• slide frame format 41x101 (RBT) or 50x50 (2"x2") compatible.
• lens diameter .... mm.
• plastic or glass lenses.
• enlargement 6x.
• fixed focus or adjustable focus.
• price.....

These specifications are good enough to get a rough impression with what kind of viewer you are dealing with. But you didn't take a look through it!!
This web page could guide you where to pay attention to when selecting a viewer. We tried to be objective and critical, even to our own viewers.

• Enlargement and lens diameter issues.
• Sharpness
• Image distortion
• Type of lens and coating.
• Lens separation
• Light diffuser at the rear side.
• Light leakage and reflections inside the viewer
• slide frame holder
• viewer body material and lens barrels.
• light attachment issues.

When you look through a viewer, whether you are wearing glasses or not, get as close as you can to the lenses and view the image. The circles in the image are standing for the lens barrels or lens edge circle. These are in most cases limiting the 'panoramic' view. You should try to keep the viewer straight horizontal, on the same level as your eyes. In this ideal situation you should see the image in the middle of the 'circle', square, sharp in totally black area.

The more shorter the focal distance of the lens is, the more enlargement you get. The enlargement is theoretically calculated by: 250 divided by the focal distance in millimeters. A lot of enlargement gives you the impression of an enormous panoramic view. But is the viewerbody able to handle this enlargement......

Assuming that the ideal situation lens is not giving any image distortion with this enlargement, other problems might arise. Assuming that the lens barrel allows individual focusing, the lens barrel itself causes the first big limitation. Especially cheap viewers are having small diameter lenses to save costs, causing small barrels at the front. In most cases the barrel will get a cone shape; small in the front, wide at the back (near the slide). When wearing glasses your eyes are not as close to the lenses then without glasses. The small entrance of the barrel will optically cut off the corners of the image. Now you are only able to view small images without cutting corners. You shouldn't need to move your head to see all corners! Sometimes it is the other way around; the lenses are big enough but the viewer body does not allow a large exit (near the slide) of the lens barrels, causing cut off corners. It is sometimes a pity that some lenses don't fit with viewer bodies. The body of a viewer should give a good lens enough space to let it come to it's full right.
A bad example is for instance the Realist 'red-button' viewer. The enlargement is to big for the size of lens and viewer..

This is one of the most important issues of the viewer. The image should be sharp everywhere. Unfortunately this requirement is not always reached in cheap viewers with short focal distances (shorter then 45 mm.)

It depends for what application you want to use the viewer; private use or public use (museum). You should imagine that the quality of eyes of the public is varying a lot. The rule is; if you want to have 95 % of the people being able to view the image sharp, you should go for 'long' focal distances (± 60 mm and more.) having a lot depth of field. Unfortunately, the enlargement is not much (4.2 and lower). But what is more important.... Tip: do not apply a viewer with adjustable focus for public use!

You can apply shorter focal distances (shorter than 60 mm.) for private use. In this case the focusing should be adjustable. Most 'private' viewers are having individual lens focusing that enables the spectator to adjust the sharpness for each eye individually. Sometimes you see viewers fit out with a 'fast sharpening' feature; a lever at the side moving the whole slide holder to- or from the lenses. You often see this facility with private medium format viewers in combination with individual adjustment. When having a long depth of field, it just takes to long with turning the individual lens barrels to see any difference in sharpness. Your eyes are also adapting their focus point during the slow adjustment. So this lever correction has to be 'faster' then your eyes can compensate, resulting in a good sharp position without feeling that your eyes are doing all the focusing work.

The cause of image distortion is in most cases the lens itself and in some cases the overall design of the viewer. Besides sharpness distortion there are two other disturbances which cannot be adjusted at the viewer but has something to do with the design of the it.

