Project: Noise

Exercise: Your tolerance for noise.

For this exercise I made a series of identical images using the full range of the ISO settings on my camera. (from ISO 100 to ISO 6400 in 1/3 stops – 19 in all) The aperture for each image was constant (f6.3) and the shutter speed ranged from 1/250 to 1/4000 sec. Noise reduction was turned off. For comparison, I have included details of only the first, tenth and nineteenth images and for each I have shown the details of the textured and untextured parts of the image to demonstrate the effect of increasing the ISO sensitivity on different parts of the image.

Here is the whole image:

2928 f6.3 1/250s ISO100

Here is a table of the images comparing the results for the textured and untextured areas of the image, in sunlight and shade, at ISO 100, 800 and 6400:

ISO 100
ISO 800
ISO 6400

Conclusion: From the images above, it can be seen that noise in the textures of the cloth is not so easy to distinguish. At high ISO the texture seems to be less sharp with some occasional coloured artifacts. at ISO 800 there appears to be no appreciable difference. On the other hand, where there is no texture, at ISO 100  there is little noise, at ISO 800 more granular structure and colour banding and at ISO 6400 there are distinct clumps although the colour banding seems to have gone. Although this exercise was a bit hit and miss as far as control over the lighting is concerned, it clearly demonstrates the relationship between ISO sensitivity and apparent noise in different parts of the image.

Project: Histograms

Exercise 3: Histogram
Objective: To increase my familiarity with the histogram by relating it to images I have just shot.
I used the manual setting on my camera and auto bracketing , 3 shots with 0, +1 and –1 EV
Highlight and shadow clipping display: Capture NX2 will show a detailed image of lost shadows and highlights using different colours for different channels but does not display a warning. My camera shows only  overexposed highlights on its LCD screen.

Here are the nine images, their histograms, exposure details and my notes:

High Contrast: –1.0 EV (this image shows a full range of tones from black to white)

	Image 0896 1/200s f11 -1.0 EV ISO 200 28mm
Lost Highlights	Lost Shadows

In this image, which is under exposed by one EV, appears darker and there are some blocked shadows as shown by the lost shadows display. The lost highlights are minimal on the display. The histogram shows the darker tones pushed up again the left, showing that here are indeed blocked shadows.

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High Contrast +1.0 EV

	Image 0897 1/50s f11 + 1.0 EV ISO 200 28mm
Lost Highlights	Lost Shadows

This image is over exposed by one EV and looks much lighter. The lost highlights are very apparent but there is no evidence of shadows being blocked. The values on the histogram are pushed the other way, right up against the right side and the darker tones cover the range almost the the black end.

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High Contrast 0 EV

	Image 0898 1/100s f11 0 EV ISO 200 28mm
Lost Highlights	Lost Shadows

This image is exposed at 0 EV i.e. as indicated by the camera’s meter, there are no blocked shadows but the highlights are clipped predominately in the blue channel. The shadow end of the histogram is fairly precisely shown not quite against the left side.

With this lens (18 – 200mm zoom) I tend to set the meter to –0.7 EV compensation and keep an eye on the clipped highlights warning on the LCD display whilst shooting. I always shoot in RAW and this gives me retrospective control of exposure and camera settings.

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Average Contrast  0 EV 

	Image 863 1/100s f11 0 EV 400 ISO 200mm
Lost Highlights	Lost Shadows

This average contrast image has the majority of its tones concentrated around the central part of  the histogram. The highlights clipped are mostly in the red channel but these are more prominent after conversion from RAW to jpeg and image resizing. There is no shadow clipping.

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Average contrast –1.0 EV

	Image 864 1/200s f11 -1.0 EV ISO 400 200mm
Lost Highlights	Lost Shadows

1 EV less exposure has moved the tones in the histogram to the left. The clipped highlights are reduced and there are no discernable lost shadows. This image is probably the most acceptable of the three.
 

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Average Contrast +1.0 EV

	Image 865 1/50s f11 +1.0 EV ISO 400 200mm
Lost Highlights	Lost Shadows

Overexposing by 1 EV has given the image the typical washed out look, the lost highlights have increased and there is some shadow clipping in two channels. The histogram has shifted to the right.

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Low Contrast  + 1.0 EV I was struggling to find a low contrast scene to shoot so I waited until dusk and took s photograph of my back garden.

	Image 0900 1/2.5s f11 +1.0EV ISO 3200 22mm
Lost Highlights	Lost Shadows

At +1.0 EV, the tones have moved further to the right and are slightly clipped. There are no blocked shadows.

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Low Contrast 0 EV

	Image 0901 1/5s f11 0 EV ISO 3200 22mm
Lost Highlights	Lost Shadows

Typical low contrast at 0 EV sees the tones predominately to one side of the histogram. There is a small amount of clipping where the sky shows through the trees but there is no shadow blocking.

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Low Contrast –1.0 EV
 

	Image 0902 1/10s f11 -1.0 EV ISO 3200 22mm
Lost Highlights	Lost Shadows

With underexposure, the tones move further to the left of the histogram but still with minimal clipping and no shadow blocking.
Conclusions: I have been familiar with the purpose and use of the histogram for some time. I regularly under expose my images using the exposure compensation control ay my camera’s settings tend to produce slightly over exposed images. I have noticed the difference between the histograms for jpeg and raw images and will make allowances for this on converting from RAW to jpeg files.
This has been a useful exercise to consolidate my knowledge

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