In an earlier discussion on the merits of megapixels, commentator and snail guru Aydin notes:
Megapixel counts matter if you need to crop out large sections of an image & still need to retain enough pixels for a large enough print.
To illustrate Aydin’s point, I’ve taken a full photo of an Australian Monomorium nest and cropped it away to show just the queen ant:
Viola! Instant magnification. I can get away with a tight crop because the original photo spans over 6 million pixels. Blog photos only need 100,000 pixels. Plenty of pixels to spare. Once I get my hands on the Canon 50D, I’ll have 15 million pixels and the resolution to crop down to a single small ant.
So I can crop, but should I? Is cropping a good thing?
Purists like Dalantech at the No Cropping Zone hold a firm line: No.
Bad habits are easy to get into, but difficult to break. If you get into the habit of cropping all your images then you’ll never put a lot of effort into composition or framing. Why should you when you can just crop it later, right?
While I am inclined to agree with Dalantech, I don’t think cropping need be detrimental to learning. I first learned composition through cropping. Once I got the hang of improving a photo by cutting out distractions and shifting the frame in post-processing, I started trying to get it right in the field using just the viewfinder. Cropping was a bit like training wheels.
Once I went commercial things changed fast. It turns out that photo buyers sometimes need a lot of pixels, especially for posters and museum displays. I can vouch for how embarrassing it is to lose sales because the image was too heavily cropped to be enlarged. Nowadays I aim to nail the composition in the viewfinder first, and if I do crop I do so sparingly, retaining at least 70% of the original pixels.
The persistent temptation in macro-photography is to crop for a bit of extra “magnification”. If you find yourself cropping as a matter of routine, it’s worth thinking about updating your optics. Think about it this way: If you are chopping half the pixels off your 10MP images, you may as well have bought a cheaper 5MP camera and spent the difference on a more powerful lens or a set of extension tubes. You’d get the same effective magnification but your images will be sharper and smoother.
Here is a shot of the same queen ant as in my example above, but with magnification augmented the old-fashioned way. I used a more powerful setting on the MP-E 65mm lens.
A crisper, cleaner image.
Myrmecos Blog 
Just stumbled across your blog and wanted to tell you that I love your photography and your perspective on things.
I agree. The magnification you get by cropping an image is like the “empty magnification” one would get if one took a photo taken thru a microscope using a 10x objective and then enlarged it to a size it would be if one had used a 40x objective.
Nevertheless, cropping is still useful if you want to get rid of the extraneous matter (30%) around a point of interest, while retaining enough pixels.
Obviously, megapixels are also good if you want to print big pictures.
Cropping is a must if you get into insect photography and all you’ve got is a 6MP point-and-shoot 😉
I really really want to upgrade, and when I do this will be good stuff to know. But I’ve got some saving to do – in the meantime, I’ll have to crop.
Ted,
True, true. I’m assuming an SLR here, of course. Point-and-shoots are constrained in their own way and only a few models have respectable macro.
While I tend to agree that cropping is often used to compensate for laziness, there are reasons why it can be useful. Sometimes you just can’t get close enough to the subject, and when you’re in the field you’re stuck with the equipment you brought along. In those cases it’s nice to know that even though you might not get as tight a photo as you’d like, you can fix it in the darkroom (whether chemical or digital).
From my perspective, I use 1:1 and 1:2 formats for my fine art work as a stylistic choice. Obviously this involves some cropping, though in response to Dalantech’s point I would point out that compose for the crop, which can actually be even more difficult than composing for a perfect crop OOC.
Just a few thoughts I had while I was reading your excellent post.
Alex,
I always strive to do as much as I can with an image “in-camera” because I certainly don’t need any more images to post-process than I already have. However, per David’s comment above, sometimes one just doesn’t have an option to move in any tighter on a subject.
One thing that helps with all of this is shooting your image in RAW mode. Since RAW files are not at all compressed, it makes it easier to crop in and increase the size of the file while losing as little of the quality of the image as possible. There are are also some Photoshop plug-ins like Alien-Skin Blow-Up which can help you make images larger in a cleaner way if you find yourself in this situation.
Clay