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I'm a Cairns, far north Queensland, Australia professional photographer specialising in travel, editorial and environmental portraiture.

Wednesday, March 25, 2009

Depth of Field

In keeping with the spirit of my fellow travel photographer Bob Krist and his recent guest blog on Photoshop Insider, this picture comes to you from just down the road. And it was only taken a few days ago.

But this post is not about Bob's great article, although you should definitely go and check it out. This post is about learning new things and re-learning old things.

When working with film on landscape images the common method of working was to close your aperture down as far as possible to get a big depth-of-field. You would be working at around f16, f22 or so to get everything from the foreground rocks to the background trees in focus.

In this shot above, although it's a bit hard to tell from this small thumbnail, everything is tack sharp. And it was shot at f8. huh? I don't get it - at least I didn't get it until I started stumbling across all these articles in magazines and on the net about diffraction and crop sensor cameras.

So in the spirit of learning together, and admitting that I certainly don't know anywhere near everything about photography and that's why I love it so much, I'm going to tell you what I found. Bear with me if you've been prowling the internet discussion groups for a while and this is old hat.

Now I always knew that any lens and film format had its optimum aperture setting - where you would get the sharpest image. For many lenses that seems to be around f8. And if you went smaller than that you would get more depth of field and a slight drop in image quality but nothing to write home about.

Well it turns out that with crop cameras (which basically means everything except the top of the range Canons, Nikons and Sony) when you close your aperture down smaller than its optimum aperture the image quality decreases rapidly due to diffraction. So f16 on your camera is not going to be anywhere near as sharp as f8 or f11 or so. Go figure.

The more I delved into this the more worried I got. How was I going to get really sharp pictures with a big depth-of-field then? Well it turns out that something I always knew in the back of my mind is the saviour. The smaller sensors of crop cameras have a larger depth-of-field at any given aperture. That means that a portrait shot at f2.8 on a crop camera is going to have a bigger depth of field than one taken on film or a dSLR with a 'full-frame' sensor.

Conversely an image taken at f8 is going to have a bigger depth-of-field than one taken on a full frame. Digging deeper I found one article that claims the difference to be about two stops or so. So f8 on a crop sensor is about f16 on a full frame. F11 would be about f22.

So putting aside any arguments about the quality difference between crops sensors and full frame, when shooting landscapes on a crop sensor camera it seems best to aim to keep your aperture in the f8 to f11 range to ensure optimum sharpness. I'm certainly going to be experimenting with this to figure out how badly the image degrades at smaller apertures and whether this is just a pixel peeping exercise, but a lot of pros that I really respect are writing about this so I'm guessing it's true.

Which just goes to show that you can never know everything about this incredible art form we all love. While the basics of light, colour and timing may stay the same, sometimes even the fundamental truths that we've held on to for decades are changing and developing.

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