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K&F ND2-32 variable ND filter review

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Review: K&F ND2-32 variable ND filter

Using a variable neutral density filter


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ND filters can have a variety of uses, Keith normally uses quite dense filters to enable longer exposures, either to blur movement, or reduce the visibility of people in some of his architectural shots.

In this short review, Keith looks at a filter that offers a continuously variable range of densities, needing anything from 1 to 5 stops of additional exposure.

More info: K&F filter:
Coupon: KF10UK (10% discount,Valid until 31/12/21)

Amazon UK: K&F filter:
Coupon: KFCUK005 (10% discount,Valid until 15/06/21)

nd32

What do you get with the filter

The filter here is a K&F XV37 82m ND2-32 multicoated glass filter.

Sizes available run from 37mm to 86mm

The 82mm filter I’m using fits my Canon TS-E24mm tilt/shift lens and is thin enough to show minimal vignetting, even at full shift. However, with strong shift, the angle of incidence in the most strongly shifted parts of the image is reduced, meaning that light is coming through two filter glass elements and there is slight deterioration of image quality at full aperture.

That said, the effect is small and only shows at full aperture – where you will see vignetting from the lens anyway.

For smaller lenses, a basic stop down filter ring is ideal. Remember a big filter is fine on a small lens, but the opposite is less often true.

nd4

Filter at ~ND4

The filter gives from 1 to 5 stops of light reduction, with the settings shown on the edge of the filter

There are hard stops to the movement at each end. This second shot is at ND32

nd32

An advantage of having the limited movement is that you only get between ND2 and ND32.

With other filters I’ve tried, the ring rotates freely, giving no clear indication of the density, and what is  more inconvenient, is that you can go past the highest usable setting and get what’s known as an ND cross.

This example from an ND 2-400 filter I looked at a while ago.max-setting-outdoor

The cross depends on the lens focal length, but is annoying if it’s there as a very slight effect, you only notice once you process the image.

Using the filter

The filter is composed of two glass disks, with the front one freely able to rotate.

Both disks are linear polarising filters, which combined together, pass an amount of light dependent on their relative orientation. The amount of attenuation in this case can vary from 1 to 5 stops.

One stop means you need to double your exposure (with aperture and ISO unchanged) or open your lens aperture by one stop (keeping shutter speed and ISO the same).

Let’s say a particular daylight scene needs 1/125s exposure at f/8 (@100 ISO)

Adding the filter (at its lightest setting – 1 stop reduction) means we now need 1/60s at f/8 (or 1/125s at f/5.6 if only changing aperture)

[Note that I’m approximating exposure values to normal camera settings]

At the darkest filter setting (5 stops), let’s step further down through exposures (from 2 stops to 5 stops)

1/30s … 1/15s … 1/8s … 1/4 second

So, I can now take a photo with a 1/4 second exposure at f/8.

In lower light, I could stretch from 1/30s at f/8 to 1 second at f/8

1/15s (1 stop) … 1/8s … 1/4s … 1/2s … 1 second (5 stops)

Do remember effect that polarising filters can have on the metering (and AF) systems of DSLR cameras. This means that such filters are distinctly more reliable to use with fully manual settings for your camera.

Colour cast

The K&F filter seemed very neutral – perhaps a slight warming. I’d make a point of including grey card in my shots, although just to be sure.

Filter comparisons

There are a number of different notations used for ND filters.

This table shows a comparison between them and how much light reduction they offer.

ND1xx Notation
ND.xx Notation
NDxx Notation
Fractional Transmittance
Optical density
f-stop reduction
% transmittance
1 1 0.0 0 100%
ND 101
ND 0.3
ND2
1/2 0.5 0.3 1 50%
ND 102
ND 0.6
ND4
1/4 0.25 0.6 2 25%
ND 103
ND 0.9
ND8
1/8 0.125 0.9 3 12.5%
ND 104
ND 1.2
ND16
1/16 0.0625 1.2 4 6.25%
ND 105
ND 1.5
ND32
1/32 0.03125 1.5 5 3.125%

More than just long exposures

Note that although I’ve emphasized long exposures, it’s just as relevant to use filters on fast lenses, allowing you to shoot wide open in daylight.

They are distinctly useful for portrait work where the benefits of a f/1.2 lens are irrelevant if your camera shutter won’t shoot fast enough. If you shoot with flash in daylight, an ND filter also allows you to work at a wider range of apertures and to crank up the flash output (to match or exceed daylight) without running into sync issues.

Video uses

You might want to use a wide aperture with a relatively low shutter speed for creative reasons. In daylight, even 100 ISO isn’t low enough to get this, even for a relatively slow lens like my TS-E24mm F3.5L.

I’ve a short video looking at this to complement this article. Do note that I make no claims to offering video services via Northlight!

Conclusions

The K&F filter is robustly made and has very good coatings. Inadvertent finger marks picked up during testing were easily removed.

I like the limits to the range and the way it makes the filter far less prone to ‘X type’ vignetting.

Summary

Effective variable density neutral filter to reduce light by between 1 and 5 stops.

More info: K&F filter:
Coupon code: KF10UK ( 10% discount,Valid until 31/12/21 )

Amazon UK: K&F filter:
Coupon code: KFCUK005 ( 10% discount,Valid until 15/06/21)

ND variable filters from B&H or Amazon.com in the USA.

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