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Ram Nanda
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According to AC Magazine April 2020 edition 

" The Pupil Marks the Focus Spot The next thing to understand about the entrance pupil is that it is at that point — not at the film plane (or sensor plane) — from which focus should be measured. It’s a long-standing practice in cinematography to take focus measurements from the film or sensor plane of the camera. "

(read the attachment)

Well, the topic seems to be fine and interesting but, the basic question is never raised nor answered - WHY FOCUS SHOULD BE MEASURED FROM ENTRANCE PUPIL?

Can some one explain these :

1) Do all of the points of light from the scene converge to a single point at the entrance pupil and then expand back out to the image plane ?

2) What role does exit pupil play in measuring focus ?

3) Can Iris be located not at aperture stop ?

4) How is DoF related to entrance & exit pupil ?

And please provide a reference website or material to understand these technical concepts clearly.

 

Thanks,

Ram Nanda.

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interesting...await replies....I always em.......focus......with my eyes.....but yes with a tape measure when shooting wide angle from the film plane on my Aaton 16mm camera...

Interesting to know where digital cameras and their 'focus peaking' measure from.....the sensor or the entrance pupil.....and whether different manufacturers differ....

Edited by Stephen Perera
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1 hour ago, Stephen Perera said:

interesting...await replies....I always em.......focus......with my eyes.....but yes with a tape measure when shooting wide angle from the film plane on my Aaton 16mm camera...

Interesting to know where digital cameras and their 'focus peaking' measure from.....the sensor or the entrance pupil.....and whether different manufacturers differ....

on larger depth of field the difference between entrance pupil plane and image plane is negligible and is covered by depth of field. but its clearly evident at extreme small DoF, like in the case of macro close ups where you have to judge by eye 

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I respect Jay’s article but I do believe he’s a bit misleading with it. With regards to the entrance pupil of a lens, technically he is correct. Where he goes astray is in two places. Firstly, it is absolutely impractical to measure focus from the entrance pupil of different lens designs since lenses are commonly collimated to the film plane/sensor. This especially come into play where different camera manufacturers have different flange depth measurements to the film plane. We need a consistent standard here and not a moving target to complicate further a complicated practice. 
Secondly, where Depth of Field is referenced, he should be talking about Depth of FOCUS. There is a big difference between the two. Depth of Field refers to the range of focus IN FRONT of the lens where Depth of Focus measures the tolerance of focus BEHIND the lens. When you look at the rear element assembly from the lens’ flange to the tip of the rear element glass, the noticeable difference is how a wide lens’ rear element is a narrow torpedo shape that extends much further away from the lens flange. Longer lenses sport a rear element that is a much wider in diameter rear glass element and is much stubbier in length. The wide lens rear element measures much closer to the film plane/sensor than the longer lens’ equivalent thus making the wide lens’ Depth of Focus much more critical for the lens’ ability to achieve proper focus. Long lenses have much more forgiving Depth of Focus while having much less forgiving Depth of Field. The two physical properties are polar opposite of each other. If the Depth of Focus is off, the lens will simply not focus anywhere within the lens’ focus range. 
 

G

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4 hours ago, Gregory Irwin said:

I respect Jay’s article but I do believe he’s a bit misleading with it. With regards to the entrance pupil of a lens, technically he is correct. Where he goes astray is in two places. Firstly, it is absolutely impractical to measure focus from the entrance pupil of different lens designs since lenses are commonly collimated to the film plane/sensor. This especially come into play where different camera manufacturers have different flange depth measurements to the film plane. We need a consistent standard here and not a moving target to complicate further a complicated practice. 
Secondly, where Depth of Field is referenced, he should be talking about Depth of FOCUS. There is a big difference between the two. Depth of Field refers to the range of focus IN FRONT of the lens where Depth of Focus measures the tolerance of focus BEHIND the lens. When you look at the rear element assembly from the lens’ flange to the tip of the rear element glass, the noticeable difference is how a wide lens’ rear element is a narrow torpedo shape that extends much further away from the lens flange. Longer lenses sport a rear element that is a much wider in diameter rear glass element and is much stubbier in length. The wide lens rear element measures much closer to the film plane/sensor than the longer lens’ equivalent thus making the wide lens’ Depth of Focus much more critical for the lens’ ability to achieve proper focus. Long lenses have much more forgiving Depth of Focus while having much less forgiving Depth of Field. The two physical properties are polar opposite of each other. If the Depth of Focus is off, the lens will simply not focus anywhere within the lens’ focus range. 
 

