The focus lies along which line?

Started Mar 24, 2013 | Discussions
fireplace33
fireplace33 Senior Member • Posts: 2,392
The focus lies along which line?

Here is a simple question with the hope that a simple answer is possible.

I'm assuming I know the answer

When focussed at some particular distance, does the focus lie more along the red, or more along the blue line,when using a typical lens on a typical DSLR?

The red line is a circle with the center at the camera, The blue line is a straight line.

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leno Senior Member • Posts: 1,257
Re: The focus lies along which line?
1

Its a good question which many people will not understand. I don't know the correct answer for sure but I don't think it is either of these. My guess is that the there should be a lens shape about the blue line on your diagram. the taper would end close to the field of view. I wait with intrest.

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Barrie Davis
Barrie Davis Forum Pro • Posts: 21,460
Blue...(edit)

The zone of sharpest focus is a plane, like a sheet of glass in front of the camera. It is at 90 degrees to the lens axis, unless "swings and tilts" are available on the camera, and are in use.

The plane is as FLAT as the lens designer can get it.

The camera does NOT focus on the inside of a cylinder, or of a dome..

.... except in the case of some specialised fisheye lenses.... where it doesn't make much practical difference to shooting because the DoF is very great.
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apaflo Veteran Member • Posts: 3,854
Re: The focus lies along which line?
1

fireplace33 wrote:

When focussed at some particular distance, does the focus lie more along the red, or more along the blue line,when using a typical lens on a typical DSLR?

A simple answer to the simple question that applies to simple lenses can be found here:

http://en.wikipedia.org/wiki/Petzval_field_curvature

Of course life is never simple, and neither are DSLRs or their lenses, so a practical discussion that is more useful can be found here:

http://toothwalker.org/optics/astigmatism.html

AnthonyL Senior Member • Posts: 2,656
Re: The focus lies along which line?

Another way of looking at it is to imagine the (rectangular) sensor projecting straight out from the camera.  Stop the projection when it hits a point you are focused at.  That whole rectangle is in focus.

Or, just stand straight in front of a straight brick wall and once you have focussed on the nearest point to you the whole wall will be in focus.

Further research - look up "focal plane" or "plane of focus"

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fireplace33
OP fireplace33 Senior Member • Posts: 2,392
Blue line wins!

Thanks for those quick answers! Looks like the blue line wins!

This is what I was assuming; you guys have just helped me "win" a little argument here at homeĀ 

...it is after all called the focal "plane".

You can't help admiring the lens designers that manage to get our lenses to focus like that, no wonder they are expensive!

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Barrie Davis
Barrie Davis Forum Pro • Posts: 21,460
Re: Blue line wins!

fireplace33 wrote:

Thanks for those quick answers! Looks like the blue line wins!

This is what I was assuming; you guys have just helped me "win" a little argument here at home

...it is after all called the focal "plane".

No.... NOT focal plane. It is the "plane of focus."

The "focal plane" is INSIDE the camera, where the image is formed, where the sensor lies....

... and immediately in front of which a "focal plane shutter" operates.

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hotdog321
hotdog321 Forum Pro • Posts: 18,704
Re: The focus lies along which line?

The blue line, though the line may be a little wavy depending on the quality of the lens. Note that a true macro lens is optimized to give you a really flat plane of focus, though. This is because they are often used to copy flat objects: books, photos, etc.

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fireplace33
OP fireplace33 Senior Member • Posts: 2,392
Re: Blue line wins!

Barrie Davis wrote:

fireplace33 wrote:

Thanks for those quick answers! Looks like the blue line wins!

This is what I was assuming; you guys have just helped me "win" a little argument here at home

...it is after all called the focal "plane".

No.... NOT focal plane. It is the "plane of focus."

The "focal plane" is INSIDE the camera, where the image is formed, where the sensor lies....

... and immediately in front of which a "focal plane shutter" operates.

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Regards,
Baz
"Ahh... But the thing is, these guys were no ORDINARY time travellers!"

Oops , thanks for the correction!

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apaflo Veteran Member • Posts: 3,854
Re: Blue line wins!
1

fireplace33 wrote:

Thanks for those quick answers! Looks like the blue line wins!

This is what I was assuming; you guys have just helped me "win" a little argument here at home

...it is after all called the focal "plane".

You can't help admiring the lens designers that manage to get our lenses to focus like that, no wonder they are expensive!

You apparently didn't read the cited URL's close enough!

The curved red line wins almost every time!

The exceptions are lenses specifically designed to have a "flat field".   Note that is not the same as a "flat field design", which can be used to describe almost every lens in use with a DSLR today.

A flat field design has a flatter field than a simple lens.   But it looks a lot more like that red line than it does the blue line!  Most lens for DSLRs fit that description.

Most, though not all, macro lenses are specifically designed to be what is described as a "flat field lens".  They are not anything near flat like a straight line though!   Instead the far outer edges  move at least somewhat towards the camera, and there are "waves" at somewhere probably a little more than half way from the center to the edge, which will comonly extend both in front and in back of the blue line.

The trick with a very good flat field macro lens is that if the center is focused on that blue line even at wide open all except perhaps the extreme edges will be within the DOF.  Hence while it is not really flat, it is flat enough.

The trick with virtually all non-macro lenses is to remember that at the edges the field of focus is going to be closer to the camera than it is at the center, which with wide apertures will definitely be significant.  (For group shots, with say 3 rows of people, focus on the center row, but make the back row half as wide, and the middle row three quarters as wide, as the front row.)

