The PROLA Lens board for Durst L138SP, L184, L1840 and HL2501AF may seem like a trivial and uninteresting item. It is not. Actually it is a small revolution because of several factors:

It is a first!
It allows you to adjust the parallelism with the negative stage or the baseboard (*1) which in return will allow you to use your enlarging lens wide open.

Why that many photographers ask, and add that they always stop down the lens to find the sweet point.

And the answer is right there, stopping down the enlarging lens to find the sweet point is a compromise, thus the term sweet point.

The sweet point is not the optimal point for any lens.

The sweet point is a compromise.

Stopping down the enlarging lens have the purpose of finding the best average between, lens coverage, depth of field and sharpness.

ALL LENSES ARE SHARPEST FULL OPEN.

All lenses produce their sharpest results full open, no bull that is how it is, just study the most basic theory about lenses and you will see that using the aperture / diaphragm stopping down introduce a number of disadvantages from more diffraction to less sharpness.

With modern shooting lenses and modern enlarging lenses of matched focal lengths the fall off in the two lenses practically cancel each other. Therefore stopping down is no longer necessary to compensate for light fall off.

Now the PROLA makes it possible to align the lens stage to an accuracy of 1/1000 of one degree and thus making stopping down to get enough depth a filed a thing of the past, which in return give you:

We offer special PROLA lens adapters for very heavy lenses,

(*1) Normally the negative stage on an enlarger cannot be adjusted for alignment. You therefore adjust the baseboard to be aligned perfectly to the negative stage. This also guarantees 0% distortion and therefore a perfectly rectangular or square image.
Then after having aligned those two planes you adjust the PROLA to gat the lens 100% aligned with the baseboard as well, and viola you have a whole new precision enlarger.

Control over the sharp and unsharp parts of the image can be of considerable importance for the success of a photograph. Traditionally, the depth of field (DOF) is a great source of confusion among photographers. To blur or not to blur, that is the question. The latter is a matter of DOF, the former not per se.

Depth of field is defined as the range of object distances within which objects are imaged with acceptable sharpness. [1]

A whole series of definitions can be found at Don Fleming's site, where a casual glance suffices to conclude that they all amount to the same thing. That is a welcome observation, because it means that there is consensus of opinion regarding the definition of DOF.

The concept of DOF
Depth of field can be treated in a theoretical framework that I will refer to as a concept. This concept is very useful because it enables extensive calculations and comparisons within the relatively simple theory of Gaussian optics. For this purpose it is assumed that the lenses are free of aberrations and that diffraction is nonexistent. The film (or digital sensor) is supposed a grainless recorder of the finest details, so that the image can be freely enlarged without loss of definition. All this is not true, but as long as we restrict ourselves to 'normal' photographic scenarios, the concept can be equated to practice with some confidence. The theory is on a separate page to preserve readability, but I will occasionally refer to an equation.

Viewing conditions
There is a difference between calculated sharpness and perceived sharpness. For the perceived or apparent depth of field, the lighting conditions are important, as a well-illuminated print will more easily reveal imperfections than the same print in a dim room. Further, an observer with a high visual acuity may reject a print that satisfies another observer with poor vision. A very important factor is the viewing distance in relation to the size of the photograph. Obviously, a large print viewed from close is much more demanding than a small print viewed from a large distance. Within the concept of DOF it is usually assumed that the print viewing distance does not depend on the picture taking lens. There is a good reason for this, for most people do not change seats during a slide presentation and at an exposition they examine the various photographs from similar distances. The concept of DOF is adapted to these habits, since the DOF scales imprinted on lenses are based on a sharpness criterion that does not depend on the focal length. Likewise, a typical DOF calculator will suggest—or may even impose—a sharpness criterion based on the film format, independent of the lens focal length. Bear in mind, however, that many of the conclusions reached in the remainder of this article would be different if we had assumed 'perspectively correct' viewing, i.e. a viewing distance in proportion to the focal length [2].

