Filtration for photographers by Andy Cross

Ever since the introduction of the true panchromatic B&W films, of the types we are so familiar with today, photographers have had a great deal of control over the tonal distribution and tonal separation of the image. This aspect of the image making process is controlled largely through the use of filtration attached to the camera lens. However there has been almost as much mis-information put into print on the subject as there has been accurate. It is the intention of this article to sort out the myths from fact and enable the photographer to use filtration effectively to produce predictable results.

Before I continue I should elaborate on what true panchromatic B&W films are. There are really three types of film used in photography. The blue sensitive, the ortho and the panchromatic types. As the name suggests the blue sensitive films are only sensitive to the blue, indigo, violet and ultraviolet wavelengths of light. These are more likely to be found as special darkroom films and not meant to be used in a camera. They can be handled under a variety of safelight conditions that are absent of blue light.

The orthochromatic types are sensitive to blue, green and sometimes yellow light. Ilford ortho film in roll or sheets are still available for both pictorial and darkroom use. These can be handled under deep red safe lighting.  Finally the Panchromatic films which includes colour films are sensitive to the full visible electromagnetic spectrum. These films must be handled in complete darkness.  

The use of filters on the camera lens most often used in B&W photography are much more extreme in colour and density than those used in colour photography. What they actually do has often been misconstrued. I have seen them called contrast filters, local contrast filters, Gamma increasing, flare reducing and toe cutter. None of these terms is correct. They are used to increase or decrease the tonal separation between subjects of different colours. In order to define what is meant by this I should explain what all the other terms used in photography are first in order to reduce the confusion.

Referring to fig 1 This is a typical characteristic curve sometimes called a tone curve. They are usually displayed on a graph which consists of a horizontal and vertical axis. The horizontal is often referred to as the X axis while the vertical is called the Y axis. In this example the X axis indicates the relative exposure the film received. The Y axis how dense the film became after processing. We need a common language to connect the two. Here the calibration language is in log units. The X axis represents log units of exposure the vertical log units of density or opacity. 

The magic number here is 0.3 log units. For every .3 log units of exposure the film receives, equates to a doubling or halving of the previous exposure. In other words a one stop increase or decrease in the amount of light that struck the film. For every .3 log units of density or opacity the vertical axis records the film is twice as dense or half as dense as the previous plot. This represents a one stop alteration in density. 

The curve itself has several points on it that have specific names. At the bottom of the curve there is the toe. This represents the point of where the film has just received enough light to begin recording some density over and above the film base plus fog value. Sometimes referred to as the speed point. From there begins the straight-line portion. Each increase in exposure relates to an increase in opacity but it is not linear. It’s on this portion of the curve all the tones of grey fall. At the top of the curve there is the shoulder. This is where development stops which is where the stop bath was introduced. Unlike papers, films are rarely developed to completion. Development is stopped at a point that prevents the highlights becoming so dense they become unprintable. 

When the exposure is altered the curve shape doesn’t change very much but is displaced to the right and left of the graph. Fig #1 Underexposure moves the curve to the right while overexposure moves it to the left. However when the development is altered gamma is altered. Gamma isn’t equal to contrast but is proportional to it. Increasing the degree of development increases the gamma of development while reducing the degree of development lowers it.

The gamma can be calculated by projecting the straight-line portion of the graph to intersect the exposure axis. When this is done an angle is formed. If you were to measure this angle and the Tan value of this was found the resulting number is expressed as G Bar or gamma. In most cases it will be a number less than one.

The mathematics of a triangle can be expressed as Sin , Cos and Tan. The Tan of the angle is used because it is the opposite angle over the adjacent angle. Another way of looking at it is to divide the number of exposure steps into the number of density steps. By altering the exposure in combination with the degree of development a characteristic curve of any shape can be created in a film to suit virtually any method of reproduction.

That’s about all the information required for photographers to make use of the tone curve information manufacturers provide in the film and developer boxes and on their web sites. However there is one thing for certain, attaching a filter to the camera lens will not alter the gamma of development or the shape of the curve in any way. It can only alter the tonal separation. 

It will be easier to show an example of increased and decreased tonal separation that describe it. See fig #3. This is a 21 step step wedge. It contains 21 steps of increasing density from FB plus fog in increments of 0.1 log units of opacity to 2.1 

It was placed in the enlargers negative carrier and printed as if it were a negative. One print was made on grade one paper the other on grade four. Although the paper is marketed as being variable contrast it isn’t. It is actually variable tonal scale paper.

The grade one print has a longer tonal scale than Fig #4 which is the grade four print. A longer tonal scale simply means it has a greater number of tones of grey between D-Min and D-Max. By default the image with the longer tonal scale has less tonal separation or less distinction between steps. The grade four print has a shorter tonal scale and greater tonal separation or greater distinction between each step. Neither print now represents the tonal separation of the original.

Variable grade paper is a photographers second chance in obtaining the tonal separation required to make a photograph. It is far less effective than printing from a negative that was made with the necessary tonal separation.

