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Doug Kerr
June 28th, 2006, 09:34 AM
It is common to describe digital camera sensors of the general size found in the Canon EOS 10D, 300D, 350D, and 20D as the "APS-C" size (sometimes, just as "APS" size).

We also sometimes hear the sensor size of the EOS 1D and 1D Mark II described as the "APS-H" size.

I thought it would be interesting to describe the basis for that nomenclature.

"APS" refers to the Advanced Photo System, a modern scheme for "consumer" film cameras developed in the 1990s by a consortium of manufacturers including Kodak.

The system is based on the use of a film 24 mm in width, carried in a special cartridge that facilitates automatic loading in the camera and automatic manipulation of the film in processing.

Most APS cameras allow the photographer to choose at the time of shooting from three different aspect ratios for each shot (more about them later). However, in every case, the actual frame exposed by the camera is the same size: 30.72 mm x 16.7 mm (the APS frame size).

The aspect ratio selected by the photographer for the shot is coded on the film. When the film is processed, the processing equipment reads that code and crops the image frame as required to produce a print of the chosen aspect ratio.

On many APS cameras, when an aspect ratio is chosen, a viewfinder mask is put in place so the finder view matches the chosen aspect ratio. Primitive cameras have "composing frames" for the three aspect ratios shown in the finder.

The three aspect ratios are called "H", "C", and "P". For each there is an equivalent frame size (the portion of the entire frame that will be used to produce the print when that aspect ratio has been chosen). (Recall again that for every shot, the full "APS frame" is taken.) Here are the details for each aspect ratio:

H (evocative of "HDTV format") - The full APS frame (30.2 mm x 16.7 mm) is used. The aspect ratio is 1.81, considered similar to the HDTV aspect ratio of 1.78 (16:9).

C ("classic" format) - The "delivery crop" here is 23.4 mm x 16.7 mm. The aspect ratio is 1.40, considered similar to the "classic" full-frame 35-mm camera format aspect ratio of 1.5 (3:2).

P ("panoramic" format). The "delivery crop" here is 30.2 mm x 9.5 mm. The aspect ratio is 3.18.

Now, what about EOS dSLR frame sizes?

The frame size for the EOS 10D, 300D, 350D, and 20D is on the order of 22.5 mm x 15.0 mm. APS-C, often used to suggest that general size, refers to an image size of 23.4 mm x 16.7 mm. What can I say.

The frame size for the EOS 1D and 1D Mark II is on the order of 28.7 mm x 19.1 mm. APS-H, often used to suggest that general size, refers to an image size of 30.2 mm x 16.7 mm. What can I say.

I discourage the use of "APS" nomenclature to describe sensor size families.

Tim Armes
June 28th, 2006, 11:58 AM
I discourage the use of "APS" nomenclature to describe sensor size families.

Fair comment. What short and concise term would you suggest?

Doug Kerr
June 28th, 2006, 03:24 PM
Hi, Tim,

What short and concise term would you suggest?

Just say what it is.

I call the general sensor size of the 10D, 20D, 30D, etc. "22x15".

I call the general sensor size of the 1D, etc. "29x19".

I call the general sensor size of the 1Ds etc. "36x24".

This is consistent with a long tradition, still used for almost format sizes bigger than 36x24: 8x10, 4x5, 5x7, 6x6, 6x4.5, 6x7, 6x9. (The units aren't usually mentioned, as there is no "8x10 cm" or "6x6 inch", for example.)

Best regards,

Doug

Tim Armes
June 29th, 2006, 12:18 AM
I call the general sensor size of the 10D, 20D, 30D, etc. "22x15".
Doug

Surely though, if APC-C isn't a good choice since it's not accurate, then 22x15 is also too "approximate". You should be sayiing 22.5x15 !

Doug Kerr
June 29th, 2006, 01:38 AM
Hi, Tim,

You should be sayiing 22.5x15

Yes, but I like to have something that makes sense for the whole family (10D as well). And of course the guys talking about "6x6" should be saying "5.6x5.6", and so forth.

But I'd be delighted to have you adopt "22.5x15" for the Canon "1.6x" family.

Best regards,

Doug

Tim Armes
June 29th, 2006, 02:27 AM
Hi, Tim,



Yes, but I like to have something that makes sense for the whole family (10D as well). And of course the guys talking about "6x6" should be saying "5.6x5.6", and so forth.

But I'd be delighted to have you adopt "22.5x15" for the Canon "1.6x" family.

Best regards,

Doug

Well, in the interests of promoting a good example, not perpetuating blatently wrong information and maintaining brevity, I've changed all references of APS-C on my site to 22x15.

So your post has done some good somewhere.

Tim

Doug Kerr
June 29th, 2006, 07:32 AM
Hi, Tim,

Well, in the interests of promoting a good example, not perpetuating blatently wrong information and maintaining brevity, I've changed all references of APS-C on my site to 22x15.

Well, I'm delighted to hear that. Thanks so much for your good work.