Circle of confusion
To calculate the depth of field, one needs a sharpness criterion. This criterion is taken as the so-called circle of confusion (COC). A COC value corresponds to the blur spot diameter, measured on the film/sensor, of an unsharply imaged point in object space. In DOF calculations it is customary to use the designation COC for the largest permissible circle of confusion. The blur disk diameter is zero for points in the plane of sharp focus and progressively grows as we move forward or backward from this plane in object space. However, as long as the blur disk is smaller than the acceptable COC it is considered sufficiently sharp and part of the DOF range. The appropriate value for the permissible COC is influenced by a variety of factors, including film format, print size, viewing distance, etc. In practice however a fixed COC is usually considered for a given format. Lens manufacturers cannot know what the viewing habits of their customers are and adhere to a certain standard. For example, the DOF scales imprinted on lenses for the 35-mm format are based on COC values between 25 and 35 µm. A comparison of such scales might suggest that the former (C=25 µm) lens gives less depth of field than the latter (C=35 µm), but this is not true. It's just that the sharpness criteria are different. For critical work the existing COC standards are superseded anyway and field photographers are better off with printed DOF tables based on preferred COC values or with a simple rule of thumb to overrule the lens DOF scale. Indeed, photographers should evaluate their own applications and demands to arrive at a satisfactory sharpness criterion, if necessary by trial and error.

A circle?
The designation circle of confusion is widely used, but it is rarely correct in the strict sense of the word. For the blur spot will be a circle only in the image center, and that only for a lens used at full aperture. Towards the image periphery, at full aperture, the shape is more like a cat's eye. When a lens is stopped down the blur takes on the shape of the diaphragm opening. A lens with a six-sided diaphragm, for example, comes with a hexagon of confusion. Fortunately, the depth of field does not noticeably depend upon the shape of the aperture. Whether the aperture is triangular, hexagonal, octagonal, ... the depth of field will not really depart from that of a truly circular aperture at the same F-number, although one may expect an influence on the rendering of out-of-focus areas. The deviant mirror lens design, which is associated with doughnuts of confusion, however, requires special attention in DOF calculations.

Visual acuity
Under good viewing conditions, many observers can resolve objects of a size equal to 1/3000 of the viewing distance [3]. Born and Wolf, and many other sources for that matter, mention an angular resolution of one minute of arc for a normal human eye [4]. This translates to 1/3400 of the viewing distance. For a photograph viewed from a distance of 30 cm, the maximum permissible blur diameter on the print would therefore correspond to ≈ 0.1 mm. This is a small number that is at the basis of many disappointing DOF findings. For the 35-mm format, a 10×15 cm print viewed from 30 cm thus requires a permissible COC on the film of 20 µm. For a 20×30 cm print viewed from the same distance, it is a shivery 10 µm. If one relies on the lens DOF scales, which are based on a permissible COC of ~30 µm (for the 35-mm format) the 10×15 cm might just pass a casual look, but the 20×30 cm print will certainly fall short of expectation. When the viewing conditions deteriorate, for instance by dimming the illumination, the visual acuity drops and matters become less critical. The eye even becomes color blind at very low light levels.

DOF versus distance
Quite generally, the depth of field depends on the object distance, the focal length, the F-number, and on the perceptivity of the observer. Scrutiny requires a smaller permissible COC value than a casual glance. In the macro regime the lens symmetry also plays a role, but I will ignore this unless specifically mentioned. A scheme for the circle of confusion versus object distance is plotted in Fig. 1—for a given focal length and F-number. Colors are used as a third dimension to indicate the size of the blur disk. For the 35-mm format, a typical value for the acceptable COC is 30 µm. So, for this format the DOF range is bounded by the yellow color. Upon focussing on a subject at 10 meters, the DOF thus ranges from 6 m to 30 m. In other words, the front DOF amounts to 4 meters and the rear DOF to 20 meters. When the object approaches the camera, or vice versa, the DOF progressively decreases and it would become truly shallow in the macro regime.

Rogonar and Trinar 50mm and 75mm, Rogonar S 50,60,75,90 and 105mm, Rodagon 50, 60, 80, 105 and 135mm, Rodagon WA 40,60 and 80mm, APO Rodagon 50 and 90mm, APO Rodagon R 75mm,

El Nikkor 40, 50 63, 75, 80, 105, 135 and 150mm, Schneider Componon S 50, 80 and 100mm, Schnieder APO Componon HM 40, 45, 60 and 90mm, Schneider Componoar S 50, 80 and 105mm, Schneider Componon 28, 35 and 60mm, Schnieder WA Componon 40, 60 and 80mm.

Rogonar S 25 mm and 35mm, Rodagon 28 and 35mm

Rogonar and Trinar 50mm and 75mm, Rogonar S 50,60,75,90 and 105mm, Rodagon 50, 60, 80, 105 and 135mm, Rodagon WA 40,60 and 80mm, APO Rodagon 50 and 90mm, APO Rodagon R 75mm,

El Nikkor 40, 50 63, 75, 80, 105, 135 and 150mm, Schneider Componon S 50, 80 and 100mm, Schnieder APO Componon HM 40, 45, 60 and 90mm Schneider Componoar S 50, 80 and 105mm, Schneider Componon 28, 35 and 60mm, Schnieder WA Componon 40, 60 and 80mm.

