How a raw shot becomes a picture

An afternoon at the garden railway rarely produces ten exposures. It produces closer to a hundred, and fifteen of those show almost the same thing: the same train in front of the same half-timbered house, only my hand wobbled once or a cloud moved on. Exactly one of them makes it into the album.

I used to click through them on screen, back and forth, until my eyes could no longer tell which one was crisper. These days I let them be pre-sorted and only look at the shortlist. Getting that to work took a few evenings, mind you.

The trap in measuring sharpness

Sharpness can be calculated: a program looks for hard edges, and lots of hard edges mean a sharp picture. The trouble is that this measures the wrong thing rather easily. A shot taken in the shade of the thuja hedge has weak edges because it is dark overall, not because it is out of focus. It can be pin-sharp and still end up at the bottom of the pile.

The fix is unspectacular. Before measuring, the brightness range is stretched, so dark frames are brought up to a normal spread first. After that you are comparing like with like, and the order suddenly makes sense. The overhead shot with the locomotive still sitting in its polystyrene box and the branches hanging into the frame would otherwise have slipped past me.

Contrast is not turned around a fixed value

The second point cost me more pictures. Raising contrast means making the bright parts brighter and the dark parts darker. The only question is where the middle sits, the point you turn around. Pick one fixed value for every picture and it works fine at midday and goes wrong in the evening. On one dusk shot, eight per cent of deep black area turned into fifteen — half the embankment under the track was simply gone.

Now the turning point is each picture’s own average brightness. An evening shot gets its own middle, a summer shot gets another. It sounds like a detail, but it is the difference between “a bit punchier” and “shadows drowned”.

Colour at the very end

What follows is little. The colours get slightly stronger, so the red of the carriages and the green of the hedge look the way they looked that afternoon. Then a measured amount of sharpening, just enough to give the roof tiles some texture. No more than that — over-sharpened figures immediately grow a white halo and look cut out.

It is not perfect. If you notice a picture where the shadows have tipped away or the colours lay it on too thick, write to me — the numbers can always be run again.