10 September 2008

Digital image processing: to crop or not to crop?

This all started when I left a comment on ant photographer extraordinaire Alex Wild's Myrmecos Blog to note that a large starting image file is good when one needs to crop the image so that enough pixels will be left to print a decent-sized final image at 300 dpi. Alex responded with this post, to explain that cropping should not be used to increase "magnification"; better image quality would be obtained by taking a more magnified picture either by moving closer to the object or by using a better lens. I agree with him wholeheartedly.

But sometimes cropping is a good thing. Here is a picture of the land snail Helix aspersa (= Cornu aspersum). This is the original uncropped image.


When I took this picture the snail was in my left hand and the camera in my right hand. Perhaps I should have moved in a little bit closer with the camera, but I didn't want to leave out any part, especially the tentacles, of the snail, which was continuously moving its head around, and at the same time, I wanted to include my fingers as a scale. So when I sold this picture to a photo agency, I cropped out quite a bit of the background. Here is the final image that will be printed in a biology textbook.


At 300 dpi, its dimensions were 16.6x13 cm (6.5x5.1 inches). I doubt they will print it that large; they may crop it some more or reduce the size. In this case, if I had moved in closer, I would have cropped less, but I doubt the final image quality would have been noticeably better.

On the other hand, if you are photographing something long and narrow, like a this slug Limax maximus, and if you want to include the entire animal in the picture there is a limit to how close you can get to it. In that case, you must crop.

Here is the uncropped image of the slug.


I could have approached it a little bit more, but not much, especially if it had had its tentacles out. The extra spaces on top and on the bottom are inevitable. So, if I wanted to print this picture in an article, I would do quite a bit of cropping.


Because I took the original with my 8 MB Olympus E-500, the cropped image retained enough pixels for a 21.7x5.6 cm (8.6x2.2 inches) print at 300 dpi. This is when megapixel cameras are indispensable.

Cropping a poor image won't make it better, but cropping a good image can improve it. Therefore, the rule of thumb is: if it's crap, don't crop.


Katie said...

I agree with you on these points. It always amazes me how some people think that they can just make do without a good macro lens and compensate with just cropping. I have two good macro lenses. I try to crop as little as possible because I don't want to lose any details.

premenopaws said...

Excellent post, especially your last rule of thumb. I'd like to add another reason. Sometimes with my macro I'm tempted to get in so close that the available light is reduced to the point where the depth of field winds up being incredibly shallow. For some things this looks nice and artistic, but in some situations I just want all the striking detail and so I intentionally back off to get a deeper DOF to get the whole critter in focus, then crop later.

Anonymous said...

If you could approach the slug more (or had a longer focal point macro lens) then cropping is not the only way to achieve a hi-res close up picture without lots of empty space.
In that case one could take 3-4 vertical images panning the camera sideways, and stitch these together for a very high resolution image. Manually setting the aperture, shutter and focus, as well as using the continuous shooting mode during panning would be preferred.


That would be a cumbersome & time-consuming operation. Also, you would have to keep the camera to subject distance the same, otherwise the magnification would change, but that would be almost impossible to achieve in a practical way with a hand-held camera & a moving subject. If pictures of parts of the subject are at different magnifications, then stitching them would be a nightmare task. Moreover, because the subject is moving, the background would change. You would end up with an unnatural image of a live animal in its natural habitat.

I'll stick to the way I am doing it.