by our availability of horizontal space. Even if want a long shot of a super hot actress, but the scenery sucks, we’ll shut out our egos and zoom in for a tighter shot – not that that’s a bad thing…ahem.
The following table shows a few camera formats, their sensor sizes and the horizontal and vertical crop factors, compared to 36mm x 24mm full frame (click to enlarge):
The coolest thing about the crop factor is you can compare equivalent sizes. E.g., the crop factor between full frame 35mm and Super 35mm 3-perf is 1.45. The crop factor between the latter and the Panasonic GH4 is 1.44. On the other scale, the same relationship applies to FF 35mm when compared to 65mm.
Play all you want.
That’s it. I hope you have a firm grasp on crop factors now. You can use it flexibly, but please use it responsibly. It is always important to explicitly identify and define the base standard that you’re going to ‘crop’ – at the very least to yourself.