Originally posted by microlight Mike, what do you mean by ‘runway style’ field curvature? I tend to use mine typically between f7.1 and f11 especially for landscapes, which may be why I haven’t noticed it.
Good question. I should have given a bit more information on my findings
So...
At 40mm, f/8, centre focusing on a relatively distant point in a landscape shot - say, a couple of hundred metres away - the centre of the frame will be in perfect focus, and as you move out from the centre towards the edges of the frame, the area in perfect focus is gradually closer and closer to the camera - to the point where it's actually just a few metres away.
Another way to think of it would be that the area in perfect focus is mapped by a cone which encapsulates the entire frame, with the wide end of the cone near your camera, and the tip of the cone in the centre of the frame and somewhere in the distance. The various points along that cone represent the distance where that part of the frame is in perfect focus.
There's a way of working around this for landscapes and similar types of shot, and that's to focus slightly beyond your subject, such that it's still reasonably sharp, whilst pulling the in-focus areas at the edges and corners further away from the camera and towards your subject.
Does that make any sense? I realise I'm not describing it too well