THE THEORY BEHIND THE USE OF CONDENSER LENSES.

 A “condenser” is a lens just as the lenses you use on your camera, a very simple lens but never the less a lens. 

Any type lens can be used in a condenser system. These are the most commonly used lens types used for condenser heads.
 


#1, PLANO CONVEX

#2, UN SYMMETRICAL DOUBLE CONVEX

#3, SYMMETRICAL DOUBLE CONVEX

#4, NEGATIVE MENISCUS, thin center and
      thick edge.

#5, POSITIVE MENISCUS, thick center and
      thin edge.

 

All Durst condensers are PLANO-CONVEX,  Type 1. 

The DeVere system is using both  PLANO CONVEX and POSITIVE MENISCUS, Type 1 and type 5. 

If parallel light rays from a distant light source, ex. the sun, is projected through a lens it will create an image of the light source behind the lens.
 

You have probably seen this effect when using a magnifier glass to start a campfire. The bright spot created by the magnifier is in fact the image of the sun that is focused as a small bright hot spot. 

In a condenser system this property is used in reverse. The lens is used to turn the globe of light emitted by a lamp filament in to a bundle of Collimated light rays. The lamp is placed at the point of focus and is projecting its light through the lens from the flat side of a PLANO CONVEX lens. 

When you focused the image of the Sun with the magnifier you may not have noticed that the image of the sun was not entirely sharp. Looking closely would reveal that it had a “halo” around it. The halo is due to Spherical Aberration. All lenses are born with a series of defects in relation to focusing an image sharply. Condenser lenses and magnifiers are very simple lenses and tend to have quite a lot of spherical aberration as their worst fault.  

Spherical aberration is a lens fault causing the light rays passing through it to focus in two different planes. 

A “condenser” is a lens-element just like the lens-elements used in any taking or enlarging lens. However it is most often a cheaply made lens and of lower quality, than a lens element used in a camera or enlarging lens. In camera and enlarging lenses Spherical aberration is minimized and controlled by using different types of glass and counter acting with several lenses of different shape and type. 

When designing an advanced condenser system the Spherical Aberration is controlled in a different manner. By placing the lamp ¼  Focal Length (FL) inside focus, which is away from the point of focus in the direction of the condenser lens, it is possible to obtain almost true Collimation with one condenser lens.  

Using two condensers in a pair further reduce the spherical aberration to 25% of the aberration created by one lens.
One would naturally think that the light would spread in a wide fan as the size of the light cone is increased. The reason this is not happening is that the light rays now has to battle the S.A and therefore is turned into a bundle of almost parallel light rays. 

Already now you can start to see the relevance of Durst using two PLANO CONVEX condenser lenses in a pair and of supplying a way of adjusting the distance from the lamp to the condenser lens. 

By using two Plano Convex lenses in a pair Collimation is increased because S.A is decreased. If we had not decreased S.A it would have caused a lot of light to spill to the sides and thus create flare and loss of light.
 

The two drawings above illustrate how S.A is controlled by moving the lamp. Looking at the drawings you may get the impression that moving the lamp will solve all problems because it is shown that the light beam are almost parallel.  

Unfortunately true Collimation is only possible with a true “point” source of light. A true point does not exist. If a point is visible it has to have a size.  The size of the point ( The light filament in a lamp with a clear cone or the size of the cone on an OPAL bulb.) creates a new problem we have to look at, in order to understand what makes a Condenser head function.

The light rays emitted from any given point on the lamp filament, or the cone of an OPAL lamp, is creating it’s own set of parallel light rays. 

This model shows how a “theoretic true point” would render the light rays.
 


The next model shows how a “theoretic true point” would render the light rays if it were placed off center in relation to the physical center of the condenser lens.
 


As you will see; the light rays created outside of the center of the lamp filament would still be a bundle of parallel rays however they would not all be seen by the next lens in the system because they are not parallel to the rays created in the center of the filament. They would instead bounce off the sides of the inside of the condenser house. That is why a condenser head and all photographic lenses are black inside – to reduce flare. 

Now – if you IMAGINE the filament as an infinitive amount of tiny points. Then you will understand; 

  1. That flare will exist even in the most perfectly designed condenser system because all lamp filaments have a size. And therefore there will be light rays leaving the primary bundle even though they are parallel.
  1. That the smaller the filament the more the final light cone will approach true Collimation. Because the smaller the filament is the smaller the amount of parallel light rays leaving the primary bundle.
  1. Why anti-reflex coating is helping in decreasing the flare level.

The optical engineers that designed the condenser systems worked with and used the fact that part of the light would be diverging. 

They used this property to create enough coverage for the negative in question with the shortest possible distance from lamp to negative. 

Let us look at how they did that:

The angle of divergence and the resulting size of the light cone can be calculated. 

Angle of Divergence     = S ∕ R Radians. 

Example:                   S = 4mm, R = 380mm and the filament needs to be projected 380mm (1 FL) to 
                                       be positioned in the aperture of the lens. 

