A fast lens such as my usual Leica Summilux is built for shooting wide open, even when the light is difficult. But in strong daylight, f/1.4 can easily let in too much light. That is where the neutral density filter — ND filter — becomes useful. It is not an effect filter in the classic sense. It is a control tool. That’s recognised by the description ND – Neutral Density. It is designed to let less light in without adding any effects at all.
It allows you to keep the aperture, shutter speed, or motion feeling you want without overexposing the photograph.

An ND filter reduces the amount of light entering the lens while ideally leaving colour, contrast, and sharpness unchanged. In practical terms, it works like sunglasses for the lens: less light reaches the sensor or film, but the character of the scene should remain the same.

ND strength is measured in “stops”, where each stop halves the amount of light.
1-stop ND lets through 50% of the light.
2-stop ND lets through 25%.
3-stop ND lets through 12.5%, and so on.
This is why ND filters allow wider apertures or slower shutter speeds in bright light.

Why use an ND filter?

There are two main reasons to use ND filters.

First, to shoot a fast lens wide open in bright light. If you want the Summilux look at f/1.4 — shallow depth of field, subject separation, glow, bokeh, and the lens signature at full aperture — daylight may force your shutter speed beyond what the camera can handle. An ND filter reduces the light so you can stay at f/1.4. As soon as you stop down the aperture to, like, f/8, the need might disappear, because less light is now entering the sensor or film.

Second, to use slower shutter speeds. For water, clouds, traffic, movement, or people passing through a frame, an ND filter lets you extend exposure time without overexposure. This is how photographers create soft water, streaking clouds, or a slight blur of human movement.

Fixed ND filters

A fixed ND filter has one strength only. For example:

Fixed ND filters are optically simpler than variable ND filters. They are usually made from one piece of coated glass or resin with a uniform density. Because there are fewer optical compromises, good fixed ND filters are generally more consistent in colour, contrast, and sharpness than variable filters, especially at stronger densities.

For high-quality lenses — Leica Summilux, Noctilux, APO-Summicron, Voigtländer Nokton, Zeiss, or similar — fixed ND filters are usually the safest choice when image quality matters most. You bought the lens for its optical quality; the filter should not become the weakest piece of glass in the system.

Left, a photo where I used an ND filter to keep the aperture fully open and create a shallow depth of field without overexposing the image,
whereas the photo at the right is shot at f/8, and therefore the ND filter wasn’t needed.

Variable ND filters and why I stopped using them

A variable ND filter gives you a range of densities in one filter. Instead of changing filters, you rotate the front ring. Most variable ND filters are made from two polarising layers. As one layer rotates against the other, less or more light passes through. I bought and tested a variable ND filter for some time and concluded that it isn’t for me. For the reasons described here. But first the good parts.

The advantage is obvious: speed and convenience. If the light changes quickly, a variable ND can be very practical. Instead of carrying ND4, ND8, ND16, ND32, and ND64, you carry one filter and adjust it by feel.

The disadvantage is also important: a variable ND is not just “a fixed ND that can change strength”. It is an optical compromise.

I also found out, on a practical level, that I tended to push the rotating ND filter when carrying the camera around, so it didn’t stay in its position. This caused me to readjust, and made me insecure if it was in the right position when lifting the camera. 

Because variable ND filters are based on polarisation, they can introduce uneven darkening, colour shifts, and polarisation effects. At stronger settings, some variable NDs can produce the well-known dark “X” pattern across the image. This is especially visible near maximum density and with wider lenses. I used it on my 50mm favourite lens, so I didn’t notice this in the period I tried it out.

The test image (left) is with the ND filter open, letting much light in, and on the right you see the colour shift when the ND filter is at maximum effect.

ND filters can also affect reflections, skies, windows, water, and shiny surfaces in ways you may not want. Since the variable ND is partly behaving like a polariser, it can change the look of the scene beyond simply reducing exposure. This can be useful — or it can be a problem if you want a neutral, natural rendering.

Why this matters

Yoy can “translate” this to other lenses with similar characteristics.

