Lectures on Digital Photography

hark

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I only watched a couple of minutes of the first video, but within that short time, he mentioned something I've never heard of previously. Based on that, this video series may have some interesting information in it.

So what did I learn? He wrote out a formula for Depth of Field which he claims he will explain further in the course. The formula (which is described at 1:25 in the first video) is this:

DoF.jpg


N is the F/stop number
C is the circle of confusion (which he claims he will describe later in the video)
U is the distance to the subject
F is the focal length

EDIT:
According to Wikipedia, the Circle of Confusion is this:

In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source. It is also known as disk of confusion, circle of indistinctness, blur circle, or blur spot.
 
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Blacktop

Senior Member
I only watched a couple of minutes of the first video, but within that short time, he mentioned something I've never heard of previously. Based on that, this video series may have some interesting information in it.

So what did I learn? He wrote out a formula for Depth of Field which he claims he will explain further in the course. The formula (which is described at 1:25 in the first video) is this:

View attachment 226874

N is the F/stop number
C is the circle of confusion (which he claims he will describe later in the video)
U is the distance to the subject
F is the focal length

EDIT:
According to Wikipedia, the Circle of Confusion is this:

I guess I need to find a new hobby that doesn't include calculus. :rolleyes: Sorry, this pretty much just sucks all the joy out of it.
 

WayneF

Senior Member
According to Wikipedia, the Circle of Confusion is this:


Not sure about your intent, if it is a question about CoC or not? It is not calculus, but it is math (mostly multiplication and division), which is often used to describe how the physical world works. Note the Wikipedia article on Circle of Confusion has several references back into the 1800s (basically, regarding camera lenses, it has always been known). It is how knowledgeable people have learned to discuss and predict what happens. Engineers and medical doctors and plumbers all have their own terms for things.

Circle of Confusion is how large the tiniest theoretical point source (of zero dimension) actually appears to us when it is not in good focus. Like perhaps a photo of a star in the night sky, but it is a theoretical concept. The star is so far away, so it should be infinitesimal size in our image, but if not in good focus, it appears as a larger blob, of size called CoC. How large is a measure of misfocus.

CoC is NOT the size of any misfocused blob in our photo. CoC is specifically the size of a misfocused point source, which otherwise should be of zero dimension.

CoC is important detail when discussing math of focus, esp including Depth of Field. In cameras, how much this CoC is actually visible to our eye depends also on the degree of print enlargement, so film size or sensor size affects what CoC diameter our eye is likely to see (to see it larger than the tiniest zero point size).

In Depth of Field, focus is of course only at one exact point. At greater or lesser distance than exact focus, focus error grows. Depth of Field computes near and far limits where we finally see and object to the degree of out of focus. Depth of Field is NOT a hard limit, it just computes where CoC becomes visible to our normal eyesight, in a standard print size (considered 8x10 inches viewed at 10 inches). Different enlargements of course computes different numbers and different CoC and different DOF numbers. And of course, there is no actual difference in a bit farther or a bit closer than the DOF number. Misfocus just gradually grows larger.

Carl Zeiss was an extremely important lens maker in Germany before WWII. His notion was CoC of 1/1730 of the film diagonal was the visible CoC limit (takes into account the standard print enlargement). Today, for 35mm film or full frame sensors, for DOF calculations, we call CoC to be 0.03 mm (for full frame. For DX, we call it 0.02 mm, because smaller DX has to be enlarged 1.5x more). This means that at the standard print enlargement (8x10 inches viewed at 10 inches), our eye can recognize a 0.03mm or 0.02mm blob to be larger than the tiniest zero point size. We then declare that to NOT be in good focus.

The purpose of the DOF charts is so we don't have do this math ourself. But for best use, we do still need to know that DOF certainly also depends on degree of print enlargement. In practice, we eagerly learn how (what factors) to increase or decrease DOF, more so than trying to assign specific DOF distance numbers.
 
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hark

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Not sure about your intent, if it is a question about CoC or not?

Lol! No, it wasn't a question but rather a comment to show how much detail the lectures will cover. I will probably watch some of them--watched more of this first video and found it to be interesting in an intellectual sort of way. But based on this first one, these videos won't be the same type that are typical of photography videos. I foresee Pete won't be the only one who steers clear of them. :beguiled:

This is one of the books he referenced. The cost is over $100, and it isn't even the most recent edition. https://www.amazon.com/Photography-...05933807?ie=UTF8&psc=1&ref_=nav_timeline_asin

The series may not be for everyone, but it's something I want to peruse further myself. ;)
 

Catherder

Senior Member
I've watched 3 episodes so far, some interesting stuff, like how the camera evolved. The math stuff flies over my head, that was one class where I didn't pay attention in school.
 

WayneF

Senior Member
I've watched 3 episodes so far, some interesting stuff, like how the camera evolved. The math stuff flies over my head, that was one class where I didn't pay attention in school.

Very interesting stuff, about the how and why it works. Stuff mostly not seen outside of schools anymore (all popular stuff is so dumbed down anymore, I suspect todays writers don't know anything either.) Maybe everyone may not be interested, but this one is very good stuff. I do wish I could hear the questions asked.
 
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