Optical flatness testingpdf

Using an ND filter to capture slow-shutter landscape photos can take careful technique, especially if you’re using a strong filter (e.g., 10 stops or 16 stops).

And hopefully, now that you’ve read this article, you love them, too! So buy an ND filter or two. Start practicing. And take some amazing shots!

The photo below shows a two-stop graduated neutral density filter in a square filter holder. The top half of the filter is dark (to block light) and the bottom is clear:

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Even optical flares beta from a robust material such as fused silica should be handled with great care in order to avoid any surface damage. For example, one should always store optical flats in a suitable box, in which it is wrapped with a soft material.

As mentioned above, it’s tough to see through a powerful ND filter. So you’ll need to identify your composition and set your point of focus before adding the filter. (You may also want to determine the proper exposure prior to adding the filter, then adjust the shutter speed to compensate once you’ve added the filter to your lens).

As you’ve probably gathered, neutral density filters are especially useful for portrait photography, especially portraits done in bright sunlight. They’re also highly useful for landscape photography – if your goal is to blur a moving subject, an ND filter can probably make it happen.

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But I wanted blur in the water, so I added an ND filter and turned that exposure time into a whopping 210 seconds. Here’s the resulting shot:

ND filters block light, yet photographers generally want more light, not less. So why should you consider using an ND filter?

That’s where ND filters come in. You can use your ND filter to block the light, which in turn allows for beautiful long exposures of landscapes.

You see, I use an ND filter kit, the circular Formatt Hitech 72mm Firecrest Joel Tjintjelaar Signature Edition Long Exposure Kit. It contains three neutral density filters, with strengths of 3, 6, and 10 stops.

Just choose a relatively weak ND filter (in the one-stop to three-stop range). Mount it in front of your lens. And take photos the way you normally would, except with greater latitude when choosing the aperture.

Different neutral density filters offer different levels of light-blocking power, generally measured in stops of light. Typical ND filter strengths are one stop (written as “0.3” or “ND2”), two stops (written as “0.6” or “ND4”), three stops (written as “0.9” or “ND8”), six stops (written as “1.8” or “ND64”) and ten stops (written as “3.0” or “ND1024”).

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Unfortunately, when the light is strong, a wide aperture will let in too much light, resulting in overexposure. So the blur-happy photographer is either forced to wait until the light dies down, or must use a narrow aperture and sacrifice their beautiful wide-aperture backgrounds.

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If you still can’t get the aperture you’re after, then you can always swap out your original ND filter for a stronger option.

Ultimately, you have to decide how much you want to spend (some ND filters are quite expensive!), then look at the options. But I can start by telling you which filters I own and recommend.

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In other words, a neutral density filter is a dark piece of glass that goes in front of (or behind, in some special circumstances) your lens.

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But to create a long exposure, you generally need a shutter speed of a second or longer, which is pretty much impossible to do in the middle of the day without dramatic overexposure.

In this article, I share everything you need to know about ND filters, including basic instructions, specific gear recommendations, plus some helpful advice along the way.

In fact, I’d say that this function – for long exposures in the landscape – is probably the most common reason to see an ND filter on a photographer’s lens.

Further, we have many interesting case studies on the same page, with topics mostly in fiber optics. Concrete examples cases, investigated quantatively, often give you much more insight!

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Optical flats are mainly used as highly flat reference surfaces in interferometers for checking the flatness of optical elements such as optical windows, laser mirrors, prisms, optical filters or laser crystals. For example, one may place an optical flat close to an inspected surface, but slightly tilted against that surface, so that surfaces with ideal quality would result in a regular pattern of straight interference fringes when the assembly is illuminated with monochromatic light. Any deviations from that pattern then indicate deviations from perfect flatness.

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Some manufacturers even make neutral density filters that block 16 stops of light or more, although these are specialty items and infrequently used by the average photographer (professional or otherwise).

Note that the higher the ND filter strength, the more light that gets blocked. So an ND2 filter, which blocks one stop of light, pales in comparison to an ND64 filter and its six-stop light-blocking capabilities.

With further production steps, such as the application of a dielectric coating, they optical flats also be turned into high-quality optical elements themselves. For example, a Twyman–Green interferometer requires a very flat reference mirror, which can be made from an optical flat.

