Doug Kerr
Well-known member
[Part 1]
I'm sure most of you are well familiar with most of this, but I thought some might enjoy being reminded of some of the history.
Introduction
The subject here is how we describe the sizes of the sensors in digital cameras.
It turns out that different conventions are used in three "bands' of sensor sizes:
A. Larger than 36 mm × 24 mm
B. In the range of about 22 mm × 14 mm through about 36 mm × 24 mm
C. Smaller than about 22 mm × 14 mm
I will discuss these in a peculiar order.
Band B sensors
The basic concept
Cameras with sensors in the "band B" size range have an "ancestry", direct or indirect, in the "35 mm" film still camera (to be more explicit, in the "full frame 35 mm" still camera). Some of these digital cameras (notably those from Canon and Nikon) are tied to their full-frame 35 mm ancestors in that they are interchangeable lens cameras (almost all the single-lens reflex type) that can use lenses in the substantial repertories from their manufacturers developed for use with the full-frame 35 mm cameras, or further lenses added to those series.
Others (notably from Sony) have no such direct ancestry, but nevertheless, even of only by virtue of their sensor sizes, occupy a parallel stable.
The "full frame" sensor size
Of course the frame size of the full-frame 35-mm film cameras was (nominally) 36 mm × 24 mm. Because that is the same as the largest frame size (thus, sensor size) of this band of digital cameras, a digital sensor frame size of nominally 36 mm × 24 mm is spoken of as the "full frame" sensor size. (Yes, of course this is sort of a circular definition!).
Smaller sensors in this band
Photographers utilize lenses of varying focal length primarily to attain varying field of view for their images. The field of view provided by a lens is the result of both the focal length of the lens and the size of the image format (sensor, in digital cameras). The larger the format size, the greater the field of view; the greater the focal length, the less the field of view.
Because of the widespread use of the 36 mm × 24 mm format in film cameras, even moderately-sophisticated photographers came to realize what common focal lengths would produce fields of view suited for various work. A focal length of about 50 mm was considered "normal" (another fascinating story), while we might use a focal length of perhaps 135 mm for "telephoto" work (that term itself being the subject of another fascinating story) or a focal length of perhaps 24 mm for "wide-angle" work.
But of course a 50 mm lens mounted to a digital camera with a sensor size of about 22.5 mm × 15 mm would yield the same field of view as would be given by a lens of focal length 80 mm mounted on a camera with a sensor size of 36 mm × 24 mm (a so-called "full frame" sensor size).
Thus came into use the often misunderstood notion of "equivalent focal length" (more thoroughly described as "full-fame 35-mm equivalent focal length").
In our example camera, with a sensor size of 22.5 mm × 15 mm, for a lens of any given focal length, its "full-frame 35-mm equivalent focal length" when mounted on that camera would be about 1.6 times its focal length.
Thus, the description "1.6 X" came to be associated with sensors of size approximately 22.5 mm × 15 mm (as their sensor dimensions were 1/1.6 times the dimensions of the "full frame 35-mm" size sensor). The number was often said to be the "focal length multiplier". A more apt, if clumsy, term would have been "full-frame 35-mm equivalent focal length factor".
"Cropped" sensors
Because a sensor of size 22.5 mm × 15 mm (for example) was smaller that a sensor of size 46 mm × 24 mm, which has come to be called "full frame" (people not bothering with the rest of the description, "35-mm"), some people came to consider such "smaller" sensors as "cropped" from the "full frame size". So we may see someone, seeing a colleague's new camera, evidently a "band B" camera, ask, "Is that full frame or cropped?" (or sometimes just "crop").
Sadly, the "full-frame 35-mm equivalent focal length factor" often came to be stated as the "crop factor" of the camera. Thus a camera whose sensor had dimensions of 0.625 times the dimensions of the "full-frame 35-mm" frame (1/1.6) would be said to have a "1.6 crop factor."
"APS" notation
"APS" refers to the Advanced Photo System, a new film system introduced in 1996 by Eastman Kodak, intended to make film use more convenient for a range of users than the traditional "135-type" cartridge used for 35-mm film. The film was nominally 24 mm wide.
