# How is the sensor size measured?

Discussion in 'Digital Photography' started by angus, Nov 30, 2003.

1. ### angusGuest

I've searched the archives and still can't figure out what an
expression like 1/2.3" means when applied to sensor size.

For example:
A 1/2.3" sensor has actual dimensions of 4.54mm x 3.42mm according to
one of the digital camera websites.

1 divided by 2.3 equals 0.313", which I had originally presumed to be
the diagonal dimension of the sensor.

But 0.313" is equal to 7.94 mm while the true diagonal is only 5.68
mm.

So what does 1/2.3" actually refer to?

Thanks,
Angus Cameron

angus, Nov 30, 2003

2. ### James AkiyamaGuest

The 1/2.3" is actually an old measurement used by the television industry to
measure the outer glass diameter of vidicon tubes. The actual active
surface is proportionally smaller.

CCDs were first used to replace vidicon tubes in TV video cameras. Even
though there's no glass surrounding a CCD, the measurement continued, partly
to make sure video cameramen could continue to use the same calculation to
determine perspective based on lens focal length.

When CCDs began to be used for digital still cameras, the metric stuck.
This, by the way, is also why most P&S digital cameras have an aspect ratio
of 4:3 (TV aspect ratio) rather than 3:2, which is normal for 35mm.

James

James Akiyama, Nov 30, 2003

3. ### jamGuest

A reasonably accurate rule of thumb that works with =most= popular CCD
and CMOS sensors is to multiply the sensor size "fraction" by 16 mm to
estimate the active diagonal. Thus 16 mm * 1/1.8 = 8.89 mm.
Unfortunately, it's off by +22% with your CCD.

The odd size designations have an arcane history going back to vidicon
tube days. Someone else will have to fill in the details.
--
Jeremy McCreary
Denver, CO
-------------------------------------------

| I've searched the archives and still can't figure out what an
| expression like 1/2.3" means when applied to sensor size.
|
| For example:
| A 1/2.3" sensor has actual dimensions of 4.54mm x 3.42mm according
to
| one of the digital camera websites.
|
| 1 divided by 2.3 equals 0.313", which I had originally presumed to
be
| the diagonal dimension of the sensor.
|
| But 0.313" is equal to 7.94 mm while the true diagonal is only 5.68
| mm.
|
| So what does 1/2.3" actually refer to?