Originally posted by Kunzite What he said ^^^^
I almost, but not quite entirely, fully agree with you.
The depth of field should be impacted by cropping and subsequent enlarging. Of course, a theory pretending that DoF is determined by the aperture is a fraud.
In photography yes....in optics, it's a little different.
Quote: The aperture determines how collimated the admitted rays are, which is of great importance for the appearance at the image plane.[2] If an aperture is narrow, then highly collimated rays are admitted, resulting in a sharp focus at the image plane. A wide aperture admits uncollimated rays, resulting in a sharp focus only for rays coming from a certain distance.
Aperture - Wikipedia
In photography you have fixed ƒ-stops at known intervals (quite arbitrarily assigned for the sole purpose of defining light intensity for the purpose of exposure.) where each f-stop is half or double thrones on either side of it. Thus, the Aperture is a set of numbers designed to help in exposure. If they were designed to double or halve Depth of Field, they'd be a photographic measurement of some value perhaps, but they aren't They were conceived and implemented as a standard to be a simple method of understanding exposure values. If at some point some guy like Northrup decides they are going to be a DoF value, the system is going to have to be completely reworked for that purpose.
The biggest problem with the way Northrup uses it, is Aperture used to describe DoF values, doesn't tell you what the DoF is. It only tells you what it is compared to some other system, for which you also don't know what the DoF is, 1 stop of Aperture, does not double or halve the DoF and it's not the same across all focal lengths. It's not a constant measure of anything, and it changes from focal length to focal lentgth. The DoF on a 50mm lens is not the same as the DoF of a 100mm lens at the same Aperture. But ƒ2.8 is ƒ2.8 for exposure regardless of focal length or sensor size. Aperture is simply not adequate as a description of DoF. Only a DoF table does that. Then you have all parameters, where does the DoF start, where does it stop, what distance does it cover. A single variable like Aperture is simply not adequate to describe it. At best Aperture describes the size of the hole, but it's different sizes on different focal lengths, so it doesn't even accomplish that adequately and there are many more parts to the equation.
If Northrup had a clue what he was talking about, he would have had to know he needs to explain all this when he says these things. His presentations present a lack of context , which suggests he doesn't himself understand the context.
If you understand the context, then you know these kinds of statements about Aperture and DoF are misleading. But if you don't understand the context, you don't even understand that you're making a mistake.