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Sensor size questions APS C vs 4 thirds

KEN JEANETTE , Feb 18, 2005; 11:03 a.m.

I read the article here by Bob Atkins concerning APS-C vs full frame, and I became curious. I went into a couple of sites specifically dedicated to the 4 thirds, and they make it seem like that is the largest sensor, although they don't actually say its size

"The Four Thirds type sensor is twice the size of the two thirds type sensor, which is commonly used in many digital cameras"

According to the Atkins article, APS-C is 22x15mm (and I think this is a "two thirds" aspect ratio. He also states that the 4 thirds sensor is 18x13.5mm, clearly smaller than APS-C.

What am I missing in translation between the article and the website. Because of their vagueness, and never having mentioned the size of the sensor, I tend to distrust the 4 thirds website. It seemed to me to be a lot of hype, with very little factual information.

So, here I am, at the only place I know I will get the real information. Thank you in advance for any input you may have.


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Bill Chiarchiaro , Feb 18, 2005; 11:15 a.m.


You're confusing aspect ratios, such as the 3:2 of 35mm film and APS-C sensors, or the 4:3 of compact digicams, with the archaic naming convention for sensor sizes. You're not alone in this. See dpreview's explanation of sensor sizes.

The name of the "Four Thirds" system is a reference to size, not aspect ratio.


Joe Beecher , Feb 18, 2005; 11:35 a.m.

The thing I am troubled by is the way DPReview refers to the new Olympus lenses in the news section. The mention a 180-500 F2.8 lens. When you click on the link you find the lens is a 90-250 F2.8. I guess since the Four Thirds lenses can not be used on a full frame camera this is bound to happen.

The issue I have is a new user might say: "Wow Olmpus gives me a 180-500 lens. Canon and Nikon don't have a lens that long." Their marketing doesn't talk about crop factor because it's both an advantage and disadvantage.

Godfrey DiGiorgi , Feb 18, 2005; 11:35 a.m.

As Bill said, FourThirds is the name of the Kodak/Panasonic/Olympus/etc sensor initiative. It was perhaps derived from the 3:4 format proportion.

For reference, the formats are...

sensor: format - hxw in mm - diagonal in mm
2/3 sensor: 3:4 - 6.6x8.8mm - 11mm
FourThirds: 3:4 - 13.5x18mm - 22.5mm
Canon "APS-C": 2:3 - 15x22.5mm - 27.04mm
Nikon/Pentax/Minolta "APS-C": 2:3 - 15.7x23.5mm - 28.3mm

If you compare a 6Mpixel Canon APS-C sensor with a FourThirds 5Mpixel sensor, and consider cropping to an 8.5x11 paper format print with either, you'll see that the difference in size is in reality very small. FourThirds is smaller but not by a very large percentage.


Bill Chiarchiaro , Feb 18, 2005; 11:46 a.m.


The sensors used in the Four Thirds systems do have a 4:3 aspect ratio, but they are named for their 4/3-inch size, not their aspect ratio.

Visit the link I posted above to see an explanation of what 4/3-inch, 2/3-inch, 1/1.8-inch, etc. mean.


Godfrey DiGiorgi , Feb 18, 2005; 01:17 p.m.

Thanks, Bill. I have been to that page many times, it is in my bookmarks for reference. :-)

What I was suggesting was that perhaps the 4/3" size designation was a part of the reason to name the format initiative FourThirds, and that the sensor proportions are also 4:3 (or 3:4) so they were trying to make it all related.

FWIW: The sensor size designations like "2/3" and "4/3" are to my mind a very odd archaism to keep around. I prefer referring to the size of the format for a digital camera as it helps me understand the relationship between optical focal lengths and field of view better ... Much easier to see 6.6x8.8 vs 24x36, for instance, than '2/3"' vs "full frame". Even "APS-C" is a silly way to describe a sensor size as APS film formats are not the same..."16x24mm" vs "24x36mm" is much more understandable. 14x18 vs 16x24 approximations makes the actual size difference between FourThirds and APS-C a lot more understandable as well.


Bill Chiarchiaro , Feb 18, 2005; 01:43 p.m.

Godfrey wrote:

'"16x24mm" vs "24x36mm" is much more understandable'

That's for sure the way I think of them!


David Bedell , Feb 18, 2005; 02:52 p.m.

"The issue I have is a new user might say: "Wow Olmpus gives me a 180-500 lens. Canon and Nikon don't have a lens that long." Their marketing doesn't talk about crop factor because it's both an advantage and disadvantage."

It would be nice if everyone spoke in terms of angle of view rather than focal length.

Bob Atkins , Feb 18, 2005; 02:57 p.m.

You can't fight city hall. The manufacturers use the terms "APS-C" and "4/3", so that's what you're stuck with.

Olympus are quite clearly and obviously making their smaller format size into a "feature", and are thus not-too-subtly marketing their lenses such in a way that emphasizes double their actual focal length and downplays their smaller format size. If I were in their advertising department, that's exactly what I'd do. They don't mention the fact that smaller formats give greater DOF, so you need faster lenses to blur backgrounds, but I wouldn't either.

The Olympus system is interesting and technically quite nice. I doubt it will get far, but it will develop a niche for itself with a band of loyal users who will get great results from it. There's no doubt that the equipment itself is well designed and well built, but given that there's no price advantage despite the smaller format and lenses, they're fighting an uphill battle against 35mm and APS-C.

Daniel Powis , Feb 18, 2005; 03:51 p.m.

However, what some people are going to be jealous of are these two new lenses, the 14-35mm f/2 (equivalent to 28-70mm f/2) and the 35-100mm f/2 (equivalent to 70-200mm f/2) which does not exist with any of the current systems.

In fact, I haven't heard of a single zoom lens with an f/2 aperture....

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