The Crop Factor Myth Explained

INTRODUCTION

There’s an idea in photography that cameras with different sized sensors have what we call ‘crop factors’. A large format Alexa 65 has a crop factor of 0,56x compared to a Super 35 camera. A 90mm lens multiplied by 0,56 is 50mm. Therefore, many people say that using a 50mm lens on this camera is going to look exactly the same as using a 90mm lens on this camera.

The truth is that this isn’t exactly 100% correct - for quite an important reason. So, let’s go over a more detailed explanation on what ‘crop factor’ is, how it works and the big misconception about it.   

WHAT IS CROP FACTOR?

As photochemical film photography emerged and cinema cameras were created, there was a push to create film with a standardised size - that could be used across different cameras from different manufacturers and be developed by different laboratories around the world. That film had a total width of approximately 35mm and therefore was called 35mm.

When digital cinema cameras started getting manufactured, they replaced film with photosensitive sensors that stuck to the approximate size of film’s 35mm 4-perf capture area.   

However, along the way some other more niche formats emerged: from smaller 16mm film that was a cheaper alternative, to large format 65mm which maximised the resolution and quality of movies at a higher cost, to tiny ⅔” video chips from early camcorders to smaller micro four thirds photography sensors.

The issue is that when you put the same lens on two cameras with different sensor sizes they will have a field of view that is different, where one image looks wider and one looks tighter.

So, for prospective camera buyers or renters to get a sense of the field of view each camera would have, many manufacturers started to publish what they called a ‘crop factor’ to determine this. 

This means you take your lens’ focal length - for example a 35mm lens - and multiply it by the crop factor of the camera - such as 2x - to arrive at a new focal length number, 70mm. This means that on this smaller sensor your 35mm lens will have approximately the same field of view or magnification as a 70mm lens on a Super35 sensor.   

Since Super 35 sensors are considered the standard size, this has a crop factor of 1x. Camera sensors larger than 35mm would have a crop factor of less than 1x and sensors that are smaller than 35mm would have a crop factor of more than 1x.

THE CROP FACTOR MYTH

So where does the myth part come in? Well, the issue is that many people interpret crop factors as saying that shooting with a 70mm lens on a Super 35 sensor is exactly the same as shooting with a 35mm lens on a smaller sensor with a 2x crop.

What’s important to note is that while the level of magnification of the image may be the same, there are still a bunch of other characteristics that lenses have that will make images different depending on what focal length is chosen. 

So what we should say is that a 70mm lens on a Super 35 sensor has approximately the same field of view of a 35mm lens on a smaller sensor. We shouldn’t say that a 70mm lens on this camera is exactly the same as a 35mm lens on this camera in every way - as different focal lengths come with other secondary characteristics beyond just their field of view.

Rather than different sensors magnifying or zooming out on the width of what we see it’s better to think about it in different terms. If you put the same lens on two different cameras with a larger sensor and a smaller sensor, the way that the light enters the lens and creates an image will be the same. 

The only difference is that the camera with the smaller sensor has less surface area to capture the image with. This makes it feel like the image is ‘cropped in’ in comparison to the larger sensor which can capture more of the surface area and therefore produce an image which feels wider.

Calculating crop factor and then changing the lens on the camera to a more telephoto lens - may make the width of the images match, but will also change the very nature of the image by altering the depth of field, the compression and the distortion in the image. 

THE EFFECTS OF FOCAL LENGTHS

The smaller the sensor is, the more cropped in the image will be and therefore the wider focal lengths you will need to use. Whereas the larger the sensor is, the wider the shot will appear which means cinematographers will often choose longer, more telephoto lenses.

One of the secondary effects from using longer focal lengths is that it will create a shallower depth of field. This means that the area that is in focus will be much narrower on a telephoto lens, which means the background will be softer with more bokeh. 

This is why movies shot on cameras with large format sensors bigger than Super 35, like the Alexa 65, which cinematographers pair with longer focal length lenses will have a much shallower depth of field, with soft, out of focus backgrounds.

It is a misconception that larger sensors create this effect. In fact, it is the longer focal length lenses that do this.

Another effect that focal lengths have is on how compressed the image is. Wider focal lengths expand the background and make objects behind characters appear further away.

Telephoto lenses compress the background and have the effect of bringing different planes closer to the character.  

For this reason, cameras with smaller sensors that need to use wide lenses, may produce images that appear a bit ‘flatter’ without much depth, especially in wide shots. While large format cameras, with their longer lenses, compress the background to create a bit more of a layered perception of dimensionality.

Wider lenses also have a tendency to distort the image more. So, shooting a close up of an actor on a Super 35 camera with a wider focal length will expand their face and make their features unnaturally larger, while using a longer focal length on a large format camera with the same equivalent field of view will compress the faces of actors a bit more which many say is a bit more flattering. 

CROP FACTORS OF DIFFERENT SENSORS

Although modern digital cinema camera sensors come in many shapes and sizes, in general they conform to a few approximate dimensions. 

Some cameras come with the option to shoot a very small section of the sensor that is equivalent to 16mm film. This has an approximate crop factor of 2x compared to Super35. 

This little format will usually be paired with wider lenses designed for 16mm - such as the Ultra 16 Primes which range from 6mm up to 50mm focal lengths, which with a crop factor applied produces a field of view of around 12mm-100mm when adjusted for Super35. As we discussed, this 6mm will produce an image with extremely limited bokeh and a deep depth of field that feels quite dimensionally flat. 

Next we have Super35 sensors which are usually considered standard, such as we find on an Alexa 35 or Red Helium. Each manufacturer produces sensors with subtly different dimensions - but most will be the approximate size of 4-perf 35mm film and produce the standardised field of view, where a 18-24mm focal length feels wide, a 35-50mm lens is about a medium, and anything longer at around 85mm starts to have a compressed, telephoto feel.

Anything bigger than Super35 size is usually considered to be ‘large format’. This includes ‘full frame’ sensors modelled on still cameras that are approximately 36x24mm. Some examples are the Arri Alexa Mini LF, the Sony Venice 6K or the Sony FX9.

These cameras will have a crop factor of somewhere around 0.67x, which bumps a wider perspective up to around 32mm, a medium feel to around 65mm and a telephoto lens to about 110mm.

65 cameras like the Alexa 65 push this even more with their approximate 0,56x crop factor that makes a 45mm lens a wide, a 90mm lens a medium and a 150mm a telephoto. As we discussed, shooting a wide field of view with a 45mm will produce much more compression, bokeh and dimensionality than using a 12mm lens on a 16mm camera - even though they’ll produce a similar field of view. 

It’s important to note that these crop factor numbers are all relative to what sensor size is considered the ‘standard’. For example, in still photography a full frame sensor is usually considered normal with a 1x crop factor, which means smaller APSC sensors which are roughly close to Super 35 will have around a 1.5x or 1.6x crop factor. 

What is much more important than getting super technical about these crop factor numbers is understanding how larger or smaller sensor sizes affect the field of view and understanding all the secondary effects that using different focal lengths will have on the image.