MTF charts are very useful, but they don’t take into account every aspect of a lens. For one, they do not measure a lens’ linear distortion, chromatic aberration, or flare. In addition, lenses are generally tested at one ideal focus distance and not through the entire range. Lastly, remember that each piece of glass can have slight differences during the manufacturing process. Check out the video below for an example of how your lenses are made.

Different manufacturers test their lenses’ performance using slightly different procedures, yet the general idea remains the same. A set of thick black dashed lines (usually around ten lines per millimeter) run from the center of the image to the edge. Similar to our example above, the lines are then analyzed to see when the white space between them begins to turn gray. These results are then plotted on the MTF chart as thick lines.

0VDCwire color

For industrial applications built to UL508A specifications, this guideline for industrial control panels suggests colors for the positive (+) and negative (-) 24 V DC signals. In these cases, including PLC input and output conductors, all (+) 24 V wires should be blue, and all (-) 24 V wires should be white with a blue stripe.

The opening scene of Lethal Weapon 3 features Riggs and Murtaugh, two police detectives, attempting to defuse a bomb. They are confronted with the typical Hollywood question involving which wire to cut. For the purposes of the movie, the assumption was that the color of wire insulation was somehow magical or that the bombmaker was kind enough to reference standards to build his bomb.

Putting this information together, we can look at how the lens handles the contrast in two separate aperture configurations with both types of lines. Typically, sagittal lines are represented as solid lines on MTF charts while meridional lines are represented as dashed lines.

Depending on the industry and application, there are many voltage levels, from 24 V up to many hundreds and even thousands of volts.  As with the DC wires, there are competing standards and guidelines, so it should be clearly stated that the jacket color sound NOT be a clear indication of function. Skepticism is a great safety tool when it comes to wiring.

Power cable colours

Color coding of wires, while popular, does have its pitfalls. The two largest pitfalls are that the wires can change color with age and that folks who experience color blindness have difficulty determining the insulation color.

Riggs: Well, I'm cutting the red wire, okay? Murtaugh: Wait? What? A minute ago you said blue. Riggs: Did l say blue? Murtaugh: Riggs, you said blue. Riggs: Well, l meant red. Murtaugh: You sure? Riggs: We can do it your way if you'd like. - Lethal Weapon 3 Script

It is also worth noting that you can tell the ‘bokeh’ or quality of blur from reading an MTF chart. Simply look at the distance between the sagittal and meridional lines. A lens that displays more natural-looking bokeh will have the two lines plotted closer together. You can keep this in mind to note how bokeh changes at different apertures.

Theoretically, high-quality insulated wire will not change color. If kept within tolerances as outlined by the code, the insulation will not be exposed to high enough temperatures to thermally degrade the color, and it will be made with dyes that do not fade with time. However, the reality is that wires become dusty, faded, or otherwise discolored. Any darkened spots indicate that the wire was exposed to high temperatures and should be replaced.

Resolution is tested and charted in a similar way, but instead of having the lens view thick black dashed lines (around 10 lines per millimeter), it views a collection of more closely spaced lines at around 30 lines per millimeter. When seen by the lens, the resolution begins to decrease as it is harder to discern the lines from each other, and they will seem to blur together.

Wirecolour codes

Now that we understand that the chart shows how a piece of glass performs from the center of the image to the edge, we can discuss exactly what is being measured and the process that takes place. Some people may not be aware, but sharpness is measured in two different ways. The first is to look at the contrast of a lens and the second is to look at the overall resolution of a lens.

The results from a resolution test are plotted on the MTF chart in the same fashion but are instead represented by thinner lines. Now you can read an MTF chart and tell at what point a lens’ contrast and resolution begin to decrease, and the conditions under which the results were obtained.

The Lethal Weapon actors were not aware that any of those wires could conduct current in the same manner as the others, regardless of insulation color. The ordinance designers in this movie probably did not consult the NEC or IEC or gain the permission of any AHJs or their employer conventions since they were not interested in preventing shocks, fires, or making it easy to troubleshoot. In the real world, however, it is important to abide by legal standards and conventional wisdom when designing and installing electrical systems.

