Copyright Michael Karbo, Denmark, Europe.
Chapter 13. Quality and ISO
The task of an image sensor is to register nuances in exposures – in as much detail as possible. It is also necessary to be able to hold a camera in our hands when we take photographs, which means the camera’s sensitivity is also important. Sensitivity is measured in ISO values, which state how fast the image sensor reacts to light. The faster, the better – which can give problems with digital noise. Read here about how you can set your camera’s ISO and which values are the best.
ISO values tell you how fast your camera reacts to light. With a high ISO you can take photographs with a slower shutter speed, which is often an advantage – in bad light, for example, or for exposures with flash or telephotos (i.e. with 6X-zoom).
All cameras have several ISO values to choose between but avoid using the highest, which give digital noise in the image, as seen in Figur 45.
A piece of good advice is, choose ISO 200 or lower for daily use. The quality of ISO 400 and 800 ought to be checked, before you use them in earnest. The image might become gritty and unsharp.
The sensitivity in Single Lens Reflexcameras is much higher because they have a larger image sensor. So here you can take photographs with, for example, ISO 800 and 1600 without this producing very much noise.
Digital noise is a problem too when taking exposures with slow shutter speeds (exposures at night, for example, without flash). Many of the better cameras have a function called reduced noise, which can remove a great deal of the noise. Measuring the noise for the same duration as the exposure itself does this; the function is selected in the camera’s menu system.
Figur 44. Selecting sensitivity via the camera’s menu system. Here with Canon, Nikon, Olympusand Fujifilm. The lowest ISO value gives images with the least noise.
Figur 45. If you look carefully, you can see the digital noise very clearly in the image on the right. This is because sensitivity has been pushed up to ISO 800.
A camera’s sensitivity
Every camera takes images with a particular sensitivity. Sensitivity expresses how the film or the image sensor reacts to the light’s energy.
Sensitivity is measured in ISO values, just as with film reels. You can, in fact, buy a “fast” film with ISO 800 or a “slow” film with ISO 50. The difference consists of how much light is needed to get an exposure. A digital camera works in the same way, the difference being that it is the image sensor that determines the sensitivity.
When the sensitivity is high, the camera is fast and less light is needed to take an exposure. It is clear that a fast camera is an advantage. You can take better exposures in bad light with a high sensitivity than you can with a low sensitivity.
For an image sensor to have a high sensitivity, it has to react to small amounts of light. This can be achieved with a large image sensor. The larger the sensitive area, then the more light energy that can be registered per unit of time.
If you cannot make the sensor larger, then you can push the sensitivity up with electronic reinforcement.
One of the big advantages with a digital Single Lens Reflex camera is that the image sensor is much bigger than those in small pocket cameras. This gives a very much better light sensitivity, because the larger the surface for the individual pixels (”electron well”) is, then the more photons (light energy masses) will be caught.
Sensitivity can be increased in the smaller cameras by reinforcement, which gives unfortunate side effects. When the camera’s sensitivity is forced, it gives digital noise in the images.
All cameras give you the option of choosing between several different sensitivities, normally with values from ISO 50 to 800.
The camera’s menu system enables you to choose an ISO value but you have to be aware of the fact that high values give noise. There is quite a lot of difference from camera to camera in the quality of sensitivity in relation to digital noise and this is, therefore, something you always ought to check. But if you keep to ISO 100 or 200, the image quality will usually be fine.
If you push a camera to its highest ISO values, it will often give gritty images. On top of this, the defects that are found on the outside edge of many objectives will be intensified. This applies especially to zoom cameras, where different sorts of imbalances are found in the objective’s processing of light rays. The optic quality is worsened when sensitivity is increased.
The value Auto is also an option; this means that the camera itself adjusts the light sensitivity to the actual photographic situation. On the face of it, this sounds captivating but it isn’t always a good idea. In some situations the camera might increase the sensitivity to avoid using the flash and this can give undesirable noise in the image.
Digital noise is also called coloured noise. It looks like undesirable “grit”, which appears in many exposures. There are two conditions, which can make noise more noticeable:
The noise is created electronically in the image sensor itself. A sensor is not perfect; the chip itself creates small electric charges, which ”pollute” the exposure. It is in this way that the sensor registers image details that don’t originate from the exposure. This is undesirable noise
A camera has a number of smart mechanisms, which try to compensate for this electric noise and they usually succeed quite well. But when you ”push” an image sensor to a higher sensitivity, then the electronic noise will also be increased. You really ought not to use the camera’s highest sensitivity at all; the images will be much too gritty.
The electronic noise will correspondingly be more conspicuous the longer the exposure takes. If you take images with three seconds of exposure, there can really be a lot of electronic noise in the image. This is one of the very few areas where an analog film reel is still better than a digital image sensor. Film can be exposed for hours (if the conditions are right), without it becoming grittier because of this.
Many cameras have a built-in noise filter, which is activated when longer exposures are taken, and which can reduce noise considerably. It is a very elegant system. First the image sensor is exposed with the desired exposure time (i.e. three seconds). Then the lens is shut so than no light can come in and then the camera takes a three second measurement of the electronic noise. Lastly the measured noise is taken out of the exposure itself and, hey presto, it is corrected for noise!
Noise can be reduced with the help of a control measurement after the exposure. By making a complete ”black” image at the same length of time as the exposure itself, the camera can measure the noise, which can then be ”taken out” of the image’s data.
So digital noise reduction doubles up the length of an exposure. Sometimes it has to be activated in a camera’s menu system. In some cameras, the function starts automatically when the shutter speed is higher than 1/25 seconds.
The best exposures, however, are taken with a camera with a big image sensor as in a Single Lens Reflexcamera. The light sensitivity is so high here that you can take photographs with ISO 800 and even 1600 without very much digital noise. Correspondingly night exposures with a long exposure time give much less noise when the image sensor is big.
Figur 46. Noise reduction is quite effective with exposure times of a second or more. It is activated in the camera’s menu system. The exposure time is four whole seconds, which is stated as 4”. This, of course, requires the use of a tripod. (Olympus 5060Z)
Figur 47. The image is exposed for three whole seconds with ISO 200. (Fujifilm S7000)