Thursday, 2 July 2015

Birds and Light - Lighting and Perspective (Part 2)

A rainbow is a reminder to us of the majesty of light.  As kids we try to find the rainbow's end until we come to realise that the rainbow always stays ahead if us, always positioned opposite the sun.  We then begin to realise that our individual perspective on the world matters.  This particularly luminous rainbow was photographed against a dark cloud, which also provided a convenient neutral grey for white balance correction.  I gave the image added saturation to punch out the colours.  Alexander's band to the left is a darker, unlit area of the sky between the primary and secondary bows.  The portion of sky inside the bow has added illumination, hence the contrast between inside and outside the primary bow.  At the outside red light at a wavelength of approximately 700nm fades off to invisible infrared, while inside violet light of wavelength roughly 400nm fades to invisible ultraviolet.  A camera's sensor can detect both IR and UV but these are filtered in most cameras before they reach the sensor (for more on IR and UV see HERE).

As an interesting aside there appears to be one or two very faint rainbows offset from the primary bow on the inside of the arc in the image above.  These might be an example of an uncommon phenomenon called supernumary rainbow, caused by wave interference, or possibly even triple-split rainbow which is rarely seen or photographed and only explained by science in the last couple of years as being due to the presence of non-spherical rain drops.  These faint bows were not obvious to the naked eye and may have been expressed as a result of increasing image saturation.  It is sobering to read that there are still new discoveries being made about rainbows - one of the most recognizable natural lighting phenomena.  Not surprising perhaps therefore there are lots people still don't know or take for granted when it comes to the subject of birds and light.

Earlier, on Lighting and Perspective
In Lighting and Perspective Part 1 I mainly focused on human binocular vision and it's implications for how we perceive the world versus how cameras record it.  I also carried out a specific lighting experiment with multiple identical multi-faceted targets to illustrate a point that subtle variations in the angle of the camera versus the subjects and light source affect the lighting and therefore the appearance of each subject.  I thought it might be useful to carry out a few more similar experiments, this time using more complex, oval and spherical shapes and to consider the possible impacts of different variables.

Right away I need to confess that I don't think I will ever totally understand and nail natural lighting in all it's complexity.  With each little experiment comes a new insight.  That is part of the challenge but also part of the real enjoyment of trying to tease out this complex subject.

Lens Comparisons
I have taken two lenses which I think most birders who use a DSLR are likely to have and use in the field.  The first is a typical camera kit lens, which often tends to be a multi-purpose zoom lens.  The kit lens of the Canon D70 is a wonderfully flexible 18-135mm, ultra-quiet zoom.  At full zoom 135mm the image is reasonably rectilinear but vignetting starts to creep in very slightly at the corners.  At 18mm the lens has a strong barrel distortion but vignetting seems to be under control (possibly due to curvilinear distortion).  The other lens I use is a 300mm fixed lens which is quite rectilinear and has no obvious vignetting.

In the first experiment I took five identical plastic eggs (raiding the kids' toy chest once again).  I arranged these symmetrically and glued them to a board.  I also placed a grey card in the scene and then took multiple images in low, diffuse evening light, spot-metering to the grey card to try and obtain consistent exposures with different lenses.  Obviously I had to adjust the distance to the subjects as I changed the lens or alternated the zoom setting.  The purpose of the experiment was to see if the targets remained similarly lit in each image and if there were any obvious differences based on lens type or focal length.

Once again I have used the versatile freeware Color Quantizer to postarize then artificially re-colour individual tones in order to map tones.  Note my choice of colours for each tone is purely arbitrary.  I explained how I did this in an earlier posting (HERE).

The most obvious thing is the major difference in perspective foreshortening between 18mm and the longer focal lengths.  The barrel distortion created by the 18mm lens is also quite striking.  The next point to make is that I didn't obtain a comparable exposure with the 18mm lens as evidenced by the paler-looking grey card.  I suspect this is due to the angle at which the grey card was lying.  So it wouldn't be fair to compare the 18mm image with the other two directly.  Obviously there would have been a delay of a minute or two between images as I needed to change lenses and realign the camera and scene between shots.  So lighting may have changed slightly between images.  Though I did pick a dull, low light and used the grey card for spot-metered exposure to try and iron out these differences.

On first impressions all targets within the same image are strikingly similar and this seems to contradict the findings of my earlier experiment HERE.  However a closer analysis of the images shows that indeed all subjects in all of the images have subtly different tonal maps.

On balance the type of lens doesn't appear to greatly influence the degree of difference between how subjects are lit and appear in the image, notwithstanding barrel distortion, vignetting etc.

Subject Size
In an experiment like this I considered that the actual size of the targets might influence the results because larger targets means working with greater distances from the camera and larger surface areas.  That said, relative angle of view doesn't change from before.  For my next experiment I switched to larger, and also more uniformly spherical targets.  This time I dispensed with the 18mm lens image and just compared 135mm and 300mm focal lengths.  The results show an even closer match between the 135mm and 300mm images than in the earlier experiment, though this may just be coincidental.  Once again, though superficially very similar looking, on closer examination all targets have a unique tonal map.

Bright Light Versus Dull Light
With the next experiment I was keen to see if there would be a difference between how the experiment would perform in low or diffuse light versus high contrast sunlight.  I was fortunate to have fast moving clouds and sunny spells so I was able to make this comparison happen in a matter of seconds.

In bright light the camera's dynamic range is challenged, contrast is increased and tonal range is reduced.  There was a small amount of clipping at the white end of the tonal range but other than that I was able to obtain a reasonable comparison.  It is interesting as an aside to see how an out of shot object managed to catch one of the targets with a long shadow in the sunlight image but the subject was entirely unaffected by this object in dull light.  One further consequence of photographing in bright sunlight I guess.  In this limited experiment I found that the more brightly lit scene produced more consistent-looking subjects.  Whether this is simply due to the reduced tonal range or a real effect of strong lighting I can't say.  In the lower light image there is considerably more variation.  

Point Source Versus Diffuse Lighting
What has come as a real surprise is that the brightest point (roughly at the centre of our concentric rings of colour) on our spherical targets above appears to change from a position of roughly 45 degrees on the sunlit image to a more vertical centre on the cloudy image.  The only possible explanation for this that I can account for is that the overall lighting on the subject may be shifting from one predominantly point source lit on the sunlit image to a scene predominantly ambient lit (i.e. lit by the sky dome) on the cloudy image.  

Further evidence for this comes in the image below where I photographed a number of targets from above.  In the diffusely lit image the centre-point in each target appears to shift with the lighting of the left hand targets pointing to the left and the right hand targets pointing to the right.  In stark contrast, with the point light source the lighting is unidirectional.  This finding has implications where we are trying to gauge lighting direction from images (for more see HERE).

Note how the shadows in the upper image appear to fall towards the centre of the image.  This is Lighting and Perspective in action. I think I am finally starting to grasp some of this subject.  Please expect a Lighting and Perspective Part 3!

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