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Nature Explored

Photography
by Chris Dunford

 

 

 

 

 

 





 

Nature Photography

Exposure - Aperture Shutter Speed and Depth Of Field

The number three problem, is getting the photo to reflect your intention as a photographer. Once you have got the photography basics sorted you have decisions to make. Do you want a sweeping 'environmental' composition showing your subject in its surroundings or perhaps you'd prefer a 'stand out' closeup with the surroundings relegated to a blurry colourwash. Your lens aperture is the creative tool you need to master for this. Plains Zebra in Ruaha National Park in Tanzania
Plains zebra about 20m away. The photo
was taken at a fairly small aperture (f/14)
giving me a depth of field from 18-23m.
The far background starts to blur a little
but it was ok for this shot. Because of the
long focal length (208mm) there was no
chance of front to back focus

Aperture

The aperture is equivalent of the iris in your eye. In low light it needs to be wider to let enough light in for the retina to register it properly. So in the camera in this situation the lens aperture is opened up wider to let more light onto the sensor. In bright light the aperture is generally much smaller to allow the equivalent amount of light onto the sensor.

Aperture is a ratio which is measured in 'f stops' as:
Widest     1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22     Smallest

In camera terms each 'stop' of the lens will either halve or double the light depending depending on whether you are opening or closing the lens aperture. (This is assuming the shutter speed is constant).

The numbers seem odd but are due the fact that light drops off proportionaly to the square of the distance it travels so if you take each number in the sequence and multiply it by the square root of two (1.414) you will get the next number which equates to halving the light.

Shutter Speed

Shutter speed is the time based regulator of how much light hits your sensor.

Shutter speed is measured in seconds from:
    Slowest 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000     Fastest.

Waves breaking on the Cornish rocks
A Fast shutter speed of 1/250s is used to
stop the action. Aperture of f/16 and a
lens focal length of 48mm make it easy to
get everything sharp right to the horizon
when I focus on the rock about 15m in
front of me
But, if you double the shutter speed ie. 1/125th of a second to 1/250th of a second this will halve the amount of light hitting the sensor, or double it if you go the other way, since the speed of light is constant (You can get much fuller explanations elsewhere).

Exposure

Variable apertures and shutter speeds give you lots of flexibility to take a correctly exposed and well focused photo. ie. 1/125s at f/11 is the same as 1/250s at f/8 because in the second example you have opened up the camera by 1 stop to let twice as much light through a bigger hole, but halved the the amount of time the shutter is open ie. doubled its speed, which haves the amount of light passing through. Net result - both exposures let the same amount of light onto the sensor.

A word here about exposure compensation. Sometimes the camera's metering system doesn't quite get it right, and may overexpose or underexpose an image. Or you may just decide that the photo looks a bit too light, or dark. You can manually tweak the exposure by adding or subtracting an offset to the metered exposure, until it looks how you would like it to.

So why bother varying apertures and shutter speeds?

Deep DOF on Rufiji river
The Rufiji river. Using a medium aperture
of f/10 to keep a reasonably quick shutter
speed when using a polarising filter was no
problem, as the focal length of the lens was
only 18mm and this gave us more than
enough scope to get a huge DOF at any
focus distance over just under 2m

Depth of Field

Well one good reason is what's called 'depth of field' (DOF) which means how much of your picture, from front to back, is in focus. Simple physics dictates that the light focusing on a sensor through a small hole (aperture) will result in much more of the front to back being in focus (DOF) than light going through a large aperture.

Shutter speed doesn't affect DOF but the focal length of the lens, the aperture and the focused distance to the subject do.

So, if you absolutely must have everything in focus from 2m to infinity then it is easiest to set your aperture quite small ie. f/16 and combine it with as short a focal length lens as you can get away with and still make the shot. The downside of this is that the smaller aperture will mean that the shutter will be open longer and so any movement may cause blur in the photo.

  • For a 35mm lens an aperture of f16 would just do this.
  • With a 24mm lens you could use a maximum aperture of f/8 to achieve this.
  • With a 17mm lens you could achieve the same focus requirements with an apertutre of f/4.

Remember smaller numbers mean wider apertures. These numbers are given for DSLRs with cropped sensors. If you have a full frame camera the numbers will be slightly different but are commonly available.

If there's nothing within 10m or so of the camera that needs to be in focus then you could go for a much wider aperture and hence, following the rule above, a much quicker shutter speed to get the same amount of light through the lens. Much better if you are trying to get a crisp shot of some action. Also it is important to try and reduce the effects of camera shake while hand holding the camera, and the longer the telephoto lens the more this shows up.

