Depth of Focus - Circle of Confusion Confusion

[Wilbur Wong]

Does the "Depth of focus" vary with focal length? And how does that vary?

In practical terms, what does that mean to a photograph, rather than a theoretical absolutely flat target and perfectly flat film - all parallel to each other.

So say I have a scene with close foreground vegetation, lily pads and vertical water reeds mid distance, and high peaks in the back. Is a longer lens more tolerant of not getting my scheimflug perfectly set up? or is it an advantage to compose to shorter focal lengths?

Short of hitting the diffraction limit, and in practical terms of stopping wind blown flowers, what are the better strategies?

[Thilo Schmid]

Wilbur,
even if you asked for a non-theoretical view, perceived DOF is dependent on exactly three parameters:

1. the magnification ratio at exposure-time

2. the magnification ratio at print-time

3. the viewing distance of the print

You cannot ignore these more theoretical aspects, because they tell you, that there is no simple answer to your question. Your setup should be derived from the desired composition, i.e. the proper perspective (viewpoint) and then the framing (focal length). DOF should not be of concern in this stage. I hate to say this in a LF Forum, but the only parameter which will actually provide more DOF is the reduction of film size (at the cost of other limits, e.g. reduced possible print size). The inpact of the magnification ratio in 1. is higher than in 2., because the nature of the first optical image is different (3D to 2D) than the second (2D to 2D).

[Dan Jolicoeur]

Focal length makes no difference on DOF. Use the focal length that gives you the composition you are looking for. The means of getting a proper DOF are the same and repetitive irrespective of focal length.

[Wilbur Wong]

Am I confusing depth of FIELD and depth of FOCUS as one and the same or as two different aspects?

In a previous thread "super symmar80 xl not sharp" Mark Woods and Michael Briggs mention this in relationship of part of the film being out of the plane of focus.

So my basic question might be is the plane of acceptable focus "fatter" with varying focal lengths? In other words can you move the film further from perfect focus given the same acceptable "circle of confusion" definition, with different focal lengths.

[Michael S. Briggs]

Depth of field and depth of focus are different. Depth of focus is on the subject side and is what photographers usually thing about, e.g., when you ask about a scene with "close foreground vegetation, lily pads and vertical water reeds mid distance, and high peaks in the back" and wonder whether all of these objects will be in focus. The equations to calculate depth of field depend on the focal length of the lens.

For a photographer with a view camera, of course you can use swing and tilt to better align the focus with the objects of interest. Rather than using depth of field tables, I perfer to use the method described in an article on the main page of this site, where you measure the focus spread in image space between the near and far objects that you want in focus and than consult a table. The advantage is that this table only depends on the focus spread and not on focal length nor object distance.

Depth of focus is the allowed tolerance in the position of the film. For non-closeup photography, the equation only depends on the relative aperture (i.e., f-number) and the diameter of the circle of confusion. It doesn't depend on the focal length of the lens, or the distance to the (assumed not close) object.

The two are conjugate, so if you use up your depth of focus by an error in film position, the depth of field won't be where you expect it to be.

People commonly say that moving to a shorter focal lenth lens increases depth of field, but it also changes your view of the scene. If you reposition the camera closer to have the same view, depth of field decreases back to essentially the same as with the original lens.

As Thilo says, for the same f-number, you can get more depth of field by moving to a smaller format and the shorter focal length lens that gives the same view. This might be a solution if you can't stop down further with the larger format, perhaps because the exposure times would become too long.

[John_4185]

Depth of FOCUS is the flip-side of Depth of Field. It is the area in front of and behind the the film that is within an acceptable degree of focus (whatever one may consider acceptable.)

Longer lens == greater degree of depth of focus. Picture it thus: imagine the target circle of confusion as diameter D, a section of a cone. The long lens has a longer cone for D than the wide lens of the same aperture, therefore, there is a greater range near acceptable focus with the long lens than a short lens. Remember, we are looking at the film plane, not the subject here.

Keep in mind that in large format photography the scale of depth of focus is relatively small in terms of outcomes when we struggle with objects near eye level with great degrees of separation in the third dimension. In such cases we may concentrate first on a solution that uses a normal to wide lens and therefore depth of FIELD, and then work with depth of FOCUS on the ground-glass when do rear swings and tilts.

Kinda makes you want to pick up a box camera when you put all this in your head. Let your eyes lead and the rest will follow.

[Wilbur Wong]

As a corollary to this, would I be correct to think that longer lenses would be more forgiving of my set up not having the plane of focus set quite exactly, due to the much smaller angle of the "cone" which contains the circle of acceptable confusion with the longer lens?

[Michael S. Briggs]

"Longer lens == greater degree of depth of focus. ...... The long lens has a longer cone for D than the wide lens of the same aperture, therefore, there is a greater range near acceptable focus with the long lens than a short lens."

This is correct if "same aperture" means the linear diameter of the aperture, for example in mm. It wouldn't be correct for relative aperture, meaning f-number, because f-number is focal length divided by aperture diameter. Two lenses of the same f-number will have the same cone regardless of f-number -- the f-number sets the angle of the cone and thus how far you can get from the film and still be within the acceptable circle of confusion. Thinking about aperture in f-number rather than in mm is probably more useful for photographers, and in this case depth of focus does NOT depend on the focal length of the lens.

The equation for depth of focus (non-closeup case) is very simple: t = 2 C N, where C is the diameter of the circle of confusion and N is the f-number. For an relatively fast LF lens, f4.5, and using C = 0.1 mm, this gives t = 0.9 mm = 0.035 inch. Since this is half in front of the film, half in back, it is 2.5X the ANSI figure of +/- 0.007 inch for the tolerance of the depth of 4x4 film holders. If you stop down to f16, t = 1.6 mm = 0.06 in.

[robc]

depth of field refers to the subject(outside the camera) and depth of focus refers to the film.

your chosen acceptable DOField detemines your acceptable DOFocus which detemines the size of acceptable Circle of Confusion.

Altenatively you can work the other way, i.e. decide on the acceptable CoC which determines acceptable DOFocus which determines available DOField.

Either way you also need to consider other affecting factors such as aperture and focal length.

There is only one plane of sharpest focus in the subject which produces very very smal CoC on the film. Anything in front of that plane focusses behind the film and anything behind that plane focusses in front of the film. This is what creates a bigger circle of confusion and when that circle becomes to big it will look blurred when printed. But this also depends on how much you enlarge the print from the negative and what your viewing distance is. i.e. your ability to resolve detail in the print from any given distance.

What is an acceptable CoC is subjective and there are no rules. Some are pedantic about sharpness and other's less so.

Just don't forget that selective focus is a creative method for emphasing some subjects and therefore using bigger CoC either side of plane of sharpest focus can be a desirable thing. i.e narrow DOField is a useful tool for some situations.

N.B. there is no such thing as a plane of acceptable focus. There is only plane of subject sharpest focus and anthing else falls into acceptable DOField or acceptable DOFocus.

[Michael S. Briggs]

That should be "Two lenses of the same f-number will have the same cone regardless of FOCAL LENGTH".