What is Crop Factor?

Field of View & Focal Length We're gonna nerd out a bit in this article, but crop factor is important and it can be a little confusing at first. Before we dive in, need to make sure we're on the same page with terminology: Field Of View What is field of view? It's what a specific camera and lens combination actually sees. More specifically, it's what a lens and camera sensor see. Camera perspective is the distance or positioning of a camera. If I move closer to a subject, my camera perspective changes. The field of view remains the same. Yes, what we see in our viewfinder will look different based on a camera perspective change, but it's still the same field of view. Make sense? Focal Length Lenses have what's called a focal length rating. Light enters a lens in parallel and it's bent and converges prior to hitting the camera's sensor. Focal length is the measurement from roughly the convergence point to the sensor hit (in millimeters). Focal lengths do affect field of view. They also have characteristics that effect magnification and such. This is a conversation for another time. But just know that focal lengths are story telling tools. Full Frame vs Super 35 Film To understand crop factor we need to begin with film as that's where it all came from. A strip of analog film has perforations on the sides called "perfs." Sprockets in cameras and projectors use these perfs to move the film. The most common film for movies was 35mm—unless of course you're Chris Nolan. If you take an old school photography camera, the roll of 35 millimeter film feeds sideways and each shot uses a length of 8-perf. Whereas many cinema cameras feed film vertically. So instead of shooting on an 8-perf frame width, they shoot on a 3-perf height and this is called Super 35. In the image above we see full 8-perf 35mm on the left (fed horizontally) and roughly 3-perf Super 35 on the right fed horizontally. You can also see sound information was stored alongside picture for the cinema film on the right. Digital Sensor Crop Factor So why do we need to know all of this? Because it directly relates to digital camera sensors. The size of a digital camera sensor is measured with a crop factor. And this crop factor is based off the size of 8-perf 35 millimeter film. Not Super 35. Does this make sense? If a manufacturer says their cinema camera has a full frame sensor this means the digital sensor is the same size as 8-perf 35mm. And this a crop factor of one because it's the same size as 8-perf 35mm film. It's a 1:1 size difference. Got it? A cinema camera with a digital sensor the same size as Super 35 has a crop factor of around 1.6 (varies by camera manufacturer). But a crop factor of 1.6 means the sensor is 1.6 times smaller than a full frame sensor. Another common format you'll hear about is Micro Four Thirds (MFT) sensor. These cameras have a crop factor of around 1.9—again this depends on the manufacturer. Above we see Blackmagic Design cameras compared beginning with the Pocket 4K (MFT) to the Cinema Camera 6K (full frame). How Does Crop Factor Affect Filmmaking? So this is the big question, right? What crop factor should you get for making your movies? I've shot films using an MFT and I've also used a full frame. Both work just fine. Both have advantages and disadvantages. I prefer full frame, but let's talk about it. Sensor Size Does Not Affect Resolution Sensor size is not tied to pixel count. You can shoot 4K on an iPhone or you can shoot 4K on an ARRI Alexa. The phone rings in around $1K and the ARRI around $30K. Huge difference in price and quality. Referencing the Blackmagic Design camera chart above you can see that both the Pocket 6K and the Cinema Camera 6K shoot the same resolution. So what does the full frame sensor do? Image Quality First, with all things being equal, a larger sensor shooting the same resolution as a smaller one can yield a better image. Larger photosites in theory can capture better images than tiny photosites. So even though you're iPhone 16 is shooting 4K RAW, the sensor cannot do what a larger 4K sensor can do. Now I can tell you with real world testing that a full frame sensor is not guaranteed to deliver a better image in every scenario. For example, the Pocket 6K does better in low light than the Cinema Camera 6K. So you have to know your cameras. But in general, the larger the sensor the better the image. This is definitely true comparing phone sensors to cinema camera sensors. But the most important story telling difference with cropped sensors? Field of view. Field of View Changes With Crop Factor A cropped sensor reduces our field of view. If I take two 50mm lenses and put them on a Pocket 4K and a Cinema Camera 6K and shoot from the same physical location, what the camera and lens combination see—field of view—will be radically different. You can see this in the image above. I had the cameras in the same spot with the same focal length. Massive difference. And this affects us as filmmakers in big ways. For example: My latest film required shooting in tight places. Barns. Old cabins. And a micro-four thirds camera required me to be on a wide lens at times. One scene in particular forced me onto a 24mm lens in order to just get a medium shot, because I couldn't back up due to space constraints. And this can be a problem if I don't want the distortion of a wider lens. It can also be a challenge for beginning filmmakers who can't afford multiple lenses. What required a 24mm lens on the MFT camera would have been a 50mm lens on a full frame. And 50mm is more true to what the human eye sees. Meaning, less distortion. A wide lens is still going to be a wide lens. A long lens is still going to be a long lens—regardless of crop factor. What About Depth of Field? You might hear someone say crop factor also affects depth of field. This is not true. Crop factor can indirectly affect depth of field in certain scenarios. I cover this in detail in our depth of field post. DOF is an important thing to wrap your head around, and there are misnomers out there about it. In Summary, What is Crop Factor? A way to measure sensors compared to 8-perf 35mm film. A digital sensor the same size as 8-perf is called full frame with a 1:1 crop ratio. Crop factor is not tied to resolution. All things being equal, full frame will deliver better images than cropped sensors. Crop factor affects field of view. Which Crop Factor Is Best? I shot 98% of my latest film (RECKONING) on the Pocket 4K which has an MFT sensor. I used 24mm and 50mm lenses for the entire shoot. This worked just fine. The most important thing to understand is what FOV your scenes require and what spaces are you shooting in? If your movie is confined to small spaces, full frame all the way. If you're shooting outside or with plenty of space, an MFT setup can do just fine. But know that purchasing lenses for full frame typically costs A LOT more than MFT. Hopefully this was helpful! If you're new to Write & Direct we are an online film school for aspiring directors. You can read more about the school here. Video Version of Article: https://youtu.be/T332LVSK6BM

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