ISO sensitivity: light sensitivity of the image sensor

Or: Where does the image noise come from?

In the analogue photography the ISO values described the film sensitivity. The more sensitive a film, the less light is needed. That means, an analogue photographer needs to know before starting his project, which film with which sensitivity is needed. In other words - what light is present on the set. The reason is that a film change is not that easy when using an analogue camera.

That is a big advantage of digital photography: with a digital camera the ISO settings can be changed between the pictures.

The ISO sensitivity of digital cameras represents the light sensitivity of their image sensor.

Sometimes the manual of a camera offers recommendations for that. The following specifications refer on a Canon full-format digital camera and require, that no flash is used.

ISO sensitivity	recording situation
100–200 ISO	Recording with sun
400–800 ISO	overcast sky, evening
> 800 ISO	at night or in dark interiors

Small number tells, that the image sensor is little light sensitive. The bigger the ISO number, the more sensitive does the image sensor get. Thereby enabling recordings with scarcely any light.

As depicted with the example in the chapter aperture and exposure time with an amount of water, time and container size, is the ISO sensitivity the equivalent to the container size.

An increase of the sensitivity causes that fewer light is needed to achieve a correctly exposed photo. In the graphic we only need a small container to fill it completely.

Therefore, we have 3 adjustment possibilities for a correctly exposed picture.

Let us assume, that the photo will be ideal exposed with following settings: aperture value f/2,8, shutter speed 1/50 s and ISO 100.

But we want to half the shutter speed to capture a fast movement: i.e. we increase the shutter speed from 1/50 s to 1/100 s. The aperture settings should stay the same. To achieve the same exposure, we simply need to double the ISO value.

Photo A: Aperture f/2,8 Shutter Speed 1/100 s ISO 200

That means, we have a linear relationship here.

The same works with the aperture:

Photo B: Aperture f/2,0 Shutter Speed 1/100 s ISO 100

For taking photo B we had a further opened aperture, so more light, therefore a less "sensitive" image sensor was required.

Picture quality and ISO values

We could now jump to the conclusion, the more light sensitive the better. But this ISO sensitivity also has its disadvantages.

A higher sensitivity (i.e. higher ISO values) corresponds with an increase of image noises. The photo gets a lower-contrast and seems blurred. Let us take a look on these effects with some photo examples.

Please be aware of: the noise behavior is also heavily dependent on the quality of the camera! And the size of the image sensor also has a significant impact on the pictures. Following footages were done with a full-frame sensor. Because of that, the outcomes with higher ISO values are still acceptable. The same ISO settings with another camera can lead to a totally useless photo. Some small compact cameras already deliver horrible results when using an ISO setting of 400. I can only recommend that you test it with your own camera!

Following photo were taken with changes of shutter speed and ISO sensitivity. Care was taken that every photo has the same brightness.

In the first picture section the photo got taken with an ISO of 25600. Here we can clearly see, how the noise behavior affects the pictures. Darker area are severely affected than lighter ones.

In the second picture section we changed the ISO value to 4000. The image noise can be clearly seen in the neck area - while nearly no image noise can be seen in the white parts, it only occurs in the darker areas.

In the third picture section with an ISO value of 250 are only minor image noises to be seen (with this full-format camera).

Thus, our image quality is much better when we use small ISO values - but also need more light. When working in a studio with studio flashes some people tend to ISO 50 to achieve the highest possible quality. This works when you have full control over the illumination.

Where does the image noise come from?

Maybe you still know it from the old analogue music cassettes - there were always background noises. Even if everything was silent while recording there were still background noises on the recorded tape. This background noise also exists when using digital image sensors!

When a recording was done too low, then you could increase the volume later on - but, that way the background noise got louder. Something similar happens when raising the ISO values. The background noise increases and becomes visible in the shape of color noise and contrast noise.

Distinction DIN & ASA and ISO

Before ISO was created photography used the terms DIN or rather ASA to describe the film sensitivity.

• DIN stands for German Institut for Standardization (Deutsches Institut für Normung)
• ASA stands for American Standards Association

The differences between those two were the size indications and that DIN values were logarithmic, while ASA values got indicated linear.

Thats why 100 ASA is equal to 21° DIN,

a halving of 200 ASA is accordingly 24° DIN

ASA: 100 200 400 800 1600 3200

DIN: 21° 24° 27° 30° 33° 36°

Today’s ISO values combine both value - on the films are specifications like ISO 100/21° - digital cameras do not use the second specification, that is why you will seldom hear somebody speak about ISO 100/21°, they are rather more likely to speak about ISO 100. This terminology can be found in photo books and sometime in journals.

Task about ISO values and image noise

Take 3 photos with a nearly identical illumination (the finished photos should be not brighter nor darker than the other ones). Vary your ISO values from a very small one towards a very big one.

Examine the image noise in the 100%-view on a computer.

Tip: Use a tripod to take every one of the three photos with the same view and photograph a non-moving object.

How to take a correctly exposed photo

Normally, the light situation is a given circumstance. We want to capture this light. There is a too much and a too little. When we catch too little light the camera sensor does not react, and we get a black area (We can speak of that as that area "got flooded").

f/2,8 | 1/80 s | ISO 125 | 50 mm | light value -2 | EOS 50D

When we catch too much light, then the pixel of the image sensor "overflow". We get in this picture area only a white area without any illustrations (we can speak of a "burned-out" area. With that I do not mean the complete image sensors, only the according pixel and areas, which light shines upon. It can be seen clearly on the example photo of the church room, where daylight hits the ground. The tiles vanish completely, and no structure of the tiles can be recognized anymore on the picture. It is a completely white spot.

Of course, we do not have a single big black area or the other way around a single big light area. Our photo scene has dark and light areas. And both areas should show their patterns and details (it should be discernible). Let us call them extreme areas. If these extreme areas differ too much (Midday sun and additional a dark house entrance, which we want to photograph), then we have a problem. The contrast range of the scene is too high. The human eye can adapt here. The brain does automatic calculations and shows it how it is. The camera cannot do that for us! We as photographers need to mind if we are in such an extreme situation, which has a higher contrast range than the digital camera can depict, or if we simply set the wrong technical settings.

Our main problem with digital cameras is that we only have a limited contrast range. That is the reason why it is possible to capture too much light on the picture and overexpose the shot or when there is too less light to underexpose the picture. In some situations, it is simply not possible to capture everything. Then the photographer must decide which death he wants to die. Do you want to lose quality in the darker areas of the picture or in the lighter areas of the picture? One way or the other - choose what feels best and always depend it on the picture's message.