Identification : Exposure


For most photographers, image exposure was traditionally the most important image quality parameter.  However modern digital image processing software has made it possible to correct exposure errors to a considerable extent at the finishing stage.  So, for me exposure is no more important than any other image quality parameter in the Image Quality Tool.

Exposure is simply the amount of light hitting the sensor or film, giving rise to the image.  It is measured as illumance times the exposure time (lux seconds).  Strictly speaking therefore the parameters which impact on exposure are those which increase or decrease the available light, namely the lens and filters, the aperture size and the shutter speed.

ISO is also generally considered part of the exposure equation and increasing ISO has a similar effect on the final image as decreasing aperture or increasing shutter speed.  However, ISO adjustment involves processing (amplification) of the image data and therefore takes place after the initial exposure has been made.

Exposure and Bird Photography

In terms of bird photography, as we are dealing with small, fast moving and often not particularly approachable subjects, image capture and exposure tends to be a particular challenge.  Bird images are often produced under the following, rather difficult conditions:-

To capture a reasonable sized image of the small, distant bird we tend to require a long lens, 300mm or longer.  The longer the lens, the lower the transmission of light through to the sensor.  Digiscoping is the ultimate in long lens photography and can be frustratingly difficult.

Digiscoping was probably at the peak of it's popularity in Ireland in 2004 when this stunning spring male Hawfinch (Coccothraustes coccothraustes) turned up in a birder's garden in County Cork.  Rarely does such a perfect photographic opportunity present itself.  Yet, I can still remember the frustration of trying to obtain satisfactory images with my Nikon Coolpix 4500 through a Leica scope.  With a DSLR an image of this quality would be relatively straightforward.  While the composition may not be great, the exposure of this image is quite good, all things considered.  There are still some telltale digiscoping clues including a  shallow depth of field and vignetting (dark edges).  This was one of only two acceptable images obtained in over an hour while having this stunning bird at near point blank range!  Still, one can hardly complain with views like this, not to mention the kind hospitality of the finder - this was photographed through a kitchen window with a cup of tea in one hand!

To capture a fast-moving subject with a long lens while avoiding motion blur the shutter speed must be very fast - perhaps 1/1000th of a second at times.  This further, greatly reduces the amount of light available to create an optimally exposed image.  

The aperture of a lens determines the depth or field of the image, i.e. the depth of the scene that appears in focus.  A greater depth of field ensures more of the subject will be in focus and becomes more of a challenge the closer the subject is to the camera.   However the greater the depth of field, the smaller the aperture and therefore the lower the light transmission.  Many photographers trade a shallow depth of field for a lower ISO and higher shutter speed in order to obtain a sharp and reasonably exposed image, only opting for a higher depth of field on brighter days or where lower shutter speeds allow.

ISO is a setting that allows the light sensitivity of the sensor to be increased.  This is done by amplifying the image data so it is part of post-production.  The downside is increased noise, though modern electronics and processing algorithms have greatly improved the quality of high ISO images.  As some modern compact cameras produce better quality images at higher ISO, plus offer the ability to shoot in RAW, this has prompted some birders to return to digiscoping as the preferred option for capturing bird images.  Not only is a compact digital camera and digiscoping setup far cheaper than a DSLR setup, it is also far more compact and convenient for the field.  On the flip side, if you have ever tried to digiscope a small, fast moving passerine or a bird in flight you will soon discover the limits of your patience!

Light Metering

Exposure metering is the brains behind automated camera exposure.  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.

Optimum Exposure

Human vision can adjust to bright sunlight and very low light conditions however we struggle to deal with both at the same time.  Our eyes must adapt by pupil constriction (in bright condition) or dilation (in dull conditions) and there is also a delay while some biochemical changes take place.  A camera can adapt to changing light more quickly but, digital cameras can only deal with light intensity within a narrow band, referred to as the camera's dynamic range.  An optimal exposure is one in which the entire range of luminance of a subject being photographed falls within the camera's dynamic range.

As discussed above, adapting for different light intensity requires the camera's on board light meter and camera exposure is adjusted accordingly.  But in high dynamic range settings the camera will simply not be able to capture all highlights and shadows within the same scene.  High dynamic range scenes consisting of patches of bright light and deep shadow tend to produce some of the least acceptable photographic results and quickly reveal the limited dynamic range of a digital camera.  In this scenario careful use of the camera's light metering is essential in order to capture a useful image of a bird in the middle of such a high contrast scene.

In the diagram above I have tried to present the overall effects of under and overexposure on highlights, mid-tones and shadows plus image artefacts.  The top row of discs represent a range of luminance patches captured at optimal exposure.  The middle row are the same discs captured in an underexposed image.  There is a net loss in contrast, there may be clipping of image data in the deep shadows and noise increases, all as a result of underexposure.

The bottom row depicts overexposure of the same range of discs.  Clipping may occur in the highlights and a bloom artefact may be introduced as electrons spill over across photosites in extreme cases.  Deep shadows are reduced and the overall image is reduced in contrast.  On the plus side, noise may be reduced during overexposure but this doesn't tend to make up for the irretrievable loss of data in the highlights.  As stated above, there tends to be much greater latitude for retrieving image data in underexposed images than in overexposed images.

This image displays and summarises the effects of exposure on image detail and colour.  It echoes the comments above regarding latitude in over and underexposed images.  Note especially the loss due to blooming of subtle colours and fine detail, an artefact caused by overexposure.  By contrast, fine detail and acutance remains reasonably good in underexposed images, though colour does suffer quite a bit.  So, given the difficulties in judging accurate exposure using a camera's on board light meter, are there any techniques for optimising exposure to minimise artefacts and data loss?

Optimal Exposure Techniques (eg. ETTR) and Exposure Bracketing

"Exposure to the right" or ETTR is photography intended for optimised exposure of the subject in which the photographer studies the histogram obtained from the most recent image or in live histogram if the camera has that function and then adjusts exposure in order to push the image histogram to the far right of the graph, usually just avoiding clipping image data.  Why to the right?  Digital images tend to allow a greater "Latitude" for recovery of underexposed detail than overexposed detail.  Hence, exposing to the right ensures highlight details are preserved, possibly at the expense of some shadow detail.

ETTR should be done with ISO set to the minimum (ISO 100) and therefore implies good lighting and a cooperative subject.  With the camera set up this way the exposure is optimal for the subject being captured and noise and other artefacts are hopefully kept to a minimum.

Despite the potential for dark, underexposed images with this method, amazing results can be obtained with this technique when processing in RAW even in high dynamic range images and low light situations, provided one is prepared to spend time working on images in post-production.  For more see HERE.

Exposure bracketing is another useful technique for improving one's chances of obtaining a preferred exposure,  It can be used in conjunction with rapid continuous shooting and basically produces of sequence of 3 or more images with different exposure setting in succession.  It is memory space and time intensive as one must be prepared to pour over and dump most of your images. Preferable, and assuming the subject is being cooperative, it is better to take the time to compose and meter for accurate exposure. For more on bracketing see HERE.

High Dynamic Range Imaging (HDRI)

It is possible to combine multiple image exposures to make allowances for the limited dynamic range of the camera.  This is done by creating multiple exposures using exposure bracketing and them knitting together the images using bespoke software.  An alternative to this is Tone Mapping, which involved adjustment of tonal curves in an image to bring out detail in highlights and shadows.  For more see HERE.

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