DoF Charts and f/stops

[Pere Casals]

Papi, I think you're confusing CoC, an arbitrary number chosen by the photographer based on how much the negative is to be enlarged, and the blur circle, which is given by the lens.

Dan, to me "In optics, a Circle of Confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source".

"It is also known as disk of confusion, circle of indistinctness, blur circle, or blur spot."

My understanding is that what a photograher picks is a maximum allowed CoC to say that something is in the DOF range... but each spot in the scene has its own CoC on film....

(https://en.wikipedia.org/wiki/Circle_of_confusion)

Is there another definition ?

_____

(of course it has to be pointed that confusion is not always in a circle, other shapes are there for coma, astigmatism...)

[ic-racer]

I never use Depth of Field charts, tables, calculators, etc., but a question had me lying in bed awake and wondering last night...

Say you're doing close up work at 1:1, and you set your f/stop to f/16. With the bellows/focal length at twice the normal extension, your effective f/stop is f/32. Which do you use for the chart/table/calculator, f/16 or f/32?

If the lens markings show f16 when at 1:1 your effective aperture is f/32. Use f/32 for calculations of exposure, CoC and Airy disk size.

[Mark Sawyer]

If the lens markings show f16 when at 1:1 your effective aperture is f/32. Use f/32 for calculations of exposure, CoC and Airy disk size.

That's what common sense tells me: figure what your f/stop actually is, and that's what your f/stop actually is, so use it. But cowanw posted:

So in your case use f 16.

...and not being familiar with how DoF calculators calculate, I'm still not sure, maybe there's a built-in compensation I don't know about. Some have a magnification factor built in, but that's more for the inherent lack of DoF close up, not an f/stop compensation.

Just a curiosity, really. Like the Scheimpflug Principle, there's always some party of reality that mucks up the theory, and you're better off trusting a good loupe, a fine ground glass, and the patience to check things thoroughly. But I'd still like to know...

[Pere Casals]

If the lens markings show f16 when at 1:1 your effective aperture is f/32. Use f/32 for calculations of exposure, CoC and Airy disk size.

racer, are you sure ?

Rodenstock DOF/Scheimpflug pocket calculator does not ask for the effective aperture... it looks...

For LF lenses effective aperture is lower (at 1:1) but perhaps we also have a more enlarged CoC because of bellows draw, so one factor may just compensate the other in the formula.
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The "pocket calculator" does not ask is the lens is "unit focus" or "internal focus"...

cowanw says we have to use the engraved aperture (I thing that's right) and you say that we have to use the effective one, have we available literature on that ?



Added:

But I'd still like to know...

me also

[Mark Sawyer]

cowanw says we have to use the engraved aperture (I thing that's right) and you say that we have to use the effective one, have we available literature on that ?

That's what I've hunted for, and found nothing. I think conanw is wrong, and the f/stop you're at is the f/stop you're at. Why would the circumstances of being focused at infinity apply to a completely different set of circumstances like being focused at 1:1? That's a whole different set of numbers. But again, I'm not sure...

[Bob Salomon]

racer, are you sure ?

Rodenstock DOF/Scheimpflug pocket calculator does not ask for the effective aperture... it looks...

For LF lenses effective aperture is lower (at 1:1) but perhaps we also have a more enlarged CoC because bellows draw, so one factor may just compensate the other in the formula.

The "pocket calculator" does not ask is the lens is "unit focus" or "internal focus"...

cowanw says we have to use the engraved aperture (I thing that's right) and you say that we have to use the effective one, have we available literature on that ?



Added:



me also

The calculator does not ask for your aperture. It tells you what aperture is required for the near and far points required and for the magnification that results in, it then tells you where to place the rear standard for those conditions and, if required, the angle of the camera to the subject and lens tilt.

[Pere Casals]

It tells you what aperture is required

Bob, OK, but what aperture tells? the effective one stated by racer ? or the engraved one stated by cowanw ?

at 1:1 we have a difference of 1 stop...

That's what I've hunted for, and found nothing. I think conanw is wrong, and the f/stop you're at is the f/stop you're at. Why would the circumstances of being focused at infinity apply to a completely different set of circumstances like being focused at 1:1? That's a whole different set of numbers. But again, I'm not sure...

Also not sure, but instead I find more plausible cowanw's way. Rodenstock does not ask if the lens is "unit focus" or "internal focus", for any format, this suggests me that there is an invariant relationship for the engraved aperture. Consider that the effective aperture variation also involves an apparent focal variation, all linked with bellows draw.

If we use the effective aperture perhaps we have to also use effective (aparent) focal in a certain formula.... and perhaps this ends in the same than always using always the engraved aperture... just guessing possibilities.

[cowanw]

Please call me Bill.
As we all know DOF diminishes as the subject gets closer than infinity. If we were to substitute a smaller F stop as our number, then the image should be as sharp (risking diffraction) and DOF as great or greater (smaller Fstop= more DOF). A lens has an effective Fstop as regards exposure at closeup because of the inverse square law of light. This has nothing to do with the optical performance of a lens.

[Mark Sawyer]

cowanw says we have to use the engraved aperture (I thing that's right) and you say that we have to use the effective one, have we available literature on that ?

The calculator does not ask for your aperture. It tells you what aperture is required for the near and far points required and for the magnification that results in, it then tells you where to place the rear standard for those conditions and, if required, the angle of the camera to the subject and lens tilt.

A note that none of the calculators or charts I've seen has "aperture" listed on them, and the raw aperture is almost never engraved on lenses, (though I often wish it were). They all deal with f/stops, which is focal length divided by aperture. Aperture is only half the f/stop ratio.

Just nit-picking the technical points in a technical discussion...

[Emmanuel BIGLER]

Hello from France!
I'm coming very late to this discussion, in the OP's question there is the notion of engraved f-number vs. effective f-number for computing depth of field (DoF) with conventional, well-known formulae.

If N is the engraved f-number, the effective f-number is simply N_eff = N(1+M) where M is the magnification ratio, M = (image size)/(object size)

Hence for a given magnification factor M, you can compute depth field as you wish, in terms of N or N_eff.

It happens that when the magnification factor M is larger than about 0.1, i.e. when the object to lens distance is shorter than about 10 times the focal lenght, general DoF formulae can be simplified under the following form, where the total DoF is p₂ - p₁ i.e. the difference between the far (p₂) and near (p₁) limits of sharpness:

p₂ - p₁ = ((M+1)/M²) 2 N c = (1/M²) 2 N_eff c

In macro work, the model based on geometrical optics suggests that DoF is distributed equally in front of the subject and behind the subject, the half- DoF beeing simply equal to (p₂ - p₁)/2.
This, of course, is plain wrong for subjects at large distances, larger that about 10 times the focal length.

So as soon as you know the magnification factor M, in the case of M>0.1, you can compute your DoF limits (according to the well-known model of geometrical optics, in its simplified form) without the help of any computer, without need of any application running on your smartphone: this can be done by hand with a pencil on the back of an envelope
The inputs are: the magnification factor M, the f-number, either engraved N or effective N(1+M), and the circle of confusion, a parameter which, unfortunately, is not intrinsic to any given lens, but depends on the film size and conditions under which you examine a final print of a given size.
The circle of confusion for a pre-WWII 6x9 cm - 2"x3"image was about 100 microns in old DoF charts for amateur 2"x3" rollfilm cameras (actually, c was taken as f/1000, hence about 100 microns for a standard focal lens of 100 mm - 4" for the 6x9 cm - 2"x3" film format), at the time, amateurs only looked at contact prints of their 2"x3" images from a distance of about 12"...