Enlarger Alignment, Negative Flatness, and Glass Carriers

[Ian C]

Enlarger alignment is a much-discussed topic. Some of the supposed “alignment problems” are actually due to the warming of the negative, causing it to temporarily change shape. This is called “popping.” The light of the lamp falls on the surface of the of the negative closest to the lamp, warming it.

Condenser lamp heads transmit considerable heat to the negative unless a heat-absorbing or heat-reflecting filter is used between the lamp and negative. These usually have to be purchased and installed by the owner, as they are not standard equipment in most cases.

Dichroic-filtered color and variable-contrast heads generally incorporate heat filters into the design, so that they transmit much less heat to the negative.

In standard negative carrier, the negative is held in a metal carrier. The carrier both restrains the periphery of the negative and acts as a heat sink, but only about the periphery. As the negative warms, the exposed part of the negative warms and expands. The expanding material must go somewhere, and it does—in a “pregnant belly” towards the lamp.

Since the DOF about the object plane is quite shallow, the displaced negative bellies out of the depth of field throwing the middle of the image out of focus. If the user sees this and adjusts focus so that the middle part of the image looks focused, the edges and corners are still held in the original plane. The corners and edges are now out of focus relative to the refocused main part of the image. Refocusing doesn’t fix the problem.

A glass carrier is the practical cure. The glass has two functions:

1. It obviously restrains the negative into a flat plane.

2. The glass acts as a massive heat sink, absorbing the heat received by the negative and helping it to stay close to its original temperature and flat.

Glass carriers have always been expensive. That’s especially true now that most enlargers are now out of production. The enlarger maker’s glass carriers can be hard to find. Some makers never offered glass carriers. Depending on the make and model of the enlarger, it might be possible to make a simple glass carrier out of two same-size sheets of unscratched window glass. I have successfully done this for several enlargers. They work the same as the commercial product and cost little to make.

Get two sheets of 1/8” (3 mm) window glass cut the same size so that they’re larger than the opening in the negative stage and that of the plate above. Slightly radius all 8 edges of each sheet of glass with a fine abrasive so that the glass won’t cut hands or damage a negative.

Place the two sheets together and use flexible cloth tape to bind them together to form a hinge along the rear edge so that the carrier can be opened like a book. It’s easy to make a black paper mask that resembles a print mat. This is a spacer that fits between the top of the negative and the top glass. It provides the necessary air gap between the shiny top of the negative and the top glass to prevent Newton rings.

The cutout in the mask can be sized to exclude spill light from around the edges of the image boundaries on the negative. You might need to make a mask for each format you enlarge. The result is identical to that from an expensive commercial glass carrier. Centering this homemade carrier under the lens takes some practice to get it right. Reference marks can be made on the edges with nail polish or modeler’s enamel to aid in centering.

Enlarger Alignment

The depth of field about the image plane at the negative is quite shallow. An error in alignment here can spoil the focus. On the other hand, the depth of field about the image is reasonably generous. A moderate error there will be unnoticed. The projection will stay crisp.

To understand this, consider the typical DOF values involved. Suppose that you’re using a 50 mm lens at f/5.6 to make a borderless 8” x 10” print from a 35 mm negative. This requires at least 9X magnification. Assuming a circle of confusion of 0.029 mm for the negative, a 9X image will result in a DOF about the plane of the negative of 0.36 mm. The average thickness of printer paper is about 0.004” or 0.10 mm, so the DOF about the negative is about the thickness of 3.6 sheets of printer paper. That’s thin.

This shallow DOF about the plane of the negative accounts for the sensitivity of the alignment between the lens axis and the negative. They must be perpendicular to each other within a small tolerance.

An angular error between the lens axis and the negative of 0.86º or greater along the 24 mm short dimension of a 35 mm negative will cause some part of the image to defocus. Along the 36 mm long dimension, the focus will begin to degrade with an angular error of 0.57º.

On the other hand, the corresponding DOF about the image plane is about 29.3 mm. If the negative and lens axis are perpendicular, then the angular tolerance between the lens axis and the image is 7.8º along the short dimension and 5.2º along the long dimension of the projection. This helps explain why the alignment between the lens axis and negative is critical, while the alignment between the lens axis and the image is much more tolerant of error (assuming that the lens-to-negative alignment is correct).

