Film has (had) a grain structure made by the emulsion crystals. Digital images can have their equivalent, known as noise. Noise needs to be considered and used when working on digital images.
Film emulsion has a grain structure created by the silver halide crystals that are the active ingredient in the film. A particle of film grain does not have a set and determined size. Films with a slow speed will have a fine grain structure, while fast films will have a coarser grain structure. The type of developer and method of processing will also have an effect on the grain.
Digital cameras contain an analogue device known as a sensor to record the image. The sensitivity of the sensor can be increased by turning up the gain, equivalent to the volume, which gives the sensor a higher ISO rating; just like using a faster film. The trouble is that sensors usually create a certain amount of interference, or 'noise'. By amplifying the signal, the noise is also amplified, so noise will increase with higher speed ratings.
Many digital camera use some form of noise reduction and spend $40,000 on your digital back and you won't get any noise. As a side issue, digital capture is inherently soft and sharpening is also applied to the image. With cameras beyond the cheapest, noise reduction and sharpening can be turned off. This has the advantage in that with the right procedures, better results can often be achieved by dealing with noise and sharpening manually in editing.
Noise comes in two forms - luminance noise in the greys and black and chroma or colour noise.
Noise can smother the signal and with noise reduction turned off, there can be more detail left in the image. Noise can then be reduced just on the channels where it is most prominent.
An image scanned from film will have grain. A digital image might also have its own noise or texture. Blurring part of an image will smooth out any grain and a new background created from nothing will have no grain. As a result, the different components of the image will not match up; their textures will differ and various sections will stand out. New grain needs to be put in so that the final image looks complete. This is done by adding Noise.
Retouching images with the Clone Stamp and patching in sections can cause small areas of softness to be put into the images. As well as blurs, the motion and radial filters will break up the pixel structure and degrade the image in some areas. The texture and feel of the image will change and quite simply look odd, especially on enlargement.
The Noise filter is the basic weapon in curing this. It can disguise faults and give a much needed consistency over the image. It can also make several images look more compatible and part of a set.
- Enlarge the image to 100%.
- Via the Layers panel and the small arrow in the top right corner, go to Flatten Image.
- Go to Filter > Noise > Add Noise.
- Select Gaussian.
Check Monochromatic which will apply the filter without affecting the colours.
The Amount of noise used should be compared in the preview with different parts of the image.
A level of 2 may be all that is needed. OK. - A high resolution image, perhaps of 50MB, might need a noise setting of 4-6 to make all the elements in the image look consistent.
With higher Amount values, also go to Filter > Blur > Gaussian Blur and enter a very low value (perhaps .3) to take the excessive crispness off the Noise filter.
If you go back and look at an image where zoom blur has been used, enlarge it to 100%, you will see the texture of the image varies. Some of the image will have grain and texture while some will not and may look rather soft. The Noise filter will cover these differences and make the whole image look more complete.
Use the minimum amount necessary to give an even appearance. A high resolution with large areas of blur will need a higher setting, perhaps 10 or more.
Duncan Wherrett is an experienced professional photographer and Photoshop instructor.
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