The reference spectra were first generated as separate standards, designated as E-891-82 and E-892-82 (for direct normal and global tilt, respectively). As of June 1999, ASTM Subcommittee G3.09 combined the two documents into the single standard "Standard Tables for Reference Solar Spectral Irradiance at Air Mass 1.5: Direct Normal and Hemispherical for a 37 Degree Tilted Surface" The relevant international standard is ISO 9845-1, 1992, based solely upon both E891 and E892. In January 2003, the G-159 standard was revised extensively, and replaced with G-173-03. The older standards E-891, E-892, and G-159 are withdrawn and no longer available except as historical standards. Downloads are provided here for reference and comparison with the G-173 spectra.

Atomic spectra

The fields in the table Direct and Global 37 Deg Tilt: ASTM G-173 are wavelength in nanometers (nm), the (Gueymard 2002) extraterrestrial spectral irradiance, direct normal spectral Irradiance (W/sm/nm), and the global total spectral irradiance (W/sm/nm) on the 37° sun-facing tilted surface for the atmospheric conditions specified above.

Rather confusingly, a smaller f-stop number signifies a wider aperture and a larger number signifies a narrower aperture. For example, f/22 is narrow and f/2 is wide.

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Nikon doesn’t mark the AF motor on their lenses. However, their F-mount lenses do have “-S” after AF (AF-S) to tell users the lens has an autofocus motor. Their older lenses just say AF, and you had to focus them manually.

What do the numbers and letters on lenses mean? You may have wondered when thinking about buying a new lens or looking at one you already own.

). A lens reading 1.8 / 55 or 1:1.8 means the lens’s maximum aperture is f/1.8.

If a zoom lens only displays one f-stop number, like f/2.8, the maximum aperture is constant throughout the zoom range. You don’t lose any aperture stops when you zoom in.

A variable max aperture is when the maximum aperture is different depending on the focal length you’re using. For example, a 70-300mm f/4.5-6.3 lens has a maximum aperture of f/4.5 at 70mm. But when you zoom in to 300mm, the aperture is limited to f/6.3.

This article covers what the numbers and letters on your camera lens mean. It may seem not very easy. You can learn to decode the manufacturers’ shorthand with a few hints. So let us look at the most common letters and numbers on camera lenses.

A camera lens that shows 0.9m / 2.96ft has a minimum focus distance of 0.9 meters or 2.96 feet. That means you can focus on anything close than that distance.

These details also tell you what the lens is capable of and what types of photography it’s good for. That’s why it’s so important to understand what the numbers and letters on a lens mean.

Myers, D. R.; Kurtz, S. R.; Emery, K.; Whitaker, C.; Townsend, T. Outdoor Meteorological Broadband and Spectral Conditions for Evaluating Photovoltaic Modules. Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference--2000, 15–22 September 2000, Anchorage, Alaska. Piscataway, NJ: Institute of Electrical and Electronics Engineers, Inc.; pp. 1202–1205; NREL Report No. CP-520-28860.

Myers, D. R.; Kurtz, S. R.; Whitaker, C.; Townsend, T. Preliminary Investigations of Outdoor Meteorological Broadband and Spectral Conditions for Evaluating Photovoltaic Modules and Systems. Program and Proceedings: NCPV Program Review Meeting 2000, 16-19 April 2000, Denver, Colorado. BK-520-28064. Golden, CO: National Renewable Energy Laboratory; pp. 69–70; NREL Report No. CP-560-28187.

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Your lens may have a number in meters and feet (#m / #ft). This is the minimum focusing distance for the lens. This number matters mainly for photographers who like to get close to their subjects.

SpectralTV stand

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Zoom lenses have a variable focal length with the limits of the range in the title of the lens. The focal range will also be marked on the lens. It would look like 70-200, which tells us the lens has a minimum focal length of 70 and a max of 200. The zoom function allows you to move between the two limits.

). This is because some zoom lenses have a variable max aperture.

Telephoto lenses often have a range of numbers on either side of the slash (

Faster lenses with wider apertures let in more light and give you better depth of field control, but they tend to be more expensive.

