There are plenty of great composition tools that we can learn and play with. I am slowly going through several, in order to understand how best to apply them. You can read more about my search HERE.
The picture above was captured in Cambridge where I was trying to get a shot of the man on a the bike in the background. I needed the man with an umbrella (I love shots with umbrellas in them) in the shot but did not want him to be the point in focus. This is the play on the compositional rule whereby your main subject is in focus. I wanted him to appear a bit mysterious so I needed him sufficiently far away as to obscure his face.
But if you look below you can see what my original intent was. The umbrella man was just going to be some well placed window dressing to the shot of a race being held in the street. I still prefer my original composition but this is because it is how I pictured it before I pressed the shutter!
The power to crop, either in digital or film workflows, is a second chance to make the composition as strong as possible. This is where the better image quality, either in terms of Mega Pixels in your digital camera, or in terms of negative size in film. Cropping a medium format film shot is easy as there is a great deal of detail there. Do the same on 35mm is possible but there are limitations as the grain will become too coarse to be natural.
I use a 16Mp OMD camera and the Leica ME which is 18Mp. Both of these give me plenty of real estate to play with however both pale in comparison with the Nikon D800. With 36Mp to play with I can recrop the image all day long and not suffer too much loss of detail.
Many people believe that a good photographer will not need to crop and will be able to get it right in camera. This is absolutely true and most of the time I do manage to get it right. But in this example, both crops work and make drastically different images. Do you want a feeling of a solitary walk in the rain or are you after the juxaposition of the man in a suit along with a bunch of runners sharing the same street?
I firmly believe that a good photograph is made when the image is taken and it is improved by delicate post processing (either digital or in the darkroom) and is finished with an appropriate paper selection for the print. Knowing how much you can crop before having a visible effect on quality for the planned print size is an important thing to know. It will give you the freedom to play with an image you already took to see what other compositions you could make.