I’d like to pick up on the issue of image sharpness. So maybe, we should begin by reviewing where we are, with references to earlier blogs.
- We discussed the pinhole camera and described how larger f-numbers (the ratio of focal length to aperture) improve depth of field and sharpness.
- We discussed the pixel limit of a given camera’s resolution.
- We considered the diffraction limit of resolution in the context of pixels.
- We described the concept of measuring resolution in terms of line pairs and line width.
- We explored the relation ship between image contrast and resolution.
- We developed a simple method for measuring image sharpness or resolution that can easily be implemented and we showed the results of some real-life lens.
- We found that with a “good lens” you can achieve resolutions close to the pixel limit.
- We found that for real lenses matched with a given image sensor there is an ideal diffraction limited f-number, where you will achieve your sharpest image, provided you don’t need to worry about depth of field.
All of this was meant to enable you to declare war on useless statements that you see all over the place like: “tack sharp,” “excellent resolution, and “very,very sharp.” Care to try to put these in quantitative order – as in is a “tack sharp” lens better than a “very, very sharp lens” or the other way around? They are really meaningless descriptions. Yet, you see them in lens reviews all of the time.
So in the next technical blog, I’d like to set the stage, for considering how image noise affects resolution. And of course, noise is an important element to understanding the dynamic range of cameras and images as well.