Maintaining the tonal separation is important if you are to separate subjects that may record as a similar tone of grey to one another even though they are different colours. Before a filter can be reliably used a speed point for that film – filter combination is usually required. This is because not all films respond to the same frequency of light in the same manner. The filter manufacturers assign a filter factor which is really just a recommended starting point for the required increase in exposure. Acquiring a film speed point for each filter is recommended to avoid underexposure. 

Armed with this information the learning process can begin in determining which filter or combination of filters will create the negative with the tonal separation you want in the final print. This can take some time and many exposures, but there doesn’t seem to be a better way of obtaining this knowledge. This will become more of an issue if a photographer often changes the type of film they are shooting. Over the last twenty five years I have remained with FP-4 for the slow speed film and Tri-X for my faster speed film.

There are however various methods you can use to allow you to previsualize the end result. The first is to record the image in colour by exposing a color transparency film or use digital capture. Various coloured filters can then be used to make a series of exposures on B&W film. The results on the light box or monitor can be compared to the proof sheets made from the negatives. 

Looking at the scene through each filter and observing how the tonalities of the subject’s change is another. In order for this to be effective the photographer has to try and ignore the colour of the subject and that of the filter. Moving the filters in front of your eyes and back again quickly is my preferred method of making these determinations. Printing out a colour patch chart and greyscale like those made by McBeth and regularly looking at it under various types of lighting through the filters is another good way of tuning yourself to how films will respond.

 Using a B&W viewing filter such as a wratten #90 has also been suggested. In my opinion this is a waste of time. Looking through this filter is supposed to allow the human eye to see the colours of the world the same as a typical B&W film. 

Each film is unique in its response to colours. It is also a very dark amber coloured filter and a person will have to exclude all extraneous light from entering their eye whilst looking through the filter. Furthermore, you will have to allow at least five to six minutes to gain what is effectively your night vision to fully appreciate the effect the filter creates. Human beings see the world in three dimensions and in colour.  It takes some time to see the world in two dimensions and in tones of grey. 

In the digital age post processing applications like photoshop and various other programs can be a real aide in learning this process. Open any image in Photoshop and click on the channels palette. In there you will find the composite RGB image and three B&W images called channels. By clicking on each one in turn you will be able to see how that subject theoretically would have been rendered, if a separation filter of that colour was attached to camera lens,  when a typical B&W panchromatic film was being exposed. By using other applications like calculations or channel mixer other filter combinations can be previewed. 

The filters I have in my kit are two densities of red, green and blue. As well as a deep yellow, orange, magenta and cyan. A couple of polarizers and three densities of neutral density as well as a set of red, green and blue notch, sometimes called enhancing filters round out the kit. 

There are two types of filters used in photography those of the absorption type and the dichroic type. The absorption filters get their name because they absorb light of particular wavelengths and transmit the remainder. Most glass filters have inorganic compounds usually metal oxides dispersed through the glass whilst it is molten. A deep red glass filter looks the colour it is because it absorbs large amounts of the green and blue wavelengths from white light and only transmits the red wavelengths. 

A red filter of the dichroic type reflects or deflects large amounts of green and blue light and transmits the red wavelengths. This is achieved by coating the glass with metallic coatings in the correct order the right number of wavelengths thick. 

Photographers rarely attach dichroic filters to their camera lenses because there are two components at work , when combined, allow the filter to transmit certain wavelengths. The angle of incidence is the other variable. In other words the angle the light strikes the filter from will alter the wavelengths it transmits. Not very useful for a photographer who has a red filter when the angle of incidence is at 45 degrees to the lens axis and purple when the light is coming from 90 degrees. Consequently dichroic filters are reserved from instrumentation like densitometers and the filters in enlargers. 

A rule of thumb when using filters is that a filter will lighten a subject that is the same colour or has a large component of a similar colour to that of the filter and darken the others. When it comes to subjects that are reflecting the subtractive primary colours i.e. yellow, magenta and cyan photographers have to think in terms of additive colour synthesis. For example what two colours of equal intensity would synthesize the colour yellow. The answer is a combination of red and green light. A red filter would absorb the green component from the yellow but lighten the red component. The overall change in tonality would be minimal. As I mentioned earlier looking at the subject through the filters is a good way of learning the films response to these changes.

The next filter I should explain is the polarizing filter. Photographers who use mainly digital capture to record their images need not concern themselves with the use of filters. Anything that can be done using them can be just as easily done with software. Except for the effects of the polarizing filter. The subject of polarization is a complex one which goes way beyond an article such as this.

Contrary to popular belief there is only one type of polarizing material made and that is of the linear type. There is no such thing as a circular polarizing filter. To make a polarizing filter a thermosetting plastic is doped with organic materials known as anisotropic compounds which are bi-refringent. This creates a filter that has an axis which looks like a grating illustrated in Fig #5.

