Sometimes You Need to Have Sensitivity.
20/12/16In previous posts I’ve talked about the “exposure triangle” and have looked at 2 of those sides, aperture and length of exposure (a.k.a shutter speed) and how they affect the image. In this post I’m going to look at the 3rd and most overlooked side: sensor sensitivity.
A great many photographers will be quite comfortable adjusting the various settings of their camera to produce the desired result, but most will not consider sensor sensitivity at all. Why is this? The main reason is that the camera usually defaults to “Auto” as far as this mode is concerned and therefore it becomes almost invisible in use. Other photographers, who know about this feature, have a misunderstanding of the tangible effects when the sensitivity of the sensor is allowed to increase and as a result positively inhibit this from happening. However, doing so denies the user from a large part of the operating potential of the camera.
So where is this sensitivity control? It is usually buried in the camera’s menu and is named “ISO Control”. ISO stands for International Standards Organisation and as far as image making is concerned, you can forget that. What is important is the 2 or more digits that follow. Usually starting at 100 (it can be lower) and going up to 6400 (or sometimes higher) these numbers represent the sensitivity level of the sensor. Why choose those numbers? They correspond to a set of numbers from “wet film” days defined by The ASA (American Standards Authority) that indicated how sensitive to light a particular film was. A film rated at 50 ASA (a slow film) needed more light than one rated at 400 ASA (a fast film). So why would you not just always use a fast film? As ever there are compromises. The slow film gives better colour saturation and much finer “grain” – particularly in monochrome. Whereas the opposite is true of the fast film. Grain manifests itself as fine “snow” in the faster rated films and while it gives a certain “grittiness” to the image, does degrade the overall quality.
Relating this to digital sensors, the lower the ISO number, the more light is required to reach the sensor to produce a correctly exposed image. Of course, the other 2 sides of the triangle can adjust to get that light to the sensor as I’ve talked about before. Just as there were compromises with wet film, there are also with digital. In order to increase the sensitivity of the sensor a higher voltage is passed through it. While that produces more heat, the main problem is electrical “noise”; this appears as speckling on the image, not too different from the grain in wet films. In the early days of digital cameras this was a tough problem to remove and therefore photographers shied away from the higher ISO settings – 800 and above. Today’s cameras have very good noise suppression algorithms, which can be supplemented by noise reduction filters in post-possessing. As a result, shooting at ISO 6400 produces perfectly useable images.
What situations would you want to do that? The main reason is low light levels: poor weather, night, low illumination etc. combined with the fact that you need a reasonably short exposure time that is unobtainable even with the aperture fully open. An obvious example is night sports where you want to freeze the action but there is not enough light to do this. By increasing the ISO number, you will effectively reduce the amount of light needed but will get some increased sensor noise as a by-product. Is that acceptable? Only you as the photographer can answer that but by using ISO Control you can access a useful part of your camera’s operating envelope and perhaps get that image that you thought was not possible.