Historically, split prism focusing aids suffered a major limitation - at smaller apertures, the prism would begin to darken. By f5.6, most traditional technology split prisms become completely black and are no longer useful for focusing. And not only would the photographer lose the ability to use the prism, they'd be faced with a black circle in the center of their viewfinder... quite an impediment to composition! This ‘blackout’ phenomenon is a function of the aperture of the lens. At wider apertures, such as f1.4 - f3.5, the traditional split prisms work very well. But as the aperture narrows, the prism becomes sensitive to the position of the eye in the viewfinder, such that if the eye is off center, one half of the prism will black out. By f5.6 or f6.3, the ‘blackout’ is complete and no amount of attention to eye position will allow the prism to function. Some split prisms were able to slightly extend the useful range of the prism, but only at the expense of large aperture sensitivity.
Note: When discussing the performance at various apertures, what is at issue is the aperture being used to compose the shot. Modern automatic lenses stay at their widest aperture until the moment the shot is taken, so it is the maximum aperture of the lens that is of concern. With an automatic lens, the "blackout" phenomenon is only visible if the Depth of Field preview function is used or if the maximum aperture of the lens is f5.6. It will also often be apparent when using a reasonably fast lens with a teleconverter. For example, a lens with a maximum aperture of f3.5 becomes f7 when using a 2x teleconverter. At this point, traditional prisms are useless and even the autofocus will struggle to find a lock. Using a full manual lens, the "blackout" phenomenon will become evident as soon as the photographer stops down to take the shot. For those users that prefer to focus after stopping down to their shooting aperture, this can be of particular concern.
The Katz Eye™ “Plus” series of focusing screens has a unique split prism design which overcomes the "blackout" problem, without sacrificing large aperture sensitivity. In our testing, the split prism of the “Plus” screens remains useful until well past f11. In fact, with careful attention to eye position in the viewfinder, users have reported that it is possible to use the prism even at f22. To characterize the performance of the prism, one must take into account the sensitivity of the prism to the position of the eye in the viewfinder. The sensitivity is described subjectively by how much attention to eye position is required to make both halves of the prism clear. This is broken down into 5 categories*:
Level 1: Little or no attention required to eye position
Level 2: Minimal attention required
Level 3: Moderate attention required
Level 4: Careful attention required
Level 5: Both halves of the prism cannot be seen simultaneously with usable brightness regardless of eye position
Note that even a ‘level 4’ rating means that visibility of both halves of the prism can be attained. Here are the results of testing with a Canon 20D and an EF 50mm f1.4 lens:
Level 1: Down to f/4.5
Level 2: Down to f/9.0
Level 3: Down to f/14
Level 4: Down to f/22, the smallest aperture at which testing could be completed with this lens
Level 5: Did not occur for any aperture which could be tested for this lens
Results are similar for other lenses and do not seem to be very dependent on the focal length of the lens or the model of camera, as long as the subject is a reasonable distance from the camera. In macro photography, where the subject is very close, the bellows factor increases the effective f-stop slightly, and can shift these points down a bit. However, even using the 50mm lens with an extension tube to provide a magnification of 1:1, the prism remained usable down to at least f11.
*Many thanks to Doug Kerr for his help in preparing this analysis.
Is the Microprism Collar “Plus” Also?
No. Due to the small size of the microprisms, the “Plus” technology cannot be incorporated into this portion of the focusing aid. The usefulness of the microprism collar is limited to viewing apertures f5.6 and larger, with optimum performance obtained with apertures of at least f4. At very large apertures, the microprisms will disappear completely when the image is in focus. With medium aperture lenses, the microprisms will still indicate focus, but some residual reticulation will remain. At apertures smaller than f5.6, the microprisms cannot be used for focusing, but will not significantly impair the view of the scene.
Which Should I Choose?
Since there are no real disadvantages to the “Plus” series of screens and they have been the overwhelming choice of most users, the majority of the Katz Eye™ models are now available exlusively in the “Plus” version.
“Plus” or OptiBrite or Both?
The OptiBrite treatment is a brightness enhancement for the matte portion of the screen (outside the prism) and is unrelated to the prism function. OptiBrite is not necessary for the “Plus” prism benefit of increased prism usefulness and the “Plus” prism does not affect the brightness of the screen. For more information about the OptiBrite treatment, please visit our OptiBrite page.
If you have any other questions about the Katz Eye™ Plus, of course feel free to