Although digital camera sensors are capable of “seeing” infrared and ultraviolet parts of the spectrum, camera manufacturers put filters in front of the sensor that filter most of this out. When you get your camera converted to infrared, the conversion company replaces these filters with ones that allow the sensor to see a specific part of the infrared spectrum and may also include part of the visible spectrum. When you look at the choices offered, you can actually choose how much of the spectrum your converted camera will see. This offers a variety of looks from which you can choose. Because of the intricate and delicate nature of converting digital camera sensors for infrared, it is very important to choose a reputable and experienced conversion company.

The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam. For more general optical systems, the focal length has no intuitive meaning; it is simply the inverse of the system's optical power.

Camera lens focal lengths are usually specified in millimetres (mm), but some older lenses are marked in centimetres (cm) or inches.

Opticallenses

Simon Weir’s tests of Fujifilm (XF) mount lenses A useful resource for Fuji shooters. He has information for both hot spots and sharpness

In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with lower magnification and a wider angle of view. On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection.

The corresponding front focal distance is:[6] FFD = f ( 1 + ( n − 1 ) d n R 2 ) , {\displaystyle {\mbox{FFD}}=f\left(1+{\frac {(n-1)d}{nR_{2}}}\right),} and the back focal distance: BFD = f ( 1 − ( n − 1 ) d n R 1 ) . {\displaystyle {\mbox{BFD}}=f\left(1-{\frac {(n-1)d}{nR_{1}}}\right).}

Why is this true? Camera lenses are designed to be optimized for visible light so that they render as accurately as possible, how we see with our eyes. Infrared light is transmitted by lenses, but normally focuses on a different plane than the camera’s sensor.

In the end, shooting infrared comes down to aesthetics and your personal preferences. Hot spots can be annoying but soft edges and/or a hazy feel are often considered part of the charm of infrared photography, especially in black and white. My aim for this article is to arm you with enough information to make informed choices based on the way you shoot or intend to shoot while participating in the amazing world of infrared photography.

The main benefit of using optical power rather than focal length is that the thin lens formula has the object distance, image distance, and focal length all as reciprocals. Additionally, when relatively thin lenses are placed close together their powers approximately add. Thus, a thin 2.0-dioptre lens placed close to a thin 0.5-dioptre lens yields almost the same focal length as a single 2.5-dioptre lens.

Sony E 18-135mm f3.5-5.6 OSS (My top choice for Sony APS-C. Excellent overall with no hot spots at any aperture and tested focal lengths)

When a photographic lens is set to "infinity", its rear principal plane is separated from the sensor or film, which is then situated at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane.

Due to the popularity of the 35 mm standard, camera–lens combinations are often described in terms of their 35 mm-equivalent focal length, that is, the focal length of a lens that would have the same angle of view, or field of view, if used on a full-frame 35 mm camera. Use of a 35 mm-equivalent focal length is particularly common with digital cameras, which often use sensors smaller than 35 mm film, and so require correspondingly shorter focal lengths to achieve a given angle of view, by a factor known as the crop factor.

Infraredlens for camera

I’m rather compulsive about testing every lens I buy, both for visible light and infrared. There have been several times I’ve gone through 2 or 3 samples to get a really good one. I’ve got the visible light testing down as I’ve been doing that for decades. In recent years, I’ve figured out some good tests for infrared. Generally the number one issue is hot spots. With my studio being located in northern Arizona, more days than not, I have a clear blue north sky to shoot hot spot tests. I am able to test resolution using the same ISO 12233 targets as for visible light, in my studio. Because resolution can change with focus distance my studio test is limited to a practical range of about 3-18 feet (1-6 meters) so I will also do field tests to check lenses at greater distances. These are the controlled tests. I will also shoot various subjects in a less controlled way just to try different situations, sun angles, etc.

For a thick lens (one which has a non-negligible thickness), or an imaging system consisting of several lenses or mirrors (e.g. a photographic lens or a telescope), there are several related concepts that are referred to as focal lengths:

A lens with a focal length about equal to the diagonal size of the film or sensor format is known as a normal lens; its angle of view is similar to the angle subtended by a large-enough print viewed at a typical viewing distance of the print diagonal, which therefore yields a normal perspective when viewing the print;[9] this angle of view is about 53 degrees diagonally. For full-frame 35 mm-format cameras, the diagonal is 43 mm and a typical "normal" lens has a 50 mm focal length. A lens with a focal length shorter than normal is often referred to as a wide-angle lens (typically 35 mm and less, for 35 mm-format cameras), while a lens significantly longer than normal may be referred to as a telephoto lens (typically 85 mm and more, for 35 mm-format cameras). Technically, long focal length lenses are only "telephoto" if the focal length is longer than the physical length of the lens, but the term is often used to describe any long focal length lens.