Common bulbul takes a drink
Common Bulbul. Aperture f/5.6 and a
400mm focal length will give a very shallow
DOF (0.031m) making the bird stand out
from the blurred background grasses. The
distance was about 5m away

Poor Light

A second reason for varying shutter and aperture speeds is that in poor light, even with a slow shutter speed the sensor may struggle to get enough light on it to register an image. The wider you make the aperture, the more light gets through to the sensor in any given time, so your chances of a good photo are improved.

Blur the Background

A third reason, if you want your subject to really stand out from the background you can do this by arranging a; good separation distance from the background, and then b; getting as close as reasonable to the subject and c; set as wide an aperture as your lens will go to, with the aim of having a very shallow DOF. Focusing on a subject at a short distance is important for this to work well. The result should be a lovely sharp subject jumping out from a very blurry background.

This is the classic technique in many wildlife shots. It also works quite well with telephoto lenses, as due to the long focal length of the lens they will usually have a much shallower DOF for any given setting anyway.

If you put your camera in aperture priority mode then all you have to do is manually set the aperture and the camera will figure out the Shutter speed for you.

A lake in March
With a 18mm lens set to f/8 and focused
only 3m into the shot - which is close to the
hyperfocal distance of 2.15m - we ensure
that everything from the branches in the
foreground right out to the far distance is
reasonably sharp

Focal Length and Focusing Distance

One thing that massively affects DOF for any given settings is the focal length of the lens (as opposed to the focusing distance to the subject). The shorter the lens the easier it is to get a large depth of field. ie. a 17mm lens will give much greater DOF than a 50mm lens at the same aperture setting and focusing distance.

The distance into the shot that you focus, combined with the focal length of your lens, affects what will and won't be in focus in the end result. A shorter lens will generally give a deeper depth of field at any given focusing distance.

Here we have the perfect front to back in focus scenario. The camera was set at F/18 with a focal length of 24mm. After checking hyperfocal distance tables I focused on the roadside plants about 3.5m in front of me and ensured everything from about 1.5m to infinity was reasonably in focus
Here we have the perfect front to back in
focus scenario. The camera was set at F/18
with a focal length of 24mm. After checking
hyperfocal distance tables I focused on the
roadside plants about 3.5m in front of me and
ensured everything from about 1.5m to infinity
was reasonably in focus
The real secret of how to get maximum DOF is to use the hyperfocal distances of the focal length/aperture as your starter. The hyperfocal distance for a given set of parameters is the minimum distance into the shot that you can focus on and still get everything in focus all the way out to infinity. Also everything from the Hyperfocal distance up to half way back to your camera will be in focus as well. These are fixed numbers and so you can easily learn the common ones or follow this link to view or print out a small table to use as reference.

One way to make more sense of all my tips is to download one of the many phone apps that deal with DOF calculations or hyperfocal distances. You can plug in the aperture and focal length of a lens and also the distance that your subject is at, and it will The hay bale is ok but the depth of field isn't enough
Aperture f/8 and focal length of 105mm.
The hay bale focused at 40m away is ok but
the DOF is only 25-90m. Had I focused at
a point 73m away then everything from
38m-Infinity would have been sharp. On the
other hand maybe my intent was a slightly
blurred background. It can be whatever
you want to create. You can get the figures
from hyperfocal distance tables
magically show you the nearest and farthest distances that will be in focus. ie. using a fixed 50mm lens at f/11 and subject (focus) distance of 10m. everything from 5.4m to 69m will be in focus.

With the same focal length and aperture but with the lens focused 30m away everything from 8.4m to infinity is in focus, and so on.

Sit and play with the app instead of watching TV and eventually you will get a feel for what settings will be needed for a given shot. This will save you much heartache and significantly boost your 'keeper rate'.

Freezing the Action

Sometimes you want to capture fast action and are not concerned with depth of field. In this case you use a higher shutter speed to capture a smaller slice of time and so reduce the effects of motion blur to as near as possible to zero.

In order to expose properly with a faster shutter speed (shutter is open for less time) the camera compensates by opening the aperture to let more light through in the smaller time frame.

The limit is reached when the aperture is fully open but the shot becomes underexposed as the shutter speed continues to be increased.

You then have one other trick in your bag. And that is to vary the ISO setting for the camera sensor.

Termite mound