# Playing around with depth of field and I think I finally get it?

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Re: A way of thinking about it

Mark Scott Abeln wrote:

bclaff wrote:

What you have described is closer to depth of focus than depth of field.

However, as small magnifications (longer distances) the differences are small.
(As are the effects of pupil magnification.)

I'm still stuck in the dark ages of the symmetric thin lens approximation.

Even so you can't use that explanation.

Although this is the Beginners Questions forum let's try this.

Here's a diagram from a currently unreleased interactive widget on PhotonsToPhotos:

Best viewed "original size"

You describe the triangle on the left from the entrance pupil (P) to the in focus object on the object plane (O). So far, so good.

But Depth Of Field (DOF) depends on the size of the blur disc on the image plane (I).
So we must also consider the triangle on the right from the exit pupil (P') to the image plane (I).

I've drawn pupil magnification of 1.2 but if this were the thin model pupil magnification would be 1; P' would be the same height as P and overlap P.

We know the right triangle doesn't really come to a point but to an Airy disc; but we will ignore that (diffraction). We also ignore aberration.

Do not extend either triangle but rather consider different points on the optical axis in object space.

A point slightly closer than the in focus point at O will focus slightly to the right of the image plane (I) and this triangle will be cut by the image plane (I) to form a blur disc.
So long as that blur disc is less than or equal to the Circle Of Confusion (COC) that point on the object side will appear to be in focus.

Similarly a point further than the in focus point at O will focus slightly to the left of the image plane (I), cross and be cut by the image plane (I) to form a blur disc.

You can find the near and far points in object space by tracing the blur disc on the image plane (I) back into object space.

The key thing is that these are different triangles, not extensions of the in focus triangles.
This is true regardless of thin versus thick model.

For an in depth treatment including the math see Optics Primer - Depth of Field

Regards

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Re: Yeah - you got it!
2

Bob wrote:

One thing, though. "Bokeh" does not mean blurry background. Bokeh refers to the quality of the blurry parts. Smooth or coarse.

Many people use the term 'bokeh' to mean a blurry background...it's a common misunderstanding of the term.

This is a common misunderstanding of the term "bokeh". Bokeh comes from the Japanese word boke which simply means blur or haze. It was introduced into the English language with the added H by Photo Techniques Magazine in 1997 to refer specifically to out of focus blur in a photograph. The idea that it can only refer to the quality of the blur came later and is inconsistent with both the Japanese definition and the original usage in English.

The original articles that introduced the word bokeh to the English language can be found here

Here is a quote from the editor Mike Jonhnston (the guy who commissioned the articles and added the H):

"Bokeh" simply means blur, specifically out-of-focus blur (as opposed to the kinds caused by subject or camera movement). It includes, but is not limited to, out-of-focus highlights." - Mike Johnston

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Re: Playing around with depth of field and I think I finally get it?
1

Gerry Winterbourne wrote:

Don't confuse bokeh - the nature or quality of blur - with blur itself and how much of it there is.

The original magazine article that introduced the word bokeh to the English language includes the quote "For bokeh by the bucketful, nothing beats long, fast lenses." which tells us that not only does bokeh have a quantity (measured in bucketfuls), but that you can get more of it by using long, fast lenses.

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Re: Yeah - you got it!
5

Bob wrote:

One thing, though. "Bokeh" does not mean blurry background. Bokeh refers to the quality of the blurry parts. Smooth or coarse.

Many people use the term 'bokeh' to mean a blurry background...it's a common misunderstanding of the term.

If it is common, then it is not a misunderstanding. This is how language works.

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Re: Playing around with depth of field and I think I finally get it?

I found this chart online which I thought was pretty interesting.

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Re: Yeah - you got it!
1

Pixel Pooper wrote:

Bob wrote:

One thing, though. "Bokeh" does not mean blurry background. Bokeh refers to the quality of the blurry parts. Smooth or coarse.

Many people use the term 'bokeh' to mean a blurry background...it's a common misunderstanding of the term.

This is a common misunderstanding of the term "bokeh".

No it isn't. The article you quote includes very early on "What we need is a word that specifically refers to the qualities that a lens imparts to objects in front and behind the plane of focus." And, indeed, the whole article is really about that.

Bokeh comes from the Japanese word boke which simply means blur or haze.

It's a common mistake to assume that words adopted from other languages must have the same meanings. If that were true then you'd be telling us that "camera" really means "room or chamber" and "lens" means "lentil".