Infraredlens glasses

Disclosure about product links and affiliations: Many of the links I provide are affiliate links which means I get a small commission, with no additional cost to you, if you click it and end up buying something. It helps keep the lights on as well as supporting this blog and my other free educational resources and articles. So if you use them, thanks! It is always my goal to report my results in a straighforward manner whether or not it favors a particular product.

Sony E 18-200mm f3.5-6.3 OSS LE (great for its absence of hotpsots at all apertures and focal lengths but only moderate performance for sharpness, as one might expect for this range. NOTE: Sony makes two other 18-200mm APS-C lenses. I only tested this one, not the PZ or older non-LE models )

Infraredlens photography

Fujifilm XF 16-80mm f4 OIS (no hot spots wide open through f5.6 at 16-24mm. Good through f8 at 35mm. 50-80mm good at all apertures)

In the sign convention used here, the value of R1 will be positive if the first lens surface is convex, and negative if it is concave. The value of R2 is negative if the second surface is convex, and positive if concave. Sign conventions vary between different authors, which results in different forms of these equations depending on the convention used.

For a spherically-curved mirror in air, the magnitude of the focal length is equal to the radius of curvature of the mirror divided by two. The focal length is positive for a concave mirror, and negative for a convex mirror. In the sign convention used in optical design, a concave mirror has negative radius of curvature, so

As s1 is decreased, s2 must be increased. For example, consider a normal lens for a 35 mm camera with a focal length of f = 50 mm. To focus a distant object (s1 ≈ ∞), the rear principal plane of the lens must be located a distance s2 = 50 mm from the film plane, so that it is at the location of the image plane. To focus an object 1 m away (s1 = 1,000 mm), the lens must be moved 2.6 mm farther away from the film plane, to s2 = 52.6 mm.

The optical power of a lens or curved mirror is a physical quantity equal to the reciprocal of the focal length, expressed in metres. A dioptre is its unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. For example, a 2-dioptre lens brings parallel rays of light to focus at 1⁄2 metre. A flat window has an optical power of zero dioptres, as it does not cause light to converge or diverge.[10]

Focal length (f) and field of view (FOV) of a lens are inversely proportional. For a standard rectilinear lens, F O V = 2 arctan ⁡ ( x 2 f ) {\textstyle \mathrm {FOV} =2\arctan {\left({x \over 2f}\right)}} , where x is the width of the film or imaging sensor.

For the case of a lens of thickness d in air (n1 = n2 = 1), and surfaces with radii of curvature R1 and R2, the effective focal length f is given by the Lensmaker's equation:[5]

I have found that potential lens issues when shooting infrared are situational. By this I mean depending on the lighting, subject matter, and the specific conversion, issues with lenses may show up to a greater or lesser degree. For example, if you’re shooting a nature scene that has lots of trees in the center area of your image, you may not see a hot spot that might be visible if that same area was clear blue sky or with different lighting angles. Hot spots, in particular, can be more or less prevalent depending on the lighting. And typically hot spots are less noticeable with black and white. Generally hot spots are the worst with cameras converted to “pure” infrared in the 830nm-850nm range. So if you get a conversion with a lower number/frequency such as a 590nm, it is possible you may not have problems or as many problems with a given lens. Additionally you may encounter different results with a “full spectrum” conversion.

I’m including lenses I’ve formally tested as well as those I’ve found to work well for infrared but prior to implementing my more formal tesing. I’ve used all of them in real world shooting. The lenses I’ve formally tested are ones I purchased more recently for Fujifilm APS-C (XF mount) and Sony full frame (FE mount) cameras. My tests were done with 665nm conversions for both. I’ve also included a few lenses for Micro 4/3 using a 590nm converted camera, that I have found to work well in real world use.