Angle of Divergence     = 4 ∕  380 Radians = 0.01052631578 Radians 

Light spread                 = 380 x  0.01052631578 =       4 mm
                                       + diameter of lens          =   250 mm
Diameter of light beam at 380mm                     =   254 mm 

Now let us look at the actual condenser lens sitting right in front of the lamp in a Durst 10x10 Condenser head and let us assume that we use a DULAMP 1200-TOP which has a filament with a size of 15x15mm. The diameter of the filament would be approx. 21mm. And let us assume that Durst had placed the lamp in the correct spot as previously determined, which was ¼ inside focus towards the first condenser. 

S = 21mm, R = focal Length of LAZUCO 181 condenser = 250mm.

The diameter of LAZUCO is 181mm.

The distance from the lamp to the first of the two LACON 380 condensers is approx. 1100mm. The diameter of the LACON 380mm is 380mm 

Angle of Divergence     = 21 ∕  (250x 0.75) Radians = 0.112 Radians
Light spread                 = 1100 x  0.112                  =   123 mm
                                        + diameter of lens            =   181mm
Diameter of light beam at 1100mm                      =   304mm 

We would need the diameter of the light cone to be exactly 380mm to cover the entire surface of the LACON 380mm lens. The light cone is obviously to small to cover the entire condenser. The diameter of a 10x10” negative is 14.14” = 359mm. Assuming that the natural light fall off created by the enlarging lens is equal to the natural light fall off created by the shooting lens (which is rarely the case in practice) we would need the condenser to be almost totally covered. 

How did Durst fix that? 

They moved the lamp closer to the first condenser! (And therefore introduced a heat filter before the first condenser). 

Angle of Divergence     = 21 ∕  115 mm Radians       = 0.183 Radians
Light spread                 = 1100 x  0.183                   =   201mm
                                       + diameter of lens              =   181mm
Diameter of light beam at 1100mm                      =   382mm

If you try and re-calculate the last example with the distance being 125mm from the filament to the first condenser you will immediately see why accurate positioning of the lamp is so important when we talk about point light sources. 

You will also understand why the PULAMP with a diameter of the filament of less than 7mm will not cover 8x10” formats. And that the PULAMP can only be used for formats with a diameter not exceeding 248mm  =  max. 7x7” neg. 

Let us try to calculate the 10x10” situation using an OPAL lamp with a 4” cone.

Angle of Divergence     = 100  ∕  75 mm Radians = 1,3 Radians
Light spread                 = 1100 x  1,3               =   1466mm
                                       + diameter of lens      =     181mm
Diameter of light beam at 1100mm               =    1647mm 

With a light cone of that size there will be a lot of stray light / flare inside the condenser head. Also, you can use any f-stop even f5.6 and still have the entire aperture filled with light. 

You have in effect created a “projected diffused light” which is an intermediate between Specular light and diffused light.

The next issue we need to look at is the actual size of the filament that are being projected into the Nodal point of the lens = right at the aperture 

Focusing the filament right in the center of the enlarging lens has the purpose of maximizing the amount of light being output to the printing surface (photo paper or film.) 

Since enlarging lenses of different focal lengths have different sizes of the Iris Diaphragm (Aperture) the same lamp filament needs to be projected with different sizes depending on the lens in use.

Ex. The diameter of the aperture in a 300mm Schneider Componon is approx. 50mm at f5.6. The diameter of the aperture in a 360mm EL Nikkor is 65mm at f5.6 and a 150mm Rodagon has an aperture with a diameter of approx. 25mm at f5.6. 

The area of the aperture opening at f8, in any given lens, will be half of the area at f5.6. The area at f11 will be half that at f8 and so on.
 

 

                    f             f5.6                                f8                        f11

 


Most condenser systems are designed to work at a certain given f-stop when using a point light lamp. Most often this f-stop will be f11. When using an opal lamp you are free to use any f-stop on the lens.

 

 

When calculating the distances for the condenser system we will always calculate with distances that are LESS than one focal length. The distances from the lamp to condenser #1 and from the Virtual Filament to the center of the main condenser lens will per our previous discussion always be less than one focal length.  

Therefore we will have to use the following formulas: 

I1         =          (M-1) x EFL
O1       =          I1 / M
 
M         =          degree of enlargement     =        Image size / Object size.
EFL      =          effective focal length of lens in question. 

When calculating the distances for the Image system (negative to print) we will always calculate with a distance from the negative to the enlarging lens which is LARGER or equal to the focal length of the enlarging lens. Only when focusing at infinity (making a print where objects are life size) is the distance (bellows extension) from the negative to the Nodal point equal to the focal length of  the enlarging lens. 

Therefore we will have to use the following formula: 

I2         =          (M+1) x EFL
O2       =          I2 / M
M         =          Image size / Object size.
EFL     =          effective focal length of lens in question.

In both cases the overall length of the system may differ slightly from the value obtained by adding the Object distance to the Image distance.  

If you use a condenser system to make prints that are the same size or smaller than the negative a whole new set of  conditions will take effect and calculations made for those situations.  