It was introduced into the English language with the added H by Photo Techniques Magazine in 1997 to refer specifically to out of focus blur in a photograph. The idea that it can only refer to the quality of the blur came later

See the extract I posted above.

and is inconsistent with both the Japanese definition and the original usage in English.

The original articles that introduced the word bokeh to the English language can be found here

Here is a quote from the editor Mike Jonhnston (the guy who commissioned the articles and added the H):

"Bokeh" simply means blur, specifically out-of-focus blur (as opposed to the kinds caused by subject or camera movement). It includes, but is not limited to, out-of-focus highlights." - Mike Johnston

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Re: Playing around with depth of field and I think I finally get it?
2

For me Bill clafs Optics Primer guide was a bit to overwhelming when it comes to its math. I found this guide for all the mortal ones out there

It doesn't rely as much on math to explain the concepts and it also explains it more concisely.

https://ascmag.com/blog/shot-craft/the-entrance-pupil-of-the-lens

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Re: Yeah - you got it!
2

Gerry Winterbourne wrote:

Pixel Pooper wrote:

Bob wrote:

One thing, though. "Bokeh" does not mean blurry background. Bokeh refers to the quality of the blurry parts. Smooth or coarse.

Many people use the term 'bokeh' to mean a blurry background...it's a common misunderstanding of the term.

This is a common misunderstanding of the term "bokeh".

No it isn't. The article you quote includes very early on "What we need is a word that specifically refers to the qualities that a lens imparts to objects in front and behind the plane of focus." And, indeed, the whole article is really about that.

You snipped that quote to make it sound more like it agrees with your misconceptions. If you don't remove the context it says:

"It's a measure of our lack of concern that we don't even have a standard term in English for the fuzzy parts of a picture. "Blur" comes closest, but covers too wide a territory (for example, motion blur, which has nothing to do with focus). What we need is a word that specifically refers to the qualities that a lens imparts to objects in front and behind the plane of focus."

Bokeh comes from the Japanese word boke which simply means blur or haze.

It's a common mistake to assume that words adopted from other languages must have the same meanings. If that were true then you'd be telling us that "camera" really means "room or chamber" and "lens" means "lentil".

I haven't made that mistake. In Japanese "boke" simply means blur or haze and can even be used to describe a confused mental state. In English it refers specifically to out of focus blur in a photograph.

If you read the articles, the term "bokeh" is used interchangeably to describe the quality of the blur, the quantity of the blur, and the blur itself, so the way the OP used the term is perfectly acceptable as it has been used that way since the very beginning of its usage in English.

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Re: Playing around with depth of field and I think I finally get it?

LillyRoseAvalos wrote:

For me Bill clafs Optics Primer guide was a bit to overwhelming when it comes to its math. I found this guide for all the mortal ones out there

It doesn't rely as much on math to explain the concepts and it also explains it more concisely.

https://ascmag.com/blog/shot-craft/the-entrance-pupil-of-the-lens

That's an above average article with small errors not worth noting, but it really doesn't cover the topic of depth of field.

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Re: Playing around with depth of field and I think I finally get it?

I agree to disagree! I I learned not only some new things about depth of field but also about a lot of other things reading that article and they have many more on their website.

"In actuality, this is a cheat and not wholly accurate. The reason it was probably chosen is because this is one of the fixed dimensions for different cameras and lenses. However, at typical shooting distances of 6' or more, a couple of inches between the image plane and the entrance pupil wonâ€™t make much of a difference and will be covered by depth of field. However, as our sensor/film-plane sizes get larger and our focal-length lenses get longer (for a given field of view compared to smaller formats), our depth of field drops, and it becomes a bit more of an issue. It is especially an issue if youâ€™re shooting extreme close-up, where the depth of field is extremely small and the difference of focal plane from the entrance pupil to the imager can make a significant difference! This is when you really need to know the entrance-pupil position and measure your focus from that point.
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To make matters worse, not all manufacturers readily provide entrance-pupil positions for their lenses, and the estimation procedure I detail here isnâ€™t scientifically perfect, but itâ€™s a good start. If you have a smart lens , it can provide metadata of the exact entrance-pupil position, or you can contact the lens manufacturer to get that information. But because it changes from lens to lens, and even within a lens as you alter other parameters, we have developed the habit of cheating. The faster your lens, the longer the focal length and the closer the object on which youâ€™re focusing, the more the distance between the film/sensor plane and the entrance pupil will affect whether your subject is in focus. "

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Re: Playing around with depth of field and I think I finally get it?

