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Area in focus, is it a plane or a sphere
When you take a photo, and assume that you are using the center point and it is in focus, is the area that is in focus a vertical plane extending up down, left and right or is it a curved surface of a sphere (where each point is equidistant from the lens)..

For example, if you are photographing a large group shot with say 50 people, should you have them arranged in a slight arc or straight across...

Comments (11)

Theoretically it's a plane..

DIPics.

Buzzphotos wrote:.

When you take a photo, and assume that you are using the center pointand it is in focus, is the area that is in focus a vertical planeextending up down, left and right or is it a curved surface of asphere (where each point is equidistant from the lens)..

For example, if you are photographing a large group shot with say 50people, should you have them arranged in a slight arc or straightacross...

Comment #1

If you focus at 10ft, then what is in focus is not a point or a sphere..

(part of). since there is something calldof(depth of field). if younuse 50mm lems and it goes wide open to f1.4 and you shoot at f1.4 then what is in focus will end up looking like a slice of bread, though at f1.4 the bread would be very thin. I would think that since the focus is actually a distance from the lens, the bread would be faintly curved in shape resembling a shere if it was big enough..

But it should be emphasized extremely strongly that the curve in the bread slice would be very very very slight. the area of what is in actual focus would be for any real practical purpose flat. and a very thin slice of bread. it would not be a flat plain due to dof; the bread slice slice would have a slight thickness.fromhttp://www.dofmaster.com/dofjs.htmlusing canon1dsmkIII(FF) 50mm f1.4 and distance 10ft.near focus is 9.52ft.far focus is 10.5ft.total infocus is 1.02ft. the bread slice above..

Put your own numbers and camera in to determine your dof...

Comment #2

Dipics wrote:.

Theoretically it's a plane..

.... except with fisheye lenses, where it is usually the inside surface of a bowl .. that isn't necessarily spherical. However, the depth of field of fisheye lenses is so great, the shape of the focused surface doesn't make a lot of practical difference..

Another case is those special large format rotating cameras.....

[the ones that can shoot a whole high school in one long strip of a picture].

..... that do need to have the staging for the group laid out in a precise arc around the centre of rotation of the camera.Regards,Baz..

Comment #3

If in my example the subject is 10ft. then the straight ahead distance is 10ft. but if the subject is 5ft wide then the straight line distance down the two diagonals down to the 2.5ft from center points on each side is not 10ft if it is actually a plain. the diagonal distance of a right angle triangle is longer than either other side..

The only way you would get 10ft down the diagonals would be if the ahead distance is les that 10ft, which would make the whole of the focus area slightly curve left to right. it would also be a curve up/down for the same reason. but as I said in my other reply the curve is extremely slight. and should for all practical purposes be thought of as a flat surfaced plain...

Comment #4

Buzzphotos wrote:.

For example, if you are photographing a large group shot with say 50people, should you have them arranged in a slight arc or straightacross..

A slight arc. Wide angle lenses don't record an especially flat plane of focus - imagine focussing a WA lens that takes in a 60 degree FOV and focussing to 10' ... in order to focus on a plane that's 10' away from the center, objects at the edges would be 11.5' away..

If you look at mtf charts, even really good WA lenses sometimes "suffer" in the tests wide open because the test chart is flat..

Tele lenses record such a narrow FOV that the plane is virtually flat, and some lenses are designed to record a "flat field" (particularly macros, and particularly short macros, like 50mm, where the FOV is still wide enough to show a curved plane of focus, but that you might want to use to photograph documents or other flat subjects)..

So you'll probably want to stop down a bit to get DOF and arrange the group in a bit of a curve. (A little trig might give you some guidelines if you can envision the setup)..

- DennisGallery at http://kingofthebeasts.smugmug.com..

Comment #5

Great explanation. I was going to ask about close-up macro lenses but you already answered that. Thanks..

Comment #6

The vast majority of lenses have a flat focal plane. This will do as a reference for that assertion:.

Http://www.zeiss.com/...b58b9/Contents-Frame/8f0189450a06c8b8c12570fb00445f0c.

So a line of dancers on a stage should be in focus across the entire width of the stage if the centre dancers are in focus despite the increased distance from lens to dancer..

This is why you see so many photographs of a brick wall as a simple test of a new lens. It should be sharp across the entire photo. Typically most lenses will lose sharpness at the extreme edges especially when wide open. A quality lens will fare much better..

The references below explain why, given the above fact, the technique of focus and recompose makes no sense:.

Http://www.mhohner.de/recompose.php?lang=e.

Http://www.outbackphoto.com/workshop/phototechnique/essay06/essay.html.

Chris Elliott.

*Nikon* D Eighty + Fifty - Other equipment in Profile.

Http://PlacidoD.Zenfolio.com/..

Comment #7

Well that settles it, it is a plane! And the articles on focus - recompose were vary valuable. Thanks for clearing this up for me...

Comment #8

Buzzphotos wrote:.

Well that settles it, it is a plane! And the articles on focus -recompose were vary valuable. Thanks for clearing this up for me..

Chris is quite right..

The plane of sharp focus is considered as flat as a sheet of glass, standing at 90 degrees to the lens axis, with that axis passing through the plane at the focused distance ... whatever that happens to be..

Which means there is absolutely NO requirement to make any compensation for the extra distance into the corners... (and it is equally true for those super-wides and fisheyes where the plane of focus isn't quite so flat)Regards,Baz..

Comment #9

Chris Elliott wrote:.

The references below explain why, given the above fact, the techniqueof focus and recompose makes no sense:.

Focus-and-recompose (FaR) is not borne of the "focal plane vs focal sphere" distinction, but because the simple focusing aids available in our cameras are almost always in the center of the frame where our subject is not..

Sure, FaR would work great if there was a focal sphere. FaR is compromised if you have a focal plane, because as you recompose, you change the focus on the subject you just locked..

However, camera tools for focusing don't support anything but FaR technique. You have a choice: (A) you manually focus if the plain matte focus screen is sufficient, or (B) you manually focus using an old-style split-prism that's in the middle of the focus screen, or (C) you let the camera focus using the limited scene positions they've installed autofocus sensors. With the most common (C) choice, note how many people distrust their camera autofocus algorithms, and force the camera to use only the center autofocus sensor. Also notice how few cameras offer autofocus points AT THE FOUR "RULE OF THIRDS" POINTS..

Until the camera offers better tools for focusing, photographers are going to use FaR and hope that the DoF will cover any discrepancy in focal plane distances..

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #10

Ed Halley wrote:.

Until the camera offers better tools for focusing, photographers aregoing to use FaR and hope that the DoF will cover any discrepancy infocal plane distances..

I'll add that historically, photographers have generally been quite satisfied with letting the DoF cover the discrepancy (which is usually only an inch or two). But over the past decade it has become fashionable among amateurs to value limited depth of fieldand the associated "bokeh"especially on portraits. This has made the FaR problem with focus distance become much more obvious..

I don't see much restricted DoF in professional photography (shots with long lenses aside). Proper choice of backgroundpossibly the use of a backdropseems to be the preferred approach for the pros...

Comment #11

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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