This is a continuation of a series of posts about blur management for landscape photography. The series starts here.
Here’s a situation in which many landscape photographers often find themselves:
- There are important objects at various distances from the camera.
- There is too much subject motion to be able to use focus stacking or stitching techniques.
- There is not enough DOF available at low-diffraction apertures for all the objects to be sufficiently sharp at those stops.
- The photographer is uncertain what f-stop to pick and at what distance to focus in order to maximize the sharpness of the important objects.
I wrote a program to assist in the aperture and focus distance decisions. It takes the code that produced the graphs in the previous post, plus two additional set of information:
- A list of the distances at which the important subjects reside.
- A list of weights that describe the relative importance of those subjects.
The program spits out the optimum f-stop and focus distance, and plots the sharpness of those settings as a function of distance. As before, diffraction and DOF are considered, but not lens aberrations.
Let’s take it for a spin. If we put a 63 mm lens on a GFX 50S, at tell it that we have equally important subjects at 10 and 100 meters, here’s what we get:
The two vertical lines mark the distances of the important parts of the image. F/16 is the optimum f-stop. That is deep into diffraction blur territory, and the resultant sharpness at 10 and 100 meters isn’t great, but anything wider — and anything narrower — would be worse.
This graph puts the lie to a couple of hoary rules of thumb:
- You should focus a third of the way from the closest thing you want sharp to the furthest.
- You shouldn’t use apertures where there is a lot of diffraction blur.
Once you decide to shoot at f/16, you could compute the distance to focus using conventional DOF calculators, but you wouldn’t have a sharpness metric that took diffraction into account.
Now let’s add a third distance, leaving the weights all the same:
That moves the optimum focal point a bit further away, and says we can open up the lens fractionally. The differences are immaterial to real-world photography.
If we double the weight of the closest part of the subject:
About what you’d expect.
Next up: taking the optimizer out into the world.