G

Thank you Mr. Gregory.

putting aside the technical difficulties related to measuring focus from entrance pupil, why is entrance pupil the right position for focus calibration ?

 

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1 hour ago, Ram Nanda said:

Thank you Mr. Gregory.

putting aside the technical difficulties related to measuring focus from entrance pupil, why is entrance pupil the right position for focus calibration ?

 

It’s not! The film plane/sensor is the right position for focus collimation.  The whole concept of the entrance pupil is more related to the calculation of F-stops and exposure. It’s an actual virtual image magnified when viewed through the front element of the lens. This is as opposed to the diminished size of the aperture when viewing through the rear element of the lens, known as the exit pupil. There’s a lot more to this and this can be researched from here on so you can go as deep into it as you desire. I’m sure it’s a bit confusing. 
 

G

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11 hours ago, Ram Nanda said:

1) Do all of the points of light from the scene converge to a single point at the entrance pupil and then expand back out to the image plane ?

No - it's the other way around. Rays from a point in object space fill the entrance pupil. Then rays going out of exit pupil converge (not perfectly because of aberrations) on the image plane (again, not exactly a plane because of astigmatism/field curvature).

11 hours ago, Ram Nanda said:

2) What role does exit pupil play in measuring focus ?

In measuring - none.

11 hours ago, Ram Nanda said:

3) Can Iris be located not at aperture stop ?

Theoretically iris can be placed anywhere - but depending on the opening size it'll be either pretty useless (it'll only contribute to vignetting) or, if closed down enough, become the new aperture stop. 

11 hours ago, Ram Nanda said:

4) How is DoF related to entrance & exit pupil ?

Depth of field is proportional to 1 / (entrance pupil size).

Depth of focus is proportional to working f/number, which depends on pupil magnification (but pupil magnification is a big deal only at close distances). 

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Practically speaking, measuring from the Φ will work for more than 99% of your shots. Additionally, if you calculate depth of field and double check everything on the monitor (assuming you're shooting digital), using Φ will be quite accurate!

@Gregory Irwin: Great breakdown!

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Mr. Michael Rodin, Mr. AJ Young & Mr. Gregory Irwin,

I texted Mr.Joy Holben, who the original article in AC mag, regarding the very same question I raised in the initial post. 

(find the conversation in the comments section of https://ascmag.com/blog/shot-craft/the-entrance-pupil-of-the-lens)

Here he accepts that the while Depth of Field covers up the minor difference in the planes between Entrance Pupil and Image/film/sensor plane but it is in the case of macro work this fails. It's in these situations eye focus is required than measurement from sensor plane.

So like Mr.AJ Young said, in 99% cases its from the Φ  (film/sensor/image plane) we measure focus. So in these 99 cases entrance pupil = film/sensor/image plane. 

and in one case entrance pupil ≠ image plane.

RIGHT ? Correct me if i'm wrong.

I know we don't often come across this in reality, but I'm just curious in knowing this stuff.

And if anyone can, please provide reference website/pdf/book.

 

Thanks,

Ram Nanda

entrance pupil.PNG

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Sorry but what a crock!

Focus marks can be factory calibrated on a lens barrel to any reference mark. You find the point of best focus for an object a certain distance away from the lens and mark it on the barrel. Whether the distance value marked is measured from the entrance pupil or the film plane doesn’t really matter except that the film plane makes much more sense for practical reasons. The main thing is to keep a consistent reference from which to measure. It’s not a “cheat” or whatever this author thinks.

