Getting off Auto - Depth of Field

I needed to take a few images to help explain Depth of Field to someone, so I thought it would be useful to utilise those pictures for another in the intermittent Getting off Auto series.

Usual disclaimer about me using Canon terms as that is what I use.  Earlier GoA threads (on Settings, Exposure+, Post-Production, Choices, Extras, Impossible Pictures & Ethics) as well as other interesting stuff about photography can be found by looking at the GoA Index.

Being able to control the depth of field in a picture is a useful thing as a photographer.  It is all about deciding exactly how much of the image is acceptably sharp and how much is blurred.  What is deemed “Acceptable” relates to some interesting physics relating to a setup’s “Circle of Confusion” – but don’t worry, having mentioned the term, I’m going to ignore it!

If everything is in focus (or nearly), you can easily lose your subject against a boring background, whereas a shallow depth of field will help to isolate the subject nicely from all that distraction.  These two images of a handy garden visitor show what I mean – the first is a bit lost against the brick wall, the second stands out much better with the blurred background.  I haven’t done anything different to the processing on these, the only difference was that one was taken at f/4, one at f/18.

I took the pictures at a distance of 80cm (the wall's around 3m behind).  With a 105mm lens on a full frame camera, that gives me a DoF of 1.2cm at f/4.  That increases to 5.4cm at f/18 and that does help get all of the bird in proper focus (the left hand side of the bird is less focussed on the f/4 shot), but I prefer the f/4 image for the way the bird is isolated.  The pictures also highlight how the transition between focussed & blurred changes – at wider apertures like f/4, the transition to out-of-focus is quite fast, whereas closed down, it’s a much more gradual affair – like this. 

How well you can use a blurred background to isolate your subject depends not only on the aperture used, but the distance to the subject and the background and even the sensor size (well, the field of view really, but sensor size is easier to get your head around!).  Big sensors make it easier to get a shallow DoF.

I keep talking about the Depth of Field I’m getting – how do I know?  I’m not completely all-knowing, I’m using a Depth of Field calculator.  I’m using an App on a phone, but you can get online ones via a web browser.  Plug in your sensor size, lens focal length, distance to subject and it will tell you what your DoF is.

The Hyperfocal distance is the distance from the camera that will be acceptably sharp should you focus at infinity.  Or, to look at it another way, if you focus at the Hyperfocal point, the image from Hyperfocal distance/2 to infinity will all be sharp.  As you change the f-stop you’ll see it alter quite a bit.  So at f/4 it’s just over 9m in my example, at f/18 it’s just over 2m.  Which explains why landscape photographers like to close down the aperture – lots of acceptably sharp bits of greenery.

So what about us wildlife photographers?  The good news is that, for much of the time, you can almost forget about Depth of Field.  That’s because a lot of our subjects are not that close and are pretty small.  This Turnstone for example was rootling around at an 11m distance.  Plugging the relevant numbers into my calculator gives me a DoF of 15cm – plenty enough to get the little Turnstone acceptably sharp, even with the lens wide open.

Larger animals at a distance?  DoF is around 2m for these Koniks (again wide open), so no issues.

However, if I try and use the same camera/lens combination on a close-up Konik, I’m in trouble.  Lots of this image is out of focus (DoF is only about an inch ) – but does it matter?  The whole point of the image was not to get it all in focus, it was to make a joke about the impossibility of shooting birds when the Koniks were being friendly – and the ear is acceptably sharp, so it works.

So in many ways we’re lucky shooting wildlife.  With few DoF limitations we can shoot wide open (allows us to get the shutter speed up, or shoot in lower light, both very handy).  When it does become an issue for us is with bugs and macro shooting.

At macro and closeup distances, your DoF can be very thin.  Even at f/13, the DoF on this little spider is just 3.5mm – you can see how quickly the image blurs in front and behind the focal point.

That can be a real problem when shooting tiny beasties.  The only options you have are to close the lens down even more – though you’ll need good light, maybe using a flash to get lots of close up illumination.  Sometimes you can move around a little bit to get the subject at a shallower angle – front on this Wasp Spider would be next to impossible to get all in focus, but from the side it’s much easier

Or you just have to accept the limitations and live with the fact the body is out of focus as long as the face is sharp, as with this Beewolf

It is possible to use a technique called Stacking to get better DoF.  Essentially, you take multiple images, each with a slightly different focus point and then use software to blend all the images together into one, fully focussed image.  Unfortunately, this works best with inanimate or dead subjects – something that won’t move in other words!

The final approach is to leave the DSLR at home and use your phone instead.  The small sensors used in phones (and various point & shoots) tend to give rather deep depths of field.  This makes them less desirable for portraits (where people like a nice blurry background) but great for bugs!