It's come up again in a few threads -- people wary of moving to the 5D because it "needs" sharper glass.

In fact, the *exact opposite* is true.

Let's consider the sensor to be a target. A FF "target" is 1.6x longer and 1.6x higher than a 1.6x "target". Which target is easier to hit?

In other words, glass has to be 1.6x sharper for a 1.6x sensor for it to match a FF sensor.

So why the myth? It's because people *inappropriately* compare at the same f-ratio rather than the same aperture. It is *critical* that people not confuse f-ratio and aperture when comparing systems.

For example, we compare 30mm f / 2.8 on 1.6x with 50mm f / 4.5 on FF: 30mm / 2.8 = 50mm / 4.5 = 11mm. If we compare 30mm f / 2.8 on 1.6x with 50mm f / 2.8 on FF, the apertures are *not* the same. The FF image will have a more shallow DOF, which will make the image appear softer everywhere off the focal plane, and it will vignette more.

Even for "zero DOF" test subjects, such as brick walls, where DOF is not a factor, the FF image at the same f-ratio will show softer edges and more vignetting. However, at the same *aperture*, this will *not* be the case. In fact, the FF image will be sharper overall.

Furthermore, we *must* compare at the same output size. Don't compare the corners of a 12.8 MP 5D image with the corners of an 8.2 MP 30D image at 100% -- that's silly. Resample both images to the same output size, then compare. If we compare at native output sizes and 100%, then the Sigma SD14 will smoke every camera out there. It's curious to me that people *require* the SD14 and 5D be compared at the same output size, but not with 1.6x and FF. One must *always* compare at the same output size when measuring sharpness.

So, no, FF is *not* harder on glass than 1.6x. FF is *easier* on glass than 1.6x. Just make sure to stop down for the same DOF (aperture).

A more detailed explanation:

http://www.josephjamesphotography.com/equivalence/

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--joe

http://www.josephjamesphotography.com
http://www.pbase.com/joemama/

Please feel free to criticize, make suggestions, and edit my photos. If you wish to use any of my photos for any purpose other than editing in these forums, please ask.