Bet regards,

Doug

Ray West
June 29th, 2006, 09:39 AM
Why not use the diagonal measurement, as in tv/vdu screens, or afaik video cameras? At some stage you are changing from imperial to metric measurement, and that may give problems with say 10 by 8 (mm's or inches?)

The safest (but lengthiest) method is to give the actual size of the useable sensor area in mm, or inches, but quoting the units. (....or is it?)

Best wishes,

Ray

Nelson Chen
June 29th, 2006, 10:23 AM
Could we just use the simplified terms of 1.6x for 10D/20D/30D/300D/350D and 1.3x for 1D/1D2?

Thanks,
Nelson

Doug Kerr
June 29th, 2006, 12:00 PM
Hi, Ray,

The safest (but lengthiest) method is to give the actual size of the useable sensor area in mm, or inches, but quoting the units. (....or is it?)

I would have no problem with doing that, but if we do, we should follow the established editorial conventions for specifying values in the International System of Units.

Thus we should say:
22.5 mm x 15.0 mm

not
22.5 x 15.0 mm

not
22.5mm x 15.0mm

and by rights, ideally the "x" should be the cross symbol, not the lower-case (or upper case) "X". (I would do that here, but it causes a problem with the forum software!)

But of course those dimensions are not the same for all cameras in the "family".

Then if we wish to describe a certain family of Canon EOS dSLR cameras, we might say, "The Thursday discussion group is dedicated to Canon EOS digital SLR cameras with sensor sizes in the range 22.0 mm x 15.9 mm through 22.7 mm x 15.1 mm (That would embrace the D30, D60, 10D, 20D, 30D, 300D, and 350D.)

But in fact, if "usable size" is to be the basis, we need to be aware that the portion of a 20D sensor from which we get outputs (in the suite of raw data) is a grid 35.22 cells x 2348 cells inm size, with a physical size on teh sensor of 22.59 mm x 15.06 mm.

(It is common to think that the "usable size is the size of the portion of the sensor grid whose photodetector count is the same as the pixel count of the largest developed image we can take. But that's jut not a reasonable outlook.)

So I guess if we wish to speak specifically of the sensor of an EOS 20D/30D (only), in "short form", we should say: "22.6x15.1". And maybe that would be a good "approximation" for then entire family.

Best regards,

Doug

Best regards,

Doug

Doug Kerr
June 29th, 2006, 12:18 PM
Hi, Nelson,

Could we just use the simplified terms of 1.6x for 10D/20D/30D/300D/350D and 1.3x for 1D/1D2?

(My snotty reply would be, "Just what quantiy is it that is "1.6x" the size of what other quantity?")

But, all kidding aside, that's fine with me so long as you don't include the word "crop" any place in the term.

That constant is a widely used (albeit arbitrary) metric for the general size of a camera format, defined as the approximate inverse of its relative linear size compared to that of the format of a full-frame 35-mm still film camera.

It's as legitimate as ""2x4" for a certain familiy of "dimension" lumber sizes, "4 inch" for a certain sizeelectrical ceiling outlet box, or "32x" for the write speed of a certain CD recorder.

And we can extend it to a lot of other realms in phtography. For example, our colleague Will Thompson has recently acquired a number of film cameras having an 0.27x format size. He even toyed with the idea of bidding on an 0.13x camera.

Best regards,

Doug

Doug Kerr
June 30th, 2006, 07:28 AM
Not that it really has anything to do with anything, but given that we have been speaking of more and less precise designations of frame sizes, I was fascinated to be reminded (by an accidental encounter with the pertinent standard during some paper shuffling) that the standard dimensions of the film frame we often call "full-frame 35-mm" are specified as 36.4 mm x 24.4 mm (0.4 mm), with the proviso that the image may have rounded corners with a radius of not over 0.4 mm.

The standard also provides for what we often call the "half-frame 35-mm" format, with specified dimensions of 17.6 mm x 24.4 mm (0.4 mm), as well as a third format (none of the three are given names by the standard) with dimensions of 24.4 x 24.4 mm (0.4 mm).

Again, nothing that should have any impact on our deliberations about "sensor size family names", but interesting nevertheless.

Best regards,

Doug

Tim Rucci
July 1st, 2006, 10:04 AM
I appreciate your detailed explanation of the APS methodology, and I feel that it basically confirms what I have said about APS from the start: it's a gimmick. If I understand your explanation correctly, the different size shots in that system are accomplished by simply masking a portion of the frame out of the photo. When the advertising was going full bore years ago promoting the APS system, I felt that the proponents were trying to get us to believe that the camera used a larger width of film to shoot a panoramic shot. But I guess in reality, the camera was just masking the top and bottom of the frame out of the photo rather than using a longer width of film.

That's the same thing my old point and shoot 35mm camera did with it's 'gimmick' panorama mode. Since geting a 3"x10" panorama shot just involved blowing up the print to 6.67" x 10" while masking the top and bottom out of the photo, it wasn't any different than ordering an enlargement and then cutting off the top and bottom of the print on a paper cutter.

Doug Kerr
July 1st, 2006, 06:30 PM
Hi, Tim,

You got it!

Best regards,

Doug