SRB Variable ND Filter Review

Features
Handling
Performance
Verdict
Specification

Neutral density filters are a popular choice for landscape photographers who want to slow down the shutter speed for creative seascapes and waterfalls, or to show movement in foliage and clouds. While architecture photographers use them to create longer exposures so any passing people magically disappear from the shot. And studio photographers can use them to reduce exposure when they have powerful lighting.

Normally you have to have several in your camera bag, but not with the SRB Variable ND. A quick rotation of the filter ring lets you adjust from an ND 2 to an ND400 value.

SRB Variable ND Filter Features

SRB's Variable ND is a neutral density filter with a rotating mount offering a variable degree of exposure reduction from ND2 (1 stop) to ND400 (8 stops). It's available in filter thread sizes from 46mm (£30) to 77mm (£40) tested here. It comes in a Hoya style hinged plastic protective case.

SRB Variable ND Filter Handling

A variable ND is two sheets of polarising filter mounted in a filter ring. The front sheet rotates and the rear stays still and by turning you reduce light transmission. Considering all this, the filter is surprisingly slim and only weighs 40g.

The rotating part is inset so you can get a good grip of the outer rim when screwing the filter onto the lens. When screwed on the exposure scale is not centred on the top of the lens which is a shame. It would be good if you could line it up, especially if you intend using the camera in portrait mode as the scale is hidden away under the lens, but that would no doubt over complicate the design.

Despite the inset control ring and the slim design it's still easy to adjust when wearing thick gloves.

SRB Variable ND Filter Performance

The first thing I noticed when taking photos at the maximum setting was uneven exposure. All variable NDs suffer from a cross pattern at the maximum setting. So I decided to start the test by shooting a white screen. This not only shows any unevenness of exposure, but also any potential colour cast that you can get when manually combining two polarising filters.

The first 13 shots below are with the lens set at 12mm, ISO200 and f/8 and the last three at 24mm. The filter was rotated until the shutter speed value changed. This was not in sync with the marker scale which actually seems quite pointless as it doesn't appear to be an indication of anything. The minimum setting was darker than the first marker, and it was only when marker 9 was reached that there was any change of exposure, then all the changes occured quite quickly from that mark to the Max setting which was accurately positioned. As you can see the cross pattern is starting to show from the second shot onwards, getting progressively worse. In a most scenes it won't be that noticeable until you reach the eighth position (in our found steps below not the actual markers on the filter) and beyond.

You can see from the last three frames at 24mm, that the cross pattern is hardly noticeable when you use longer focal length lenses - the last dark frame was, like all other frames, on auto and the camera meter has been fooled so you need to remember to increase the exposure when shooting at the maximum setting.

The colour is quite neutral with just a slight warm hue that is easily corrected in the image editing software if you feel it necessary.

1/20sec - No filter	1/8sec - Minimum Setting	1/6 sec	1/4 sec
0.3 sec	0.5 sec	0.7 sec	1 sec
1.5 sec	2 sec	3 sec	4 sec
20 sec - Maximum setting	24mm lens 0.3 sec	24mm lens 1/5 sec	24mm lens 0.8 sec

Here's what happens when the cross pattern occurs in a real life scene.

Left is near minimum setting with an exposure of 1.3 sec at f/8 on the 12mm wide angle. Right is the same scene with the filter on maximum setting, resulting in a 20sec exposure and the cross pattern causing darkening of areas of the scene.

Another aspect of this type of filter is the potential reduction of sharpness. To check this out a sheet of stamps were illuminated with a flash modelling light and photographed with an 80-320mm lens set at 320mm on a Pentax K20D. The first is a straight shot without a filter attached (below left), the second shot (below middle) is the filter attached and set to minimum strength and the third shot (below right) is the filter set to maximum strength.

Click on the thumbnails to see a larger version and note the significant quality loss. This is a sign of cheaper material used. If you hold the filter to your eye and move it up or down you can see the view wobble slightly.

1/30 sec - no filter

1/15 sec - minimum setting

2 sec - maximum setting

While using the filter it was also noted that the polarising aspect is working, so the filter will reduce reflections. This was not the case with a Tiffen version tested at the same time. There are two issues with this. One is that the photographer wishing to reduce the exposure may not also want to reduce reflections. And with a normal polariser you have the option to rotate and adjust the strength of the polarising effect, but with this one it's fixed and the only way to adjust strength is unscrew and use the filter loose on the lens thread. It's all a bit messy.

Value For Money

The SRB Variable ND is more competitive than some rivals such as Kenko and Tiffen who offer ones at hugely higher prices. The Tiffen Variable ND is 6x the price at £240 and the Kenko Variable ND is an incredible £360! We've tested the Tiffen and it's optically far superior when shooting with longer lenses than the SRB. There are other products in the same price range as the SRB. Hama's Variable ND, for example, has an RRP of £65, but is currently selling on Amazon for £38, and it looks remarkable similar to the SRB! As do a selection of unknown names that are appearing on ebay and now selling for less than £20, which starts to make the SRB less good value.
 

SRB Variable ND Filter Verdict

Someone looking for an ND filter may not be looking for polarising filter characteristics and this one does reduce reflections with no control over that aspect. Also the cross effect is bad at the stronger settings, but this is normal for this kind of filter - even with the expensive varieties. The affect on sharpness when using telephotos is not acceptable, so overall that's quite a downer for this filter. You really have to weigh up the convenience of light weight, lack of bulk and no need to switch filters when changing ND values, as well as low price, against the quality issues. It's a quick fix, but a standard ND filter would be a better bet, unless you can justify the cost of the Tiffen.

SRB Variable ND Filter Pros

Slim design
Easy to adjust - even with thick gloves
Low cost
Wide exposure range

SRB Variable ND Filter Cons

Cross pattern at the maximum settings
Scale's position cannot be change
Scale not accurate to filter's exposure steps
Sharpness reduced when used on longer lenses

FEATURES
HANDLING
PERFORMANCE
VALUE FOR MONEY
VERDICT

SRB 77mm Variable ND Specifications

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