2 Mar 2011

Exercise: Higher and Lower Sensitivity

In the old days my camera bag was cluttered with various rolls of film from luscious colour slide, through bog standard ASA200 colour print to highly sensitive and big grain Black and White.  Many rolls were half used and rewound waiting for an opportunity to finish them off.

Of course nowadays my camera can replicate many of the results I obtained with my 'wet' medium at the flick of a dial.  Using a good digital has, however, made me a bit lazy and it was good to go through this exercise to remind me of the flexibility available.

Most of the time I shoot with as low an ISO setting as I can get away with so that my photos have as little noise as possible and can be easily blown up to full size without deteriorating.  When I am faced with a low light situation I usually try to get away with using my image stabiliser or a tripod if I am lucky enough to have brought one.  Some situations, however, need good control of depth of field or speed and I need to wind up the ISO sensitivity to get the speed and aperture settings that I need.

1/30  f5.6 ISO 1600

For example, this shot of the Guggenheim Museum in New York needed to be hand held so I selected the lowest speed I could get away with, an f stop that would just about give me enough depth of field and an ISO that exposed correctly.  All were a compromise as I would have loved to have shot at 1/200, f11 and ISO 100 but without a tripod and shooting quickly as compositions presented themselves that was not an option!

1/350  f11 ISO800
 Here I was shooting in the early morning light, working fast to get what I wanted before the sun rose too high so I preset ISO800 and worked without worrying too much as I knew that I would have enough sensitivity to cope with a reasonable shutter speed and f stop.  As can be seen I could easily have lowered the ISO as I had plenty of light at this point.

At ISO800, as the closeup shows, noise wasn't a problem as my camera copes well with quite high ISO setting.  The following exercise asked us to shoot scenes at high and low ISO settings to compare the pros and cons.

 20sec f14 ISO200
 This image of a Miami Beach hotel is taken on a tripod using an insensitive ISO of 200 with a good f stop for depth of field and an appropriate time to expose properly.  The long exposure produces the smooth water on the fountain and the light streak of a passing car.  The depth of field is great and it makes a good image.

  The 100% closeup doesn't show any noise and it is a nice clean image that could be enlarged to poster size without any concerns except for a number of artefacts that would need to be removed before printing.  These artefacts, seen below, show up as bright specks of white or coloured light are a result of the long exposure required for the shot.  With low light some pixels on the sensor prove to be inappropriately sensitive and display themselves as bright spots, they are often referred to as 'hot pixels'.  Their output is interpreted as real light and the camera's sensor logic highlights the area.  The appearance of these artefacts can be reduced by using the camera's long exposure or low noise settings or they can be manually removed post production.


1/25 f4 ISO6400
Shot again off the tripod, the hotel looks very similar, particularly when displayed at this size and only at 72 DPI.  The shutter speed is much faster having gone from 20 seconds to 1/25th of a second which has given a different effect to the water fountain, partly freezing the water.  The depth of field is much smaller but since most of the image is approaching infinity it all looks well focussed.  It isn't until we examine the close up that the differences can really be seen.
Now we can see that the smooth and even colours of the image look mottled with noise and false artefacts when the sensor is set to such a high ISO.  This is the trade off between exposing for a long time with a low ISO setting and exposing for a short time with a high ISO setting.  It is the equivalent of using fast film which employs large silver halide salts to absorb the light quickly enough but has a large and visible grain when enlarged.   In a digital camera the noise comes from the amplification of the sensor signal (Gaussian noise) needed to increase its sensitivity to allow for the low light levels. The problem with noise on digital images is that the natural and uneven look to a grainy print is replaced by an much more regular and multicoloured effect.  We have grown used, over the years, to accept a grainy black and white image as a natural, indeed full of atmosphere and emotion which can enhance an image.  Noise on a digital image just looks like an electronic failure and detracts from the images purity and quality.  

  25sec f13 ISO200
Taken across a busy street, the exposure duration has much more impact on this image as  25 seconds with the shutter open gives rise to ghost images and streaks of light that add to the feelings of activity and light in the composition.  Some cars appear less blurred than others as they were waiting at traffic lights and moved late on in the exposure.

The lack of noise at ISO200 is apparent here in the closeup and if people remain still enough they appear as normal as opposed to a blurred ghost.
 1/30 f4 ISO6400
 The advantages of shooting at a high ISO and therefore a faster speed is that the motion blur becomes manageable and the cars are frozen allowing us to include a limo into the image which would otherwise have been just a streak of light.
The drawback is, as I have previously mentioned, the level of noise that builds up at high ISO settings.
15sec f14 ISO200
Again here a long exposure has allowed good depth of field but given a strange look to the sitting man's feet as he moved them during the exposure.
No noise or artefacts which makes this an excellent choice of exposure for an enlargement.

 1/50 f5 ISO6400
At ISO6400 the sensitivity allows the speed to rise to 1/50th which gave me the chance to freeze the movement of some passers by, something impossible when the exposure is measured in seconds.
The level of coloured noise that occurs at a high ISO setting is very apparent against the white of the church front.

1/60 f5 ISO6400
Although noise is ugly and rarely acceptable in a colour image, when the image is converted to black and white the noise resembles our old friend film grain which we love.  This gives an acceptable alternative to a high ISO blow up that might otherwise be unusable.

Although in this case I rather miss the blue palm tree!

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