The so-called cushion effect is related to the quality of the lens. There are two types of lenses most used in slide viewers; meniscus (plan-convex) lenses (one glass element) and achromatic lenses (two glass elements glued together). A too strong enlarging meniscus lens easily causes cushion effects. Achromatic lenses are better in compensating the image distortion but does also have it's limitations. Meniscus lenses are rather cheap, especially when they are made of plastic. Cheap viewers often have meniscus lenses and do not enlarge that much for distortion reasons. An overall rule here is; the more you enlarge, the more cushion effect is introduced. Every lens has a bit cushion effect. Very rare is the solution of using a triplet anastichmat (three lens elements) that is able to turn the distortion down to a very low level.

Spherical aberration is a form of distortion that sometimes occur when using meniscus lenses. You see a small band of colors of the rainbow close to borders of the picture and strong black-white contrast areas. You see it more often when using plastic lenses and molded glass lenses which are poorly grind.

The choice to be made what type of lens the viewer should have depends on:

• how much enlargement do you require.
• how much distortion do you allow.
• where will the viewer be used; private or public.
• how much money do you want to spend on this.

There are two types of lenses most used in slide viewers:

The Meniscus is made in plastic as well as glass and is the 'cheap' solution. But don't get this wrong: Meniscus lenses with a average enlargement of 4 x, made of glass and viewing 23x21 mm. slides are doing fine! Every lens has its limit of acceptable image presentation.

The achromatic lens allows more enlargement without a lot of distortion. They are in most cases about the double price compared with the glass meniscus lenses. The slightly curved surface at the left side allows the spectator more miss-alignment of the eyes with regards to the center of the lens. Therefore they are suitable for public environments; fixed focus and fixed lens separation.

Technically it is not possible to make one single achromatic lens having a focal distance shorter than 50 mm. (enlargement 5x) and diameter larger then 30 mm. without a lot of distortion. For that reason, short focal distance achromatics are having diameters smaller than 30 mm. Or you have to make combinations of lenses like we did with the Combi viewer; Two achromatic lenses in one lens barrel, making a focal distance of 50 mm. and 33 mm. diameter. The distortion is still between the boundaries of acceptance

The only problem is that the one of the biggest expenses of the viewer are now the lenses. But that is the most important viewer part......

Coating is sometimes seen as luxury on a viewers lenses. But there is a good reason. In most cases is the spectator located in a illuminated area with white walls. Inside the viewer it is dark. Nothing is more irritating then seeing your own eye reflected in the front lens and parts of your eye lash. You often see people trying to cover the head and lenses to eliminate the area light. These reflections are gone for about 75% when having a good coating. When the viewer has more lens elements, be sure that they all have a coated surface.

Lens separation is a relative expensive feature for a viewer; it requires more moving parts. For lens separations counts: enlargements stronger than 50 mm. and diameter 25 mm. or smaller, require adjustable separation. Of coarse, when only one person is making use of the viewer and the separation is fixed, the configuration could be just right for her/him without adjustments.

When a viewer designer chooses not to allow lens separation adjustment, he has to be sure that the lens diameter is at least 30 mm. The spectator will get problems with viewing the corners of the picture for sure when the lenses are too small! Strong enlarging lens configurations are also getting more sensitive for un sharpness when not looking straight through the center of the lens. Grown up people are having eye separation distances varying from about 57 up to 70 mm. Children are of coarse smaller.

All viewers have some kind of light diffuser at the back of the viewer body. Without it, you would have a very unpleasant disturbed background of the picture. There are two materials most used as diffuser; frosted glass or milk-glass. Both are available in glass and plastic.

The best material to diffuse light is milk glass. Almost every source of light is diffused to a smooth illuminated surface (spotlight, daylight, etc.) But there are some (dis) advantages:

• Milk glass requires rather a lot of light power to reach an acceptable level of illumination.
• Milk glass often turns 'white' light into a kind of yellow. That causes pictures not to represent the original color when viewed.
• Due to the light consuming property of the material, it is not very suitable for mobile light sources to diffuse light.
• Milk glass is rather cheap.
• Milk glass is often used for public viewers, the light sources are having enough power and cool-white tube lights can compensate the yellow tint.

