17-40 L and 5D Hyperfocal - Can anyone explain this?

[paul_taylor7]

Using Canon 5D body with 17-40 Lens

The background (actually 250 meters away) is out of focus using the hyperfocal

distance:

Here's the link to 100% crop (not my webspace)

http://www.promediasport.com/hyper1740canon5D.jpg

 

 

Using the DOF calculator, at 17mm - by focusing at 2 feet will give: from 1 ft

to infinity in focus. I've took 3 shots at f16, at 2.5ft 4ft and thirdly

lining up the mark with infinity line.

 

I've done this test about 6 times, with exactly the same results each time

(and at f22 at focusing 1.4ft 3ft and infinity etc). Only when focused at

infinity is the background ever sharp?

 

Has anyone else experienced this or have any clue as to what's going on here?

 

Paul

[ekoppel]

If you plug those values (17mm, f/16, focused at 2 feet) into a DOF calculator, you get 127 feet at the far end of your limit for acceptable sharpness. But f/22 should give you sharpness to infinity.

[gdanmitchell]

I may be off base here, but there is "focus" and then there is "focus." If I'm not mistaken,

subjects at the near and far edges of the hyperfocal range will be in "acceptable" focus but

not "perfect" focus. I wonder if that is what is going on here?

 

You wouldn't notice this at less than 100% magnification.

 

Dan

[paul_taylor7]

Thanks Eric but not according to DOF master see attached illustration. Where do you get that information from ?

 

Surely thats not correct Dan.. is it?

[paul_taylor7]

p.s.

I get the same results at f22 (as mentioned focusing at 1.4ft 3ft and infinity)

 

Paul

[paul_taylor7]

heres the same results at f22 attached

[bennyboy]

What circle of confusion value is dofmaster using?

[Ken Katz]

Only the actual point you focused on is really in focus, and increasing DOF by stopping down the lens simply expands the zone of "acceptable" sharpness in front and behind that point. "Acceptable" sharpness is dependant on the degree of enlargement. Viewing 5D files at 100% represents extreme enlargement and I am not surprised that a far distance image looks better focused at infinity than at a shorter distance irregardless of the aperture. I would generally use an an aperture 1 stop small than what was recommended by DOF charts for a shot where I wanted to expand DOF. More important than how it looks at 100% on screen, how does an 8 x 10 print look, or a 12 x 18 print?

[eosdoc]

<blockquote><i><small>

Only when focused at infinity is the background ever sharp?

</small></i></blockquote>

Yes. Exactly right. Read:<BR>

<a href=

http://www.trenholm.org/hmmerk/DOFR.html

>http://www.trenholm.org/hmmerk/DOFR.html</a>

[phyrpowr]

Kenneth hit the point, I've always read that you'll get "acceptable" shaprness at infinity, which to the writer might mean "you can pretty much tell what it is if you already know"

[silvergull]

Paul: assumptions have been made in the creation of the very concept of "depth of field".

The "circle of confusion" that is acceptable to one person may not be to another. In fact,

the laws of Physics and the mathematics used to describe the focal plane show that depth

of field does not actually exist.

 

There is a plane, that is often curved to some small degree, upon which the image is

focused, that has no thickness. That plane is what one tries to place on the film or chip

surface. Small errors cannot be sharply recorded. The amount of error that is acceptable,

is what is at the heart of the "depth of field".

 

So, it is no wonder that a lens focused at infinity produces the sharpest image of an object

250 meters away.

 

Also, that plane of focus contains image elements that are most sharply rendered only at a

couple of f-stops. The 17 - 40 is an 1:4 lens, wide open. Its best f-stops are probably f5.6

- f11. Certainly by f16 diffraction has raised its ugly head. DOF calculations ignore

diffraction! In fact, I would expect diffraction to show-up even at f11.

 

Hyper-focal distance is a crutch from the era of news camera people trying to capture

moving subjects before auto-focus was invented; long before. If one wants front to back

sharpness, one needs to use equipment that will permit front tilt. For that to work, the

subject plane must be visualized and found to be quite flat; that is, able to be defined by a

shallow box.

 

It all comes down to what purpose one has for the image; a newspaper print at 150 line

screen, or a 24"x36" print that will be viewed at 18"?

 

A final thought: one ought to select the best tool for a given task. The landscape, urban or

natural is still best done with large format, adjustable cameras. For extractive landscape

work using selective focus, small and medium formats seem best. For action or candid

work, the small format and to a lesser degree, medium format, seems best. In the end, it

is what seems best to you given your degree of happiness with a given result.

 

Cheers,

 

Jim

[paul_taylor7]

Well, much appreciated advice all - thanks very much. Leaves me much to ponder (a can of worms worth almost).

I'll study this more now and experiment too course.

 

 

Regards,

Paul

[matthias_meixner2]

> Paul: assumptions have been made in the creation of the very concept

of "depth of field". The "circle of confusion" that is acceptable to

one person may not be to another. In fact, the laws of Physics and the

mathematics used to describe the focal plane show that depth of field

does not actually exist.

 

Yes and No. There are more things to consider: Resolution of the camera and diffraction limit. If the circle of confusion is well below the pixel size of the camera and/or within the limits of diffraction, you will not be able to see any difference between an in focus area and a slightly out of focus area. However, this also means that with each increase in resolution of the camera, the DOF gets smaller and smaller.

[radu_diaconu]

Paul,

 

Do not forget that when you are photographing, you are focusing only a single plane of the image. I explain; take a picture, you have the foreground, the background and all that composition that stands in the middle. When you focus your image, you are focusing exactly and precisely on only one of these planes, the rest will be, like other said, curvature of field and circle of confusion (that's why your picture at F.22 focused at infinity looks sharper, or more sharp at infinity than the ones focused on "planes" that are relatively closer to you.

 

Depth of Field is only an illusion, and you can never get sharp results over the entire field, precisely because you are focusing on a 2D plane.

 

Hope this helps.

 

Radu D.

[bobatkins]

The limits of the DOF, by definition, represent points which are not unacceptably blurred when viewed in an 8x10 print from a distance of about a foot. That is to say they are acceptably blurred, or, if you like, acceptably sharp.

 

How are you defining sharp? There is only one focal plane. Only one distance at which points will be at their minimum size (maximum sharpness).

 

You might want to look at some of the articles here http://www.bobatkins.com/photography/technical/index.html