depth of field: its effect and cause...

[DeanTaylor]

hello

A beginner asks about DoF...


We know that stopping down--i.e., increasing the f number/reducing the aperture--serves to increase the depth of field. Okay--but how does it actually occur that a smaller aperture (i.e., letting in less light) will serve to increase DoF?

A novice (me) might intuit that a smaller aperture = less light = less everything, e.g., less objects in focus since less image-bearing light is reaching the film.

Another respondent noted that objects out of focus appear as small circles, and that with a smaller aperture the circles are smaller as well--his conclusion is that the smaller circles appear as having greater clarity, therefore, the greater DoF.

But, is this the entire explanation?


thank you


Dean Taylor

[Mike Buckley]

It has nothing to do with the amount of light reaching the film or digital sensor. It has to do with the physics of light pertaining to the optics of a lens. It's far too detailed for me to be able to explain but I've seen explanations.

As for the amount of light reaching the film or digital sensor, keep in mind that the aperture is not the only control of that; the shutter speed also controls it. Also keep in mind that the greater the sensitivity of the film or sensor (the higher the ISO value), the less light that is required to achieve the same exposure as when using a lower sensitivity. When the sensitivity varies (when the ISO value varies), the varying amount of light required to make the exposure has no impact on the depth of field.

[Manfred M]

Dean - CiC has a good tutorial on this subject at the following link:

https://www.cambridgeincolour.com/tu...h-of-field.htm

[jcuknz]

The CiC page on DoF is not that clear and I suggest that you make a drawing if you are reasonably competant in that direction and trace the lines from an object/point to the film as the CiC one does and then superimpose a smaller aperture and draw the lines to the iris and immediately you see that they extend further back and closer forward from the film.
I have somewhere a drawing which I did this as my explanation to help newbies but I cannot find it and offer my drawing similar to the CiC version but simulates the light coming from points outside the camera.
If you make a copy of that drwg and then reduce the Iris opening and draw the lines maintaining the same refraction/bending angle you find that the lines inside the camera have a much smaller angle and stretch further in front and behind the film/sensor plane than when using the fully open lens. The limiting factor for apparent sharpness is the diameter of the Circle of Confusion as explained on one of my webpages.

edit You may find these links to my webpage helpful ....
Why
and
DoF
The second drwg on this page shows with red and green lines the effect of closing the iris and the resulting increase in DoF with the narrower lines at the sensor point. I missed it at first because my monitor appears to be a little bright and doesn't show the green lines as well as I would like.

edit 2 The good teacher is tolerant and continues with a different tack until the pupil understands .... my wife makes this point at me often to my irritation at her failure to understand photo points or lack of interest in detail ... but I understand the validity of her point when I am less worked up.

[jcuknz]

One of the 'trick' aspects of this subject that strictly speaking Depth of field refers to the range of apparent sharp focus outside the camera while inside the camera it is Depth of Focus and most diagrams illustrate Depth of Focus and not depth of field.

But if you draw both, even if not to scale, the result of using a smaller iris becomes immediately obvious and answers the question ... we also normally draw a simple single meniscus lens and it gets rather complicated when you consider multi element lenses normally in use ... but thankfully we are not discussing them.

I hope the water is clearing for you Dean

[pnodrog]

Have you ever taken something out into the bright light to see it more clearly?

Part of the reason it works is that the size of your eyes iris reduces giving you better depth of field. It is particularly apparant as you get older and the eye looses some of it's ability to focus on close objects and the increased depth of field helps compensate.