Hey there. I'm using a sinar 4x5 camera with a 210 mounted on a flat lensboard. When using a depth of field chart like: http://www.rbarkerphoto.com/DOF2.html where does one measure from? For example, if using a tape measure, should I have the starting point (zero) be the front of the lens (i.e. the UV filter on the lens), the lensboard, or maybe (though I couldn't see why) the film plane?
Thanks,
Serge
None of the above.
DoF is measured about the plane of best focus. The focused distance (= distance to the plane of best focus) is, by convention, measured from the film plane.
That's why, for example, many 35 mm SLRs have a mark on the top plate to indicate where the film plane is.
Serge,
DOF systems for view cameras normally measure the distance the rear standard moves when focusing and the difference in the position between the near and far points as recorded on the ground glass. Then the rear standard is moved half way between the near and far distance as mearsured on the rear standard focus track.
It sounds like you are using a DOF calculator for non view camera use.
Just be aware that the reproduction ratio and the angle the camera is positioned at change the DOF requirements.
Such tables give depth of field when there are not tilts or swings. There are two ways it is commonly done.
The table may give both the near depth of field, which is the distance in adequate focus in front of the plane of exact focus and the far depth of field, which is the distance in adequate focus in back of the plane plane of focus. The total depth of field is the sum of the two.
Or, it may be given by specifying the nearest point along the lens axis which is in adequate focus and the furthest point along the lens axis which is in adequate focus. Planes perpendicular to the lens axis at these points delineate the region in adequate focus. The distances of these points are specified in terms of distance from the lens or sometimes some other point associated with the lens, such as the front principal point of the lens or even the film plane, but except for close-ups, it doesn't make much difference precisely from where you measure it.
The web page you refer to appears to use the second method. I can't tell which formulas they use since the link doesn't get me anywhere useful.
There are in fact different formulas that one could use, but they give approximately the same results, at least for distant subjects. There is one exact formula which works at any distance and some approximations which are easier to use but which apply specifically for distant objects or for close-up objects. You can find these formulas in Jacobson's Lens Tutuorial, elsewhere at the photo.net website.
It should also be noted that the results depend critically on the coc chosen. The values specified here appear to be plausible values, but some people may be more demanding.
It is probably a mistake to use a tape measure to estimate depth of field. Everything you need to know can be determined by measurements along the rail or bed of a view camera. This is the so-called focus spread method. It is described elsewhere in the Large Format web page.
I looked at your question again.
As I said, it usually doesn't matter much, except for close-ups, precisely where you measure from. I believe that the usual formulas are based on an analysis which measures from the so-called front principal point. For most lenses, like your 210, this would be close to the front of the lens board. For telephoto lenses, it could be well in front of the lens. Of course, the formula being used could partially compensate for that by choosing a different point to measure from and incorporating that in the formula. Since we don't know just which formulas are being used, it would require trying to reverse engineer the results to find the formula, and honestly, it isn't worth the effort. If you insist on using a tape, which I think is a big mistake, then measure from the lens board. It should be close enough, at least for distant objects.
For close-ups, it is probably a big mistake to use subject distance in the formulas. Instead, you should use image magnification. This can be calculated by measuring the image size on the ground glass, and then dividing by the subject size. Or, it can be determined by finding the additional bellows extension beyond the position when focused at infinity, and dividing that by the focal length. Then both the near and far depths of field (from the plane of exact focus) are given by the formula
f-number times coc divided by the square of the magification all multiplied by one plus the magnification.
It is independent of focal length. Let me do an example. Suppose the magnification ratio is 1:2 so that the magnification is 1/2, and your aperture is f/22. Choose a coc of size 0.1 mm, a common choice. Then the
formula gives
22 x 0.1/ (1/2)^2 all times (1 + 1/2) = 13.2 mm
So there would be 13.2 mm in focus in front of the plane of exact focus and 13.2 mm in focus in back of the plane of exact focus.
I looked at the Barker web site and table but unfortunately when you click on the link to the formula being used it just takes you to a page having to do with home values so I don't know what exactly he's doing. However, since the table includes all formats up to 8x10 it's clear the calculator is in fact intended for view camera use.
There are a lot of variables and assumptions involved when constructing any depth of field table and so you don't need perfect precision in working with one. I think tables like this are practical to use primarily in a studio where it's relatively easy to measure distances. When you're outdoors photographing landscapes, buildings, etc. it just isn't feasible to make the kinds of measurements this table calls for. However, FWIW I believe that the distance he's probably talking about is the distance from the lens, assuming you're focusing by moving the lens back and forth. Since many large format cameras don't allow focusing with the back Idoubt that he's talking about the distance from the film. I don't think exactly where on the lens you measure from is critical given all the assumptions and variables that went into constructing the table in the first place.
Brian Ellis
Before you criticize someone, walk a mile in their shoes. That way when you do criticize them you'll be
a mile away and you'll have their shoes.
The following formulae are correct for a quasi-symmetrical lens (i.e : non-retrofocus and non-tele with a close-to-one pupillar magnification ratio) at all distances within the model of pure geometrical blur by defocusing, the distances d, d1 and d2 are measured with respect to the front principal plane H which is located somewhere between the lens board and the front lens vertex for many quasi-symmetrical view camera lens formulae. f is the focal length. Most DOF charts are based on those formulae. Jacobson gives more general formulae valid for non-symmetric lenses. H = f*f/(N*c) is the classical hyperfocal distance, N is the f-number and c the diameter of the circle of least confusion.
1/d1 = 1/d + (1/H)(1-f/d)
1/d2 = 1/d - (1/H)(1-f/d) total DOF = d2-d1.
When focused to a distance d equal to H+f measured from the front principal plane, DOF extends from d1 = (H+f)/2 to d2 = infinity.
This rule for hyperfocal setting is also valid for non-symmetrical lenses whatever their pupillar magnification ratio might be. Only in the close-up range will retrofocus or tele lenses substantially differ in terms of DOF formulae with respect to quasi-symmetrical lenses.
Taking into account the fact that most users ignore where the principal plane H is located, it is a tradition among small and medium format lens manufacturers to engrave their focusing helicals with distances measured form the film plane.
For view camera use referring simply to the lens board position will be accurate enough for practical use except if you use a telephoto design, where the principal plane might be located.. anywhere ;-) but the exact position of H is always documented by view camera lens manufacturers.
The DOF table on my site is one written originally by Michael Gillett. I simply expanded it (with Mr. Gillett's permission) to include LF formats and additional apertures at both ends, to make it more useful for my Leica Ms and LF work. (Eventually, I'll get around to adding f/1.0 for the Noctilux. DOF? What DOF? ;-) ) Mr. Gillett's web site may well have changed since that time.
The chart is straight HTML, however, so one can do a "view page source" to examine the formula used, and to modify the CoC assumptions, if one wishes to do so.
Serge, I don't know which model of Sinar 4x5 you own/use, but all "modern" Sinars (I think from year 1972 or 73 till today) have depth of filed scale on the focusing knob. I have found that to be quite enough for me. O.K... there maight be a reason not to use that aid, but I don't know it...
I have placed scan on my site for anyone with used Sinar and without summary instruction card. Sure, © held SINAR AG...
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