Sensor Equivalence: Size Does Matter. Or Does It?

The recent spate of smaller and smaller and more and more powerful cameras has brought forth new talk about sensor sizes. Let’s assume sensor size does matter — or doesn’t it? Or only conditionally? The bigger the better?

Greetings from Dr. Freud. The whole crowd will just stand back to make way for you with the ginormous 300mm telephoto lens. You’re all powered up. No one messes with a pro photographer lugging around a giant camera.

Honestly, what’s the real benefit of carrying much larger gear than may be necessary? Bigger sensors are apparently better, but smaller sensors require lighter lenses — a Gordian knot the industry hasn’t cut so far.

What’s good for you? It all boils down to “equivalence,” understanding how different sensor sizes relate to each other. Problem is, F4 doesn’t necessarily equal F4.

Mirror mirror on the wall, who is the fairest of them all |

But first, what the heck is equivalence.

Equivalent photos are photos of a given scene that share the following five parameters:

  • Perspective
  • Framing
  • Depth of field
  • Shutter speed
  • Display dimensions

Say you’re shooting with Micro Four Thirds gear and following settings: 40mm F4 1/100 ISO 200.

Shooting the exactly same scene with the exactly same settings will give you a different image on full-frame.

To get an “equivalent” image on full-frame taken from the same position and displayed at the same size you would require 80mm F8 1/100 ISO 800, and 50mm F5 1/100 ISO 320 on a 1.6x crop sensor.

Alternatively, if one used a 50mm F1.4 on a full-frame sensor, this would produce a depth of field so shallow it would require an aperture of 0.9 on a camera with a 1.6x crop factor — not possible with consumer lenses.

Or more simply: 50mm F2 on Four Thirds is equivalent to 100mm F4 on full-frame.

So why does this matter.

Because this is the 21st century, and there’s still no common standard making clear how different sensor sizes relate to each other.

It’s like talking yards and meters, Celsius and Fahrenheit. You’re shooting a 12mm lens on Four Thirds, that’s a 24mm on full-frame. A 12mm is reasonably wide on Four Thirds, really wide on 135 format, a.k.a. 24x36mm sensor/film.

Four Thirds and full-frame shooters wouldn’t understand each other. Both say they’re using a 12mm lens — with completely different results.

To have a common ground the 135 format is generally referred to as as “full-frame,” the base format. Even though every format is “a full frame.” Compared to Four Thirds 135 format is even “double-frame.”

There have been full-frame cameras and half-frame cameras for decades (not to mention other sizes of 135 film). Half-frame 135 film cameras for example.

And it gets more complicated: Take Olympus claiming to have reinvented full-frame.

They quote viewfinder magnification as 1.15x referring to a 50mm lens on a Four Thirds sensor… It’s in fact 0.575x referred to 25mm lens on Four Thirds format. So even Olympus refers to something they actually wish away.

And the marketing of the Ricoh GXR modular camera system, for instance, states focal lengths in full-frame terms, a.k.a. the 50mm F2.5 is in reality a 33mm.

To make this yet more confusing, 135 format is in fact the only format that has crop-sensor modes. Nikon full-frame cameras have a crop mode to shoot at a reduced resolution for faster speeds or with smaller-format DX lenses.

In reality though every sensor is full-frame. The only time a smaller sensor has a crop factor though is when you use a lens designed for a larger format.

Crop factor means the sensor’s diagonal size compared to a full-frame 35mm sensor.

And cropping a.k.a. throwing away information does have its advantages. Nearly all lenses are sharpest at their centers, while quality degrades progressively toward to the edges.

This means that a cropped sensor effectively discards the lowest quality portions of the image.

Meaning Four Thirds is a specifically full-frame format with a telecentric design that ensures vertical incidence of light on the image sensor.

All lenses made for (Micro) Four Thirds are full-frame relative to the Four Thirds sensors that they were designed for.

So on the one side you have, if I may, conformist romantics relating to 135 format as the key standard to understanding the law of focal length, on the other side you have revolutionaries adopting new, smaller sensor sizes and calling their own the real thing.

Both can be dogmatists, et voilà you have an equivalence war… How insignificant do the world’s larger problems suddenly look like.

Well the technically incorrect “full-frame” standard is here to stay for a while. Even honorable DP Review refers to “equiv” on previews and reviews.

But then again, let bygones be bygones. Who cares what 135 format was. Camera and lens makers should have never have marketed lenses for a any “cropped” sensor in 135 terms.

To speak with Ken Rockwell: 35mm film marked with digital camera sensor sizes. Green: Canon 1.3x; Red: Nikon DX; Blue: Canon 1.6x; Nikon and Canon full-frame are the same size as the image in the film. Depth of field of every "frame" is the same. Just the picture is cropped. So the field of view is cropped from a 35mm equivalent |
There is a reason why the Panasonic Lumix 12-35mm F2.8 and the Olympus Zuiko 35-100mm F2 zooms have generated massive hype. Because they give a similar angle of view to Canon’s and Nikons 24-70mm F2.8 and 70-200mm F2.8 pro zooms. So the zooms for the smaller sensors can be conveniently called equivalent to 24-70mm and 70-200mm on full-frame, with the exact same amount of light on the sensor for a given shutter speed as f/5.6 on full-frame.

It’s not that simple though. The focal length is the focal length regardless of the size of the piece of film you put behind it. It’s an optical property of the lens. It just happens that a 35mm lens provides you with one angle of view on a 135 camera, and another angle of view on a cropped sensor camera.

