Shallowdepth of field

F number depth of fieldphotography

ii.     The Aperture of the lens, as F-number: a system with a large F-number would have a larger DoF. However, less light would enter the system, which would then be at the expense of the integration time.

F number depth of fieldnikon

iv.     The focal length of the front objective: the DoF decreases while the focal length increases. In practice, a lens with a wider FOV has a larger DoF.

All the above shows that high spatial resolution with large DoF can only be achieved at the expense of a high F-number (lower throughput and longer integration time).

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F number depth of fieldcanon

Every optical imaging system has a depth of field (DoF). It is defined as the distance between the nearest and the farthest objects in sharp focus. The DoF should be distinct from another optical property, called the Depth of Focus, which refers to the tolerance of placement of the image plane concerning the lens position (Figure 1).

Where u represents the working distance (WD), N is the f-number, c is the Circle of Confusion, and f is the focal length. Note that equation (1) does not take into account lens aberrations. In some cases, when the circle of confusion is negligibly small relative to the Aperture (term c² tends to 0), and the working distance is significantly larger than the focal length (𝑢 − 𝑓 ≈ 𝑢), equation (1) can be approximated by [1].

An imaging system’s DoF is crucial when measuring non-flat samples. It could be in a laboratory, an industrial system, and remote sensing applications. The DoF can be estimated as follows [1].

F number depth of fieldcalculator

iii.     Circle of confusion (CoC): this is often corresponding to the pixel size, in the spatial dimension, along the slit, at the slit level (also taking into account the magnification of the system). A camera with large pixels will have large DoF. It also means that spatial binning would increase the DoF. PS: in practice, the (CoC) is limited by the system’s optical resolution but not the detector’s pixel size. Using the pixel size as CoC gives a minimum approximation for the DoF.

i.     The Working Distance: DoF increases with the measurement distance. Notice that this is at the expense of the spatial sampling of the object.