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National Offset Warehouse

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National Offset Warehouse



Guide to Pressroom

Optics, Microscopes, and Magnifiers


National Offset Warehouse offers the most diversified line of magnifiers and microscopes for the graphic arts and printing professional. The optics presented are industry benchmarked for excellence. National Offset Warehouse optics are the highest quality obtainable. Many feature achromatic and color corrected lenses as well as flat fields to reduce distortion.

Listed below is some helpful technical information about optics. Use this guide in addition to the magnifier Recommendation Chart, located below, to help you choose the correct magnifier for your needs.

Types of Lenses

  • 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.

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 that 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 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.
5X Round Stand Magnifier Large 60 mm field-of-view is ideal for viewing color transparencies on a slide sorter or light box
12X Plastic Base Loupe Excellent working distance, wide field and clear base makes this unit best suited for darkroom and stripping areas.
10X & 12X LithoMags Sufficient working distance to be used in stripping department for touch-ups.  Achromatic lens system is perfect for press checks.
20X LithoMag Higher magnification shows some spherical aberration.  Reduced working distance.  Best suited for camera focusing, registration checks, slur and gain checks.
15X Swivel Base Loupes Moderate working distance allows unit to be used in stripping department.  Use in pressroom for registration checks.
22X Swivel Base Loupe 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.
25X, 50X, 75X and 100X Pocket Microscopes Used for close-up inspection of small areas.  Pen-style makes them handy to keep in your shirt pocket.
60X - 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.
4X Peak Loupes Large format for viewing 120/220 film, focusing on ground glass of cameras