In practicality you do not need to worry about all this theory. Durst has already provided a table showing all the necessary condenser and enlarging lens combinations. Refer to you enlarger manual for details. 

Calculating the EFL (Effective Focal Length) of a lens (set of condensers) is quite simple.
 

The Effective focal length, (EFL), of a set of identical lenses will always be equal to ½ focal length of one of the two lenses.

 

 

 

 

The effective focal length (EFL) of three or more lenses, in contact or nearly in contact, can be calculated with the following formula: 

Three lenses
1/f combined = 1/f1 + 1/f2 + 1/f3
1/f combined = 1/100 + 1/200 + 1/300 = 11/600
invert
f combined =  600/11 = 54.54mm effective focal length

 

When calculating the size of the projected lamp filament for an a-focal condenser system things grow a bit more complicated. 

In order to understand this model in relation to the other models shown it is very important to be aware that any of the two lenses (F1 and F2) can be a set of condenser lenses in contact or near contact. It can be a set of three or four condenser lenses for that matter. If that is the case then the EFL of this lens is first calculated with the previous formula. 

In this case M = F2 / F1  =  570 / 250  =  2.28X 

Therefore, if we used a DULAMP 1200-TOP with a 21mm filament we would produce a projected filament with a size of approx. 48mm in diameter. This would fit the 300mm Schneider Componon at f5.6. 

Now let us assume that the negative is positioned 70 mm under the last condenser lens which would position the negative approx. 500mm from the Nodal point of the enlarging lens. With a 300mm lens and a 500mm bellows extension we calculate the print size in the following manner. 

We know three parameters: 

Effective focal length of the enlarging lens is 300mm

The long side of our negative is 10” = 254mm (8x10”)

The bellows extension is 500mm 

Then we need to find M, which is the degree of enlargement. 

M         =          EFL / (O2 – EFL)
M         =          300 / (500 – 300)
M         =          300 / 200
M         =          1.5 X 

Thus we can print a 12x16” print from an 8x10” negative with this specific combination of lenses and still maintain all parameters optimal. 

The following is a drawing of a Durst 10x10” condenser head. The drawing is true to scale. The blue frames in the head indicate the two surface coated mirrors. The larger of the two mirrors can be tilted down to cut the light path short and concentrate the light on smaller formats.
 

With their two condenser heads – LACO 57 and LACO 1010  Durst has taken “Condenser-printing” to the highest possible level obtainable within reasonable means. 

The heads are extremely easy  and comfortable to work with and they are of very high mechanical and optical quality. 

The Durst system standard configuration is three condensers and two mirrors. The use of two mirrors makes it possible to change the distance from the lamp to the nodal point off the lens. The Durst system offers 9 condenser-lenses of different focal length accommodating almost any focal length lens on the market. 

The most usual condenser-lens is Plano convex. For each negative format Durst is using THREE Plano Convex lenses. With a few exceptions the convex side is facing the convex side when two condenser-lenses are paired.
When using two identical condenser lenses, as a pair in contact, you reduce the TOTAL focal length to half of the focal length on one condenser while maintaining the diameter of the “longer-focal-length” condenser lens.  This is a very important and largely overlooked aspect of constructing a condenser head.
Ex.  A Durst Condenser head is using a pair of two identical condenser lenses each with a focal length of 570mm for 8x10” and 10x10” printing. This focal length of the condenser lenses are necessary to make the diameter of the two lenses large enough to cover a 10x10” negative (254mm x 254mm). When pairing the two lenses the Effective Focal Length (EFL) is reduced to 285mm which is very important in order to fit the optical system inside a compact condenser head and to make it possible to make LARGE prints from 8x10” negatives. Please see the calculation on the previous page.

Had the focal length not been reduced the lamp would have had to be placed almost four feet away from the condenser lenses and the enlarging lens would have had to have been placed another almost four feet away from the condenser lenses. Thus making the whole system at least nine feet long.  

When designing a real effective condenser head the challenge is to be able to maintain the image of the lamp filament in the center of the lens while at the same time being able to focus the negative on the paper. Therefore only certain focal length enlarging lenses work correctly with certain combinations of condenser lenses. 

The Durst Condenser model 250 has a usable diameter of  250mm and a focal length of 375mm.

The Durst Condenser model 380 has a usable diameter of  380mm and a focal length of 570mm. 

Durst has further introduced a third condenser lens quite close to the lamp. The function of this lens is both to enlarge the size of the lamp behind it but also to increase the degree of parallelism (Collimation) of the light rays. Enlarging the lamp or the lamp filament is equal to creating a light cone large enough to cover the format in question as discussed earlier. 

One of the mirrors, the large mirror, can tilt from its top position to a 45 degree angle and thus reduce the distance from the first condensers to the two second condensers. This feature is used when the 10x10” head is used to print smaller formats, 5x7 and smaller.
 

 

Continue....to Article about Film Exposure and processing for Condenser & Diffused Light heads.


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