The angular tolerance at the image plane is sometimes exploited to intentionally tilt the easel to correct converging lines on simple enlargers that lack the movements needed to use Scheimpflug geometry.

When making a 16” x 20” print from a 35 mm negative with a 50 mm lens, we’d need at least 17.8X. At this magnification, the DOF of about the negative is still small at 0.34 mm. But the DOF about the image increases to 109 mm. In each of the DOF values cited throughout these comments, ideally the negative and image lie approximately at the center of their respective DOFs.

My Experience

My first enlarger was an Omega B66XL. The original owner had no baseboard, as it was mounted to a countertop. I bought a scrap sink cutout from a custom cabinet maker for a small price. It has a plastic top glued to a particleboard base. It’s not particularly flat. It has some curvature from front to back and the column is not square to the base from front to back. The column leans forward somewhat but is reasonably perpendicular from side to side. The alignment between the negative and lens axis is good. Nonetheless, it makes crisp prints as it is. That’s why I’ve never bothered to make a new baseboard and adjust the lens-to-baseboard alignment.

My other enlargers have lens axes that are square to the baseboards in addition to proper alignment between the negative stage and lens axis. The prints I make on these accurately aligned enlargers don’t look any different than the ones I make on the Omega B66 with its imperfect front-to-back alignment relative to the baseboard.

Any extra leaning of the column from the weight of the head raised to the highest position that might cause the lens axis to lean marginally rearward is trivially small. Additionally, when the head is in a high position, it’s raised high to obtain a large magnification. But, as noted above, at such large magnifications, the depth of focus about the image plane increases considerably. There will be no discernible loss of resolution in the image due to any additional column lean due to the weight of the head raised to a high position.

My Conclusions

Perfect alignment is a good idea. Accurate alignment between the lens axis and negative within a small tolerance is critically important for good enlarging.

Good alignment between the lens axis and the image plane is also desirable but isn’t nearly as critical as that between the lens axis and negative plane.

Also, the negative must be kept flat within the shallow depth of field surrounding it during focusing and projection. The best way to do this is by restraining the negative in a glass negative carrier.

 

[ic-racer]

Some other observations. A typical 50mm 2.8 (El-Nikkor) has a curvature of field at 16x20" enlargement magnification opposite the curvature of a typical 35mm negative in a glassless carrier.

However, it is possible to get it all in "reasonable" focus.

The focusing equation of Hansma (which takes into account both Airy disk size and CoC size) predicts that at f16 on my enlarger, the whole image can be pretty well in focus.

This comes from a focus spread (on the enlarger column) of about 10cm. That is if one needs to move the head about 10cm up to get the corners in focus after focusing on the center, then at f16 it should all be reasonably in focus if the enlarger head is set at the half-way point between the center and edge focus points.

Again, the Hansma equation below assumes a certain reasonable viewing distance for the 16x20"

N = (20/(1+M)) * square root of 'dv'

N = Aperture number
20 = constant (circle of confusion 0.15mm)
M = magnification
'dv' = millimeters of focal depth on the baseboard.

 

[Hilo]

Ian, thanks for taking the time to put this on paper, er the screen.

English is not my native language and this probably has to do with my confusion. But I must also admit that I am rather a-technical.

I do not understand this: "the depth of field over the image plane at the negative is quite shallow" and I also do not understand: "the depth of field about the image is reasonably generous" .... Apparently there are 2 depth of field, can you explain?

What I would like to understand most of all is how much depth of field there is when we print.

I have been printing 35mm to 6x9 negatives for over 40 years, sometimes slightly larger (not quite 4x5). Paper sizes are 24x30cm to 50x60cm (roughly A4 to 20x24in.) I use a 50mm for 35mm negatives and a 100mm for medium format. Then only sporadically, I use a 135mm for some Polaroid negatives and a 38mm for half frame negatives.

