Friday, 19 September 2014

Birds and Light - Metering and Exposure

In an earlier post (HERE) I explored some of the challenges facing a birder trying to obtain a decently-exposed image of a bird, including the difficulties of working with long lenses, wide apertures, fast shutter speeds and high ISO.  I also looked at related image artefacts, noise and blooming.  Lastly, I looked at some methods for optimising image exposure including ETTR and exposure bracketing.

Here I am going to take a closer look at light metering, the brains behind automated camera exposure.  Firstly why do we need light metering at all?  In an ideal world the camera would be sensitive enough to record exactly what it sees and with minimal fuss.  Unfortunately digital sensors and indeed film stock are less sensitive to light than for example the human eye, and digital cameras are also incapable of capturing the entire dynamic range of every scene we encounter.  Dynamic range is the contrast between the brightest parts of the scene and the darkest parts.  Outdoors, on a bright day, the light contrast can far exceed the dynamic range of any camera.  On a dull, overcast day, chances are, everything in a scene falls well within the dynamic range of the camera and image exposures tends to be far easier to get right.  Metering is used to adjust camera exposure in order to capture a certain contrast range so that at least the part of the scene we are interested in (i.e. the bird) is well exposed.  The key is to be able to meter just the subject which we wish to capture.

A photographic image is possible because incident light coming from the sun, the sky and surrounding objects all reflect off a subject and that reflected light then enters the camera to be recorded by the sensor.

There are two important and distinct components at play here.  Firstly we have the reflectance of the subject we are photographing, and, secondly we have the intensity of the incident light hitting the subject and reflecting off of it. Reflected light from the subject is therefore a result of both of these components working together.  It stands to reason that a highly reflective surface will reflect more incident light and will therefore appear brighter to the camera.  The light meter however merely measures the amount of reflected light reaching the lens.  This presents a big problem.  Because a camera cannot distinguish a bright day from a brightly reflective object there has to be a trade-off, and the trade-off is this. A camera's default is to consider the world and everything in it as being of a uniform pale grey reflectance (approx. 18% grey).  When a camera meters light it treats the reading as though it were measuring incident light, i.e. the light hitting the subject, not the light reflecting off of it.  Hopefully the illustrations below explain this more clearly than I can.

As the illustration above demonstrates, there are alternatives to using a camera's on board light meter.  Handheld light meters and grey cards both work by giving a proper measure for incident light intensity.  Armed with this information a useful image exposure can be worked out.  This approach is somewhat more reliable than on board exposure metering measuring reflected light, but obviously there is a limit to it's practicality for most bird photography.  So, lets assume for the most part, light metering in bird photography is mostly done by the camera and involves reflected light only.

At it's most basic, camera metering using one spot can lead to results that are way off the mark and this is entirely down to the camera's inability to measure reflectance.  However, if the scene is highly variable and highly contrasting it may help to spot meter and then use exposure compensation to make up for an expected discrepancy.  For example when photographing gulls, spot metering of the grey mantle of a European Herring Gull (Larus argentatus) might produce a reasonable exposure but trying to use the same method to photograph a Lesser Black-backed Gull (Larus fuscus) will tend to produce over-exposed images, because the back of a LBBG is much darker than 18% grey.  The solution might be to meter off the white instead and apply a standard exposure compensation, or else use an evaluative light metering method that takes into account far more points in the scene, including background.  Again however, none of these metering methods may produce perfect results.

Metering options include spot-metering as illustrated above, evaluative metering, based on a large number of metered points in the scene, partial which has fewer points and centre-weighted, which meters more points but they are all around the centre of the image, where the subject is most likely to be.  None of these options are fool proof.  One of the big advantages of digital cameras is that the photographer has the ability to check and adjust metering and exposure and hopefully go back for more images until a satisfactory exposure is nailed.

There is obviously a lot more to metering and exposure but at the moment I am only touching on the highlights, primarily to illustrate just how difficult it is to get exposure just right.  For those interested in reading on check out THIS nice tutorial on the Cambridge in Colour website and also check out links HERE and HERE.

Having split up camera exposure into a number of it's complex components, how do exposure time, light intensity and dynamic range, subject reflectance and camera dynamic range all relate?  Here is an attempt at presenting all these elements together.

Our ambient lighting is ever changing.  In dull conditions it is low in contrast and dynamic range but on bright days it's dynamic range far exceeds that of the camera.  Depending on where the subject is positioned in it's environment (directly in sun, or in partial or total shade) a certain light intensity will be shining on the bird.  The reflectance in the bird's plumage and bare parts will determine how much of this incident light reflects towards the camera.  While the subject's reflectance is fixed the actual intensity of light reflecting from the bird is in direct proportion to the intensity of incident light hitting it.  The camera records this reflected light using the on board light meter and uses this information to try and gauge an appropriate exposure time.  If the exposure is correct then subject's reflectance will be in line with the camera's dynamic range.  Otherwise there will be a mismatch, with the result that the image will be either under or overexposed.  If this is severe enough there will be an irretrievable loss of detail and colour and image artefacts like noise and blooming will be introduced.  So, accurate metering of the subject is the critical component in all of this.

No comments:

Post a Comment