In some cases, it’s obvious why an episode needs to be capped again. Take The Fires of Pompeii for example. There are four (4) images total. Or blatant image quality problems. Any of the images in Space and Time will tell you that.
Sometimes, though, it’s about more minor differences in image quality. You might have to click for the bigger versions of these images to see what I mean. These are from the recently re-capped The Curse of the Black Spot, which I first capped just this past spring. In each pair, the first is the new and the second is the old. I don’t know how obvious it is through the webpage, but when one of those old images pops up in the desktop or screen saver, it makes me want new spectacles.
The more visually dark an episode is, the worse the effect, so as you may expect, The Rebel Flesh and The Almost People really suffer badly with this, as does The Doctor’s Wife. The brighter eps are mostly okay and so are not immediately slated for re-capping, although I would like to anyway.