TeleFragger wrote in post #10738070
im just curious so i know for future what that means...
The "crop factor" is a reference number that relates to the difference in film or sensor size (known as the camera's "format") between two cameras like the Canon 40D and a 35mm film (or a so-called "full-frame" digital) camera. Let me list the facts:
35mm film cameras and so-called "full frame" DSLRs have a film frame or sensor size of approximately 24mm X 36mm, while the Canon 40D has an APS-C sized sensor, measuring approximately 15mm X 22.5mm. The other Canon APS-C format cameras - starting with the D30 in the year 2000 and progressing through all of the "digital Rebel" xxxD series, the xxD series, and today's 7D - all have sensors that are sized similar to that in the 40D.
When camera manufacturers started designing digital SLRs (DSLRs), they decided that the DSLR bodies should be about the same physical size and configuration as their 35mm film SLRs. For that reason, they concluded that they could use the line of lenses they already had for their 35mm SLRs on the new DSLRs.
All lenses designed for 35mm cameras project an image circle onto the film that covers a 24mm X 36mm rectangle. The 35mm camera records the portion of that image circle that is defined by the opening behind the shutter for the film (24mm X 36mm in size). A digital SLR with an APS-C sized sensor only records the smaller area (approximately 15mm X 22mm) of the image circle projected by the same lens.
When you put a 100mm lens on a 35mm film camera and make a photograph, then put the same lens on a DSLR such as the Canon 40D and make a similar photograph - same subject, same position for the camera, and same focal length - and then enlarge both photographs to the same size print (4 X 6 inches, for example), it will appear as though the photo from the Canon 40D was taken with a longer lens. That is because the image recorded by the Canon 40D was of a SMALLER PORTION of the image circle projected by the lens - cropped, if you will - compared to the image recorded by the 35mm camera.
The special lenses for the Canon 40D (and other Canon APS-C cameras starting with the 300D - the first Digital Rebel) are called the EF-S series. These project a smaller image circle, making the lenses less expensive to design and produce in wide-angle and extreme wide-angle formats. The EF-S lenses also project deeper into the camera than the EF specification allows (the "S" referring to "Short back focus), allowing for less expensive wide-angle lens designs. However, an EF-S lens set to 40mm will produce the exact same image as an EF lens set to 40mm if both lenses are used on the same APS-C format body and both lenses are focused at "infinity". Focal length is focal length, period.
Now to the primary point that I want to make: NOTHING about lens EVER CHANGES when you put it on different format cameras. Focal length never changes. Aperture range never changes. The only thing that would change is the apparent field of view, and that change is not a function of the lens but it is a function of the size of the sensor or film that will record the image.
The "crop factor" calculation for "35mm equivalent focal length" has only one valid use. That is for comparing lenses used on two different format cameras.
Here's one common example: Joe took a photo of Mount Rushmore with a 35mm camera from a particular place using a 200mm lens. You want to replicate that photo with your Canon 40D. What focal length do you need to do that from the same location that he took his photo? Divide the 200mm by 1.6 and you get the answer - 125mm.
Here's another popular example: Mary Sue has been using a Canon SX120 IS point-n-shoot camera and is wanting to use a Canon 50D DSLR. She is, of course, interested in what focal lengths she would need to keep the versatility of the SX120 camera's 10X super-zoom lens. The SX120 lens is actually a 6.0mm to 60.0mm lens, but the advertising also shows the "35mm equivalent" focal length range as 36mm to 360mm. To know the focal lengths needed for the 50D, merely divide the "35mm equivalent" values by 1.6. In other words, Mary Sue would need 22.5mm on the short end and 225mm on the long end for the 50D to have the same field (angle) of view coverage as her SX120 IS camera.
The "crop factor" is NOTHING MORE than a REFERENCE between the two camera formats that lets you compare the field of view of particular focal lengths between the two formats.
The "crop factor" (as related to using lenses essentially designed for 35mm SLR cameras) is always given assuming that the 35mm format (24mm X 36mm) is the reference master. Something to realize, though, is that the 35mm film format is not, never has been, and never will be the master" format against which all other camera formats are referenced. It is simply the format of the cameras that have also evolved into today's commonly used digital SLRs.
I hope this lengthy post helps you understand the facts.
