Within depth of field range just as sharp as exact focus?

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68degrees

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I was wondering. My gut says that exact focus will be sharper but lets say the subject is exactly 5 feet away and I am at f8. Will my subject be just as in focus and sharp if I focus at 6 feet as if I focus at 5 feet because of the depth of field of f8?

Thanks

Gary
 
The plane of critical sharpness is very narrow. But the "acceptable focus" is that is used in the scale on a lens. Your most sharp area would be exactly where you adjust focus, but the band before and behind will be almost as sharp, and difficult to tell with a 35mm lens.
 
goamules said:
The plane of critical sharpness is very narrow. But the "acceptable focus" is that is used in the scale on a lens. Your most sharp area would be exactly where you adjust focus, but the band before and behind will be almost as sharp, and difficult to tell with a 35mm lens.
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So the answer is no. Exact focus is always sharper. The dof scale on the lens is for 'acceptable' sharpness. So dead on balls focusing has real value and not just ocd.
 
For any lens. It's physics. The long for focal length will have a narrower band, the wide angles a wide band of acceptable focus.
 
goamules said:
For any lens. It's physics. The long for focal length will have a narrower band, the wide angles a wide band of acceptable focus.
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Well you specified a 35mm lens not being very noticeable so I was asking if a 75mm on medium format, would it be about the same or more noticeable. That was the nature of my questions.

Regarding the relatonship between focal length and depth of field band that much I knew but I didnt know if it was a gradient or if DOF kept everythign within a range in the same degree of focus and then outside the range it falls off. Now I understand that it is a gradient and 'acceptable' is a subjective term of the person that made the depth of field scale on the lens. What is acceptable to one person is not to another and so forth.
 
Another thing to factor in is film/sensor resolution. Higher recording media resolution allows greater discrimination between what is acceptably sharp and what is not along the focus gradient. For a given aperture, a low res digital sensor will render a deeper dof than something like a D800, when the print size is large enough to perceive the difference.
 
Hi, depends on the focal length of the lens....

In a given Depth of filed sharpness decreases as it goes away from the exact focus point....the decreasing is more abrupt or more soft depending on the foal length you use...

Also depends on the quality of the glass involved...for instance the VC 21mm f4 lens is so sharp that even at a huge depth of field you can actualy see perfectly where the exact fcus was achieved....
In that respect quality involves diffraction, as you close the aperture you´ll be able to get more of that flaw...

Cheers!
 
lynnb said:
Another thing to factor in is film/sensor resolution. Higher recording media resolution allows greater discrimination between what is acceptably sharp and what is not along the focus gradient. For a given aperture, a low res digital sensor will render a deeper dof than something like a D800, when the print size is large enough to perceive the difference.
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Is that the same thing as saying that small prints it will be acceptably sharp but in huge blow ups it wont? Or am I missing something more?
Thanks
 
Most sharp would be a point ie. infinitely small; but the imperfections of the lens, the resolution of film or sensor etc mean that that point will be more or less a blob. DOF is essentialy a judgement of how big and out-of-focus that blob gets before it is noticed. It will depend both on how big the print and how close you view it.
 
If I understand what you are asking, then here are the facts:
The sharpest point is the exact plane of focus for the lens. This is in fact infinitesimally thin, and some lenses with curvature of field may have a curved plane of focus (as if you're inside a dome).

Beyond this plane of sharpest focus, object points are not rendered as points anymore, but as larger and larger blur circles the farther away from the plane of exact focus that they are. Because the eye doesn't have infinitely fine resolution, below a certain value the eye will accept a sufficiently small blur circle to look "sharp". This zone is what is marked on your lens depth of field scale.

The MANY things which affect the apparent depth of field include:

Your own eyesight (worse = better apparent DOF)

film to subject distance (very shallow in macro photography)

lens focal length (longer = shallower)

shooting aperture (wider = shallower)

film size (larger formats tend to have shallower DOF with the same lens angle of view)

image magnification: viewing a contact print gives a different impression than if you project the image on a big screen. The bigger you magnify the image, the shallower the apparent depth of field.

Picture viewing distance: the closer you are to the image, the less the apparent DOF. Something that looks sharp from far away may be blurry up close.

You should be aware that the DOF scale of many 35mm film format lenses is based on the assumption that you have average good eyesight, and you are viewing an 8x10 print from 1 foot away.

Regarding your question of the 75mm medium format camera, I can tell you that with 6x6 and 6x9 cameras, the DOF is very shallow at f/3.5.
 
I was applying the following logic to the resolution of digital sensors in particular (e.g. a 6Mp vs 36Mp 35mm digital sensors, with the same lens mounted):
If an image is viewed on a low-resolution display medium such as a computer monitor, the detectability of blur will be limited by the display medium rather than by human vision. For example, the optical blur will be more difficult to detect in an 8″×10″ image displayed on a computer monitor than in an 8″×10″ print of the same original image viewed at the same distance.
(from Wikipedia: Circles of Confusion - Photography)
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Substitute capture for display in the above and you'll see what I'm getting at. I think a low res sensor will resolve a slightly defocused blur circle as sharp, while a hig-res sensor will be able to accurately record the defocus.

I'm assuming the same would apply to large-grain vs very fine grain film. A fine grain film would be able to more accurately record a defocused blur circle than a large grain film, which would record it as sharp. So the fine grain film would, theoretically at least, record a shallower dof than a coarse grain film.

Perhaps others here may have done this comparison? Theoretically, it makes sense to me.
 
One thing is missing :): Focus shift.

For example, if you use a ZM 50/1.5 Sonnar (optimized for f1.5) at f8, the center point of focus will be at the beginning of the DOF window marked on the lens, not the center.

All lenses shift and have field curvature (meaning the center point of focus is different than in the picture corners). It's just a matter of degrees. Some a lot, some negligible.

The DOF marks on the lens not only indicate acceptably sharp focus due to aperture, but also the window in which the focus point can be, and as my example above shows, this can be extreme for some lenses.

This is all theory. Best is to get to know your lenses by using them ....

Roland.
 
I just checked ... it seems it's a point rather than a range, so no ... and not only that it looks like even the photos that look OK are in fact a bit off

... surely focus shift is just a function of these old-fashioned Range-finders ... nothing to do with the lens, and it doesn't even seen to effect modern cameras at all
 
68degrees said:
So the answer is no. Exact focus is always sharper. The dof scale on the lens is for 'acceptable' sharpness. So dead on balls focusing has real value and not just ocd.
Click to expand...

"Exact focus" as you're interpreting it (as in: "always sharper") doesn't exist. DOF will always be "in force", when using a (proper) camera lens, etc etc etc:


http://en.wikipedia.org/wiki/Depth_of_field
 
Sparrow said:
I just checked ... it seems it's a point rather than a range, so no ... and not only that it looks like even the photos that look OK are in fact a bit off

... surely focus shift is just a function of these old-fashioned Range-finders ... nothing to do with the lens, and it doesn't even seen to effect modern cameras at all
Click to expand...

Irony or Scotch.
 
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