They work by blocking polarized light trying to pass through them. For light to be polarized it must be vibrating in one plane only. In effect for a polarizing filter to work the light being reflected from the subject must be polarized. Light can become polarized in nature in various ways. Polished surfaces like glass, plastic polished wood etc often reflect light polarized. This is because to polish a surface like glass you actually polarize it. Placing thousands of minute scratches all over the surface all of which are the same depth, width and running ibn the same direction polarizes the surface and makes it transparent.

A polarizing filter will now be able to absorb the light being reflected from the surface when the filters axis is rotated to be at 90 degrees to the plane of polarized light. You can’t polish water and it too can reflect light in a polarized fashion. Here gravity works on the waters surface tension stretching it out and aligning all the surface molecules. It now reflects light in a polarized manner. Break the surface tension with wind or ripples and this effect is lost.

Polarized light can be transmitted from the sky even though the air is in constant motion. In this case polarization occurs by atmospheric scattering. Light in this case becomes polarized more as it approaches Bruisters angle which is usually at 90 degrees to the light source. The more the lens axis is moved away from bruisters angle the less effect the filter has.

Next comes the issue of the circular polarizing filter. To make one of these a linear polarizing filter is made first. A piece of linear polarizing film is laminated between two thin pieces of crown glass. To one side of the filter a quarter wave plate is attached. This is a dichroic type of coating which is applied to the glass one quarter of one wavelength thick. I won’t go into the details of what happens here but the interference patterns it creates bends the wave fronts back on themselves a bit like a snake eating its tail.

A plate is a plate not a filter but they inappropriately market them as circular polarizing filters. These filters are directional in that the plate always has to face the camera lens. Turning it around means it will not work as a polarizing filter just a neutral density filter. They are necessary for some camera auto focusing systems to work correctly. 

Photographers often make a mistake when it comes to calculating the required increase in exposure when attaching a polarizing filter. These filters have a neutral density component which is around one to one and a half stops. When it is blocking polarized light it my double this amount. 

You only need to increase the exposure by the neutral density component. If the photographer doesn’t want to darken the scene that much then all that is required is to back the filter off by rotating it. Increasing the exposure beyond the ND component will only result in overexposing areas of the scene that weren’t reflecting polarized light.

Neutral density filters are another handy tool to have in your kit. As the name suggests these filters are neutral grey in colour and only serve to block light of every wavelength equally. When using fast films or opting for shallow depth of field with a wide open lens aperture these filters will allow you to use slower shutter speeds allowing motion blur and other time related effects to come into play. 

There are the graduated types of ND filters available as well as coloured with varying densities and graduation transitions. I avoid using them because the results look fake unless you have a clean unobstructed horizon. The graduation obviously can’t navigate its way around obstacles protruding above the horizon. I have found them more useful in making smooth transitions when combined with burning in operations in the darkroom.

The last type of filter I will cover is a type that isn’t usually found in a photographer’s kit. The correct optical name for them is the notch filter but are often just referred to as POP or Enhancing filters. These filters are absorption filters made by doping glass with rare earth elements. Because of the way they are made they are only available in glass. 

The manner in which they work is complex. In the simplest terms they absorb a very narrow band width of frequencies of light in the region of the spectrum they are supposed to enhance. When the necessary exposure compensation is used the remaining colours in that region become lighter and appear more saturated. Until 2005 the only enhancing filter you could get was the red enhancer. The first company to introduce the same for the green and blue wavelengths was Marumi in Japan. Since then Hoya and others have also brought out these filters.

There were two versions produced by Marumi. The enhancer and the enhancer light. The first version although they can be used with colour films was primarily intended for use in B&W. Shooting with colour film required a colour correcting filter pack to be used in combination with the filters. The light versions were less severe in their effect but were suitable for use with colour film without any correction.

The advantage with these filters in B&W photography is that you can lighten one hue without the expense of darkening the other colours. I found that increasing the exposure for the filter factor, in other words exposing normally, and giving a slight push in processing of about half a stop was the most effective way of using these filters.

In a final note about filters most of them can be obtained in either multicoated and un-coated forms. The reason multi-coatings are used is to reduce the flare produced within the lens at each glass to air interface. Flare is non image forming light which reduces contrast and reduces the effective sharpness. Whenever possible always use a coated filter.

Looking back at my negative files and reading the data I have recorded on most of them I realise a filter of some description was in place on the lens in approximately 70 percent of them. My perspective is that they are almost as essential as the lens they are attached to when making images that have the required tonal separation.

Andy Cross has a BSc in optical engineering  and a Ba in photography with 26 years’ experience in optics and 40 years in photography. I am a proof reader for two photography book publishers. Am one of the few remaining photographers in the world still making dye transfer prints and tri-colour carbon prints. When shooting for my own art work I prefer large format film. I have  integrated digital with analogue with the use of  film recorders. Does image restoration and replication , runs workshops in both digital and analogue, hires out darkroom etc. I have works in many fine art museums private and public collections throughout the world.