SpectralAudio

The first number is the f-stop. Some manufacturers express the f-stop as a ratio (1:

Some telephoto and macro lenses have a focus limiter switch. The label will say something like “Full / ∞ – 3m”, though the noted distance will vary depending on the lens.

The focal length of your lens is shown in millimeters (mm) on the lens. Your camera lens may or may not have “mm” on it. You see a number, a slash, and another number (# / #). The first number is the max aperture, but the second tells you the focal length of the lens.

You will also find an abbreviation that tells us what type of motor the lens has for driving the autofocus. Some brands have the same type of motor of all their lenses, but other use different ones on different lenses.

Gueymard, C. "Parameterized Transmittance Model for Direct Beam and Circumsolar Spectral Irradiance," Solar Energy, Vol 71, Issue 5, November 2001, pp. 325–346.

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You tend to find image stabilization on telephoto or high-end lenses, so if you can’t see one of these on your lens, it doesn’t have built-in stabilization.

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You’ll find a switch on the side of the lens to activate the IS. It most cases it will say “IS On / Off”. If it has more than one mode it will say ” IS Off / 1 / 2″.

Access the ASTM G-173-03 tables as a spreadsheet or in a compressed format. By accessing these files, you agree to abide by the NREL data disclaimer.

The letter “F” does not always refer to the aperture when marked on a lens. You will often see the letter F on Canon, Nikon, Fujifilm, and Sony lenses, but this is to tell you which mount system the lens belongs to.

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United States Committee on Extension to the Standard Atmosphere, "U.S. Standard Atmosphere, 1962: ICAO Extension to 32 Kilometers. Committee on Extension to the Standard Atmosphere." U.S. Government Printing Office, Washington, D.C., 1962

The air mass zero (AM0), or extraterrestrial spectrum used to generate the terrestrial reference spectra was developed by Gueymard and is a synthesis of several air mass zero data sets. (See Gueymard 2004 for an historical perspective.) Note the spectrum used in conjunction with SMARTS to produce the reference spectra is not the air mass zero spectrum in ASTM E-490-99, as there are slight differences in bandpass and spectral resolution for the two spectra.

If the ratio is 2:1, then the reproduction on the sensor is twice the size of the original. If the ratio is 1:2, the sensor image is half the size of the real thing. You may have the option to change the ratio on your lens.

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Gueymard, C. "The Sun's Total and Spectral Irradiance for Solar Energy Applications and Solar Radiation Models," Solar Energy, Vol 76, Issue 4, April 2004, pp. 423–453.

The American Society for Testing and Materials (ASTM) G-173 spectra represent terrestrial solar spectral irradiance on a surface of specified orientation under one and only one set of specified atmospheric conditions.

USM stands for Ultra-Sonic Motor. Using ultrasonic vibrations to move the elements and focus ring, the USM system is quicker and quieter than the STM driver. Canon use the USM system on their higher end lens models.

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Each lens manufacturer has a unique way of labeling lens features. But you can expect to see the maximum aperture (f-stop) and focal length labeled on the lens. You may also see other lens features, including the lens mount.

Let’s go over the main numbers and letters on lenses to give you  better understanding of your gear. We cover everything from f-stop numbers to magnification ratio. If you want more information, you can see our guide to cameras lenses in this link.

Every lens has a focus ring for manual focusing. Zoom lenses also have the zoom ring for zooming in and out. Newer lenses often have a third control ring, a custom ring you can assign other settings to.

For example, if you have a Sigma 24-70mm f/4 lens with EF on it, you know that it’s compatible with Canon DSLR cameras. If you see Z, you know it’s compatible with Nikon mirrorless cameras.

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The spectra are modelled using the SMARTS2 (version 2.9.2) Simple Model for Atmospheric Radiative Transmission of Sunshine of Gueymard. (See Gueymard 2001 and Gueymard 2003).

Emission spectrum

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On a zoom lens, the focusing distance may be a range of numbers. A reading of 0.5m / 1.64ft – 0.8m / 2.63ft means that as the lens zooms, the minimum focusing distance increases.

