...
But what baffles me is that if the stops change from f/11 towards f/32 how come image start becoming softer!!!! (I always get confused that from f/11 towards f/22 is called step up or down).
Confusing? Yes,
raising the f-Number from 11 to 22 (ie going from an aperture of f/11 to f/22) is 'stopping
down' (two stops in this case). The aperture becomes smaller, and less light passes through it.
I swear I saw many razor sharp images at f/32 in the books (learning to see creatively). How does one achieve that?:er: How to achieve the best compromise? On what factors does it depend on - focal length or there are some universal rules?
My search will continue...
Well, keep reading and keep asking questions about things that are unclear.
Why can a picture taken at f/32 look sharp?
The rule of thumb that I gave above suggests that at f/32 the nearest anything in the image will be to a true point is a smear about 32 μm in diameter. Just suppose that you were using film, and you made a contact print (ie the print is the same size as the piece of film). A smear 32 μm in diameter would look like a point if we viewed the print from normal viewing distance. If you want to equate that to a digital print, it would be something like an 800 dpi print (these numbers are intended to give an indication of magnitude, not for film vs digital mudslinging).
If you go along with the idea that a 300 dpi print is good enough to appear smooth - ie you aren't seeing the individual dots at normal viewing distances* then you might estimate that you could double the diameter of the f/32 smear and still be just below the size at which we would see it as a circle instead of a point. If you double the f-Number, you double the diameter of the smear - so f/64 would be OK to use if you were going to make a contact print. And it is. If you use a large format 8x10 camera with the lens stopped down to f/64 and then make 8x10 contact prints they will be razor sharp.
Now suppose that you are making a 2x enlargement. Using a piece of 4x5 film to make an 8x10 print. To keep the size of the smear on the print to 64 μm, the smear musn't be more than 32 μm on the film - so f/32 will look razor sharp.
And so on, as the format size comes down. It's not quite that simple, but it is a start, to accompany the other stuff you are reading.
I use f/32 with full-frame 35 mm when I absolutely need depth of field above all else (or I used to, until focus stacking became practical). One thing you do in that case is to ensure that the entire picture is at equal sharpness or softness, and not to have sharper images nearby.
What is the practical limit?
With a three-dimensional subject such as your rock, the overall sharpness can increase as you stop down. The parts that were in perfect focus at f/2.8 may get sharper at first as lens aberrations are reduced then softer as diffraction takes over. The parts that are way out of focus just get sharper as you stop down as the depth of field increases. They may reach a balance where the focus blur equals the diffraction blur. That would be the limiting aperture in that particular case. Then it is a matter of whether or not you can accept that degree of blur. This is something to learn from experience. Every case is different - the best f-stop depends on the situation. It is not a fixed value, unless you are taking pictures of flat objects all the time.
Photo.net discussions can be lively, surreal, entertaining, arcane, confusing, informative and embarrassing all at once. Here are some threads on diffraction, in no particular order:
One
Two
Three
Enter at your own risk.
*'Normal viewing distances' - there is a can of worms.
Best,
Helen