The ruler of thirds - a new way to describe sensor sizes

Doug Kerr

Well-known member
You've seen me make fun of the "Vidicon bottle diameter" convention for describing digital camera sensor sizes - especially the "inverse" form that leads to descriptions such as "1/2.8 inches".

But, upon reflection, I realize that we can make use of this system to get a handy way to readily compare sensor sizes over a range that is of considerable interest to us. This scheme is largely propelled by the wide acceptance of the "four thirds system", for which the standard sensor size can be described as "four thirds". I don't say "inch", as that suggests that some dimension of the sensor size is "4/3 inch".

But if we start by acceptance of that convention (perverse as its origins are), we find that we can describe a lot of sensor sizes of interest to us in "thirds". Now, "thirds" of what? Well, just "thirds".

And I will spell out the number names involved to further get rid of the notion that this is actually a dimension in some recognized unit.

Of course, these designations are "nominal", and do not express a precise sensor dimension.

So let see how that works out for some formats of interest to us. I will mention a camera model or format size family; in some cases, afterwards, I will state in square brackets other familiar names for this format family.

"One inch": Three thirds

(Micro) Four Thirds system: Four thirds. (Duh!)

"APS-C": Five thirds [1.6x]

"APS-H": Six thirds [1.3x]

"Full-frame 35-mm": Eight thirds

Just thinkin'.

Best regards,

Doug

Tom dinning

Registrant*
You've seen me make fun of the "Vidicon bottle diameter" convention for describing digital camera sensor sizes - especially the "inverse" form that leads to descriptions such as "1/2.8 inches".

But, upon reflection, I realize that we can make use of this system to get a handy way to readily compare sensor sizes over a range that is of considerable interest to us. This scheme is largely propelled by the wide acceptance of the "four thirds system", for which the standard sensor size can be described as "four thirds". I don't say "inch", as that suggests that some dimension of the sensor size is "4/3 inch".

But if we start by acceptance of that convention (perverse as its origins are), we find that we can describe a lot of sensor sizes of interest to us in "thirds". Now, "thirds" of what? Well, just "thirds".

And I will spell out the number names involved to further get rid of the notion that this is actually a dimension in some recognized unit.

Of course, these designations are "nominal", and do not express a precise sensor dimension.

So let see how that works out for some formats of interest to us. I will mention a camera model or format size family; in some cases, afterwards, I will state in square brackets other familiar names for this format family.

"One inch": Three thirds

(Micro) Four Thirds system: Four thirds. (Duh!)

"APS-C": Five thirds [1.6x]

"APS-H": Six thirds [1.3x]

"Full-frame 35-mm": Eight thirds

Just thinkin'.

Best regards,

Doug

But does it fit into my handbag, Doug?

For us lowly colonial convicts who use metric measurement none of this has any consequence or even understanding.

But thanks for the mental doodle. I find myself looking for things to do from time to time.

PS. Isn't the four thirds thing referring to the diameter in inches of the projected image on some video cameras? I read that somewhere.

xx

Doug Kerr

Well-known member
Hi, Tom,

PS. Isn't the four thirds thing referring to the diameter in inches of the projected image on some video cameras? I read that somewhere.

Not that I know of. But then there are many things I don't know. For example - well, I just don't know what that one is.

But it is rare to project an image onto a video camera. Still, if one didn't have a wall . . .

Best regards,

Doug

Tom dinning

Registrant*
Hi, Tom,

Not that I know of. But then there are many things I don't know. For example - well, I just don't know what that one is.

But it is rare to project an image onto a video camera. Still, if one didn't have a wall . . .

Best regards,

Doug

Smart arse! I meant the other way. I.e., the image diameter projected by the camera lens on the sensor plane.
I thought you knew everything. Make up a believable answer for me. I need an authority just in case someone asks me.

Xx

Doug Kerr

Well-known member
Hi, Tom,

Smart arse! I meant the other way. I.e., the image diameter projected by the camera lens on the sensor plane.
I thought you knew everything. Make up a believable answer for me. I need an authority just in case someone asks me.

The "four thirds" notation (and in fact, such notations as "1/2.8 inch", goes back to the introduction of the Vidicon TV camera pickup tube. There were shortly two sizes, one with a target (we would today say "sensor") of dimensions 8.8 mm × 6.6 mm, and one with a sensor about 1.5 times those dimensions.

To distinguish the two, technicians (not really aware of the target dimensions) spoke of the nominal envelope diameters of the two types of tube, 2/3 inch for the smaller one and 1 inch for the larger (in much the way that the sizes of monitor cathode ray tubes were described).

