These flat discs feature highly polished surfaces with flatness options of λ/4, λ/10, and λ/20. When these optical flats are placed in contact with a test surface, the presence of light and dark bands provide a straightforward visual determination of the surface quality of the test piece.

Neutral density filters can be used to create any combination of (i) a longer exposure time, (ii) a shallower depth of field and (iii) a sharper photograph. Of these three, the first is by far the most common application, so that's where we'll start.

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Variable ND Filters & Stacking. With these, the strength can be adjusted without having to switch filters, but they're also much more expensive — often even more so than a few separate filters. For more flexibility on a budget, many photographers therefore just stack multiple ND filters when they need to reproduce the light-reducing ability of a single stronger (but unavailable) filter.

Filter Mounting Systems. Since ND filters have the same effect regardless of how they're shifted or rotated, using a filter mounting system isn't as critical (unlike with GND filters). However, these still have some advantages, including quicker insertion and removal, or freeing up a hand from holding the ND filter in place.

Neutral density (ND) filters reduce the amount of light entering the camera, enabling a longer exposure time than otherwise possible. This can emphasize motion, or make an otherwise tumultuous scene appear surreal and quiescent. Alternatively, an ND filter also enables larger apertures, which can produce a shallower depth of field, or achieve a sharper photo. Either way, this is a useful and often under-appreciated filter that deserves a deeper look. ND filters are also one of the easiest filters to use, and their effect cannot be replicated digitally — at least not with a single shot.

Photo courtesy of alex campos. Note: above example doesn't use an ND filter, but does illustrate a situation where one could potentially help (depending on the camera's max shutter speed).

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Technical Note: Recall that each "stop" of light-reduction corresponds with a halving of light. A given filter strength therefore passes only 1/2strength of the initial incoming light, where "strength" is the filter strength in stops. For example, a 3-stop ND filter therefore only passes 1/8th the incoming light (since 1/23 = 1/(2*2*2) = 1/8).

Most of our windows are coated with an anti-reflective coating that increases durability and efficiency. Since these AR coatings allow the window to maximize transmission of the desired wavelength of light, it is wavelength specific. The full spectral range of your system must be considered before a selection is made.

In general, better results are achieved when (i) more photos are averaged, and (ii) the exposure time of each photo is long compared to the interval between them. The above example required about 16 averaged shots to adequately mimic an ND filter, in part because each individual exposure was so brief.

Another important specification, the Abbe Number, describes how the refractive index varies with wavelength.  The lower the Abbe number, the higher the dispersion. The Abbe number of sapphire is 72.24, and for BK7, 64.17.

Our state of the art factory is able to manufacture both large optical windows and micro windows for nano-sized optical assemblies. A wide variety of optical substrates are available to choose from, and since production is carried on in-house our design team will be able to match your exact specifications when carrying out your order.

Substrate properties and optical surface specifications are two attributes you will want to keep in mind as you select custom optics for your application.

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On the other hand, these situations are rare, and can usually benefit more from photographing the subject under dimmer (and likely less harsh) lighting. These situations are also unlikely to require anything stronger than a 2 or 3-stop ND filter.

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Our high quality optical windows are suitable for a wide range of applications, from military defense to scientific experimentation, manufacturing, lasers, and high precision photography.

Many other intermediate strengths exist, but high precision typically isn't needed with ND filters. One can often instead adjust the aperture, ISO or shutter speed by one stop without substantially changing the image.

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In the above example, note how the water appears unusual when only two photos were averaged. Note the appearance of a double image from the distant wave crest. This effect is less pronounced with 4 photos, but hasn't been eliminated entirely.

During daylight, achieving this effect usually requires having a 10-stop or greater ND filter strength, which blocks all but a staggering 1/1000th of incoming light. Try revisiting the exposure settings from some of your past images to see what ND filter strengths would've been needed to achieve multi-second exposures. For example, if a given series of landscapes used a 1/50 second shutter speed at optimal aperture and ISO speed, then these would've required a 10-stop ND filter to extend this exposure time to 20 seconds.

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The index of refraction quantifies how much the light is slowed down as it is transmitted through the substrate. It is calculated as the ratio of the speed of light in a vacuum to the speed of light through the substrate. For instance, the index of refraction of Zinc Selenide, 2.631, means that light travels through a vacuum 2.631 times faster than it does through ZnSe.

However, let's focus on a specific waterfall example. Without a filter, one would use both the smallest aperture and the lowest ISO speed available. With a waterfall under soft daylight, f/22 and ISO100 might yield an exposure of 1/10 second. Unfortunately, not only is this duration insufficient, but the f-stop also had to be increased to the point of reducing sharpness due to diffraction.

Although ND filters cannot be replicated digitally, certain scenes and exposure times can be adequately emulated by employing a technique called image averaging. This works by combining several separate photos in a way that simulates a single, longer exposure:

Although ND filters are primarily used to achieve longer exposures, a less common application is to enable a shallower depth of field in very bright light. For example, most SLR cameras have a maximum shutter speed of 1/4000 second, so a subject in direct sunlight might therefore require an f-stop greater than about f/4.0 (at ISO100). With a 2-stop ND filter, one could reduce this to f/2.0 — yielding a dramatic improvement in background blur and subject isolation.

Filter Color Casts. Some cheaper ND filters can introduce a color cast. Luckily this isn't a major problem with digital cameras, since this can usually be corrected by adjusting the photo's white balance in post-processing. However, in extreme cases, cheaper filters can introduce color casts which don't correlate with white balance settings — making color cast removal much more difficult.

