Monday, 21 May 2012

Higher and Lower Sensitivity

Exercise - 12 images

The purpose of this exercise is to study the effect changing ISO sensitivity has on an image comparing the overall effect on appearance and any change in ease of shooting.

It is suggested to photograph a street where there is a mixture of light levels and subject movement in order to access these changes.

I am expecting to see a marked increase in noise as the ISO increases and also a faster shutter speed which would result in less motion blur in moving figures. My current camera, the Nikon D7000, generally has good reviews when it comes to the effects of noise on an image due to its internal noise reduction system but this is something I haven't tested out myself so it will be interesting to see the effect.

ISO 100 - ISO 640
All of these images were taken on Aperture Priority  - f/8. At ISO 100 the shutter speed was 1/350sec which jumped to 1/2000sec at ISO 200, 400 and 640. I had expected a faster shutter speed at increased ISO levels but am surprised by such a marked increase between ISO 100 and ISO 200.

With the slowest shutter speed recorded at 1/350sec there is no motion blur in the image. In hindsight using a narrower aperture to carry out this exercise would have demonstrated the effect on shutter speed better. Viewed at 100% there is very little noise visible in the images.

ISO 800 - 3200

At ISO 800 the shutter speed increased to 1/3000sec at f/8. Between ISO 1250 and ISO 6400+ this increased again to 1/8000sec. Some detail is beginning to appear in the shadow areas at the higher ISO however the brighter areas are also becoming brighter and the colour in the sky less saturated as a result.

ISO 5000 - ISO 6400+
The camera now captures much more detail in the shadow areas but the scene is generally overexposed. Adjusting aperture, shutter speed or using exposure compensation would correct this but generally I can't see a reason I would use such a high ISO value in this type of daytime setting anyway. In low light or night photography high ISO values are much more useful.

Comparing Visible Noise

These are the images take at ISO 100 and ISO 6400+ viewed at 100%.
ISO100  viewed at 100%
ISO 6400+ viewed at 100% 
The speckled appearance of the noise is clearly visible in the image taken at ISO 6400+. More noise is visible in the shadow area than the cobbled street area where it is disguised by the general pattern of the paving. How detrimental this is in the image probably depends on how large the final image will be viewed at and the purpose of the image.

Higher ISO values have their uses and are particularly useful in low light and night photography where the use of a tripod is restricted or not permitted. As a general rule using the lowest possible ISO setting is recommended.

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