Image formation in a Camera - The lens image
A lens will form an image in space at the prescribed focal distance from the lens, and this image, though not visible in its entirety, may be observed in pieces by looking into the back of the lens. The portion of the image observed will be that portion of the subject which lies in a direct line described through the center of the lens from the eye point to the subject. The surface of the lens, acting as a light gathering device, will gather an infinite number of rays emanating from each point of the subject, and direct them to their correct, corresponding place in space to form the image. The light exiting the back of the lens is organized and directional. Thus, the rays forming the image, and therefore, the image, may only be seen by placing the eye-point in such a position that the rays are intercepted. In other words, the light of the subject, which is diffuse light, is organized into light which converges at the image plane in space as non-diffuse light. If uninterrupted, this light will cross paths at the image plane, and then keep right on going.

This organized light requires the action of a diffusing surface in order to recreate the diffuse image so that an entire diffuse image of the subject is again visible. This is the job of the viewing screen. A truly diffuse image may be seen from almost any angle. In photography, however, we rarely use a completely diffuse image (except, of course, in the final print.)

The Hot Spot
Let's return to the lens forming an image in space, in which all of the light is directional and organized. Without a ground glass, the image can only be seen in
small sections by positioning the eye appropriately. The image will appear to reside in the back of the camera lens. Now let us place a ground glass at the focal plane. A ground glass may be described as translucent, but the term translucent does not adequately describe the true nature of a ground glass. The tiny facets of the ground surface have an ability to diffuse light, but the diffusion is not complete. The glass retains some of its ability to transmit light directly, and directionally. The fact that ground glass images are NOT completely diffuse images is the source of all the confusion. By placing the ground glass in the path of the converging rays behind the lens, two things will be apparent. First, a large image is now visible, beyond the small image section which was visible in the back of the lens. Second, you will be able to see that the portion of the image which was visible in the lens as an aerial image is now visible on glass as a "hot" spot, brighter than the surrounding image on the ground glass. If you move your eye-point this way and that the hot spot will also move. What you are experiencing is the tendency of rays emanating from the lens to continue in the same direction through the ground glass. Therefore, wherever you move your eye-point, the image opposite the camera lens will be the brightest part of the image on the ground glass.

Going back to to the statement at the beginning of this section, that a truly diffuse image is visible at virtually any angle, if you look at a picture in a magazine, regardless of the viewing angle, you will still be able to see the picture. It stands to reason that on a flat surface, the maximum viewing angle would be very close to 90o from vertical, or close to 0o from the surface. As many of you know from experience, if you viewed a ground glass from this angle, you would not see an image. If you replaced the ground glass with an opal glass (a white, texture-less translucent material), you would be able to produce virtually a fully diffuse image with no hot spot. This would mean that the image could be viewed from any angle, but it would be a much dimmer image, with much lower contrast. The reason for this is the wide distribution of light. In this case the image is completely diffuse, and the eye only sees a small percentage of the light of the original image. In a ground glass, the bundle of rays, or hot spot, has a much higher concentration of light visible to the eye. This scenario describes what is very familiar to photographers who use large format cameras. Although the photographer can move his eye-point around on the ground glass in order to view a bright image, it is not possible to see a bright image all over the entire ground glass simultaneously. The problem can be compounded or ameliorated by a couple of factors.The diffuse image of a traditional ground glass is darker farther from the viewing axis. It is also darker in the corners of the ground glass with wide angle lenses than with long focus lenses.. (In fig.2a and 2b, the difference in angle and the distribution of rays show that more rays reach the eye-point in the middle viewing position with the long lens than with the short lens.)