For your next experiment, you should try keeping the f-number the same but changing the lens (if you have more than 1 lens for the same camera). Try to keep the battery the same size in the frame for the experiment.

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Re: Playing around with depth of field and I think I finally get it?

LillyRoseAvalos wrote:

I agree to disagree! I I learned not only some new things about depth of field but also about a lot of other things reading that article and they have many more on their website.

"In actuality, this is a cheat and not wholly accurate. The reason it was probably chosen is because this is one of the fixed dimensions for different cameras and lenses. However, at typical shooting distances of 6' or more, a couple of inches between the image plane and the entrance pupil wonâ€™t make much of a difference and will be covered by depth of field. However, as our sensor/film-plane sizes get larger and our focal-length lenses get longer (for a given field of view compared to smaller formats), our depth of field drops, and it becomes a bit more of an issue. It is especially an issue if youâ€™re shooting extreme close-up, where the depth of field is extremely small and the difference of focal plane from the entrance pupil to the imager can make a significant difference! This is when you really need to know the entrance-pupil position and measure your focus from that point.
Live in the sunshine where you belong

To make matters worse, not all manufacturers readily provide entrance-pupil positions for their lenses, and the estimation procedure I detail here isnâ€™t scientifically perfect, but itâ€™s a good start. If you have a smart lens , it can provide metadata of the exact entrance-pupil position, or you can contact the lens manufacturer to get that information. But because it changes from lens to lens, and even within a lens as you alter other parameters, we have developed the habit of cheating. The faster your lens, the longer the focal length and the closer the object on which youâ€™re focusing, the more the distance between the film/sensor plane and the entrance pupil will affect whether your subject is in focus. "

This quote is mostly to do with cinematography not our usual photography.

We don't "measure distance" the way that they do.

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Re: Playing around with depth of field and I think I finally get it?

I cheated and used my 12 60 lens on my Gx80 same f stop  similar size of battery different focal lengths but I assume if it was that easy you would not have told me to swap lenses :-). I cant see anything out of the ordinary..

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Re: Playing around with depth of field and I think I finally get it?

Hi Lily! I can't pick they all look very cool in your focus is spot on although on, although I'm a beginner I thought you might find the below interesting or fun since you like experimenting. thanks for posting this thread it has been very interesting to follow.

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Re: A way of thinking about it

bclaff wrote:

For an in depth treatment including the math see Optics Primer - Depth of Field

I'm going to need some time to digest this, and this isn't the first time you pointed me in this direction!

So how *wrong* is my model, in terms of actual results?

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Re: A way of thinking about it

Mark Scott Abeln wrote:

bclaff wrote:

For an in depth treatment including the math see Optics Primer - Depth of Field

I'm going to need some time to digest this, and this isn't the first time you pointed me in this direction!

So how *wrong* is my model, in terms of actual results?

Not too wrong regarding results but definitely the wrong model to teach about what is actually happening.

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Re: A way of thinking about it

bclaff wrote:

...

Although this is the Beginners Questions forum let's try this.

Here's a diagram from a currently unreleased interactive widget on PhotonsToPhotos:

Best viewed "original size"

Mark,

Not to scale but this crude annotation might help.

Best viewed "original size"

So it's whether the red and blue cross the image plane (I) inside the Circle Of Confusion (COC).

FWIW, your approach would imply equal distances behind and in front for Depth Of Field (DOF) and we know that definitely not so.

And your approach can't explain how to get the hyperfocal distance when focused at infinity.

Regards

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Re: Playing around with depth of field and I think I finally get it?
1

I was pretty happy with the result especially considering my Canon dslr doesn't have that amazing focus peaking that myÂ  mirrorless cameras do or anyÂ  other manual focusing aids whatsoever for that matter

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Re: A way of thinking about it

bclaff wrote:

FWIW, your approach would imply equal distances behind and in front for Depth Of Field (DOF) and we know that definitely not so.

It would be the size of the blur as seen from the point of the view of the camera. So a blur of any given size will appear to be smaller the farther away it is.

And your approach can't explain how to get the hyperfocal distance when focused at infinity.

Merklinger recommends finding the closest significant detail in the scene and then adjusting the aperture width accordingly. If I recall, this leads to a one f/stop decrease in aperture width compared to hyperfocal focus (not when focusing to infinity) but with notionally better definition for distant subjects. The main problem with traditional hyperfocal focus is actually focusing at that distance: especially these days when lenses often don't have focus scales.

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Re: Playing around with depth of field and I think I finally get it?

You're right that is disappointing! I will try and take some battery photos with more variation in focal length and upload tomorrow

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