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Below are three photographs of the same building, taken from the same spot, with the Sony FD97 camera, autofocus and spot metering. Below top centre - full wide angle, below left - 10x optical zoom, below right - 20x digital zoom (10x optical and 2x digital).
These photographs have not been straightened up nor the fringing removed, which I would normally do before displaying them.
For the first experiment, the area in the 10x photograph corresponding to what is shown in the 20x photograph was selected and the image cropped (below left). In order to make a effective comparison, the 20x photograph was resized to the same size as the cropped selection (below right). There are a few pixels difference in the two images because I found it difficult to select the exact same area. Both of these photographs were sharpened using Ulead PhotoImpact 7 focus once, because that is what I would do with any cropped or resized image.
My own conclusion on comparing these two images is that the resized 20x image is clearer than the cropped 10x. Neither is now capable of giving a 10x8 print.
The final experiment was to blow up the cropped 10x (below left)to the same size as the original 20x (below right). I sharpened the cropped 10x after the blowing it up. The 20x original has not been sharpened.
It is quite difficult to see any difference in these two images, but the 20x untreated photograph is perhaps a tad sharper than the 10 cropped blowup, but so little as to be of no significance. The difference might have been more marked had I not sharpened the blowup, but since one would always sharpen a resized photograph, I felt it would have been unfair not to do so.
My own conclusion is that the 20x digital zoom is as good as, if not better than, a cropped blowup from a 10x zoom photo. I would certainly not go to all the trouble of cropping and enlarging just to avoid using the digital zoom.
But I have presented all these photographs so that people may make their own judgements. Please feel free to download any of the photographs if you wish to experiment.