An "estimate in image" dof calculator

The basics

This is a new dof calculator which gives the same results as an usual dof calculator. The main difference is that instead of estimating distances on the scene, it requires that you simply estimate directly in the image.

The method works in use cases where you have a reference plane, like a street, floor of a building, flat landscape, river... The focus plane, and the near/far limits of the dof must have positions in this plane typically represented as horizontal lines... You must also be able to easily estimate the altitude of the camera above this plane.

The simple formula in the image dimension

• f is the focal length (real not equivalent)

• r is the portion of the total image (in the height dimension) in terms of ratio (1/3, ..) which represents the area between the focus line in the reference plane and the dof limit still in this reference place.

• k is a number of pixels, representing the resolutions that you have at your limits. k =800 in landscape mode or 1200 in portrait mode gives results 100% consistent with an usual dof calculator (similar to Coc = 0.03mm for FF)

• h is the height of the camera above the reference plane

assumptions

A few constraints to see if the method can apply:

1. A reference plane

You must have a reference where you will have the different positions of these distant planes:

• near dof limit plane

• focus plane

• far dof limit plane

This plane works with straight plane, but this is used as an estimate method so you have some latitude. It does not necessarily have to be at the same altitude, it can have a constant slope. The positions of these planes along the plane of reference is displayed with horizontal lines.

2. A known altitude above this plane

You must be able to estimate the altitude above the reference plane taking into account the tilt if any:

A few remarks

• Your height does not change, if for instance you shoot at eyes level you will use mainly the same h everytime.

• in many use cases, the tilt is 98% neglectible. In landscape photography, streets, inside buildings, the tilt is often minor.

• If this is not the case, you can either estimate directly the distance h (see the schema above) or apply an adjustment with the formula:

3. Do not focus and recompose

This is a normal assumption for better accuracy.

Step 1: Select near + far focus limit method

Here is the first important point

With the dof calculator, you select both limits and it automatically draw a horizontal line at the focus distance which gives equal sharpness for both limits.

Step 2: compute f_#

The dof calculator computes f_# based on the formula.
It also gives the f# taking into account diffraction.

An alternative method (Select focus + limit)

Sometimes you want to focus closer to one or the other limit. The dof calculator allows to first set where the focus line is and then to set the limit that you want to be acceptably sharp. The other limit being closer, it will be sharper. Some people prefer to give priority to the background of foreground, using this method is certainly preferable.

A few tricks

Easy calculations

If h=1600mm, which is common, the ratio k/h = 0.5 which makes the equations really easy.

You can use this number is adjust slightly the result after you used this simplifcation

estimate the portion of the image with the grid lines

If you display the grid lines (3x3, 4x4), you can better estimate the ratio of the image for the corresponding area.

Infinity + hyperfocal

This is at the end of your plane, just a bit above. Imagine somebody having the same height as you, draw a line at the same "altitude" above this end point.

Hyperfocal is made easy with this method.

estimate tilt with camera level

mirrorless displays the camera level with the tilt up/down, it can help to adjust h.

diffraction taken into account

You have also the optimal f_# (called f# diffraction) to use for maximum sharpness at the near/far limits. This is calculated using this equation

This will give a good resolution at these limits but will often reduce the resolution near the focus plane. Using the first f_# as a minimum value and the diffraction f_# as the maximum value can be of a great help for the photographer.