Making Sense out of Sensor Sizes

--by David Cardinal

It has become almost impossible to actually figure out how big a sensor is, or how long a lens is, just by glancing at the specs. For most of the photography industry, for many decades, 35mm was more or less the norm. By definition that meant a standard size film negative, about 35mm by 24mm – corresponding to today’s “full frame” sensors – and a simple measurement of focal length for lenses could be read right of the lens as a result.

Early in digital most of the DSLR sensors were between 60% and 80% as large as a 35mm film negative. That introduced a little complexity, but they were still measured in millimeters, and multiplying focal lengths by 1-1/2 worked well enough most of the time, and since all the sensors were almost the same size, no one worried too much about how exactly how they were measured.

Even re-introducing full frame in digital, with the advent of the Nikon D3, didn’t rock the boat too much, as we were all familiar with 35mm nomenclature. But then came micro-4/3 with its oddly measured ‘4/3”’ sensor, along with high-end point and shoots which are evaluated by sensor sizes measured in a similar crazy way, and now the Nikon 1 series featuring another alternative size – back to being measured in millimeters. So now we have sensors measured in millimeters, inches and ratios, along with lenses labeled with their absolute focal length which means almost nothing to anyone, unless they are shooting full frame.

Full-frame sensors like the one in the D700 used for this photo make high ISO images possible.
Do you know what size your sensor really is?

As an example let’s take the specs from Nikon’s own website:

  Sensor Size Lens Measurement Nikon Explanation
Nikon 1 Series 13.2mm x 8.8mm / CX 10-30mm Approx. 2.7x lens focal length
Nikon P7100 1/1.7 in. 6.0-42.6mm (angle of view equivalent to that of 28-200mm lens in 35mm [135] format)
Nikon D7000 23.6 x 15.6mm / DX 17-55mm DX (example) none – but really 26-82 equiv.
Nikon D3S 36.0 x 23.9mm / FX 14-24mm FX (example)  

Just among these few cameras, we’ve got some sensors measured in millimeters and others in an odd form of fractional inches – some of which have been named FX, DX, CX to at least help the numerically challenged – and lens focal lengths measured in ways that require you to do the math to figure out what you’re actually shooting.

Personally I review and compare a lot of cameras so I see the problem more than most, but I have to believe many Nikon DSLR shooters also own Nikon point and shoots and are looking to purchase one of the excellent, new Nikon 1 series cameras. So wouldn’t it be great if they could look in their bag and when they saw a camera or lens with a focal length they knew what it meant? And when they went to look for a new camera the sensor size was in a consistent format so they could be compared?

Camera Sensor Size Comparison Chart. Courtesy of Jinho.JungLet’s take the simple matter of the P7100, labeled as 1/1.7 in. Pulling out a calculator I started by finding out that 1/1.7 in. was 15mm (1/1.7 = .58 inches * 25.4 mm/inch = 14.9mm) –  hmmm, larger than that of the Nikon 1 series. So I figured that couldn’t be right. It turns out it is actually 7.6mm x 5.7mm, about half the size (in each dimension) of the Nikon 1 sensor. That’s because the fractional inch sizes are actually based on an antiquated system invented for measuring the usable area on TV camera tubes in the 1950s! So there is no way to directly compare any of the sensors measured in millimeters with those measured in inches without a reference table. How can that make any sense for consumers unless the camera companies are just nervous about telling them exactly how small the sensors really are in most point and shoots.

Sorting it Out: Table of Common Sensor Sizes

For your reference here are some common sensor designations and actual sizes:

Designation Aspect Ratio Width (mm) Height (mm) Approximate
Focal Multiplier
1/2.7” 4:3 5.37 4.03 6
1/2.3” 4:3 6.16 4.62 5
1/1.7” 4:3 7.60 5.70 4
2/3” 4:3 8.80 6.60 3.5
Nikon 1 (CX) 3:2 13.20 8.80 2.7
4/3” (micro-4/3) 4:3 18.00 13.50 2
APS-C (DX) 3:2 approx 24mm approx 16mm 1.5
35mm (FX) 3:2 36.00 24.00 1

In fairness to the vendors, I don’t have a perfect solution for how to label lens focal length – although I think if they came up with some nice abbreviation like “E” for equivalent, they could label lenses with something like “24-85mm E” or “24-85 E,” instead of forcing us through mental gymnastics every time we change cameras or lenses.

Sensor size is a simpler problem. There is really no customer value in the oddly-measured ‘2/3”’ and ‘4/3”’ designations or the 1/1.7” or 1/2.3” for that matter. They should all be ripped up and replaced by a human usable measurement – even one as simple as giving the actual size in millimeters, the way it has been done for the larger cameras all along.

If you use multiple cameras and have a pet peeve about sizing, or a tip for how to quickly know how to move between them, let us know. Or if you use multiple cameras and it just doesn’t bother you, let us know that as well. As always, we welcome you, along with whatever size sensor camera you have, to join us for a !