For true color images you should choose for frosted glass. Although the diffuse capabilities are not that good. Using other production techniques you can increase the qualities of this material:

• Frosted glass provides the spectator a bright illuminated background.
• Frosted glass does not change the light color temperature of the light source.
• Frosted glass has to be grind very fine to prevent the spectator to see the grain of the frosted glass.
• Fine grind frosted glass is not cheap and isn't available everywhere..... when breaking it.

The size and position of the diffuser is not always optimal, due to commercial reasons. There are two locations where diffuser can be placed:

• A few centimeters behind the slide
• Right behind the slide/ close to the slide.

Left image show diffuser location behind the slide.
The right image shows the space between slide and diffuser.

The location just behind the slide has some advantages. The construction of the viewer body can be quite simple. The slide is sometimes fixed with a spring against the diffuser. This construction makes the viewer body 'box shaped'. There are no extra walls or partition walls needed. Doing it this way saves costs. A big disadvantage is that dust and scratches on the diffuser are within the sharp focusing field, so they are as sharp visible as the slide! Especially with frosted glass, the grain will be good visible.

Mounting the diffuser just a few centimeters behind the slide will make the construction of the viewer body more complex en expensive. Although dust and scratches are out of focus and nearly visible! However, this solution requires a larger diffuser and viewer body for optical reasons. The wider viewer body prevents the side walls to be visible when looking through the viewer.

This type of image distortion has a lot to do with the construction of the viewer and the costs to make the viewer.

The ideal image is presented as you are sitting in a dark cinema looking at a projected image. Without any disturbing surrounding light. Light leakage means ray's of light coming inside the viewer through grooves of the viewer body.

• Deze lichtstralen kunnen een witte streep over het beeld heen laten vallen waardoor een 'grauwsluier' over het beeld ontstaat en het diaraam materiaal door de lenzen zichtbaar wordt. De lichtstralen kunnen ook de 'lelijke' wanden van het viewerhuis laten zien.
• Bij eenvoudige viewers die 50x50 mm. en 41x101 raampjes kunnen verwerken is de diahouder zodanig geconstrueerd dat het eerste type raampje aan de voorkant en het tweede aan de achterkant van de houder geplaatst kant worden. Hierdoor onstaat een spleet aan de voorkant van de houder als daar geen dia in geplaatst is. Hierdoor komt ook licht naar binnen.

• Bij eenvoudige kijkers komt het ook wel voor dat er geen goede geleiding voor de dia aanwezig zijn. De wanden van het huis bevat geen gleuf waarin de dia geschoven wordt. Een kier naast en onder het raampjes is onvermijdelijk. Erg storend bij een felle lichtbron.

Ook al heeft een kijker geen lichtlekkage, reflecties kunnen ook hinderlijk zijn. Het is niet altijd eenvoudig om reflecties tot 0 te reduceren, je zal ze altijd in meer of mindere mate houden. Tenslotte komen reflecties door licht wat door de dia zelf komt! Zwart schilderen of anodiseren van oppervlakken helpt niet altijd maar is wel het minimum om de meeste reflecties kwijt te raken. De hardnekkigste reflecties treden op in de lensbussen. Licht reflecteerd nu eenmaal sneller op oppervlakken die in het verlengde liggen van de 'kijkas'.

• Schoolbordverf is een goed materiaal om reflecties te reduceren. Het droogt mat op en laat weinig kwast sporen achter in het gezichtsveld.
• zwart flueel op plakstroken absorbeerd veel licht en reflecteerd nauwelijks. Nadeel is wel dat het materiaal dik is en op den duur los kan laten.
• Bij het ontwerp er voor zorgen dat onderdelen helemaal niet in het zicht komen, er valt dan weinig te reflecteren.