How do you plan to use your neutral density filters? Which filters do you plan to buy? Share your thoughts in the comments below!

Different degrees of surface quality can be obtained. A typical flatness specification would be λ/10, but even higher qualities are available. Usually, the surface quality should be substantially better than the specifications of the objects to be tested, so that obtained deviations from flatness are largely dominated by the quality of those objects.

Just make sure to watch the light carefully. If the sun goes behind clouds, you may need to take the filter off the front of your lens to get the shots you want.

The photo below shows a ten stop neutral density filter mounted on the front of a lens. As you can see, the filter is nearly opaque; once it’s added to your setup, you won’t be able to see through the camera viewfinder.

Note: If you plan to use your filters on multiple lenses, buy the filter size you need for the largest lens, and get step-down rings to adapt the filters to fit the smaller lenses – or get a square drop-in filter kit instead.

So if you want to know how to use a neutral density filter, when to use an ND filter, or why to use an ND filter, you’ve come to the right place.

Typically, an optical flat has a cylindrical shape, a diameter of a couple of centimeters, a thickness of a few millimeters, and is made of fused silica or some other clear optical glass. It thus looks like a mirror substrate. However, different types of optical flats are available:

Artifex Engineering offers precisely flat reference plates for optical testing. Our optical flats are available as single surface and dual surface options. We will gladly customize to your requirements. Additionally we offer standard optical flats in N-BK7 and fused silica. Visit our product page for more information. We look forward to your inquiry.

Opticalflat interferometer

Optical flats are plates with one or two surfaces of particularly high optical quality and flatness. The degree of flatness is quantified with the distance between two parallel imaginary planes, where one is tangential to the highest point of the surface and the other one to the lowest point. That flatness measure is normally at least λ/10, and sometimes even substantially better – e.g. down to a few nanometers. The surface roughness is also usually very low.

During interferometric measurements, both the optical flat and the tested surface should be very clean; see the article on cleaning of optics.

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It’ll fit over the lens and block out some of the bright light. That way, you can use a wide aperture to achieve your desired blur effect without overexposing the shot.

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Neutral density filters block light evenly across the frame. Graduated neutral density filters, on the other hand, block light across just part of the frame. Half the filter is clear, half is opaque, and you get a graduated area in between.

As you may be aware, wide apertures create beautiful blurry background bokeh that makes the subject pop off the page (portrait photographers love this!).

In general, graduated filters are used by landscape photographers to deal with tricky sky-foreground lighting conditions. And unlike neutral density filters, their purpose isn’t to slow down the shutter speed or widen the aperture – instead, it’s simply about blocking out a too-powerful sky.

Make sure you’ve mounted your camera to a tripod (you don’t want to end up with any camera shake!). Then trigger the shutter using a remote release or your camera’s self-timer function.

A neutral density filter blocks light, which is why it’s sometimes referred to as sunglasses for your lens. The result? Less light passes through the lens to reach the camera’s sensor.

Used as a reference surface, Knight Optical's optical flats are provided in single-or double-sided Zerodur and quartz varieties with λ/10 or λ/20 flatness available. Our stock catalogue includes 26 mm to 302 mm diameter optical flats which are packaged in a protective wooden box. Custom optical flats are also available with differing dimensions and flatness specifications. We also stock monochromatic light units which are used to view the fringe patterns on the surface of the optical flat when testing a surface.

However, you can certainly use ND filters in other scenarios. For instance, an architectural photographer might want to blur the clouds when photographing buildings or even home exteriors. An abstract photographer might want to blur water when photographing on the beach. And a videographer might want to keep their aperture wide while filming in the sun.

For checking the quality of optical flats, one may inspect them in interferometers, ideally using an even better reference surface. In some cases, one uses reference surfaces based on a liquid like mercury, which can be extremely flat but is difficult to handle.

Well, as it turns out, more light is good…most of the time. Occasionally, though, you’ll find that you have too much light to work with, which is where ND filters come in handy.

GND filters are designed to handle uneven exposures. If you’re photographing a sunset with a bright sky and a dark foreground, the darker portion of the GND filter will dial back the sunset, while the lighter portion will do nothing. That way, you get a well-exposed sky and a well-exposed foreground in the same photo. Make sense?