Through a "marketing" process I won't describe again here (as I described it at length in my recent series of notes), the convention came into place of describing sensor sizes with a number that had the same ratio to 2/3 inch that the dimensions of the sensor had to 8.8 mm × 6.6 mm.

Thus the "four thirds (inch)" sensor size has dimensions essentially twice 8.8 mm × 6.6 mm. And a "one inch" sensor has dimensions essentially 1.5 times 8.8 mm × 6.6 mm.

We trust the above to be satisfactory.

Best regards,

Doug Kerr

Doug Kerr

Well-known member
Hi, Tom,

I quite here from my longer notes about sensor size description the history behind the adoption of the "2/3 inch" reference for sensor sizes:

************

Embarrassingly-small sensors

When the first "consumer" digital cameras emerged, marketing folks were reticent to state, in advertisements and the like, their sensor sizes, which might be on the order of 4 mm × 3 mm (0.16 in × 0.12 in) or even less. They were concerned that this would seem very tiny to those with even a rudimentary knowledge of photographic technology.

So somebody got the bright idea of drawing upon the notion used for the size of vidicon tubes and their targets. Since we called a vidicon tube whose target dimensions were 8.8 mm × 6.6 mm a "2/3 inch" (0.67 inch) size, then a digital camera sensor with dimensions 4 mm × 3 mm could be called a "0.30 inch" sensor.

But that still sounded pretty wee. So the next step was based on the fact that 0.30 could be expressed, mathematically, as 1/3.3. So then, reasoned the marketing guys, we could speak of this camera as having a "1/3.3 inch" sensor. And "3.3 inch" (of course that was not what the notation said) didn't sound all that small.

So this is how we end up with cameras whose sensors are described as "1/2.8 inch" or "1/1.8 inch" sizes.

************

We trust the above to be satisfactory.

Best regards,

Doug

Tom dinning

Registrant*
Hi, Tom,

I quite here from my longer notes about sensor size description the history behind the adoption of the "2/3 inch" reference for sensor sizes:

************

Embarrassingly-small sensors

When the first "consumer" digital cameras emerged, marketing folks were reticent to state, in advertisements and the like, their sensor sizes, which might be on the order of 4 mm × 3 mm (0.16 in × 0.12 in) or even less. They were concerned that this would seem very tiny to those with even a rudimentary knowledge of photographic technology.

So somebody got the bright idea of drawing upon the notion used for the size of vidicon tubes and their targets. Since we called a vidicon tube whose target dimensions were 8.8 mm × 6.6 mm a "2/3 inch" (0.67 inch) size, then a digital camera sensor with dimensions 4 mm × 3 mm could be called a "0.30 inch" sensor.

But that still sounded pretty wee. So the next step was based on the fact that 0.30 could be expressed, mathematically, as 1/3.3. So then, reasoned the marketing guys, we could speak of this camera as having a "1/3.3 inch" sensor. And "3.3 inch" (of course that was not what the notation said) didn't sound all that small.

So this is how we end up with cameras whose sensors are described as "1/2.8 inch" or "1/1.8 inch" sizes.

************

We trust the above to be satisfactory.

Best regards,

Doug

More than, Doug. I can always rely on you for the best answer possible.
So, there was even a bit of size envy back then. It's funny how blokes get a bit nervous when inches are mentioned.

Doug Kerr

Well-known member
Hi, Tom,

More than, Doug. I can always rely on you for the best answer possible.
So, there was even a bit of size envy back then. It's funny how blokes get a bit nervous when inches are mentioned.

And, as they say in the auto engine business, "inches count".

But if we only speak of "thirds" . . .

Best regards,

Doug

Doug Kerr

Well-known member
I feel it would be useful to add two more often-referenced small sensor size classes to the "ruler of thirds" taxonomy.

I have also decided to use a numerical expression (e.g., "4/3") rather than a written-out expression (e.g., "four thirds") for the size class designations.

I also note that I do not attach the unit "inch" to these size designations. To do so would incorrectly suggest that these designations are an actual dimension. They are just "names" (as in the case of that finished lumber size class called "2x4").

The expanded table is here:

"1/2.7 inch": 1/3

"1/1.7 inch": 2/3

"One inch": 3/3

(Micro) Four Thirds system: 4/3 (Duh!)

"APS-C" [1.6x]: 5/3

"APS-H" [1.3x]: 6/3

"Full-frame 35-mm": 8/3