Regretably, a misconception presented on this site and others state/imply/hint that mounting a ‘regular’ lens such as a Canon EF 85mm f1.8 lens (or EF 85 f1.2L) on a 1.6 crop body such as a Canon 60D “magically” transforms it into EF 136mm lens (85 x 1.6). This is completely false.
When the above lens is mounted on the crop body, there are no internal changes that take place in the lens. The glass does not change, the internal mechanisms don’t change, the electronics don’t change, NOTHING changes. Nor does the image projected by the lens change in any way when mounted on a crop body!
What DOES CHANGE, however, is the image “seen” by the sensor, as only the center 60% or so of the full, lens-projected image is “sensed” by the smaller APS-C sensor. THIS is the crop factor. It is not a multiplier factor, but a CROP factor.
Consider a printed 8”x11” family picture taken using a Canon 1Ds (full-frame) camera using the 85mm lens above. Now place a 5”x7” piece of paper over the printed picture so it is exactly centered, and cut away all of the 8x11 picture that is outside the 5x7 piece of paper. The remaining 5x7 picture is the cropped portion of the 8x11 picture. The people in the original picture haven’t been enlarged or magnified, they have not gotten ‘closer’ or ‘further away’, they are no brighter or more dim than before. Just the “periphery” of the original picture is no longer there…that’s all that has changed.
What has changed is the angle of view, which has been reduced by a factor of 1.6 when the EF 85mm lens is mounted on a crop body. Thus, the 28 degree diagonal angle of view of the EF 85 f1.8 lens (per Canon web site) is reduced to about 18 degrees (17.5x1.6=28), similar to that of the EF 135mm f2.0L lens. It did not BECOME a 136mm on the crop body, but has the same angle of view as would a 136mm lens. Using the 8x11 picture mentioned above, to get the same image with all the periphery using the 85mm lens on a crop body, it would be necessary to back up some distance until the entire scene was visible in the viewfinder.
One other misconception also presented/inferred is that when an EF-S lens such as 18-135mm is mounted on a crop body, it, too, is magically ‘zoomed’ into a 29-216mm zoom lens (18x1.6, 135x1.6). Nope! Nor is the angle of view altered/reduced/cropped by a factor of 1.6 either. All EF-S lenses are designed so that their full, projected image is completely sensed by the smaller sensor. If it were physically possible to mount an EF-S lens on a full-frame body, the diameter of the circle of light projected on the sensor would be noticeably less than the height or width of the full frame sensor. What the full frame sensor would see is a circular image surrounded by a rectangular black border.
So please…stop treating the 1.6 CROP factor as a 1.6 MULTIPLER factor. It’s no different than using a paper cutters on an 8x11 picture to produce a 5x7 result.
