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Variable Neutral Density Filters and Bokeh Artefacts

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neutral density filter sm

A few weeks ago, I came across an article by Jonas Rask photography which kinda freaked me out a little.

The gist of Jonas’s article was that we shouldn’t buy variable ND (neutral density) filters as they cause artefacts in out of focus areas (the bokeh) and also within specular highlights. And that we should stick to the non-variable variety instead.

I have personally never paid super close attention to the bokeh by pixel peeping at the out of focus regions, but his article really brought this to my attention. I decided to try to reproduce these artefacts using my Singhray variable ND filter, to see what happens for myself.

For those of you who are interested in reading his article, follow this link to his blog post.

Pro’s & Con’s of Variable Neutral Density Filters

Almost all variable ND’s have downsides to them, such as: the degredation of image quality and also the dreaded “X” shadow that you get when you push turn your ND filter too far whilst shooting with a wide angled lens. In my opinion, that is fair enough and nothing is perfect. What you get from a variable ND instead, is convenience out in the field, as opposed to carrying multiple filters, and changing from one to the other in order to cater to the ever-changing conditions of ambient light.

In terms of image degradation however, it isn’t bad with high quality ND filters. In fact, the Singhray variable ND filter that I have does not exhibit any color casting nor does it affect the sharpness of my images (not that I can see from pixel peeping anyway). This filter has also been recommended by David Hobby along with many other photographers. It isn’t cheap though and it is probably one of the most expensive variable ND’s you can buy on the market.

Singhray Variable ND

The following images were shot with the Fuji X-T1 and the Fujifilm XF 35mm F1.4 Lens – shot wide open.

singhray variable ND filter
Singhray variable ND filter
no ND filter
no ND filter
No ND Filter Crop
No ND Filter Crop
ND Filter Crop
ND Filter Crop

Photos provided by Mark Soon Photography

As you can see from these crops, there aren’t any artefacts in the bokeh regions at all – nada, zip, none. These are straight out of camera JPG’s and I couldn’t spot any bokeh differences between the images, with and without the neutral density filter attached.

It does what it’s supposed to, which is to cut down ambient light enough to shoot wide open at whatever flash max sync speeds your camera is capable of obtaining.

How Did These Artefacts Happen?

So the question remains: how did Jonas Rask manage to get his bokeh to look so messed up?

It seems like he’s a Fujifilm shooter as well sporting similar gears setups as myself, judging from the featured image on his blog post. I can’t be 100% sure however. He also writes that he had tried many different ND variables such as Singhray, Heliopan, Hoya etc. and they all suffered from the same issues.

To be honest, I don’t really know what happened in Jonas’s image samples.

My best guess would be that his shutter speed was a little too slow when the ND filters were in play, which caused motion blur – this can happen even when a camera’s mounted on a tripod. The bokeh artefacts do look smeared and have a “directional” quality to them which is very strange. This is just one theory, but there could be also issues at play here.

Perhaps some of you might like to post some thoughts on what you think caused the artefact – I’d really like to hear from you.


 

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One Comment on “Variable Neutral Density Filters and Bokeh Artefacts

  1. Mike

    Why did I only now come across this. Jonas’ article almost completely put me off buying a variable filter however your article has good grounding and helpful reassurance. I’ll be continuing my research however your write up was quite informative.