Explaining Sensor Sizes (strange units)

[Meysha]

I'm sure other people have found it difficult to understand how big their sensor size is in their camera, So here's some info.

When the camera companies list sensor size something ambiguous like 1/2.7" you could be forgiven in thinking that that is a description of the diagonal of the sensor. ie: 9.407mm. (or just under a centimeter). But infact, this sensor size 1/2.7" is only 6.721mm across its diagonal.

Huh???? In the great words of Julius Sumner Miller,,,, Why is it so???

This unit measurement dates back to when they first started using video cameras... like the old-school ones back in the 1930-1980s. They didn't use sensors like we use now, instead they used a camera tube. Which I like to think of as a CRT TV in reverse. ie: Light (electrons) comes in and excites the phosphor (sensor). - please excuse me if my electronics is wrong - I'm just explaining the 'idea'.

But they found that when you create this "sensor" (or camera tube) you can't use the entire diameter of the tube to capture image data. So this weird 1/2.7" unit is referring to the entire diameter of the tube - when infact you can only use about 2/3rds of that size for capturing data.

Weird hey.

It gets weirder. As you change tube size and get bigger or smaller - you'd think the ratio of 2/3rds usable data would stay the same. It Doesn't. So the descriptions of size of 1/2.7" and 1/1.7" aren't directly relatable!!!!!! However, they are similar, and are around 2/3rds but they're not mathematically the same.

So whenever you want to know what sensor size you're actually getting in your digital camera. You need to carry a little table around with you to figure out what all the sizes are. Frustrating hey.

Why won't the camera industry just describe sensor size as it is physically and not in an outdated, uninformative unit?!?!?!


ps. If I didn't explain that very well check out this site, it has pictures to explain and a table of sensor sizes.
http://www.dpreview.com/learn/?/Glossary/Camera_System/sensor_sizes_01.htm

Thanks to my Grandpa who used to work for Phillips back in the early days and to this website for helping me understand: http://www.dpreview.com/learn/?/Glossary/Camera_System/sensor_sizes_01.htm

 

[HatMan]

If it's any consolation, my Pentax K10D (10.2MP) clearly states the sensor size in the manual as 23.5mm x 15.7mm.

 

[Meysha]

Yep, when I bought my Canon 350D it was listed clearly as having dimensions in mm's. But I'm looking at buying a small P&S now and got so confused by these measurements.