Durst HOTUB 2500 recessed lens board.
100mm / 4.0" deep recess, the lens is fully accessible
Price: $880.00

USED $350.00, as long as stock last.

Durst LARATUB N recessed lens board.
40mm / 1.5" deep recess, the lens is fully accessible
Price: $465.40

Red filter assembly is not included.

USED $350.00, as long as stock last.

Durst UNIPLA lens board.
(Requires LAPLA lens adapters)

Price: NEW $278.00
USED $99.00

Durst TRIPLA lens 3-lens lens turret, w 50mm / 2" red filter.
(Requires LAPLA lens adapters)

USED $145.00

Red Filter assembly w/ 125mm / 5.0" red filter.
Price includes installation on PROLA, TRIPLA, HOTUB and LARATUB.

USED $234.00

Rogonar and Trinar 50mm and 75mm, Rogonar S 50,60,75,90 and 105mm, Rodagon 50, 60, 80, 105 and 135mm, Rodagon WA 40,60 and 80mm, APO Rodagon 50 and 90mm, APO Rodagon R 75mm,

El Nikkor 40, 50 63, 75, 80, 105, 135 and 150mm, Schneider Componon S 50, 80 and 100mm, Schnieder APO Componon HM 40, 45, 60 and 90mm

Schneider Componoar S 50, 80 and 105mm, Schneider Componon 28, 35 and 60mm, Schnieder WA Componon 40, 60 and 80mm.

39mm x 1/26"

$80.00

LATUB N .

Rogonar and Trinar 50mm and 75mm, Rogonar S 50,60,75,90 and 105mm, Rodagon 50, 60, 80, 105 and 135mm, Rodagon WA 40,60 and 80mm, APO Rodagon 50 and 90mm, APO Rodagon R 75mm,

El Nikkor 40, 50 63, 75, 80, 105, 135 and 150mm, Schneider Componon S 50, 80 and 100mm, Schnieder APO Componon HM 40, 45, 60 and 90mm, Schneider Componoar S 50, 80 and 105mm, Schneider Componon 28, 35 and 60mm, Schnieder WA Componon 40, 60 and 80mm.

39mm x 1/26"

Rogonar and Trinar 50mm and 75mm, Rogonar S 50,60,75,90 and 105mm, Rodagon 50, 60, 80, 105 and 135mm, Rodagon WA 40,60 and 80mm, APO Rodagon 50 and 90mm, APO Rodagon R 75mm,

El Nikkor 40, 50 63, 75, 80, 105, 135 and 150mm, Schneider Componon S 50, 80 and 100mm, Schnieder APO Componon HM 40, 45, 60 and 90mm, Schneider Componoar S 50, 80 and 105mm, Schneider Componon 28, 35 and 60mm, Schnieder WA Componon 40, 60 and 80mm.

39mm x 1/26"

Rogonar and Trinar 50mm and 75mm, Rogonar S 50,60,75,90 and 105mm, Rodagon 50, 60, 80, 105 and 135mm, Rodagon WA 40,60 and 80mm, APO Rodagon 50 and 90mm, APO Rodagon R 75mm,

El Nikkor 40, 50 63, 75, 80, 105, 135 and 150mm, Schneider Componon S 50, 80 and 100mm, Schnieder APO Componon HM 40, 45, 60 and 90mm, Schneider Componoar S 50, 80 and 105mm, Schneider Componon 28, 35 and 60mm, Schnieder WA Componon 40, 60 and 80mm.

39mm x 1/26"

Rogonar and Trinar 50mm and 75mm, Rogonar S 50,60,75,90 and 105mm, Rodagon 50, 60, 80, 105 and 135mm, Rodagon WA 40,60 and 80mm, APO Rodagon 50 and 90mm, APO Rodagon R 75mm,

El Nikkor 40, 50 63, 75, 80, 105, 135 and 150mm, Schneider Componon S 50, 80 and 100mm, Schnieder APO Componon HM 40, 45, 60 and 90mm Schneider Componoar S 50, 80 and 105mm, Schneider Componon 28, 35 and 60mm, Schnieder WA Componon 40, 60 and 80mm.

39mm x 1/26"

Rodagon 240 and 300mm, Rodagon 210 and 240mm, EL Nikkor 180 and 210mm, and other lenses with a rear diameter not exceeding 80mm.

Componon S 300mm and other lenses with a rear diameter not exceeding 80mm.