Higher and lower sensitivity

The aim of this exercise is to consider the use of changes in ISO settings on digital images (impossible using film camera).

ISO is basically the fourth tool within the camera itself the digital photographer has to influence the amount of light contained within a shot. ISO measures the sensitivity of the image sensor; increasing the ISO value increases the sensitivity, thus allowing the photographer to either increase the aperture setting (therefore increasing depth of field); or to increase the shutter speed, possibly vital where the camera is hand held, or there is movement in the frame; or a mixture of both.

My Canon 450d has an ISO range of 200 to 1600. I set ISO to 200 normally so as to reduce noise, and tend to increase ISO only in low light situations where the shot cannot be taken hand held without an increase in ISO, almost as a last resort. Another possible use of ISO increase is where one requires greater depth of field, though personally I usually prefer to accept a low depth of field ahead of the noise caused by increasing the sensitivity of the image sensor.

I decided to take a few shots in the garden for this exercise.

First, however, I picked a situation while cycling home where increasing ISO is the only way to get a picture.

f4; 1/30; ISO 1600

f4; 0.3 ISO 200

Here is a good example of where ISO make a shot possible. Having no tripod, I had to hand hold the camera; even at 1/30 it is difficult to avoid camera shake; once the ISO is down at 200, it becomes impossible with a shutter speed of 0.3 seconds. This pair of images does I think demonstrate well the power of increasing ISO: the effect of increasing from 200 to 1600 has decreased the shutter speed by 7 stops.

f4; 1/20; ISO 200

f4; 1/60; ISO 800

There is no discernible difference between the two images here. I used a tripod to demonstrate that there is minimal benefit for some subjects in changing ISO.

f13;1/4; ISO 200

f13; 1/30; ISO 1600

There is a greater difference between these two shots of a leaf resting in a potted shrub. At a high zoom (800%+) one can see the higher degree of noise in the high ISO shot.

f13; 1/4; ISO 200

f13; 1/15; ISO 800

As with the birds, there is no discernible difference in noise between the two shots of a smart phone reflecting the tree above. What is becoming clear is that ISO 800 adds little discernible noise.

f13; 1/10; ISO 200

f13; 1/90; ISO 1600

Here the effect of maximizing ISO provides the opportunity to use a shutter speed six stops higher. There is more noise in the second shot, but hardly discernible.

f13; 1/30; ISO 1600

f13; 1/4; ISO 200

The last pair of shots was taken when preparing for the time of the day sequence at Barrs Court Moat near my home for a later exercise. (the second shot is used also as the first image of the sequence). Accidentally, I had failed to revert to ISO 200 after the last sensitivity shot so got a high ISO to commence with. Took the same shot at ISO 200 with a slower shutter speed and was amazed to discover a very discernible difference in the amount of low lying mist in the centre of the image, there being significantly more mist with a higher ISO. The shots were taken at 0709 and 0710, as close together as is possible so we can discount any change in the atmospheric conditions as being the cause f the difference.

A closer inspection zoomed in reveals that the greater amount of mist can be explained by the significantly higher amount of ISO noise in the first image, and that seems to have created a misty effect.

Learning outcomes from this exercise:

1. Increasing ISO can be a very helpful tool to make an image possible - with a constant aperture, my camera can take images at about 6-7 faster shutter speed stops by changing the ISO setting from lowest (200) to highest (160).
2. There was little discernible ISO noise added by increasing to ISO 800, however noise was discernible when increasing to 1600;
3. There can be a very noticeable impact of this in certain types of image, as seen with the last pair of images above.