The Best Tool to Carry is a Magnifier

Visually inspecting your plants on a regular basis is an important component of an effective pest monitoring program. Careful, systematic inspections will help keep track of the health of your plants and identify any potential weed or pest problems.
Magnifier In-use
A magnifier is an important part of visual inspection.
Magnifiers let you see small details that are helpful in
identification of weeds and pests.
Click on the tabs below for more information
How do magnifiers work?

Magnifiers work by bending light rays to produce an image of the object that is larger than life. The more the surface of the lens is curved, the more the light is bent and the greater the magnification. The degree of magnification is therefore determined by the curvature of the lens surface. Greater curvature limits the lens size, which means that higher powered magnifiers are going to be smaller.
How is magnification expressed?

Magnification is expressed as "x." A 20x magnifier shows you an object 20 times larger than life. The lower the magnification, the greater the depth of field and "field of view", so be sure to choose the lowest magnification needed for your task. For example, at 2x you may be able to see an entire insect clearly, from the tips of its antennae down to its tarsi (toes). At 10x, you'll see either antennae or tarsi in greater detail, but not both at the same time.

10x or less is a good choice to start with for most pest scouting. Once a pest is located, a larger power lens may come in handy. With practice and increasing skill in scouting, you will eventually want to equip yourself with a wider range of magnifiers to be able to complete nearly all of your sampling and identification tasks in the field.

You Can...
2 - 5x
Scan large areas for insects, eggs, mites, disease.
6 - 10x
View details on larger insects and plant parts.
11 - 25x
Identify insects and some eggs and mites.
26 - 45x
Identify small insects, eggs, mites and many diseases.
46x +
Identify most diseases, nematodes and other organisms barely visible or invisible to the naked eye.
What different types of magnifiers are there?

There are many forms of magnifiers; loupes, linen testers, watchmaker's loupes, aspheric, lighted, folding pocket, etc. One style is not necessarily better than another. It depends on the style of magnifier you feel comfortable using.
What are the different type of lens systems?

When using magnifiers, be aware of distortion, chromatic and spherical aberration errors that can occur. Distortion is a defect in a lens where images of straight lines appear curved. Chromatic aberration errors occur when the lens cannot focus light of different colors at a single point, causing a blue-red image. Spherical aberration errors cause rays to focus at different distances.

Different types of lens systems offer varying levels of correction for these problems.

The simplest type of lens is the single aspheric lens. This lens is designed for small powered magnification because it is a single lens with a series of different curves on the surface for sharper focus over the entire surface.

Spaced Doublet consists of two lenses with air space in between them. This lens system is inexpensive but does not offer much distortion or color correction.

Coddington Lenses are made of a single lens with a grooved diaphragm around the circumference. This groove allows for sharp images at higher magnifications.

Cemented Triplet is the last type of lens system and is also called a Hastings. The Hastings lens consists of three lenses cemented together, providing the sharpest color-corrected image.

What is the proper way to use a magnifier?
To use a loupe, hold the lens close to your eye and move the object you are looking at close until it's in focus. For linen testers, simply place the magnifier on top of the object and move your head closer to the lens for larger viewing.
Do's and Don'ts of Using Magnifiers

Use the proper technique for your specific magnifier.
Think all lenses are the same.
Select the proper magnification for the job.
Only use the largest magnification.
Know how to select a lens system for your needs.
Forget to visually inspect your plants.