Also at the advise of my tutor I have followed the following tutorials. These notes are in order to help me with my studies. I have sometimes quoted and sometimes abbreviated the text within the tutorial in order to remember its contents.
Note if there is noise in the image then you must be very careful with sharpening not to increase the noise so apply sharpening only to edges. Noise is particularly noticeable in the darker areas of the image. First reduce noise and then proceed to sharpen.
Selective sharpening using masks to apply only to part of an image. Creating a mask layer new layer in CS4. You can apply more aggressive sharpening to an image on duplicate layer and then adjust the opacity or only allow opacity in certain areas and other areas can be painted black or white. Or as with Kelby change the blending mode to luminosity so it is only applied to certain areas of the image. Another option is to apply softening to the parts of the image you do not need sharp which makes the rest of the image appear sharp.
Sharpening for web and output sharpening. It is impossible to tell from the print on a screen what it will look like when printed often what is effective for printing will not look good on your computer.
The type of paper, printer are influences. For example matt paper requires more agressive sharpening then on glossy. I have noticed this as here in thailand glossy only exists on small prints however I printed small on glossy and then large on matt and the matt looks much less sharp almost with more noise.
So an estimated calculation is the viewing distance typically the length of the diagonal of the image for example 25cm and the ppi for example 300 means the sharpening radius should be around 1.2 pixels. Or sometimes its just trial and error. Perhaps at this point for myself as I do not print. I should start to always use the same printing company so as to compare this.
The key is that it should be small enough to be at the limit to what our eye can resolve but large enough to visible improve sharpness.
Noted in this tutorial that for the web if the image is downsized to 50% then it is difficult to see the halos of sharpening.
Sharpening is irreversible so save copies and work on that. RAW files react much better than JPEGS. I have been working on my JPEGs so I should start using my RAW files.
Blurring due to movement of subject may require advances sharpening techniques.
There is more information within this tutorial however for more selective sharpening and effects of different lenses I am going to hold off and return at a later date when I have digested all of the above.
Light sharpening Halos are more objectionable than dark ones.
How the unsharp mask works. It creates a blurred copy of the original and places it over the original, it is then subtracted from the original to detect the presence of edges. Contrast is then increased along the edges leaving a sharper image. The unsharp mask is actually using a trick of how our own brain perceives what are called Mach Bands. Again be careful not to over do it.
Confusion circle not influenced by focal length greatly. Longer focal lengths appear to have a shallower depth of field as they enlarge the background compared to the foreground.
Note how a wide angle lens provides a more gradually fading DoF behind the focal plane than in front, which is important for traditional landscape photographs.
On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will frame the subject entirely differently). This is more representative of everyday use, but is an effect due to higher magnification, not focal length.
Why not just use the smallest aperture (largest number) to achieve the best possible depth of field? Other than the fact that this may require prohibitively long shutter speeds without a camera tripod, too small of an aperture softens the image by creating a larger circle of confusion (or “Airy disk”) due to an effect called diffraction — even within the plane of focus. Diffraction quickly becomes more of a limiting factor than depth of field as the aperture gets smaller. Despite their extreme depth of field, this is also why “pinhole cameras” have limited resolution. So to sum it up a large aperture creates a larger circle of confusion.
For macro photography (high magnification), the depth of field is actually influenced by another factor: pupil magnification. This is equal to one for lenses which are internally symmetric, although for wide angle and telephoto lenses this is greater or less than one, respectively. A greater depth of field is achieved (than would be ordinarily calculated) for a pupil magnification less than one, whereas the pupil magnification does not change the calculation when it is equal to one. The problem is that the pupil magnification is usually not provided by lens manufacturers, and one can only roughly estimate it visually.
See also reflections after assignment four regarding sharpness of image.
See also Hyperfocal Distance.