Saturday, 13 September 2014

A closer look at Image 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 just about as important as some of the other image quality parameters in the Image Quality Tool.

Exposure is simply the amount of light hitting the sensor or film, giving rise to the image and 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!

Optimum Exposure, Light Metering and Dynamic Range

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.

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.

Put another way, if the camera is set up to judge camera exposure based on the average lighting throughout a whole scene, consisting of bright sunlight and deep shadows, it is probably unlikely that the chosen exposure setting will be suitable to capture the subject properly.  The bird will either be too light or too dark in the final image.  More than likely the photographer will have to select spot metering and aim the metering spot directly at the bird to have some chance of obtaining a correct exposure.

Lastly, it is important to be aware that a light meter is calibrated to a grey patch of approximately 18% grey.  If the meter is aimed at a white gull or some other bright white object the result will be an dark, underexposed image.  Similarly, if the meter is pointed at a black crow or other very dark object, the result is a bright, overexposed image.  So, if spot metering is being used, one must also use exposure compensation to adjust for the luminance of the actual subject being photographed.  All, in all it should be pretty clear that obtaining a perfect image exposure of a bird is no mean feat!  Very often, when we are dealing with a difficult ID involving a poor quality image, the subject has not been optimally exposed.  This is very commonly the case with digiscoped / phonescoped images, images of contrasting scenes or birds with contrasting plumage markings.

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 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.

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.  ETTR encourages slight underexposure in order to preserve highlights.  Using the lowest ISO setting (ISO 100) during ETTR minimises noise, an artefact associated with underexposure.

See also
Lighting and Composition
Metering and Exposure

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