In all these years I have rarely ever seen blurred grain in the corners. I was trained by a professional printer in Paris, whom I assisted for about five years. We never talked much about the underlying reasons, he only told me "to use the grain focuser in the corners as much as possible."

 

[L Gebhardt]

I do not understand this: "the depth of field over the image plane at the negative is quite shallow" and I also do not understand: "the depth of field about the image is reasonably generous" .... Apparently there are 2 depth of field, can you explain?

What I would like to understand most of all is how much depth of field there is when we print.

There are two planes that each have different depths of field. One around the negative and one around the paper. The closer to the lens the plane is the shallower the depth of field will be. In the case of enlarging the negative plane’s depth of field may not be great enough if the negative is not flat or if the negative plane is tilted too much. At the paper plane there is usually enough depth of field to allow you to tilt the paper to make minor perspective corrections. But in most cases you want the negative, the lens, and the paper planes all flat and parallel to each other.

 

[Ian C]

In answer to post #3:

The Wikipedia article “Depth of Field” is useful. It used to be much better, with many equations, explanations, and carefully rendered accompanying diagrams. One of these diagrams was particularly useful. It showed the object plane at the left, the projected image plane on the right, and the lens between them, with straight lines illustrating the depth of field about the object plane and the resulting depth of focus about the image plane.

Much of this was removed from the Wikipedia article after having been there for several years. I believe that the removed material had been excerpted from one or more copyrighted books without permission, necessitating its removal.

https://en.wikipedia.org/wiki/Depth_of_field

The related articles “Circle of Confusion” and “Hyperfocal distance are useful too and complement many of the ideas in the first article.

https://en.wikipedia.org/wiki/Hyperfocal_distance

https://en.wikipedia.org/wiki/Hyperfocal_distance

Here are 4 diagrams from a different site that are similar to the one that used to be on the Wikipedia “Depth of Field “article. I’ll refer to the second diagram.

https://physicsandphotography.wordpress.com/2014/10/02/depth-of-field/

Take the vertical red line to the left as the object plane and the red line on the right as the image plane. At the far left, the depth of field about the object plane extends from the blue line to the green line.

At the right, the corresponding depth of focus about the image plane extends from the green line to the blue line. The black “T-shaped” vertical lines illustrate the aperture.

These ideas work for any object-lens-image system. Older literature about optical systems used the term “depth of field” when discussing the fields of acceptable resolution about both the object and image planes. Later, “depth of field” was reserved for the object side and “depth of focus” for the image side fields. This is the usual practice now.

The entire article is useful and reasonably easy to understand, especially with the aid of the accompanying diagrams.

The depth of field about the image plane in enlarging can be calculated from a standard formula. It’s somewhat long. I find it tedious to key in the values into the equation with a calculator. I think it most practical to write a program. Then all I need do is enter the four values separated by spaces and press a key to execute the program.

I use a 1993 HP48GX calculator for this. The values I need are:

N = aperture number

c = COC diameter for the film format in millimeters

m = magnification

f = focal length in millimeters.

The argument list (which must be entered in the given order) is

DOF(N, c, m, f) for camera DOF about the object plane.

For enlarger use to find the DOF about the image plane, I input the data as

Image DOF(N, c, 1/m, f)

What happens is that every occurrence of magnification m in the formula is replaced by 1/m.

Using the example of post #1 above, the DOF about the image plane is

Image DOF(5.6, 0.029, 1/9, 50) = 29.3 mm

In use, I key in: 5.6 SPACE. 0.029 SPACE 1/9 SPACE and press the key I’ve chosen to execute the program. The answer is displayed almost immediately.

For the field about the negative (the “object” in this case), the argument list is

Negative DOF(N, mc, m, f)

Here, c is replaced with the product mc.

Using the example,

Image DOF(5.6, 9*0.029, 9, 50) = 0.36 mm

In this way, I can use the same program for camera or enlarger DOF on the object side or image side as needed.

