🎬 Sensor Size and Crop Factor

Your lens projects its circular image onto your camera’s sensor. The image circle could be smaller or larger than your sensor–the lens doesn’t care. So what would happen if your sensor is significantly smaller than the image circle projected by the lens? You’d see only the center of the image. And vice versa, what happens if the sensor is larger than the image circle? In this case, you see the entirety of the lens’s image, and the black area around that image circle where the sensor is not exposed to light. This darkening around the edges usually isn’t this extreme in real life, but it’s what we call vignetting. Lens manufacturers generally try to find the sweet spot where their lens is small and light enough to just cover the entirety of the sensor without too much noticeable vignetting. It is quite normal for a lens to fall off both in brightness and sharpness towards the edges of the field. So it makes sense that a lens would be designed for a specific sensor size.

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Sensors come in many different sizes. Many people assume the bigger the better since large sensor cameras are generally more expensive. Larger sensors mean larger photosites (the little photo receptors that actually gather light on the sensor) and that means improved light sensitivity and dynamic range, but the difference is not so large as it once was. Larger sensors make shallow depth of field more easily achievable. We’ll talk more about what that means later, but it’s essentially how narrow your area of focus is which relates to how blurry your background can be. Low-budget productions often benefit from the ability to blur the background when sets are less-than-high-end, and it’s an easy way to direct focus to a subject. That being said, in some situations these “advantages” can be undesirable. Larger sensors require larger lenses making them comparatively less portable, they provide a wider angle of view meaning “less zoom”, and they can make deep depth of field (think macro photography) more challenging. The tricky part about crop factor is that lenses are classified according to their “focal length”: a value (usually in millimeters) that refers to the distance between the back of the lens (where light rays converge) and the sensor. The focal length is what you’ve probably known as the “zoom” of the camera. The problem is this: what if your lens is projecting this displayed image onto your sensor but the sensor is only wide enough to capture the blue box in the center? Your true focal length hasn’t changed, but your “effective focal length” is now much greater because the sensor is only picking up the center of the image making the image appear more “zoomed.” The recorded image you see will be very zoomed in or “telephoto” looking because you’re only seeing part of it. For this reason, we need a baseline to determine this “effective focal length” or “field of view” and we’ll compare all others against it. That baseline is 35mm “full frame” film. Anything smaller is a “crop” of that and will have an associated “crop factor”. Multiply the focal length of the lens (in mm) by the crop factor and you have your effective focal length. For now, just know that it’s important to know the size of your sensor because it determines how much of the scene you see, which we’ll call your “field of view”.

35mm “Full Frame” is about 36mm wide. Again, that’s the gold standard. In photo cameras, this film was run horizontally through the camera, meaning the perforations run along the top and bottom. This leads to an image area of approximately 36mmx24mm or “full frame”. Many of today’s digital cameras still use 36mmx24mm sensors based on this initial 35mm sizing. Examples include Canon’s 5D line, and Sony’s popular A7 series cameras. It’s also become quite popular for motion picture video cameras to use this same full frame sized sensor. High end cameras like the Sony Venice, Arri Alexa LF and Red Monster all cover something similar to this full frame size. However, this historically has not been the case, and a lot of people shooting on their Canon 5D Mark II cameras at the start of the “DSLR revolution” didn’t realize that they were shooting to a larger full frame ’sensor’ than most of their favorite Hollywood movies had used. In motion pictures, that same 35mm film was run vertically, rather than horizontally. In this case the area between the sprockets becomes the width, instead of the height of the image like it was in a stills camera. This makes the full frame image approximately one and a half times larger than the image from the 35mm film run horizontally, and it’s very close to the format still photographers know as APS or “cropped sensor” because it’s a ‘cropped’ version relative to the full frame size. Popular cameras include the Canon Rebel and SL series (e.g. T6i, SL2, 80D, etc.) , Nikon’s DX line (D3500, D5600, D7500 etc.) and the Sony alpha 6X00 line (e.g. a6500). “APS” is what most people refer to if they talk about “cropped” cameras vs “full frame” cameras. Again, the crop factor of most APS sensors is around 1.5X. APS is about 25mmx19mm. MFT (micro four thirds), another popular sensor size is about 22mm wide. iPhone XS is about 5.6mm wide (but classified as a 1/2″ sensor-see the bonus section for more information there.)