To render closer objects in sharp focus, the lens must be adjusted to increase the distance between the rear principal plane and the film, to put the film at the image plane. The focal length f, the distance from the front principal plane to the object to photograph s1, and the distance from the rear principal plane to the image plane s2 are then related by:

Joel Wolfson is an internationally published photographer who loves teaching as much as shooting. He shares his 30 years of experience as a working pro with other photographers and enthusiasts by way of his workshops, 1 on 1 training, video tutorials, articles, blog and speaking engagements. His technical articles have been translated for use in more than 30 countries yet he is best known for his artistic images of nature’s fleeting moments and unexpected views of everyday places around the globe. He is one of the pioneers of digital photography having conducted digital photography seminars for Apple and other corporations starting in the early 90s.  His roster of notable clients includes numerous publications and fortune 500 companies. He currently works with great affiliates like Arizona Highways, Topaz Labs, ON1, and Skylum to have more avenues for working with those wanting to pursue their love of photography. His goal is to make learning and improving one’s photography easy, fun and rewarding.

LifePixel hot spot testing database The testing was done by LifePixel themselves, of various Canon, Nikon, Fujifilm, and Sony lenses

The most common issues or problems that show up with infrared that you won’t see in the same lens with visible light photography are “hot spots”, loss of acutance (sharpness), and sometimes excessive flare.

While it’s quite easy to find multiple reviews for any given lens, they rarely cover infrared performance. For this reason I have done a number of exacting tests myself on numerous lenses using infrared. In some cases I have sought out third party lenses when the manufacturer doesn’t have one in my desired focal length that works well in infrared. Sometimes I just end up buying a lens and trying it because I can’t find enough (or any) information about its infrared performance.

For an optical system in air the effective focal length, front focal length, and rear focal length are all the same and may be called simply "focal length".

If you’re lucky, you find a lens that will work well for both visible light and infrared. For example the Sony 100-400mm f4-5.6 GM is stellar for both as long as you don’t try to use the 1.4X teleconverter for infrared. For a lot of my photography (both visible and infrared) I like to have a walkabout lens that covers a broad range of focal lengths so I don’t miss shots while changing lenses. For my Fuji system I keep the Fuji XF 18-135mm on my infrared body and the Fuji XF 16-80mm on my visible light body. The XF 18-135mm happens to be superb for infrared. In the case of the Sony full frame system, I really like the FE 24-240mm. It’s certainly less than perfect for infrared but the range is so handy I just try to avoid focal lengths and apertures that are troublesome. Though it isn’t ideal, with the overlap of the Sony/Zeiss 35mm f2.8 prime, 12-24mm, and 100-400mm I can cover most situations where the 24-240mm might fall short.

For a thin lens in air, the focal length is the distance from the center of the lens to the principal foci (or focal points) of the lens. For a converging lens (for example a convex lens), the focal length is positive and is the distance at which a beam of collimated light will be focused to a single spot. For a diverging lens (for example a concave lens), the focal length is negative and is the distance to the point from which a collimated beam appears to be diverging after passing through the lens.

Something else I’ve noticed but isn’t widely reported is loss of acutance with the same lens compared to its visible light performance, especially as you go out from the center of the image. Because people use different terminology for resolution or acutance, for ease of explanation, let’s call is apparent sharpness. It is not always strictly how well the lens resolves that leads to loss of apparent sharpness. It can also be lack of contrast, other optical factors or a combination of these. For example, a subtle broad and gradual hot spot will cause a loss of apparent sharpness in the central area by reducing contrast there.

When a lens is used to form an image of some object, the distance from the object to the lens u, the distance from the lens to the image v, and the focal length f are related by

Hot spots are usually bright areas in the middle of the image and tend to get worse and more defined as you stop down. Some lenses don’t exhibit hot spots at all and thus are preferable for infrared. Or hot-spots may not appear at wider apertures but show up as you stop down (example below.) With zoom lenses you may get hot spots at some focal lengths and not others.

The distinction between front/rear focal length and EFL is important for studying the human eye. The eye can be represented by an equivalent thin lens at an air/fluid boundary with front and rear focal lengths equal to those of the eye, or it can be represented by a different equivalent thin lens that is totally in air, with focal length equal to the eye's EFL.

I have several recommendations (see My Recommended Lenses below) based on my tests and real world use. I’ve also included a resource section for tests done by others.

Finally, the flare issue is highly variable and dependent on the situation much like visible light. In my experience, some lenses may be more prone to flare using infrared than with visible light. For me it’s not a deal breaker if the lens is otherwise good for infrared. I treat it as I would with visible light: I try to avoid shooting directly into the sun or other light sources. For those times I want a sun star I just try it and see if I get excessive flare or not.

