You’ve probably come across the term ’35mm equivalent’ before. If not, brace for it. Another variation of the term is ‘full frame equivalent’. This article will tell you why you need any equivalent for anything.
Why we need an ‘equivalent’ at all
When you compare two things, one of them will be a standard, a known quantity.
E.g., if you travel abroad, you’ll be faced with the prospect of finding the ‘equivalent’ rates of the foreign currency to your local currency, so you know exactly how much you’re spending. In this case, the ‘standard’ is your local currency, the one around which your whole life revolves.
Similarly with cameras, we have different sensor sizes. There are full frame cameras (36mm x 24mm), APS-C cameras (e.g., 22.3mm × 14.9mm among others), Micro Four Thirds (7.3mm × 13.0mm), iPhone 5 (4.54mm x 3.42mm), and so on. Every camera needs a lens in front of it, and often, you can use a lens designed for one sensor on another sensor. This is where the problem starts.
Think of windows. A larger window will show more scenery than a smaller one. Yet, the focal length of your eye remains the same. Sensors behave like windows. A larger sensor will always show you more scenery than a smaller one.
Now, there are three ways to measure the scenery ‘projected’ on a sensor:
Depending on how you use your camera, you might find yourself preferring one of the three. The outer box is a larger sensor, while the blue outline is the smaller one. Even though the angle from the lens is the same (which is dictated by its focal length), the ‘projection’ changes. Parts of the scene are cut off in the smaller sensor.
E.g. let’s say a 50mm lens on a full-frame DSLR camera (sensor size: 36mm x 24mm) covers a scene in this fashion:
- Horizontal angle of view: 39.6o
- Vertical angle of view: 27o
- Diagonal angle of view: 46.8o
Now, let’s say someone uses this lens on a smaller sensor, say, APS-C, which has a sensor size of 22.3 × 14.9 mm, and parts of the scene that were visible with a full-frame DSLR sensor are cut off. This person shows you the final image, but lies to you and says it was shot on a full frame sensor.
He gives you the distance to the subject and the width of the subject, and asks you to guess the focal length. Using a bit of simple trigonometry you arrive at the conclusion that an 80mm lens was used. Obviously, the lens used was a 50mm, but the same lens on an APS-C sensor gives the approximate equivalent of an 80mm lens on a full frame sensor.
Here’s another way this information is helpful: Let’s say you have two camera bodies dangling around your neck – one a full-frame DSLR and another a Micro Four Thirds camera. You want to shoot the exact same scene with both cameras. You quickly shoot the first with the full frame DSLR, using a 24mm lens. Now, you can’t use 24mm on the smaller camera because it would give you a different scene. To get the same scene, you realize you need a 12mm lens.
This is how the lens equivalent works.
Problems with ‘equivalent’
We’ve already seen one scenario where you couldn’t get the exact same equivalent and had to compromise. In fact, looking for the exact equivalent is really stupid. Here are some factors that make it so:
- Not only are sensor sizes different, but they might have different aspect ratios. So, the equivalent along one dimension (say, horizontal) will not translate into the equivalent on another (the vertical), and so on.
- Sensors are not always precise. E.g., a Canon 5D Mark III has a horizontal size of 36mm, while the Nikon D800E, another full-frame sensor, has a horizontal size of 35.9mm.
- Lenses are not always precise! A 50mm lens from Canon will not match a 50mm lens for Nikon or Zeiss.
Therefore, whenever you’re looking for equivalent lenses, getting a rough idea is enough. The only way to know for sure if the lens will work for your scene is to use it.
Whose 35mm are we talking about here?
The title of this piece asks: Why is it confusing? ‘It’ here does not refer to ‘equivalent’, which is simple enough to understand. It’s for the other word: ’35mm’.
It’s like saying ‘dollar’. What do you mean: American, Australian, Singaporean, Namibian? Similarly, for better or for worse, there are two kinds of 35mm in the world of video and cinematography (only one in stills though, which is the full frame 36mm x 24mm sensor):
- 36mm x 24mm, simply known to the photographic world as 35mm or full frame (neglecting the niche market of medium and large format).
- Super 35mm
Cameramen brought up on film might only use the word ’35mm’ in one of its cinematography avatars (just showing the horizontal sizes and aspect ratio for simplicity):
- Academy – 22mm (1.375:1)
- Widescreen – 21.95mm (1.85:1)
- Cinemascope – 21.95mm (2.39:1)
- Super 35mm 3-perf – 24.89mm (16:9, 2.39:1)
- Super 35mm 4-perf – 24.89mm (4:3)
There are people who think one should use Super 35mm when calculating the 35mm equivalent. After all, for video, why should anyone consider a standard photographic sensor size anyway?
Fair enough. Which one of the above five should one pick? You see, Super 35mm is a relatively new standard (if it can be called that) even in the film world. In the digital world, even if sensor manufacturers use the term ‘Super 35mm’, they don’t actually mean the film size that it refers to. E.g (horizontal sensor sizes only):
- Arri Alexa – 23.76mm
- Red Epic – 27.7mm
- Red Epic Dragon – 30.7mm
- Blackmagic Production Camera 4K – 21.12mm
- Canon C500 – 24.6mm
- Sony F55 – 24mm
While the Blackmagic 4K Camera has a smaller than Super 35mm size, the Red Epic Dragon has a bigger than Super 35mm size. In fact, not a single manufacturer makes the exact same size as Super 35mm 3-perf!
Just for your understanding, here’s a diagram comparing the two versions of Super 35mm:
See the problem? There is no single standard called Super 35mm.
Cinematographers, traditionally, never use the term ‘full frame’ unless they’re looking for a fight. They also don’t use the term ‘Super 35mm’ by itself simply because there are different versions like 4-perf, 3-perf, 2-perf and so on. They would have to get more specific than that. Some cinematographers prefer anamorphic exclusively, so what would ‘full frame’ or ‘Super 35mm’ mean to them? Nothing.
Super 35mm, then, is a term ‘up for grabs’. Digital camera manufacturers have taken advantage of this ambiguity even though they offer different sensor sizes, as you can see from the above list.
To make my life (and yours) easier, I use the full frame 35mm size of 36mm x 24mm when calculating the ’35mm equivalent’ or ‘full-frame equivalent’. Here are my reasons:
- Full-frame 35mm is common and refers to a fixed size and aspect ratio, both horizontally and vertically. There is no ambiguity.
- The full frame 35mm size is uncorrupted by bad marketing!
- DSLRs and photography cameras are ubiquitous.
- Modern cinematographers and filmmakers are brought up on DSLR video shooting.
- Even traditional cinematographers shoot stills, so they know what full frame 35mm means.
- Lens manufacturers like Canon, Zeiss, Schneider, etc., have made cine lenses that cover this larger area, so it is only reasonable to expect more cameras with full-frame sensors in the future.
- Stills and video are converging!
Therefore, for the sake of world peace (and peace of mind), at least on this blog, when I say ’35mm equivalent’, I am referring to the currency of 36mm x 24mm.