So what’s the takeaway? Know your camera’s sensor size and what that means to you. Know how it relates to your lens options, especially concerning field of view and depth of field. It’s nice to know how it compares to other cameras, but don’t obsess over it. You likely don’t need “full frame” any more than your favorite directors of photography didn’t need it when shooting your favorite films.

This great video from Apalapse describes focal length and sensor crop very clearly.

When An Inch Isn’t An Inch

The keen observer will notice a discrepancy in the actual sensor size and the “categorized” sensor size of smaller sensors. Small sensors, like those in cell phones, will often be sized based on a fraction-of-an-inch standard. The iPhone XS, for example, is a 5.6mm x 4.25mm Sony sensor (it’s active area is this size at least). But this is classed as a 1/2″ sensor. Now on these smaller sizes, the measurement is diagonal, unlike the width measurement of the 36mm film, but even so, the actual physical sensor size is much smaller than 1/2″. This is because this technology is based in old video sensor tubes which were measured in inches and not by their actual active image area. So to figure the actual crop factor of an iPhone relative to full frame, use the 5.6mm width figure for an approximate 6.4x crop factor. This means the 4.2mm focal length wide angle lens of the iPhone XS is between 26–27mm.

Converting Angle of View and Focal Length

These two formulas are more advanced than we will cover here, but they come in useful: Field of View to Focal Length

ANGLE OF VIEW (IN DEGREES) = 2 ARCTAN( SENSOR WIDTH / (2 X FOCAL LENGTH)) * (180/Π)

Other Sensor Sizes in Photo and Video

There are other sensor sizes as well. The popular Micro Four Thirds cameras use an even smaller sensor (close to a 2X crop of full frame). Other high quality cameras use a 1” sensor, like the popular Sony RX100 series or the DJI Mavic 2 Pro drones. Many cell phones in 2019, center around a 1/2” sensor.  Going the other direction, there are medium and large format stills cameras with sensors as large as 9×11” (This would be consider “Ultra Large Format” and isn’t really practical in 2019). On the motion picture side there’s IMAX. IMAX film is not only 65mm wide (much larger than the aforementioned 35mm), but it’s run horizontally through the camera like the stills cameras do. Each frame occupies 15 perforations  meaning the film has to move at over 6km/h through the camera. This makes for an imaging area 9 times larger than that original 35mm format, at the cost of extreme bulk, noise, and a high price. The world of IMAX cinematographers is not large and I’ve had the privilege of working with some of the few cinematographers and directors who still shoot this format. There’s a reason they love it so much, but it’s not a practical solution for most shoots. So what’s the digital equivalent to IMAX? Well, it’s been slow to come, but you may have seen the first Hollywood film to be shot entirely in digital IMAX if you witnessed the micro budget, independent film “Avengers: Endgame”. ARRI Alexa IMAX digital camera (2015) Avengers: Endgame was shot completely with this camera. That being said, Vision Research also made a 65mm camera which you likely never heard of so this really isn’t a new idea. And, even the IMAX-labelled Arri is a 65mm camera, so in terms of surface area of its sensor it’s drastically smaller than its IMAX film counterpart. So remember how we talked about lens and sensor sizing working in tandem. Not only are cameras like this rental-only items, the lenses are rebranded Hasselblads designed to cover medium format size. And you can’t afford either of them. But the good thing is, you don’t need to. And you don’t need to care about the information presented in the latter half of this video for anything more than information’s sake. If you consider what the high-end cinema industry has gone to to get to this level, and that the majority of your favorite films have likely been shot on that traditional super-35 or APS crop sensor size, isn’t it a marvel to thing what you can do with a full frame A7 series Sony camera? In my view, IMAX as a format is little more than a brand labeling that has little to do with an audience’s experience when watching a film. If resolution and size were the most important factors, there likely would have been more public outcry over the “LieMAX” maneuver when IMAX replaced their high resolution film projectors with 2K “low resolution” equivalents. Nobody I know personally cared much, and many still paid the additional premium for a regrettably inferior technical experience. If they cared about this during the movie though I doubt the movie was well made.

Steve Yedlin’s resolution demo is a very insightful look at comparing the resolution of several of these larger formats, both in film and digital varieties: http://www.yedlin.net/ResDemo/ResDemoPt1.html

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