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Descriptions, Definitions, Recommendations

Types of Lenses

Referenced below is some helpful technical information including lens types and terminology definitions. Use these guides in addition to the magnifiers recommendations below to help you choose the correct magnifier.

Simple Positive Lens - A single piece of optical glass or acrylic with two convex surfaces (curved outward). Designed for low magnification.

Simple Negative Lens - A single piece of optical glass or acrylic with two concave surfaces (curved inward). Used in conjunction with positive lenses.

Achromatic Lens - A positive simple lens cemented to a negative simple lens. Achromatic lenses are corrected for two colors and produce flatter fields of view at higher powers.

Double Lens - Two simple lenses used as a system but not cemented together. This system produces a far superior image than a simple lens.

 

Technical Definitions

Power: The number of times a lens or lenses multiplies an image with respect to the original size. Low power is recommended for scanning large surfaces. Higher powers are generally used for inspecting small areas.

Field of View: The maximum area seen through a magnifier. The entire field of view may not be in focus. See Flatness of Field.

Flatness of Field: Due to the physical laws of optics, the outer part of the image formed by a lens may be out of focus. The greater the power and curvature of the lens, the more pronounced this problem becomes. Magnifiers with multiple lenses can reduce this problem. The visible area which appears in focus is the flat field.

Working Distance: The distance from the magnifier lens to the object being viewed. As magnification power increases, the working distance decreases.

Aberration: This is a distortion caused by the lens. Spherical aberration is distortion of the image, as explained in Flatness of Field. Chromatic aberration is distortion of colors being viewed, as explained in Color Distortion.

Color Distortion: Lenses produce a prism effect which causes the image to develop false color fringes. This is due to the fact that different colors focus at different points. Achromatic lenses correct this problem by focusing many colors at the same point.

Parallax: Parallax is the apparent misalignment of two different items when viewed from an angle. It is the cause of improper registration when stripping multiple color jobs. Parallax is eliminated when viewing straight down on registration marks instead of from a slight angle.

Coated Lens: A coated lens helps to minimize or eliminate color distortion caused by different colors focusing at different locations. Optimum color viewing is obtained from coated achromatic lenses.

 

Magnifier Recommendations

Magnifier Usage
1" x 1" Linen Tester (6x) General purpose work. The graphic arts most popular magnifier.
12x Plastic Base Loupe Excellent working distance, wide field and clear base makes this unit best suited for darkroom and stripping areas.
10x & 12x Folding Loupes Sufficient working distance to be used in stripping department for touchups. Achromatic lens system is perfect for press checks.
20x Folding Loupes Higher magnification shows some spherical aberration. Reduced working distance. Best suited for camera focusing, registration marks, slur and grain checks.
12x & 15x Swivel Base Loupes Moderate working distance allows unit to be used in stripping department. Use in pressroom for registration marks.
20x & 22x Swivel Base Loupes Achromatic lens system. Reduced working distance due to high magnification. Lens shows some spherical aberration. Adjustable focus lens with locking ring. Best suited for critical camera focusing, registration checks, dot gain and slur checks.
7x, 10x & 12x Collimating Magnifiers Built in Collimator feature narrows field of view to eliminate edge aberration (parallax). Perfect for checking registration.
20x, 50x, 75x & 100x Pocket Microscopes Used for close-up inspection of small area. Pen-style design makes them easy to keep at hand.
25x - 200x Stand Microscopes Used for the most critical checking of register and dot structure. Should be used to view printed circuit boards and bar codes.
8x Econo Loupes General inspection work in pressroom and camera department. Clear plastic skirt prevents working under lens, so they are not recommended for stripping.
8x Peak 35mm Format Clear plastic skirt is 24mm x 36mm for a perfect fit to view 35mm transparencies.


Choosing the Right Loupe for the Job, a.k.a. Not All Magnifiers Are Created Equal

We are asked the question, “What is the best loupe?” The answer is always the same, “For what purpose?” This usually creates more confusion but it is easily corrected. It is definitely subjective, but it is necessary to qualify the usage before recommending the loupe. All references can be seen www.peakoptics.com
GENERAL INSPECTION LOUPES
Loupes that are used for general inspection purposes are usually equipped with a larger diameter, lower magnification lens. This combination allows the viewer to quickly scan a large area. If a defect is noted, it is not unusual to do a closer inspection of that specific area with a higher power loupe. General inspection loupes are usually in the 4X to 6X range. Their most important characteristics are a flat, bright field and a large flat field-of-view to lens diameter ratio.
PRESS-CHECK LOUPES
The most common press-check magnifier is the 1" X 1" Linen Tester. This may be OK for small formats or single colors to give a quick look, but for process color printing a stronger, higher quality loupe is needed. These loupes need to have a large, flat
field-of-view to lens diameter ratio, they should be achromatic and color coated. The magnifications for loupes of this type range from 10X to 20X. Several factors to remember with these loupes are that as the magnification increases, the lens diameter generally decreases, as the magnification increases the working distance between the lens and the subject decreases and as the magnification increases the flat, field-of-view decreases. Some excellent examples of Press-Check Loupes are Lithco Litho Mags, the Lithco 12X Plastic Base Loupe and the Lithco 12X Swing-Base Magnifier.
STRIPPING TABLE LOUPES
There are two primary types of loupes designed for the stripping table. The first is an anti-parallax loupe and the second may well be called a “touch-up” loupe. The first type facilitates alignment of registration marks. The optical design requires that you look perfectly straight down on the marks in order to see them, thus allowing you to align them in exact registration. Prime examples of anti-parallax loupes are the Octagon Anti-Parallax Loupes. The main feature of the touch-up loupe is the ability to get between the lens and work surface to do scribing or opaquing. These include the Lithco 12X Plastic Base Loupe 6X Wide Field Magnifier and 5X Round Stand Magnifier.


Regardless of your magnifier needs, GWJ co (Peak Optics)  has the right product for the job.

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