Color blindness strikes many people, and some may not even realize they are colorblind until they are faced with a question like, “is this brown or red?” Red-brown and blue-green color blindnesses are common, as are several other varieties. If either the installer or the electrician servicing the line experience color blindness, dangerous misunderstandings can occur. Because of this, wire insulation color should not be relied on as the only method of testing a wire’s origins and meaning.

24 vdcWire

Some European color conventions (commonly found on NPN and PNP 3-wire devices from all suppliers, EU or Int’l) use brown for (+) DC connections and blue for (-) DC connections.

The horizontal axis tells you the distance of the measurement in millimeters from the center of the frame to the edge. On a standard full-frame camera body, the distance from the center to the edge of the sensor is around 21.6mm. When a lens is measured for a smaller sensor type, such as an APS-C sensor, the distance will decrease.

Purchasing a new lens for your kit can be an expensive endeavor and the more information you know before making a purchase is always useful. You may have noticed that lens manufacturer sites tend to include what is known as a Modulation Transfer Function (MTF) chart. These charts help to give shooters a better idea of the glass’ performance, but they’re not the ‘end-all’ answer for knowing whether a lens is good or not. Here’s an introductory guide on how to accurately read an MTF chart and how to take their readings into account for your purchase.

Green insulation indicates ground wires. Think “Green-Grass-Ground”. However, green insulation is not the only way to determine a ground wire. According to the NEC, green insulation with a yellow stripe is also an indication of ground. Either of these is acceptable, and there is no real difference between them. Both of these colors are to be used explicitly for ground connections.

You will also notice that the lens’ contrast measurement at both wide open and f/8 is represented by two lines and not one. For a lens, lines that are running parallel to the diagonal from the sensor are easier for the lens to reproduce accurately. As a result, the tests are run with two different dashed lines: one set that consists of parallel diagonal lines (sagittal) and one that consists of perpendicular lines (meridional).

24VDC brown blue

Designing, installing, and maintaining control systems involves a lot of wires. Each wire has a purpose, often indicated by the color of the insulation. But what are the common colors, and what do they mean?

3-phase systems vary with geographical location and voltage level, so it is wise to rely on current local guidelines before maintaining and installing them. Any personnel with permission to access 3-phase locations should be knowledgeable about the equipment and the guidelines before proceeding.

The NEC only specifies a few mandatory color codes. The IEC suggests others, and likely the AHJ and certain conventions may specify a few more. However, when performing a repair, engineers and technicians should not simply rely on the color of the insulation alone.

The basic key idea to keep in mind when viewing an MTF chart is that the closer to the top of a chart the line is, the better it will perform. Most manufacturers and photographers agree that a rating of above 0.6 is acceptable, while a high-performance lens will score about 0.8.

MTF charts give photographers a decent indication of how a lens will perform under certain conditions; they compare real-world lenses with a hypothetical ‘perfect’ lens. Even lenses designed with the highest level of precision have minor imperfections and aberrations that are naturally present in the glass. Learning to read an MTF chart will help you understand how sharp a lens is from its center to the outer edges.

All About Circuits outlines the wiring color code standards and conventions, showing how different parts of the world incorporate colors for single-phase, three-phase, and direct current (DC) systems. It is worth noting that some of these colors have changed, so the age of a building and its wiring can play a part in troubleshooting procedures, as the color conventions may have changed since its original construction.

24VDCwire color codeIEC

The resolution of a lens, on the other hand, is the overall ability of a lens to render very fine pieces of detail. A lens can be very high in resolution, but low in contrast, thus displaying a blurred or ‘soft’ image.

Bare metal conductors (copper, copper-clad aluminum, aluminum) can be used as a ground. In fact, bare metal conductors should ONLY be used as a ground. If they were used for conducting current during normal operation, they would be very unsafe and risk shock. Aluminum and copper-clad aluminum cannot be used where they are in contact with concrete or masonry due to corrosion issues. These requirements are reiterated throughout NEC Articles 200-250 in various contexts.

