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I came across a youtube video that discusses some aspects of depth of field in photography/videography. Some things that have been said collided with my understanding of DOF. I then had a long discussion with the creator of the video that unfortunately lead nowhere. I'm no scientist, so I lack a proper understanding of the subject matter. If what is said in the video is correct, I would like to know, so I can learn something from it and correct my misconceptions. The segment in question starts at 8mins 28s and ends at 9mins 10s (just over 30s in total)

https://www.youtube.com/watch?time_continue=507&v=xlnciK2cyto&source_ve_path=MjM4NTE&feature=emb_title

He says "the format size, whether it is full frame, super35, any of that, does not change depth of field", he also shows a lens projecting an image on a wall and then says "it doesn't matter how big the wall is, it hasn't changed the depth of field" because "the image has already been formed, as it goes through the lens, before it hits the sensor" (wall in this case).

Are those statements actually accurate?

My understanding of DOF is that you have to take into consideration the properties of the picture as it arrives on our retina. In order for us to consider a part of the image "in focus", the circle of confusion has to be under a certain threshold. Many factors have an influence on the circle of confusion: When recording the image, the main factors are size of entrance pupil of the lens and distance to subject but also the size of the recording format because it influences how much we have to enlarge the image when viewing it on our TV. When we view the picture on a TV, we have to take the distance from the viewer to the TV into account, the size of the TV etc.

We can't really say anything regarding to the DOF by looking at a picture projected to a wall without taking the other factors into account. The DOF we get in a picture is different if we record it on a small piece of film or on a large piece of film because we need to enlarge the small piece of film more than we enlarge the large piece of film to display it on our TV (the two screenshots of a DOF calculator illustrate that, the only parameter that has ben changed is the sensor size FF Sensor, MFT Sensor). Therefore the statement "it doesn't matter how big the wall is, it hasn't changed the depth of field" because "the image has already been formed, as it goes through the lens, before it hits the sensor" seams to be nonsensical.

Am I actually miss-understanding something here?

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Your understanding is correct; the author is confusing depth of focus with depth of field... it's a fairly common confusion as the two are interrelated, but only one is common terminology.

The depth of focus is the relative sharpness of details at the image plane, and depth of focus is a fixed characteristic of an image. It is the non-variable component of depth of field; and it is dictated only by the physical size of the aperture opening/entrance pupil (not the f/#).

Depth of field is not a fixed aspect of an image; so when people talk about depth of field as a fixed aspect, they are really talking about the depth of focus.

Depth of field is dictated only by magnification... it is how apparent the depth of focus is made to the viewer. Magnification includes all of the other variables... focal length, subject distance, sensor area/cropping/enlargement, viewing distance, and even the viewer's visual acuity.

If the final/total magnification causes the image to have the same composition and relative size to the viewer, then the depth of field will remain the same and be dependent only on the depth of focus recorded. That's why the standard for image sharpness (and the calculators), assumes a standard viewing distance approximately equal to the image diagonal; and to where the image occupies the human's ~ 45˚ primary field of view, of a person with standard visual acuity (the "standard" circle of confusion).


Edited to add for completeness, and because I already had the image:

Many think that the depth of field is fixed once the image is recorded due to subject distance and FL... that's what the author is stating. But it is not, only the depth of focus (relative sharpness) is fixed.

This shows a 6x crop/enlargement in post. Note that the depth of field as measured linearly within the scene decreases with the magnification factor; because depth of field is variable. However, the percentage of the image area that remains acceptably sharp remains the same; because the depth of focus remains constant.

enter image description here

The exact same reduction in the depth of field would occur if I had used a longer FL, shorter distance, or crop sensor instead; because they all cause the same increase in magnification/relative size.

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  • \$\begingroup\$ You might explicitly say that the term FOV assumes same image size for the viewer. \$\endgroup\$ Commented Apr 11 at 11:31
  • \$\begingroup\$ @EuriPinhollow... because of some of the stuff in the video? I'll add another blurb to tie it all together. \$\endgroup\$ Commented Apr 11 at 13:00
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    \$\begingroup\$ I don't think the author is referring to Depth of Focus. \$\endgroup\$
    – Rafael
    Commented Apr 11 at 21:52
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    \$\begingroup\$ @Rafael they literally says in the video "the only things which affect DOF are focal length, aperture and distance to subject". That's not true about DOField. \$\endgroup\$ Commented Apr 12 at 7:13
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    \$\begingroup\$ Thanks for the picture, illustrates the concept really well.... \$\endgroup\$
    – Oldhouse
    Commented Apr 16 at 17:58

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