F stands for f-stop or f-number and relates to the “speed” of your lens. We use the f-stop scale to denote the lens’s aperture. The size of the aperture is denoted by an “f” followed by a / then a number. It looks like this, f/4.5.

At the widest focal length, the focusing distance is 0.5 meters. But when zoomed in, the focusing distance is 0.8 meters.

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You’ll be able to find the maximum aperture, but your lens may or may not have the “F” on it. You may see a number, a slash, and another number, like this # / #.

We’ve already seen that some lenses have a switch for the image stabilization. But others might have a custom function button. This is labelled L-Fn, and you can assign different settings to it.

Prime lenses have a fixed focal length, meaning there’s no zoom function. A 50mm prime lens with have 50 or 50mm noted on the lens.

Line spectrum

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Macro lenses can take photos of small things or photograph very fine details. A 1:1 magnification ratio means the reproduction on the sensor is the same size or larger than the real life object.

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You will always see the maximum aperture, that is the widest aperture, marked on the lens. This will tell you if the lens is “fast” or “slow”, as fast lenses have wider max apertures.

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All lens manufacturers note the mount system on the lens. This is usually with one to three-letter labels. As we’ve seen, Sony uses FE for it’s full frame lenses.

The receiving surface is defined in the standards as an inclined plane at 37° tilt toward the equator, facing the sun (i.e., the surface normal points to the sun, at an elevation of 41.81° above the horizon).

When buying a new lens, look carefully at the numbers and letters on lenses. Each holds valuable information. One letter (or lack of a letter) may change the lens’s value by hundreds of dollars!

/

STM stands for Stepping Motor, and this AF system uses rotating gears to move the elements and focus ring. This is a more basic system found on Canon’s more affordable lenses.

This is important if you need a new lens cap. You also need to know the diameter if you want to use lens filters. Screw-on filters come in different sizes, so you need to match the filter size to the lens diameter.

or 1:

A lens with a 20mm focal length gives you a wide angle. A 50mm lens is considered standard angle, which is similar to the human eye. A 100mm focal length gives you a tighter, more magnified view.

The symbol that looks like a circle with a line through it (Ø) tells us the diameter of the lens. This is the measurement, in millimeters, around the outside circle of your lens.

Lenses with built-in image stabilization have it noted in the lens’s name and somewhere on the physical lens. All lens brands do this, but they don’t all use the same acronym.

Sigma makes lenses compatible with Canon, Nikon, and other camera brands. By finding the lens mount label, you will know which cameras the lens is compatible with. The same goes for Tamron and Tonika.

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Kurtz, S. R.; Myers, D.; Townsend, T.; Whitaker, C.; Maish, A.; Hulstrom, R.; Emery, K. "Outdoor Rating Conditions for Photovoltaic Modules and Systems," Solar Energy Materials, 2000, Vol 62 #4. pp. 379–391. See also NREL Report No. JA-520-27160.

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Manufacturers stamp lenses with details about the glass, aperture range, focal length, and other specifics. It’s all important information, so it’s a good idea to take the time to learn what all these number and letters stand for.

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For instance, “FE” for Sony identifies the mount system it’s compatible with. In this case, FE tells us the lens is compatible with Sony’s full frame mirrorless cameras.

These distributions of power (watts per square meter per nanometer of bandwidth) as a function of wavelength provide a single common reference for evaluating spectrally selective PV materials with respect to performance measured under varying natural and artificial sources of light with various spectral distributions. The conditions selected were a reasonable average for the 48 contiguous U.S. states over a period of one year. The tilt angle selected is approximately the average latitude for the contiguous U.S.A.

Helium spectrum

The numbers are equal but expressed in two different measurement systems as different countries use either metric or imperial measurements.

Gueymard,C.; Myers, D.; Emery, K. "Proposed Reference Irradiance Spectra for Solar Energy Systems Testing," Solar Energy, Vol 73, Issue 6, December 2002, pp. 443–467.

For both EF and RF lenses, Canon has it’s L series. The L means it’s one of Canon’s top-quality lenses. For example, you have the Canon RF 70-200mm f/2.8L IS USM lens.