Depth-of-field with telephotos

I’m still working through material from last month’s trip to Lord Howe Island. Here’s one example, a Red-tailed Tropicbird (a species which took up a lot of my time and let me refresh my birds-in-flight skills).

Red-tailed Tropicbird

This photo was taken in mid-afternoon light near the top of some sea-cliffs (200m above the sea). We’d been at this location since the morning, and it was a wonderful experience spending so much time with the birds. By the end we had nicknames for most of the individual birds who’d pass us.

These birds have very dark eyes, and this is one of the small number of shots where I managed to get a glint in the eye to give the eye some definition. But while processing this image (in Lightroom of course) I realised it also illustrated a technical issue.

This photo was taken with an EOS 5D MkIII with 100-400mm lens, at 350mm and f/8 (obviously it’s a square crop from within the frame). ISO 400, 1/1600s shutter. I had the lens stopped down one stop from wide-open to optimise the sharpness (although I was often shooting at f/6.3 or f/7.1, even with f/8 the depth-of-field is very limited). The 5D MkIII’s auto-focus is wonderful, but when tracking a bird flying past you at probably well over 40 km/h it can be hard to pick exactly which bit of the bird you want to be in focus, and sometimes you end up letting the camera choose. Here it’s locked on around the chest and thus the wings are in clear focus. At this size the beak may look sharp as well, but when examining the image in detail, it’s obviously that the beak is not quite as sharp as the wings. Similarly the feet are “ok”, with the tail trailing off into more blur. For a web-sized image it’s acceptable, but for a big print it’s challenging. At 350mm, the closer you get to your subject the less of your subject will be in focus. Unfortunately in these scenarios there’s little scope to use a smaller aperture for more depth-of-field, or to use things like “focus stacking”.

We (bird photographers) tend to like long and “fast” (bright) lenses, which can let us shoot with faster shutter speeds or in darker conditions, and these are both advantages. But often we need more depth-of-field than a 500mm/2.8 lens would give us. Of course, the other advantage of a bright lens is that more light gets to the auto-focus system. So we can always find a justification for the faster glass!

A2_122120Incidentally, to give you some context of our environment for this day, here’s a shot looking back towards where the above photo was taken from (about where my friend Kim is standing: that’s my backpack on the left). Lord Howe Island is a wonderland for bird photography. I’m really looking forward to going back!

One Comment:

Leave a Reply