Almost invariably, the camera took a frame of dimensions 30.2 mm × 16.7 mm. But the user, at the time of taking the shot (usually), would instruct the camera that the frame size he really wanted to use was one of there, designated by three letters;
H (high definition): 30.2 mm × 16.7 mm. This has an aspect ratio of almost exactly 16:9, consistent with the format then coming into use for "high-definition" television.
C (classic): 25.1 mm × 16.7 mm. This has an aspect ratio of almost exactly 3:2, the same as for the "classic" full-frame 35-mm cameras.
P (panoramic): 30.2 mm × 9.5 mm. This has an aspect ratio of almost exactly 3:1, and is of course intended for "panoramic" format images.
But regardless of the format chosen by the photographer, the camera (almost always) exposed a frame of dimensions 30.2 mm × 16.7 mm. The desired frame size, set by the photographer, was encoded magnetically on the film. In the laboratory, prints of the selected aspect ratio for each frame (if it were marked for the C or P format) were produced by cropping the actual exposed frame (here a proper use of the term "crop"). (For the H format, the full frame was printed - here a proper use of the term "full frame".)
Thus the three aspect ratio options in the APS system came to be known as the H, C, and P "crops" (even though the H format did not require cropping).
Now back to our digital camera sensor size story. A popular sensor size in "band B" cameras was either
about 22.5 mm × 15 mm (Canon) or about 24 mm × 16 mm (Nikon). Because of the anxiety of writers about digital photography for ways to express these sensor size "ranges", it became common to speak of wither of these sizes as "APS-C" (since the "C crop" of an APS frame had dimensions of 25.1 mm × 16.7 mm, and these were "something like that").
Now, for quite a while, a common digital sensor size in the Canon camera repertoire was in the area of 27.7 mm × 18.5 mm. (That would be a "1.3X" size.)
And in the same vein I discussed above, this general sensor size range became known as the "APS-H" size (since the "H crop" of an APS frame had dimensions of 30.2 mm × 16.7 mm, and these were "almost something like that").
************
The reader with a sensitive stomach may wish to take some Pepto-Bismol here.
************
As for me, I'm going to have breakfast.
[to be continued]
I'm sure most of you are well familiar with most of this, but I thought some might enjoy being reminded of some of the history.
Introduction
The subject here is how we describe the sizes of the sensors in digital cameras.
It turns out that different conventions are used in three "bands' of sensor sizes:
A. Larger than 36 mm × 24 mm
B. In the range of about 22 mm × 14 mm through about 36 mm × 24 mm
C. Smaller than about 22 mm × 14 mm
I will discuss these in a peculiar order.
Band B sensors
The basic concept
Cameras with sensors in the "band B" size range have an "ancestry", direct or indirect, in the "35 mm" film still camera (to be more explicit, in the "full frame 35 mm" still camera). Some of these digital cameras (notably those from Canon and Nikon) are tied to their full-frame 35 mm ancestors in that they are interchangeable lens cameras (almost all the single-lens reflex type) that can use lenses in the substantial repertories from their manufacturers developed for use with the full-frame 35 mm cameras, or further lenses added to those series.
Others (notably from Sony) have no such direct ancestry, but nevertheless, even of only by virtue of their sensor sizes, occupy a parallel stable.
The "full frame" sensor size
Of course the frame size of the full-frame 35-mm film cameras was (nominally) 36 mm × 24 mm. Because that is the same as the largest frame size (thus, sensor size) of this band of digital cameras, a digital sensor frame size of nominally 36 mm × 24 mm is spoken of as the "full frame" sensor size. (Yes, of course this is sort of a circular definition!).
Smaller sensors in this band
Photographers utilize lenses of varying focal length primarily to attain varying field of view for their images. The field of view provided by a lens is the result of both the focal length of the lens and the size of the image format (sensor, in digital cameras). The larger the format size, the greater the field of view; the greater the focal length, the less the field of view.
Because of the widespread use of the 36 mm × 24 mm format in film cameras, even moderately-sophisticated photographers came to realize what common focal lengths would produce fields of view suited for various work. A focal length of about 50 mm was considered "normal" (another fascinating story), while we might use a focal length of perhaps 135 mm for "telephoto" work (that term itself being the subject of another fascinating story) or a focal length of perhaps 24 mm for "wide-angle" work.