The Fresnel Lens
In the foregoing section it may have sounded that the hot spot is a serious impediment to viewing the ground glass image. The seriousness is often overplayed, and many photographers have no problem at all seeing and composing on their plain ground glass.For many photographers, however, the fresnel lens is the solution to the uneven ground glass. The Fresnel is a true lens, flat, about one sixteenth inch thick, divided into extremely small segments in the form of concentric rings. Its function is to redirect the ray bundles of the ground glass, especially the edges of the glass, to the central viewing position. In this manner, the ray bundles which would be largely divergent, causing dark ground glass corners, are now visible. The effect is almost startling, in that the whole ground glass is, in a real sense, turned into one big hot spot. The Fresnel is not without its problems, however. Because the fresnel is a true lens, it has a true point of focus. Combined with the ground glass, the fresnel will have a beam which resembles the collimated beam of an enlarger condenser. In both cases, the beam is convergent, which means there is an ideal viewing position of some distance from the screen.This distance will vary with the format, because the average focal length of "taking" lenses on smaller formats are shorter than on larger formats. Therefore, the focal length of fresnel lenses is longer with larger formats in order to match more accurately the angle of light approaching the ground glass.With the fresnel in place, the viewing position is now in a central position. Moving off center from this position, you will find that the fresnel image will start to go dark. This makes viewing the corners with a loupe difficult. Because viewing the corners or extreme edges of a ground glass is a likely activity for the large format photographer, this reduces somewhat the usefulness of the fresnel. Some manufacturers have a removable fresnel which may be put in place for general viewing and composition, and then removed for critical focusing with a loupe. The added advantage of this is the absence of fresnel texture which would otherwise obscure detail in critical focussing.

What is the Proper Placement of  the Fresnel? One subject which comes up often is the correct position of the fresnel lens. In years past, several arrangements have been used, including placing the fresnel behind the ground glass, in front of the ground glass, and incorporated into a ground glass made of plastic. However, there is only one correct arrangement. The ground glass surface should face the camera lens, and the fresnel is placed behind it, on the outside of the camera, toward the photographer. The textured surface of the fresnel should be placed against the ground glass. There is a particular reason for this arrangement. In manufacturing cameras and film holders, one overriding concern is the correct position of the focal surfaces of the respective parts. In the film holder this is the position of the septum, against which the film rests, and in the camera this is the position of the diffusion surface, or ground side of the ground glass upon which one focuses the camera. Nothing is more important than the proper registration and agreement of these two elements. If the fresnel were placed in front of the ground glass, interposed between the lens and the ground glass surface, even if it were still physically in the same position occupied by the film plane, a lack of registration can occur. This can be explained by considering the effect of a parallel sided glass plate such as a glass filter upon a beam of light (fig.?). Rays passing through such a plate are displaced by about one third the thickness of the plate, and depending upon the angle when passing through the plate, will be displaced laterally.  .Furthermore, the greater the angle, such as in a wide angle lens, the greater the effect, resulting in an apparent curvature of the image when no such real curvature exists. The result will be erroneous focusing of the edges of the image.

"Bright" Screens
One device which gets a lot of press and is usually misunderstood is the so-called bright screen. These are usually proprietary ground glass type screens which claim to transmit a brighter image with greater contrast. The explanation is really quite simple. Going back to our discussion of the ground glass, we said that it possessed two characteristics, that of diffusion, and that of residual transmission. The less diffusion, the greater the transmission. This effect can be seen readily by smearing a little water on the ground side of the glass. The glass will become nearly transparent, and any image viewed through the wet area of the glass will appear to be brighter and higher in contrast. The hot spot will be greatly exacerbated. The less diffusion, as we have seen above, the worse the hot spot. Commercial bright screens used a permanent means of reducing the diffusion of the glass, usually by some sort of coating, thus creating a glass which has these characteristics. These glasses MUST be used with a fresnel lens because the hot spot is so much increased. Regarding the claims for brightness, no glass can be brighter than the original aerial image emanating from the camera lens. One proprietary focusing screen uses a micorcrystalin wax for the diffusion medium, and the result is a very sharp image with all the same qualities of the combined diffusion and transmission that a good screen requires. This screen uses a very thin cover glass as part of the wax lamination. In consideration foregoing discussion of interposed glass in the image path, this glass is probably too thin to make a difference, but you should be aware of the effect in the event of a critical application.