An ND filter is nothing more than a semi-transparent piece of glass that gets placed in front of your lens. What makes it special, however, is that it obstructs a precisely controlled fraction of incoming light, and does so uniformly — thereby not altering image contrast or sharpness. The obstruction also aims to be equal across the visible spectrum, thereby not introducing a color cast (although this isn't always the case). This last characteristic also happens to be why it's called a neutral density filter.

Surface quality refers to the presence or absence of surface imperfections: scratches and digs. It is quantified with a two-part scratch dig number, as specified by the United States Military Performance Specification MIL-PRF-13830B. The lower the number the better the surface quality.

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Perhaps the biggest problem with averaging is that moving objects aren't always rendered as continuous streaks — no matter how many photos are averaged. Photos containing people, a visible sun, stars or other moving objects are all particularly susceptible.

We can also customize optical filters for your device, as requested. These are optical windows with dyes injected into the substrate in manufacturing, or with special coatings, and they transmit selective bands or colors of light.

If your specifications application is weight-sensitive, you may need to pay attention to substrate density. Although the refractive index of optical materials tends to increase as density increases, this is not always the case, and the relationship is not always linear.

Even though they might appear gray or even opaque to our eyes, this isn't how your photo will appear; the camera's metering automatically compensates by letting in more light. However, the viewfinder will still appear very dark, so photographers often compose their image prior to placing the filter in front of the lens.

Shanghai Optics offers wedged windows in addition to our range of parallel window offerings. These optical components, which feature a controlled wedge in the optical path, are ideal for laser systems because they prevent common problems that a parallel window setup might entail. For instance, power spikes through unwanted reflections, interference effects, and mode-hopping can all be eliminated or control through the use of wedged windows.

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A scratch-dig number of 40-40 or even 80-50 is appropriate for most optical windows, especially those used for imaging systems. Some precision applications may require 40-20. High power laser systems require high surface quality, perhaps 20-10 or 10-5. The lower the scratch-dig number, the higher the manufacturing cost, and there is a high cost premium associated with 20-10 or 10-5 windows.

Surface flatness is described in terms of the deviation from a completely flat surface, and is often measured with a precise reference piece called an optical flat. Deviations from perfect flatness can be quantified and are given in what is called waves, abbreviated λ.  Lower λ implies higher flatness. While flatness of 1λ is sufficient for most applications, precision optics such as high power lasers may require surface values of as low as λ/20.

ND filters are specified by their light-reducing ability, where stronger filters appear as darker shades of gray. Some common specifications are summarized below:

Longer exposure times can achieve a wide variety of artistic effects, including softening the appearance of turbulent water, blurring waves of blowing grass, or emphasizing motion within a crowd of people. For a full discussion of these and other examples, see the tutorial on using shutter speed creatively.

Optical windows are flat plates made of optically transparent material, designed to allow light into an optical instrument. They can also be used to protect a light source from an outside environment.

How long of an exposure does one typically need? For full effect, multi-second exposures are usually a must. These can render clouds as streaks in the sky, blur moving people beyond recognition, or make waves appear as a uniform, low-laying mist. However, this depends on the nature of the motion, the amount of subject magnification and the desired effect. The key is lots of experimentation.

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Alongside our regular range of high quality optical windows, Shanghai Optics offers optical flats for measurement purposes or for use in applications where precision is at a premium.

Our optical flats are carefully tested with precision metrology equipment to ensure they fully meet the flatness level specified.  Do be aware that optical flats do deteriorate over time and with use, and should be periodically recalibrated.

These windows are designed to minimize both reflection and absorption while maximizing transmission over a target wavelength range. When choosing an optical window, you will have to keep in mind optical surface specifications, material transmission properties, and the mechanical properties required by your application.

MgF2 windows are hard and durable, with a refractive number of 1.378 and an Abbe number of 106.22. They have a high laser damage threshold, and very good broadband transmission from 120 nm to 8 μm. Our MgF2 windows are often used with UV radiation sources and receivers.

The material properties of the substrate used will determine transmission, refractive index, and hardness. Our Potassium Bromide substrates, for instance, will transmit UV visible and infrared light. They have a density of 2.75 g/cm3 and an index of refraction of 1.527. Zinc Selenide, on the other hand, blocks UV as well as some visible light and transmits higher wavelength visible light and infrared. It has an index of refraction of 2.631. Fused silica has a density of 2.202g/cm and a index of refraction of which varies from 1.55 to 1.40.

Using an ND filter allows you to address both of these problems, but the extent to which either is improved depends on how you choose to allocate its effect. With a 5-stop ND filter, the same settings would yield a 32X longer exposure time — giving the water a much silkier appearance. Alternatively, one might feel that a 16X (4-stop) exposure time increase is sufficient when weighed against the potential for having a sharper photo, and could instead also decrease the f-stop to f/16.

Step-Up & Step-Down Adaptors. These prevent you from having to purchase a different screw-on filter size for each lens diameter (usually listed as 77 mm, 72 mm, etc.). One could then just purchase an ND filter that fits their largest lens, and use what's called a "step-up adaptor" to have this same ND filer fit on any smaller lenses.

Substitutes. Sometimes other filter types can be used in place of an ND filter. Depending on the size of your GND filters, these can sometimes be used as ND filters if the darker region is large enough to cover the entire lens. Alternatively, polarizing filters can function as 2-3 stop ND filters, but only if one also wants the polarized look, and only when this filter is rotated for maximum effect. Some photographers don't purchase 2 or 3-stop ND filters for this reason, and instead opt for stronger varieties (such as a 10-stops).

The refractive index of our sapphire windows is 1.76-1.77. For optical windows, the refractive index is typically specified at 587.6 nm, the Helium d-line wavelength.  Optical glasses with a high index of refraction are sometimes called ‘flints’, while windows with low index of refraction are called “crowns”.