Infraredlens for iPhone

The focal length of a thin convex lens can be easily measured by using it to form an image of a distant light source on a screen. The lens is moved until a sharp image is formed on the screen. In this case ⁠1/u⁠ is negligible, and the focal length is then given by

Keep in mind that your results could be different, particularly with different camera models and IR conversions but this should be a good guideline. It is always best if you can try before you buy and if you can’t do that, then rent or make sure the store has a good return policy.

For an optical system in a medium other than air or vacuum, the front and rear focal lengths are equal to the EFL times the refractive index of the medium in front of or behind the lens (n1 and n2 in the diagram above). The term "focal length" by itself is ambiguous in this case. The historical usage was to define the "focal length" as the EFL times the index of refraction of the medium.[2][4] For a system with different media on both sides, such as the human eye, the front and rear focal lengths are not equal to one another, and convention may dictate which one is called "the focal length" of the system. Some modern authors avoid this ambiguity by instead defining "focal length" to be a synonym for EFL.[1]

A fantastic lens for normal visible light photography may be terrible for infrared (aka “IR”) photography. Choosing the right lens can be tricky. If you’re planning to get an infrared conversion for your camera, make sure you have lenses that will work well for it or be ready to buy one or more additional lenses, depending what you plan to shoot with your infrared camera.

The list of lenses below are ones that range from partially useable to excellent for infrared. Please read the comments to guide you.

Panasonic Lumix G Vario 100-300mm f4-5.6 ver 1 (hot spots at 100mm but does well above 180mm. This version 1 lens is discontinued but may be available used.)

The focal length of a lens determines the magnification at which it images distant objects. It is equal to the distance between the image plane and a pinhole that images distant objects the same size as the lens in question. For rectilinear lenses (that is, with no image distortion), the imaging of distant objects is well modelled as a pinhole camera model.[7] This model leads to the simple geometric model that photographers use for computing the angle of view of a camera; in this case, the angle of view depends only on the ratio of focal length to film size. In general, the angle of view depends also on the distortion.[8]

Kolari Vision hotspot database This is a compilation database they put together of numerous major equipment brands and various third party lenses.

1 f = ( n − 1 ) ( 1 R 1 − 1 R 2 + ( n − 1 ) d n R 1 R 2 ) , {\displaystyle {\frac {1}{f}}=(n-1)\left({\frac {1}{R_{1}}}-{\frac {1}{R_{2}}}+{\frac {(n-1)d}{nR_{1}R_{2}}}\right),} where n is the refractive index of the lens medium. The quantity ⁠1/f⁠ is also known as the optical power of the lens.

Infraredlens material

In addition to my testing that I’ve provided here, the links that follow are a decent starting point to further help you choose lenses. One of the links is specific to Fujifilm X-mount lenses. The other two include several brands. For the most part they are all geared towards hot-spot performance. Simon Weir’s tests (Fuji mount only) provides some comments on sharpness too.

Fujifilm XF 27mm f2.8 R WR 2nd gen (only useable at f2.8 no hot spot wide open but hot spot from f4-f16. More defined as you stop down. It is so well defined at the smallest apertures, it could be spotted out in many cases)

Many infrared shooters like the dreamy look of softer edges and don’t seek to have the same apparent sharpness they’d expect with visible light. With the infrared image that I used for my example below I had good success getting sharpness comparable to visible light using Topaz Sharpen AI. I don’t show the sharpened results here because I want viewers to be able to see the infrared versus visible light difference. However, with such amazing tools available to us, it can now be an artistic choice.

Determining the focal length of a concave lens is somewhat more difficult. The focal length of such a lens is defined as the point at which the spreading beams of light meet when they are extended backwards. No image is formed during such a test, and the focal length must be determined by passing light (for example, the light of a laser beam) through the lens, examining how much that light becomes dispersed/ bent, and following the beam of light backwards to the lens's focal point.

Panasonic Lumix G Vario 14-45mm f3.5-5.6 (excellent throughout range of focal length and apertures. This lens is discontinued but you may find them used)

Note for the technically minded IR shooters: I think the 665nm conversion used in most of my tests is a useful one as it sits between the 590nm which tends to be more forgiving and the 830nm which tends to be more unforgiving regarding infrared issues.