But of course a 50 mm lens mounted to a digital camera with a sensor size of about 22.5 mm × 15 mm would yield the same field of view as would be given by a lens of focal length 80 mm mounted on a camera with a sensor size of 36 mm × 24 mm (a so-called "full frame" sensor size).
Thus came into use the often misunderstood notion of "equivalent focal length" (more thoroughly described as "full-fame 35-mm equivalent focal length").
In our example camera, with a sensor size of 22.5 mm × 15 mm, for a lens of any given focal length, its "full-frame 35-mm equivalent focal length" when mounted on that camera would be about 1.6 times its focal length.
Thus, the description "1.6 X" came to be associated with sensors of size approximately 22.5 mm × 15 mm (as their sensor dimensions were 1/1.6 times the dimensions of the "full frame 35-mm" size sensor). The number was often said to be the "focal length multiplier". A more apt, if clumsy, term would have been "full-frame 35-mm equivalent focal length factor".
"Cropped" sensors
Because a sensor of size 22.5 mm × 15 mm (for example) was smaller that a sensor of size 46 mm × 24 mm, which has come to be called "full frame" (people not bothering with the rest of the description, "35-mm"), some people came to consider such "smaller" sensors as "cropped" from the "full frame size". So we may see someone, seeing a colleague's new camera, evidently a "band B" camera, ask, "Is that full frame or cropped?" (or sometimes just "crop").
Sadly, the "full-frame 35-mm equivalent focal length factor" often came to be stated as the "crop factor" of the camera. Thus a camera whose sensor had dimensions of 0.625 times the dimensions of the "full-frame 35-mm" frame (1/1.6) would be said to have a "1.6 crop factor."
"APS" notation
"APS" refers to the Advanced Photo System, a new film system introduced in 1996 by Eastman Kodak, intended to make film use more convenient for a range of users than the traditional "135-type" cartridge used for 35-mm film. The film was nominally 24 mm wide.
Almost invariably, the camera took a frame of dimensions 30.2 mm × 16.7 mm. But the user, at the time of taking the shot (usually), would instruct the camera that the frame size he really wanted to use was one of there, designated by three letters;
H (high definition): 30.2 mm × 16.7 mm. This has an aspect ratio of almost exactly 16:9, consistent with the format then coming into use for "high-definition" television.
C (classic): 25.1 mm × 16.7 mm. This has an aspect ratio of almost exactly 3:2, the same as for the "classic" full-frame 35-mm cameras.
P (panoramic): 30.2 mm × 9.5 mm. This has an aspect ratio of almost exactly 3:1, and is of course intended for "panoramic" format images.
But regardless of the format chosen by the photographer, the camera (almost always) exposed a frame of dimensions 30.2 mm × 16.7 mm. The desired frame size, set by the photographer, was encoded magnetically on the film. In the laboratory, prints of the selected aspect ratio for each frame (if it were marked for the C or P format) were produced by cropping the actual exposed frame (here a proper use of the term "crop"). (For the H format, the full frame was printed - here a proper use of the term "full frame".)
Thus the three aspect ratio options in the APS system came to be known as the H, C, and P "crops" (even though the H format did not require cropping).
Now back to our digital camera sensor size story. A popular sensor size in "band B" cameras was either
about 22.5 mm × 15 mm (Canon) or about 24 mm × 16 mm (Nikon). Because of the anxiety of writers about digital photography for ways to express these sensor size "ranges", it became common to speak of wither of these sizes as "APS-C" (since the "C crop" of an APS frame had dimensions of 25.1 mm × 16.7 mm, and these were "something like that").
Now, for quite a while, a common digital sensor size in the Canon camera repertoire was in the area of 27.7 mm × 18.5 mm. (That would be a "1.3X" size.)
And in the same vein I discussed above, this general sensor size range became known as the "APS-H" size (since the "H crop" of an APS frame had dimensions of 30.2 mm × 16.7 mm, and these were "almost something like that").
************
The reader with a sensitive stomach may wish to take some Pepto-Bismol here.
************
As for me, I'm going to have breakfast.
[to be continued]