The DOF formula I use is

dof(N, c, m, f) = 2Nc(m + 1)/[m^2 – (Nc/f)^2]

The HP48 code that evaluates this equation is

dof: << 4 ROLLD 3 DUPN 1 + * * 2 * 5 ROLLD SQ 4 ROLLD * SWAP / SQ - / >>

Using a program makes this quick and easy. There are shorter DOF formulas. Be careful. These shorter versions are usually applicable only to specific situations.

If you need to know the DOF for a specific enlarging setup and don’t have the tools to calculate the answer, you can ask here. The required information is: aperture, circle of confusion diameter (or format size), magnification (or image size), and the focal length of the lens.

 

[ic-racer]

Depth of field when enlarging can also empirically be derived by observation of the grain with a grain magnifier. Again, to quantify it, the enlarger head has to be moved to focus (to get the numbers off the scale on the column.

 

[MattKing]

I've made this a "Sticky" thread, to keep it always at the top of the Forum.

 

[Sirius Glass]

I've made this a "Sticky" thread, to keep it always at the top of the Forum.

Great move. Thank you.

 

[Hilo]

Thanks again Ian, and the other posters. Although I no longer see the forest because of all the trees, I know this will change while re-reading.

 

[cliveh]

Depth of field when enlarging can also empirically be derived by observation of the grain with a grain magnifier. Again, to quantify it, the enlarger head has to be moved to focus (to get the numbers off the scale on the column.

Don't you mean depth of focus (just being pedantic).

 

[Hilo]

If you need to know the DOF for a specific enlarging setup and don’t have the tools to calculate the answer, you can ask here. The required information is: aperture, circle of confusion diameter (or format size), magnification (or image size), and the focal length of the lens.

Thanks again Ian, I always get lost with math and numbers. So, if you don't mind, I would like to give you some examples. I narrowed it down to the smallest and largest size of my prints. And the lenses (50 and 100mm Focotar-2) stopped down twice. I hope I understood well your 'format size' is about the size of the negative. Regards, Michael

35mm negative to 24 x 30cm paper:

- f8
- 24x36mm
- image size on paper: 18,7 x 28cm
- 50mm enlarging lens

35mm negative to 50x60cm paper:

-f8
-24x36mm
-image size: 37 x 55,7cm
-50mm enlarging lens

------

6x7 negative to 24 x 30cm paper:

-f11
-negative size 56 x 69mm
-image size on paper: 21,4 x 27cm
-100mm enlarging lens

6x7 negative to 50x60cm paper:

-f11
-negative size 56 x 69mm
-Image size on paper: 42,8 x 54cm
-100mm enlarging lens

 

[Ian C]

For accuracy, we should divide the width of the actual projection (whether the projection overlaps the paper or not) by the corresponding width of the negative seen by the lens to determine magnification. By "width", I refer to the minor dimension of the rectangle as recorded on the film and of the projection, as we must raise the head to size the projection to cover the paper.

Q1: f8, 24x36mm, image size on paper: 18,7 x 28cm, 50mm enlarging lens

I’ll assume that you’re using a metal carrier that masks the periphery of the format rectangle on the negative to a width of 23 mm. I’ll also assume that the corresponding image width you gave is the actual width of the projection. Even if these dimensions are actually somewhat different, the result will still be approximately the same.

So, for a 23 mm width of a 35 mm negative projected to a width of 187 mm, the magnification is

m = 187 mm/23 mm = 8.13X.

Image DOF(N. c. 1/m, f) = Image DOF(8, 0.029 mm, 1/8.13, 50 mm) = 34.5 mm (about print)

Object DOF(N, mc, m, f) = Object DOF(8, 8.13*0.029, 8.13, 50) = 0.52 mm (about negative)


Q2: f8, 24x36mm, image size: 37 x 55,7cm, 50mm enlarging lens

m = 370 mm/23mm = 16.1X.

Image DOF(N. c. 1/m, f) = Image DOF(8, 0.029 mm, 1/16.1, 50 mm) = 128.5 mm (about print)

Object DOF(N, mc, m, f) = Object DOF(8, 16.1*0.029, 16.1, 50) = 0.49 mm (about negative)


Q3: f11, negative size 56 x 69mm, image size on paper: 21,4 x 27cm, 100mm enlarging lens

I’ll assume a metal carrier masks the 56 mm negative width to 55 mm.

m = 214 mm/55mm = 3.9X

I’ll assume a COC of 0.059 mm for the 6 x 7 cm format. This can be chosen somewhat larger or smaller, depending on your requirement.

