Reading EXIF data--what's the "Circle of Confusion?"

[Quicklabs]

Wouldn't that be a great name for a garage rock band?

But I know it can't be that! LOL Can anyone enlighten me?

 

[Gdad]

I know what they are- All the little circles in the bokeh- I'm sure someone much smarter than me can explain what causes them and changes them.

 

[jann1033]

the least confusing definition i have heard is the cut off point where something would no longer considered to be sharp ...then usually they start adding things that throw me off

to get it for 35mm they took the smallest point some photographic high court decided was discernible by the average sighted person ( 1/16 of a mill. roughly ) then divided it by 5 ( they figured a print was enlarged 5x from a negative to a 5x7) and came up with .0333 for the cof BUT the monkey wrench is i've also seen it as .25 (micron i think, that little dropped down backwards u symbol) so thinking that might have something to do with a crop size/vs 35mm maybe????

so if you think of a circle that is fuzzy rather than sharp on the edges which would be the circle of confusion vs a tack sharp point which would be the circle/point of least confusion, i think the confusion being how your eye views it as sharp vs a little unsharp

 

[Quicklabs]

OK--that makes sense. Kinda like jnd--just notable difference. Thanks!

 

[MarkBarbieri]

I have no idea what Circle of Confusion has to do with EXIF data. That strikes me as odd.

Imagine that you are taking a picture of a star. For all practical purposes, a star is a single point of light. If you are focused properly, the star will be a single point on your sensor or film. If you are not focused on the star, the star will no longer be a point. It's light will be spread into a small circle. That is the "circle of confusion." The more out of focus the star is, the larger the circle of confusion.

Circle of confusion and DOF are related. The true focal plane is always just a plain. Anything even slightly in front of or behind that plain is out of focus, no matter what aperture you use. If something is very close to the focal plain and you are using a fairly small aperture, the thing won't be much out of focus. The circle of confusion is a measure of how out of focus it is. If the circle of confusion is smaller than a certain limit, we consider it to be effectively in focus (close enough) and so it is within the depth-of-field.

What's an acceptable circle of confusion for something that is in focus? That's what Jann1033 was talking about. There are lots of ways of calculating it. It depends on how much you enlarge the image, how close you view it, and how well you can see.

So why would CoC be listed in the EXIF? It varies for objects in the picture based on how far they are from the focal plane. Maybe they also list a DOF range and the CoC that they list is a reference to the acceptable CoC used to determine the DOF.

 

[Master Mason]

I have no idea what Circle of Confusion has to do with EXIF data. That strikes me as odd.

Imagine that you are taking a picture of a star. For all practical purposes, a star is a single point of light. If you are focused properly, the star will be a single point on your sensor or film. If you are not focused on the star, the star will no longer be a point. It's light will be spread into a small circle. That is the "circle of confusion." The more out of focus the star is, the larger the circle of confusion.

Circle of confusion and DOF are related. The true focal plane is always just a plain. Anything even slightly in front of or behind that plain is out of focus, no matter what aperture you use. If something is very close to the focal plain and you are using a fairly small aperture, the thing won't be much out of focus. The circle of confusion is a measure of how out of focus it is. If the circle of confusion is smaller than a certain limit, we consider it to be effectively in focus (close enough) and so it is within the depth-of-field.

What's an acceptable circle of confusion for something that is in focus? That's what Jann1033 was talking about. There are lots of ways of calculating it. It depends on how much you enlarge the image, how close you view it, and how well you can see.

So why would CoC be listed in the EXIF? It varies for objects in the picture based on how far they are from the focal plane. Maybe they also list a DOF range and the CoC that they list is a reference to the acceptable CoC used to determine the DOF.

For some of us the Circle of Confusion is the place we enter when such things get explained to us

 

[zackiedawg]

The circle of confusion is when a person in the know tells a person who isn't in the know what they know in a way the not knowing person doesn't know, so they mention they don't know what the person in the know knows, and the person in the know tells the person not in the know the same thing they don't know again, not knowing that they're still not going to know. And the not knowing person still doesn't know what the knower knows, and asks again explaining that they don't know to the knower, who of course knows, but doesn't know the not-knower can never know what he knows. And it keeps going around in a circle.

 

[WDWFigment]

I may not know much, but the relevant parts of this discussion seem to me to involve science. As a liberal arts person, I can unequivocally say that science is a dark art in which one should not engage under any circumstances. Nothing good can come from it, and if you guys keep it up, I'll report you all as being witches.

 

[Master Mason]

I'll report you all as being witches.

so you know my ex wife then?

 

[boBQuincy]

I may not know much, but the relevant parts of this discussion seem to me to involve science. As a liberal arts person, I can unequivocally say that science is a dark art in which one should not engage under any circumstances. Nothing good can come from it, and if you guys keep it up, I'll report you all as being witches.

Although I must admit that witches (even wicked ones) are generally held in higher regard than engineers and scientists they don't know squat about circles of confusion. For information on that you must turn to a darker force, those who understand physics!

While we are at it, why all of a sudden the increased interest in COC? Because high megapixel sensors have raised the bar.
COC was pinned down at a size that would produce a reasonably sharp 8x10 print *but* that only requires 3000 x 2400 pixels, not much more than 6 MP. Enter 8, 10, 12, 15 MP APS-C sensors and 13x19 printers! Now we have enough pixels to easily produce high quality large prints *but* the diffraction at f/16 is now too much, we must limit our lenses to f/11 or even f/8 to get enough sharpness for large prints and viewing at 100%.

It gets worse, most lenses we mortals (and witches and engineers) can afford don't get real good until about f/8! That doesn't leave much wiggle room between reducing aberrations and the onset of diffraction softness! So what to do?

I have read a lot about this, understand it, and mostly ignore it and go out and take pictures!

 

[jann1033]

I may not know much, but the relevant parts of this discussion seem to me to involve science. As a liberal arts person, I can unequivocally say that science is a dark art in which one should not engage under any circumstances. Nothing good can come from it, and if you guys keep it up, I'll report you all as being witches.

the science part is ok, the math i agree belongs in the realm of the supernatural . but it wouldn't be the first time i was called a witch, I've been called that a lot. however those people all have had some kind of speech problem and when they said it ,the first letter sounded more like a "b" than a "w"

 

[ColleenG]

I am now spinning in the middle of the circle of confusion

 

[spinetnglr]

Some days I think the Circle of Confusion is my address. And after reading the posts in this thread, mine is not the only house on the block that is occupied

 

[JoeDif]

I hope this will help to simplify the Circle of Confusion for us that are not so scientifically inclined:


So subject (object) to front principal point distance.
Si rear principal point to image distance
f focal length
M magnification

1/So + 1/Si = 1/f
M = Si/So
(So-f)*(Si-f) = f^2
M = f/(So-f) = (Si-f)/f


Now....doesn't it all make sense

 

[ColleenG]

Now it is clear as mud!

 

[Sewfun]

I hope this will help to simplify the Circle of Confusion for us that are not so scientifically inclined:


So subject (object) to front principal point distance.
Si rear principal point to image distance
f focal length
M magnification

1/So + 1/Si = 1/f
M = Si/So
(So-f)*(Si-f) = f^2
M = f/(So-f) = (Si-f)/f


Now....doesn't it all make sense

God I hate math!!

 

[Evad]

Ok I'm lost..........