Guidenet
Guidenet Forum Pro • Posts: 15,748
Plane of focus

Note the plane of focus may not extend as wide as your blue line may suggest. At some point the difference in distances become too great and the plane of focus no longer remains very flat. It's also why very wide lenses can become more and more expensive as they get wider.

I agree with you that it seems to be a miracle of optics that this is accomplished at all. Pretty cool.

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edu T Senior Member • Posts: 1,174
Re: Blue line wins!

leno wrote:

Its a good question which many people will not understand . . .

and

fireplace33 wrote:

. . . Looks like the blue line wins! This is what I was assuming; you guys have just helped me "win" a little argument here . . .

I think I can see what the "red line party" is about: for their credit, when we focus the camera to, say, 10ft, we tend to intuitively assume that ALL objects 10ft away will be in focus -- well, why not?

But if we (mistakingly) think that way, the blue line or the notion of a "plane of focus" becomes COUNTERintuitive (as it implies that the nearer an object is to the corners of the image, the longer the focusing distance, as measured from the camera, gets...)

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edu T Senior Member • Posts: 1,174
Re: Blue line wins!

apaflo wrote:

fireplace33 wrote:

Thanks for those quick answers! Looks like the blue line wins! . . .

You apparently didn't read the cited URL's close enough! The curved red line wins almost every time!

. . .

Nevertheless, both sources deal with the deviation from an ideal/flat plane of focus as an "optical aberration", so the lens makers' goal is supposed to be the blue line indeed.

In fact, the OP somehow made a provision for real-word variations when he asked whether "the focus lies MORE along" one line or another.

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Limburger
Limburger Veteran Member • Posts: 7,834
Re: The focus lies along which line?

fireplace33 wrote:

Here is a simple question with the hope that a simple answer is possible.

I'm assuming I know the answer

When focussed at some particular distance, does the focus lie more along the red, or more along the blue line,when using a typical lens on a typical DSLR?

The red line is a circle with the center at the camera, The blue line is a straight line.

Shoot a portrait of several people in rows. Use a big aperture lens at big aperture and there is your answer. You have to change the aperture till everybody is in focus.

Focussing and recomposing can result in oof subject due to how a focal plane and DOF work, in particular noticable with big apertures (shallow depth of field).

Your depth of field is the red line and the focal plane is the blue line. By focussing and recomposing (you pivot on a 45 degree angle) the blue line will move the same 45 degrees following the red line.

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Hank3152
Hank3152 Forum Pro • Posts: 13,499
Maybe this is clearer.......
2

This is often why a properly focused subject is slightly out of focus when the frame is recomposed, therefore lies the argument for benefits of using an off center focus point.

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nelsonal Senior Member • Posts: 2,464
Re: The focus lies along which line?

Let's ask the same about the earth.  It's the red line, but the radius is usually relatively large as that's one of several things optical designers aim to make as close to the blue line as possible (when the radius of a circle is large enough it's approximately flat, think of how rare it is that you "see" the curvature of the earth).

Limburger
Limburger Veteran Member • Posts: 7,834
Re: Maybe this is clearer.......

Thanks Hank,

Exactly what I tried to explain.

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Cheers Mike

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Barrie Davis
Barrie Davis Forum Pro • Posts: 21,460
Re: The focus lies along which line?

Limburger wrote:

fireplace33 wrote:

Here is a simple question with the hope that a simple answer is possible.

I'm assuming I know the answer

When focussed at some particular distance, does the focus lie more along the red, or more along the blue line,when using a typical lens on a typical DSLR?

The red line is a circle with the center at the camera, The blue line is a straight line.

Shoot a portrait of several people side by side. Use a big aperture lens at big aperture and there is your answer. You have to change the aperture till everybody on the sides are in focus.

Really? How unfortunate!

Which lens are you using that requires a noticeably different focus distance for the centre compared to the edge? How do you manage with lens test charts?

Or do you think you have a lens which is not FOCUSED on a different distance at the edge, but just isn't very SHARP at its edges at open stop? That would be a different fault.

Focus and recomposing can result in oof subject due to how a focal plane and DOF work.

If I understand you correctly, what you're saying suggests the subject plane is (indeed) flat, and NOT curved.

If you focus on a subject that is at the edge of the intended compositon using a focus point on lens axis, and then swing to recompose 'till it is no longer on the sharp plane, it must be expected to go out of focus when shallow DoF prevails... (it will be back-focused)

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Baz
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Limburger
Limburger Veteran Member • Posts: 7,834
Re: The focus lies along which line?

I was editing while you replied.

It should make sense now

BTW Hank has a perfect example.

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Cheers Mike

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scorrpio
scorrpio Veteran Member • Posts: 3,595
Re: The focus lies along which line?

The correct answer is:  blue line.

Set your camera on a tripod pointed at a finely textured wall.     Lens axis should be perfectly orthogonal to the wall.    Focus on the wall.   Entire frame should be in focus, from center to the corners.   If corners are softer than center, it means focus plane curvature and indicates a poor quality lens.

Read any serious lens review - the sharpness is judged by taking pictures of a flat resolution chart.

Macro work tends to make focus plane curvature especially obvious, which is why having a flat focus plane is especially critical for macro lenses.

There definitely are lenses with more of a red line focus plane.   But you are highly unlikely to see a red (or gold) ring on such a lens, or a four-digit price tag for that matter.   Probably one notable exception would be the fisheyes.

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