If we look at the Leica Summilux, it is a high-resolution, high-contrast, characterful lens. The filter in front of it should be chosen with the same seriousness as the lens itself. Leica’s own technical data for the Summilux-M 50mm f/1.4 lists an E46 filter thread, and the Summilux-M 35mm f/1.4 ASPH also uses E46 in the current version.

That does not mean every Summilux uses E46 — older versions and other focal lengths can differ — but it does mean that many modern M Summilux lenses use small, compact filter sizes. Always check the exact lens version before buying.

For these lenses, I would recommend:

1. Use high-quality fixed ND filters for still photography. I use B+W and Heliopan.
2. Choose slim, multi-coated filters to reduce flare, reflections, and vignetting.
3. Avoid cheap variable NDs if you care about colour, microcontrast, and lens rendering.
4. Use a variable ND mainly when speed matters more than absolute optical purity.
5. Do not stack too many filters on small Leica lenses, as this can increase flare and mechanical vignetting.

Schneider-Kreuznach/B+W describes its higher-end filters as using polished and finely ground glass, MRC anti-reflective coatings, and brass mechanics, with the MASTER and T-Pro lines aimed at professional demands. Those are exactly the types of qualities worth looking for on a Summilux-class lens. Heliopan has the same qualities.

What quality should be used?

For Leica Summilux and similar lenses, the filter should be:

• Optical glass, not cheap resin.
• Multi-coated or nano-coated.
• Colour-neutral.
• Slim-mounted, especially on wider lenses.
• Mechanically precise, preferably brass or high-grade aluminium.
• From a reputable maker.

B+W’s MRC Nano Master ND filters, for example, are listed with Schott optical glass, multi-resistant nano coating, and brass rings; their 3-stop and 10-stop filters are specifically described as reducing exposure while allowing wider apertures or slower shutter speeds.

Which ND strengths should you buy?

For a Leica Summilux shooter, I would start with two fixed ND filters:

ND8 / 3-stop

This is the everyday Summilux filter. It is useful when you want to shoot at f/1.4 in bright but not extreme light. It gives enough reduction to keep the lens wide open without making the view too dark or exposure times too long.

ND64 / 6-stop

This is the stronger creative filter. It lets you shoot wide open in harsher daylight, or slow the shutter enough for visible motion blur. It is also useful when photographing water, sea, trees moving in wind, or people passing through a street scene.

Optional: ND1000 / 10-stop

This is for long exposure work. It is less about shooting a Summilux wide open and more about transforming time: soft water, blurred clouds, empty streets, or dreamlike movement. A good 10-stop filter must be very colour-neutral, because flaws become more visible at high density.

Fixed vs variable: which should you choose?

If your priority is still photography, image quality, colour consistency, and preserving the lens character, choose fixed ND filters.

If your priority is speed, travel convenience, or video, a variable ND can be useful. But, as I mentioned, I had the experience of the ring being pushed out of position, and therefore you have to be aware that it may not stay in position when carried around, or put in the bag and taken out again.

The danger with variable ND filters is not that they are always bad. Good ones can be very good. The danger is that they may quietly change the image in ways you did not intend: uneven density, colour cast, vignetting, polarisation effects, or the dark X-pattern at high settings. Shooting a series of pictures and changing the ND-filter position during the shoot can easily change the colours in an unwanted way.

For a Leica Summilux, I would personally treat variable ND filters as a convenience tool, not the primary choice for critical still photography.

Variable ND filters are not wrong

My practical recommendation

For a Summilux-M 35 or 50, or similar high-end f/1.4 lens, I would build a small, simple ND kit:

• One excellent 3-stop fixed ND.
• One excellent 6-stop fixed ND.
• Optional 10-stop fixed ND for long exposure.

Buy fewer filters, but buy better filters.

A cheap ND in front of a Summilux is a false economy. The lens is designed to give you beautiful contrast, clarity, colour, and rendering. The filter should protect that character, not flatten it.

The best ND filter is the one that lets the lens disappear from your thoughts. It should simply give you control over light, so you can keep the aperture, shutter speed, and visual intention you wanted in the first place.