24VDCwire color codeUSA

Now that you are ready to go out and begin comparing lenses for their performance, there are a few notes that you have to be aware of during your journey. First, manufacturers may use different methods to measure their lenses’ performance, so comparing MTF charts between different lens manufacturers may not give you an accurate idea of performance. Also, side-by-side comparisons of lenses with different focal lengths will not give you an equal comparison.

For the most part, in the US and Canada, 120 V 1-phase AC systems use black for the line wire (L1) and white or gray for the neutral (N). In a 240 V system with a second line wire (L2) uses a red wire. In Europe, the convention is similar to DC, with L1 being brown and L2 (or N) being blue.

Finally, there are informal standards that may suggest or permit certain wire insulation colors. These conventions may be required by a particular employer but are not legally recognized. For the sake of consistency, it may be that a new housing development will use a certain color to indicate all travel wires for three-way and four-way switches across all of their newly constructed homes. This is just for ease of troubleshooting and not legally binding.

The National Electric Code (NEC) specifies standards for all things electricity for residential, commercial, and industrial building projects. It gets updated every three years for such purposes as adding content for electric vehicle charging stations or clarifying language on grounding electrodes. However, the NEC only specifies a few conductor insulation color codes. Instead, the NEC focuses more on how wiring is to be performed: what gauge of wire is required, what types of raceways and conduits should be used, how to support wiring and wire coverings, and how receptacles, branch circuits, luminaries (lights) and protective devices are to be wired.

Next time you are on the market for a lens, be sure to compare your two favorite ‘same-brand’ lenses with each other to see which one is rated as having better performance — but remember that not everything can be taken into account. Read up on each lens and view sample photographs to give yourself a better idea of what you are purchasing.

According to Canon, “there are a good number of optical engineers who feel that higher contrast will result in a lens that more users will regard as ‘sharp’.” The answer is to find a good balance of contrast and resolution when developing a lens.

Due to the way lenses are crafted, their sharpness and contrast tend to be strongest in the center of the image and weaken as you move toward the edges. When looking at an MTF chart you will notice that there are two labeled axes. The vertical axis is always marked from 0 to 1 and is read as a percentage. The higher something is charted on the vertical axis, the higher the lens’ performance in that area.

Ground wires are not to be confused with neutral wires, present in single-phase and three-phase AC systems. These neutral wires have white or gray jacket colors.

Iecwire color

At this point you may be saying, ‘but wait, there are four thick lines!’. Each thick line you see on an MTF chart depicts contrast testing under different conditions. In our example MTF chart, the company tests the lens at its widest aperture (blue) and at f/8 (red). Most manufacturers tend to test once at wide open, then at a ‘middle aperture’.

Following this example of two opposing guidelines for various circumstances, it can be seen that subtle differences in the voltage level, application, and the country of origin may dictate conductor insulation colors. The overall answer? Learn to use your voltmeter and NEVER assume a voltage based on a wire color alone.

Tracing wires, using a multimeter, or other such methods can be used to verify wires are connected as expected. Even though the NEC does not say much about color coding, it is essential for showing electricians how to properly size conductors, calculate load requirements, choose proper fittings and conduit, and many other important considerations.

The contrast of a lens is its ability to show a clear distinction between dark and light levels of a subject. If we were to line up a collection of black dashes and take a photograph, the point at which the space between said dashes starts to become grey instead of white is when the lens’ contrast performance decreases.

Direct current wires, such as those found in automotive wiring, typically use red wires for a positive voltage and black wires for a negative voltage. From there, either a black or green wire bonds the negative terminal of the battery to the car's frame as a ground.

The NEC speaks of the Authority Having Jurisdiction (AHJ), meaning any local, city, county, or state authority has the final say in how wiring is to be performed. In general, these are more strict than the NEC and may specify a color coding system for wiring.

Besides NEC, there is the International Electromechanical Commission (IEC), which also provides a set of standards. These standards are used throughout most of Europe and other parts of the world and are generally in accordance with the NEC. IEC 60445 provides guidelines for color coding wires and devices.