Image DOF(N. c. 1/m, f) = Image DOF(11, 0.059 mm, 1/3.9, 100 mm) = 24.8mm (about print)

Object DOF(N, mc, m, f) = Object DOF(11, 3.9*0.059, 3.9 100) = 1.63 mm (about negative)


Q4: f11, negative size 56 x 69mm, Image size on paper: 42,8 x 54cm, 100mm enlarging lens

m = 428 mm/55 mm = 7.8X

Image DOF(N. c. 1/m, f) = Image DOF(11, 0.059 mm, 1/7.8, 100 mm) = 89.3 mm (about print)

Object DOF(N, mc, m, f) = Object DOF(11, 7.8*0.059, 7.8, 100) = 1.46 mm (about negative)


Note: Had you used an 80 mm lens to enlarge the 6 x 7 cm negative at the SAME MAGNIFICATION, you would have gotten the same DOF.

 

[Hilo]

I don't think it changes the parameters much, but the negative is projected on the paper a tiny bit larger than the image sizes I gave. And then masked off to the precise sizes I gave using an easel. That results in white borders all around the image.

I need to sit down and understand the outcome of the various options.

I appreciate your input a lot!

 

[Hilo]

I am specifically interested in the depth of field around the surface of the paper. Since I focus on a sheet of paper which is the same as the paper I use to print, I asume the DOF will have the emulsion side of the paper in the middle of the range of this DOF. Is that correct?

When I understood this correctly, we then look at my Q1 and your outcome, concentrating only on the DOF by the paper on the easel. Which is 34,5 mm. This means roughly 17 mm above the surface of the paper and as well roughly 17mm under the surface of the paper.

It is important to understand I got this right, before I look at the outcome of the other questions and their calculations.

(here Ian's calculation that I refer to in this single post)
So, for a 23 mm width of a 35 mm negative projected to a width of 187 mm, the magnification is

m = 187 mm/23 mm = 8.13X.

Image DOF(N. c. 1/m, f) = Image DOF(8, 0.029 mm, 1/8.13, 50 mm) = 34.5 mm (about print)

Object DOF(N, mc, m, f) = Object DOF(8, 8.13*0.029, 8.13, 50) = 0.52 mm (about negative)

 

[Ian C]

Yes. You understand correctly. The object and image planes lie at approximately the center of their respective fields.

You can see this in the second diagram in the Wordpress article from post #5.

https://physicsandphotography.wordpress.com/2014/10/02/depth-of-field/

The red line to the far right represents the image at the emulsion surface of the paper. The blue line is the near limit of DOF above the paper. The green line is the far limit of DOF below the paper.

The red line at the right side is the object plane at the emulsion of the negative. The blue line represents the upper limit of DOF above the negative. The green line is the lower limit of DOF below the negative.

 

[ic-racer]

The diagram Ian C linked above to is actually a good diagram for the relations in photographic enlarging, as the field depth is about equally distributed on either side of the point of sharp focus on both sides of the lens at the distances usually encountered in the darkroom.

When diagramming this concept for a camera, in which the subject gets closer to infinity, the depth at the subject side extends farther behind the point of sharp focus as seen below:

 

[Ian C]

IC: Thank you for the diagrams. That makes the discussion much easier to understand and visualize.


For the example at the bottom of post #14 we get

Negative Plane:

Rounded to the nearest 0.01 mm, the DOF about the negative extends from 0.26 mm above the plane of the emulsion and extends to 0.26 mm below.


Image Plane:

The DOF about the print begins at 16.60 mm above and extends to 17.90 mm below the top surface of the paper.

 

[Hilo]

Thank you Ian, for your patient and precise explanations and thanks as well to ic-racer for the diagram!

