Tuesday, 25 October 2011

Exercise: Colour cast and white balance

In this exercise we are exploring the varying colour temperatures given off by the sun in different circumstances such as time of the day, clouds, shade, sunshine and the interaction with artificial lighting. A key point I want to note is that although temperature increases with heat (degrees kelvin) the 'colour cast or colour temperature' of blue (around 10,000k) is a higher temperature that that of a setting red sun (3000k), and white light (12 mid day in sunshine) being somewhere in between the two at 5500k. An interesting comparison made in the notes was certain elements as they increase in temperature turn from red to white to blue.

We are to find scenes in sunlight, shade on a sunny day and cloud and take four images of each with the camera set for white balance settings of auto, sunlight, shade and cloud noting the difference.

The first set of four images were taken on a cloudy day.

I chose these coloured pegs as I thought varying settings would make a difference but there is very little. If anything the Cloudy setting is less intense and therefore probably more true  in terms of colour but I like the brighter more intense colours of the Shade WB.

The next set were of those in sunlight, taken at around 11 am in early Autumn.

Again the relevant setting seems to be more accurate, that is the daylight setting, or sunlight setting does not have a red'ish cast compared to the others, albeit warming up the scene nicely. As it can be seen although the sun was shining there were clouds about and the Cloudy setting has also worked reasonably well, perhaps better than the Auto setting.

The final set were taken on a sunny day in the Shade or part shade. Here there are seen more dramatic differences.

The Auto setting definitely has a blue cast to it and is not the best version by far. I was surprised to see that the Shade version has tried to compensate for the Blue Cast but warming up the image, but a little too much, almost as it it were in sunshine itself. The Cloudy WB probably reflects the most accurate and pleasing image here.

The second part of this exercise is to take three images using a white balance of auto, sunlight and tungsten at dusk where both the inside and outside are visible with the interior lit by incandescent (tungsten) lighting. This required a tripod as the shutter speeds were around 1/4 second.

The daylight setting certainly has warmed up this image, a little too much though. The auto setting has too warmed up  the image but not quite as much. The tungsten setting has good neutral inside tones but at the cost of producing a blue cast to the outside particularly on the edge of the panes of glass.

Finally we are asked if using RAW to look at adjusting the white balance sliders to see if there is a compromise. If found that by adjusting the temperature from 3150 to 3400 and the tint from +4 to -12 gave the best results. This would warm up the image and increase the green of the background foliage respectively.

Monday, 24 October 2011

Exercise: Your camera's dynamic range

This was an interesting exercise to determine the dynamic range of our camera, mine is a Canon EOS 40D. To measure the range we are to set up a scene in sunlight with a piece of white card to provide the highest relfectance and hence the highest highlight. The scene should also have a dark area  with detail, I chose a summerhouse in sunlight where the windows were almost black but able to see some detail, I used a piece of A3 card and placed it in both shade and direct sunlight.

With the camera set at 1/800 exposure at ISO 100 with all noise reduction off, I found that an aperture of f/4.5 just prevented the camera's clipping warning on the white card, signifying the brightest part of the scene. This image is shown below.

We were then asked to measure the brightest part of the image and two or three dark parts. I changed the metering of the camera to spot and zoomed into the card which measured f/11. The lowest part of the image I measured was f/4, this however was also on the limit of the lens I was using but seemed to be a good reading.

With the image open in Photoshop I used the colour sampling tool to measure the RGB channels on the card, these were almost at the maximum with typical readings on all channels in the 25x range showing that there was no burn out in the image and it was indeed exposed as best as possible.

To find the lowest exposure capable of the camera I opened the image in RAW and turned off any noise reduction and zoomed into the darkest part of the image. Adjusting the exposure slider so that the dark areas were lightened until the level of noise introduced could not be distinguished from the detail. Here is a snapshot of that scene. The noise introduced on the wood is not not distinguishable with the detail.

This adjustment gave a further 3.5 stops. This only gave my camera a total dynamic range of 6.5 stops, reading online literature it is actually around 8.5 stops, but I think that although this exercise shows how to judge the dynamic range it is probably not as accurate as the testing methods I have reviewed. If time permits I will repeat this again with a lens that has a lower f-stop, I think that is probably where I lost a couple!

Monday, 10 October 2011

Exercise: Sensor linear capture

This was an interesting exercise to see how light that falls on the camera's sensor is processed within the camera. Light is treated linear for the camera's sensor and most of the captured light is towards the the left of a histogram in the dark areas. To compensate for this the camera applies a gamma correction to boost the highlights and mid tones, effectively shifting the histogram to the right. This gives an image which is close to what the human eye would have seen.

Here is the original with a copy of the histogram.

The following curve was applied to this image to bring it back to what it would have looked like before the camera applied its gamma correction.

This has made the image a lot darker with the histogram bunched up to the left.

The key part to note here is that the majority of the available levels above in the histogram are used in the brighter part of the image, this part of the histogram that is in the right half. This means that there is less available for the darker tones to work with that are all scrunched up on the left, this leads to lesser sampling which will introduce artificial effects such as noise.

By applying the following curve we are emulating the processing of the camera to the linear image. This has restored the image to something near the original, but during the process has introduced additional noise in the areas that it has boosted, notably the shadows.

Although these changes were made with a 16 bit resolution an interesting point to note that is not mentioned in the notes is that the number of samples on the left (shadows) is now significantly reduced, and will therefore inherently introduce unwanted effects.