A factory can just as easily mark macro distances too according to where the point of best focus is with respect to any reference point, so it’s no different.

It’s as if the author thinks lens distance marks are somehow unchangeable intrinsic values, rather than simply reference marks determined by testing the actual lens. 

The advice to use eye-focus for macro work is pretty funny since surely most people would eye-focus anyway - at very close distances the depth of field becomes so shallow that marks are virtually useless and the idea of attempting to measure the mm to a ladybird is sort of ludicrous.

Also, who uses distance marks with a fish-eye lens? The whole article is nonsense.

 

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46 minutes ago, Dom Jaeger said:

Sorry but what a crock!

Focus marks can be factory calibrated on a lens barrel to any reference mark. You find the point of best focus for an object a certain distance away from the lens and mark it on the barrel. Whether the distance value marked is measured from the entrance pupil or the film plane doesn’t really matter except that the film plane makes much more sense for practical reasons. The main thing is to keep a consistent reference from which to measure. It’s not a “cheat” or whatever this author thinks.

A factory can just as easily mark macro distances too according to where the point of best focus is with respect to any reference point, so it’s no different.

It’s as if the author thinks lens distance marks are somehow unchangeable intrinsic values, rather than simply reference marks determined by testing the actual lens. 

The advice to use eye-focus for macro work is pretty funny since surely most people would eye-focus anyway - at very close distances the depth of field becomes so shallow that marks are virtually useless and the idea of attempting to measure the mm to a ladybird is sort of ludicrous.

Also, who uses distance marks with a fish-eye lens? The whole article is nonsense.

 

Please don’t sugar coat it Dom! Tell us what you really think! 😂

G

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......the phrase 'can't see the wood for the trees' comes to mind.....pointless, confusing article to me....put the lens on....focus with the eye....mark the lens.....you're done.....wtf...its just so pointless to even consider unless your a lens designer/manufacturer.....
 

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There’s a motto within my camera team that we believe in and is stated several times a day:

“Keep it stupid simple!“
 

I believe that applies here.

G

 

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@Ram Nanda: It's not a matter of right or wrong; everyone is right. What matters is what works for you and your team. Measuring from the image plane works just fine for 99% of camera angles.

The most important thing you need to do is check the focus marks of each lens during camera prep, which is standard practice. If your marks match the distances you measure in prep, then measuring from Φ will do you just fine. If measuring from the entrance pupil works for you, then go for it!

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this 'focus distance' part of the article was maddeningly confusing to me as well, so i had to ask a lens technician to explain.
I will try to paraphrase his words below, and perhaps Dom can explain a bit further if this is still confusing.

The explanation and the theory of the entrance an exit pupils in the article are correct. Entrance pupil is an image of the aperture stop in front of the lens. The size of that image and position controls the amount of lite going through the optical system (lens).

Lenses have nodal points, which are important here because nodal planes or conjugate planes can be inside or outside of the lens depending on the design. Therefore they may or may not be aligned with the position of the entrance or exit pupil.
 
In theory, the distance to the subject is measured from first nodal plane and not from the film plane. But, it is not practical for the end user to measure from nodal point. You can't put a tape hook on the lens, some lenses have nodal point outside the lens body, zooms have different nodal point because of the different focal lengths, etc. 

Therefore, as Dom has explained, the lens footage scale is calibrated as a final distance between an object and its image on the film or sensor, because this is what is important for end user - final, sharply focused, image of an object at given distance.
Measuring from object to the nodal point is for design and calculation purposes of the optical system.

For example, when a designed and manufactured lens is set on MTF or large collimator, it is focused at a given distance that is measured from a FILM PLANE, not from nodal point. The best image obtained on that film plane is engraved as the distance on lens. This measurement avoids the size of the lens and its internal optical movements.

in essence, the article author is flexing lens design knowledge on the readers 🙂
but we do learn something out of it!

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