I have a condition called Dyscalculia, which in my case basically means that numbers throw me off, losing concentration very quickly. Over the years, when printing, I have asked myself this question but I never went much further. It was also not really neccesary because the grain in the corners of my prints was and is sharp. With the exception of some very old and very dense negatives that I must reprint now and then (five minutes exposures / 250 watt bulb).

I must still take more time to go through Ian's answers with the help of the diagram, but I already know that I have stayed well within the margins. I have also made sure the enlargers are without any problems.

 

[DREW WILEY]

Forget all the math. You don't need it. I have some exceptionally precise industrial-grade enlargers, including one completely designed by myself, and didn't spend 10 seconds worrying about that kind of thing. What you do need is a really good quality level with a machined edge, or set of levels, in order to get all you planes consistent. Then you need a good glass carrier preferably with two sheets of real Anti-Newton glass.

If you're only dealing with small negatives, from 35mm up to 6X7 cm format, you could seek out some old Gepe glass slide mounts with AN glass in them. Otherwise, some sort of precision neg carrier with real optical quality glass in it would constitute money much better spent than the next fancy camera lens.

Then get a good quality enlarging lens preferably of a little longer than "normal" focal length per format, and a serious quality grain magnifier too. That's nothing you want to skimp on.

 

[L Gebhardt]

I agree with Drew that in practice all the math isn’t needed if you have everything setup correctly. But it is good to understand it.

I will add that I find a diy laser alignment device and mirror is much cheaper than a good machinist’s level, and probably quicker especially for a quick confirmation that nothing has shifted before printing. Someday I’ll spring for a Starrett and test the two methods.

 

[DREW WILEY]

Well, when it came to lasers as well as levels I amid I had a big advantage as a dealer. We were the largest Stabila dealer in the Western Hemisphere, and also stocked Starrett. I borrowed an expensive industrial laser from our own inventory, and was given some really serious levels as free samples. But you can get a pretty good little German-made Stabila torpedo level with an actually machined edge for maybe 50 dollars. Don't assume cheap hardware store or home center levels are consistently level. They aren't.

I went ultra-precise more for fun than is really necessary. I like making precise things. Even the big 3 by 4 foot vacuum easel on one of my units is precisely machined and pin-registered. My cost? Zero. I cannibalized it from a giant industrial process camera left behind in a commercial building demolition. Took the lenses too.

If you find the math fun too, by all means, indulge in it.

True machinists levels, like from Starrett's US mfg precision tool division, can be just too precise and fussy for work like this, since few enlarger surfaces are themselves truly machined flat enough. But if one had a Science Fair mentality as a kid, they could easily make a mirror collimator from components sold by Edmund Scientific. At one time, Salthill offered a simplified version of that concept specifically for enlarger alignment, along with easy instructions. I think it was called the Salthill Enlarger Alignment Kit or something like that. Once in awhile these turn up used; and in my opinion, they were the most practical thing out there.

 

[Michael Heller]

I dont think enlarger alignment is as critical as it is sometimes made up to be. It seems like it takes a whole lot of mis-alignment to make a difference.

 

[Philippe-Georges]

I have a test image from PENTAX (see below) printed and reproduced it then on a graphical film 4"x5" and 6x6cm (Hass. back on a LF camera), with a Schneider G-Claron lens, and print it with the enlarger (D3) to check parallelism and lens/aperture sharpness.
Then I, if really needed, tune the enlarger's parallelism with the Hasselblad reproduction mirror system mounted on the enlarger.

A Siemensstern is verry picky...

 

[DREW WILEY]

Michael - depends one one's expectations. It takes relatively little misalignment to turn an otherwise impressive print into an amateurish-looking one. Yeah, you can get down the road on an unbalanced set of tires too, or even on a flat tire, but how efficiently?

And it's not like you have to align an enlarger over and over again. A little extra effort up front is a wise investment of your time and effort in the long run.

 

[ic-racer]

It seems like it takes a whole lot of mis-alignment to make a difference.

True on the baseboard side, but not true on the negative side. Camera and enlarger physics are the same. One would not expect a sharp image from a Hasselblad that has the negative held 1mm askew in the film gate. Likewise there is no need for a laser to align a correctly functioning Hasselblad to photograph artwork in a museum.