0.0f wrote in post #12181868
could someone please explain why the focal range on a lens is multiplied by the crop camera of the camera you are using?
i'm new to photography and have only just come across this.
in process of buying a 17-55mm and i thought i would be essentially getting this focal range however, i understand that the actual focal range will actually be 27-88mm why is this?
This should help you understand the issues:
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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 7D and a 35 mm film (or a so-called "full-frame" digital) camera. Let me list the facts:
35 mm film cameras and so-called "full frame" DSLRs have a film frame or sensor size of approximately 24 mm X 36 mm, while the Canon 7D has an APS-C sized sensor, measuring approximately 14.9 mm X 22.3 mm. The whole line of 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 approximately the same size (± 0.2 mm).
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 35 mm film SLRs. For that reason, they concluded that they could use the line of lenses they already had for their 35 mm SLRs on the new DSLRs.
All lenses designed for 35 mm film cameras project an image circle onto the film that covers a 24 mm X 36 mm rectangle. The 35 mm camera records the portion of that image circle that is defined by the opening behind the shutter for the film (24 mm X 36 mm in size). A digital SLR with an APS-C sized sensor only records the smaller area (approximately 14.9 mm X 22.3 mm) of the image circle projected by the same lens.
When you put a 100 mm lens on a 35 mm film camera and make a photograph, then put the same lens on a DSLR such as the Canon 7D 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 7D was taken with a longer lens. That is because the image recorded by the Canon 7D was of a SMALLER PORTION of the image circle projected by the lens - cropped, if you will - compared to the image recorded by the 35 mm camera.
The special lenses made by Canon for the 7D (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 40 mm will produce the exact same image as an EF lens set to 40 mm 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" is NOTHING MORE than a REFERENCE between two camera formats that lets you compare the field of view of particular focal lengths between the two formats. For the photographer who started with an APS-C format DSLR and has never used a 35 mm format camera (at least enough to have developed a feel for what certain focal lengths provide him/her), the "crop factor" calculations can be completely forgotten for day-to-day lens selections. Only when comparing two camera formats is the "crop factor" useful.
The "crop factor" calculation for "35 mm equivalent focal length" has only one valid use. That is for comparing the field of view of lenses used on two different format cameras.
Here's one common example: Joe took a photo of Mount Rushmore with a 35 mm camera from a particular place using a 200 mm lens. You want to replicate that photo with your Canon 7D. What focal length do you need to do that from the same location that he took his photo? Divide the 200 mm by 1.6 and you get the answer - 125 mm.
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 6.0 mm to 60.0 mm lens, but the advertising also shows the "35 mm equivalent" focal length range as 36 mm to 360 mm. To know the focal lengths needed for the 50D, merely divide the "35 mm equivalent" values by 1.6. In other words, Mary Sue would need 22.5 mm on the short end and 225 mm on the long end for the 50D to have the same field (angle) of view coverage as her SX120 IS camera.
The "crop factor" (as related to using lenses essentially designed for 35 mm SLR cameras) is always given assuming that the 35 mm film format (24 mm X 36 mm) is the reference master. Something to realize, though, is that the 35 mm 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.
Beginning photographers are often first confronted with the crop factor puzzle when choosing their first DSLR camera. Intuitively, "Full Frame" sounds better than "Cropped", as if one is getting a complete camera instead of a partial camera. There are very few really significant differences (other than features) between similar-generation cameras of different formats. The fact is that both format cameras can be used to make essentially identical images, though different focal lengths will be needed on them to keep the framing the same.
Beginning photographers are also confronted with "crop factor" issues when buying lenses. Focal length (translated to how big or small of a field of view you want) is the first factor to consider when asking the “which lens?” question. The beginner doesn't have to convert every focal length to its "35 mm equivalent focal length" value but they should know that, on their APS-C camera, a 28 mm lens isn't going to be wide angle but instead is a "normal" focal length and that a 250 mm lens is going to be a rather long telephoto.
Many photographers who are new to DSLRs have acquired some very wrong ideas about "crop factor" issues. For example, they sometimes expect the focal lengths of EF-S lenses to be different (as in the focal lengths being pre-converted for the "crop factor") than the focal lengths of EF lens. This is completely false, as all SLR lenses are marked with their actual focal lengths. In addition, many new photographers who use APS-C format DSLRs seem to have been fed with the idea that they need to employ the "crop factor" calculations whenever thinking about using lenses on their cameras. This is generally not necessary at all as I have outlined above.
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