If you want a photograph on a 1.6x camera to have the same angle of view as a 35mm lens on a 135 camera, you need a 24mm lens.

In the end equivalence only matters when you’re comparing cameras. Equivalence is useful to create a common language for talking about cameras and understanding their characteristics.

Full-frame has become the common linguistic expression to describe a 135 equivalent format, even though strictly technically speaking it is pretentious — because it minifies smaller “full-frame” formats.

Full-frame is a bit of an accidental term that just stuck.

Full-frame simply means lens coverage with respect to sensor or film.

The ensuing equivalence debate comes from the desire to compare formats in ways only gearheads can do.

For anyone who buys a Micro Four Thirds camera as their first serious camera, all this equivalence stuff doesn’t matter at all. They will simply think of 25mm as a normal lens, 14mm as wide angle, 45mm as a good focal length for portraits. However, they’d be surprised when shooting a different sensor.

Having a standard therefore is very useful. At least for me.

But remember, we’re talking photography. So what on earth does equivalence matter.

A good picture may depend on low noise, a suitable exposure, a reasonable amount of pixels and well-rendered edge detail. All things that small sensor cameras are supposed to be unacceptable for. But that’s the past. Even the iPhone’s mini-sensor produces excellent photography.

You can do serious photography of demanding subjects with a double handful of megapixels and a sensor the size of your pinky nail. Yes, size doesn’t matter. To some extent.

On the other hand, sometimes there is no substitute for larger sensors with more pixels, even though larger sensor sizes do not necessarily have a resolution advantage.

Whether equivalence a.k.a sensor size matters or not completely depends on your shooting style and preferences.

If you’re one of the minority who’s passionately in love with ultra-shallow depths of field, you absolutely need the largest possible sensor and fast lenses. The larger sensor though require you to get closer to the subject/object, or to use a longer focal length in order to fill the frame.

Larger sensors generally also have larger pixels with generally lower image noise and higher dynamic range.

If life for you starts at ISO 1,600 and goes up from there, your sweet spot is somewhere in the 2/3 to full-frame sensor range.

For macro photography you’ll prefer a smaller sensor, a.k.a. the less shallow depth of field. As sensor size increases, the depth of field will decrease for a given aperture. And you’ll also enjoy the benefit of smaller, lighter gear.

The smaller you go however, the more a compact camera struggles with portraits because of wider depth of field.

So sensor size does say a lot about photography. It does matter. It just doesn’t really matter for most of us.

And equivalence only deals with cold technical imaging parameters. Equivalence doesn’t tell you so much of what’s important. Equivalence gives you no clue about the size, form factor and cost of a camera or how it will feel in your hand.

In the end, it’s not the technical perfection that makes a good photograph.

Better spend your time taking photos than trying to understand a lifeless concept.

  • Bob

    Well, I haven’t read the entire article, but the errors in the text down to “why does this matter” makes it moot. You, and almost everyone else who discusses DOF equivalence, ignores one significant factor:  The circle of confusion, how CoC affects DOF, and how the degree of enlargement to the final image affects CoC and DOF.  Maybe that’s more than 1 factor.

    Anyway, the range of acceptable sharpness, or DOF, is defined relative to a specific circle of confusion and a specific degree of enlargement. For 35mm systems, the DOF scale was typically calibrated for a full frame enlargement to 8×10 inches. Make a smaller print, and DOF appears greater.  Make a larger print, or crop, and DOF appears smaller. (And appearance is all that matters–DOF is entirely a subjective phenomenon, dependent on our individual determination of what is “sharp.”)

    What does this have to do with your article?  Simple.  To make the same size image from an m43 capture requires more enlargement than to make the same size image from an APS-C or FF camera. This greater enlargement reduces DOF and partially offsets the simple rules you state up front. Not totally, but if all other parameters (including final image size) are the same an image taken at 50mm f2 on an m43 camera will have less DOF than an image taken at 100mm f4 on a FF camera. It won’t be as shallow as f2 on the FF, but it’s not the simple equivalence you posit.

  • Appreciate your valid pointer Bob.

    Equivalence is a slippery territory indeed, although the lack of circle of confusion in this article doesn’t make it erroneous per se. Also didn’t go into diffraction.It may be found that what is one concept is, in fact, two or more distinct concepts.We’d end up with the theory or relativity.

  • Edouard de Castro

    Don’t forget the MTF advantage:

    There is no real equivalence! larger images (projected by the lens) will have better micro-contrast and maximum sharpness – before even being “sampled” by the sensor – than “equivalent” (dof and fov – wise) images on a smaller system!
    On a larger format system the image produced by the lens is better to begin with : Larger size = shift to lower spatial frequencies = using the lens in a better “zone” of its MTF!
    You could say that there are more “analogical details” in a larger image (between “equivalent” lenses for different formats)!

    To have real equivalence you would need the smaller lens to have a proportionally better MTF / quality… which is difficult as you quickly reach the diffraction limit… We can’t fight against the law of physics!

  • Thanks for this Edouard, one bottomless pit this equivalence complexity…

  • Bengt Nyman

    Forget the techno babble guys. It does not matter. Nobody in his right mind chooses a camera and lens based on the “circle of confusion”.
    Those choices are made by where your head is at, what “circle” you run with and what they use.

    Technically, and from an image quality point of view there is no contest. Large format and high resolution wins every time. However, a smaller format and lower resolution is right for a lot of people.