What Should I Expect of a Variable ND Filter?

Hello all.

I'm a very amateur photographer with too much internet theory and not nearly enough field experience. I'm hoping someone with real experience can help me.
A few weeks ago I purchased a Cokin Pure Harmonie ND-X Variable circular Filter for my Nikkor 50mm 1.8 ( shooting with d750). This is my first ND filter and I planned on using it while on holiday recently. While it hardly got used while away, I have been using it since I have been back. Mostly down at the beach with my family, lens wide open, 1.8, sunny days.
The problem is, the images captured with this filter on are absolutely terrible (in my opinion, and no, I don't have any samples) . All of them have been very blurry bar none. I have tested the lens with and without the filter. Without the filter photos are always sharp, put filter on and..blurry. during testing camera was always manually focussed, on a tripod, different apertures used with multiple ND filter settings also used. Same result everytime.

Ok, so that was very long winded, sorry. I guess my point is, what Should I expect out of a $145 ND filter?. Should I expect a lot more than I am getting? As it is now I won't use it again. Should I just return it?. Being a novice I don't have the confidence that this filter is operating outside of the parameters of a $145 filter to tell the store that I want my money back/exchange .

Any thoughts?
Thanks
Grant

Hi Grant,

What exactly are you trying to capture?

I'm a novice shooter as well, and bagged a 10-stop ND filter a few weeks back. My intention was to capture fall colors reflected in water. My method of operation was as follows,

1. set the camera on a tripod. Compose the scene and achieve focus with the desired f-stop and note the shutter speed for the correct exposure without the filter
2. then go to manual mode and manual focus, dial down the shutter speed by 10 stops
3. put on the filter, drape the camera with a sufficiently dark cloth to avoid any light seeping in from the anywhere other than the lens.
4. put the camera on a 2 sec timer and shoot.

This worked relatively well for my use case.

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I hadn't heard of variable ND filters before. At first I thought you were talking about a graduated neutral density filter. The variable filter has an outer ring that adjusts the density of the filter. If you make it denser than conditions warrant, it will have the effect of your camera opening the aperture, slowing the shutter speed, and increasing the ISO if you're using auto-ISO. A wider aperture will lead to a shallower depth of field. Things outside of the focal plane will be out of focus. A longer shutter speed would lead to movement blur or camera shake if you're hand-holding the camera. It sounds like you need to experiment a little with this filter.

fiwi wrote in post #18173594
The problem is, the images captured with this filter on are absolutely terrible (in my opinion, and no, I don't have any samples) . All of them have been very blurry bar none. I have tested the lens with and without the filter. Without the filter photos are always sharp, put filter on and..blurry. during testing camera was always manually focussed, on a tripod, different apertures used with multiple ND filter settings also used. Same result everytime.

Grant, the variable density filters are actually two polarizing filters mounted in one frame (so that one of the two can be rotated relative to the other).

The problem is likely the polarization of the light (by the filter) affecting your auto focus.

Try manually focusing instead of using auto focus and see if your images become appropriately sharp.

.

SkipD wrote in post #18173894
The problem is likely the polarization of the light (by the filter) affecting your auto focus.

Try manually focusing instead of using auto focus and see if your images become appropriately sharp.

Hey, Skip! Perhaps you missed the following info that Grant told us in the initial post:

fiwi wrote in post #18173594
. . . . . . . during testing camera was always manually focussed, on a tripod, different apertures . . . . . . .

.

Assuming pretty long shutter speeds:
Have you used mirror lockup?
Have you tried adding weight to the tripod, because even a little wind could effect the stability of the tripod? Many (lightweight) tripods have a hook on the center column to hang weights from.

fiwi wrote in post #18173594
Ok, so that was very long winded, sorry. I guess my point is, what Should I expect out of a $145 ND filter?.

You should expect blur.... that is what ND filters are primarily used for. It is very difficult to give an accurate answer without sample images but there are two likely causes of "blur".

When you add a filter, you block light from reaching the sensor. That means that, in order to get the same exposure as you would have without the filter, you need to set the camera to a wider aperture and/or slower shutter speed.

Wider aperture (especially something as wide as 1.8) will result in very narrow depth of field. This means that anything/anyone not exactly on the plane of focus will be out of focus. This may be mistaken for blur.

Shooting at a slower shutter speed will result in the sensor recording any movement in the scene, including trees blowing in the wind or movement of any people. This will result in motion blur.

So the question is, if you don't want blur and/or narrow depth of field why are you using an ND filter?

Tom Reichner wrote in post #18173933
.

Hey, Skip! Perhaps you missed the following info that Grant told us in the initial post:

.

Yep - you're right....

Thanks for the replies guys.
I should have some sample shots to show you but I don't. I deleted all of the beach shots And the sample shots I took. On the weekend I will reshoot.

The shots that I have used the ND filter for so far have mainly been at the beach, very bright sunny days, taking photos of my kids. I wanted the aperture wide open, 1.8 for the background blur. Even at max shutter speed, 1/4000 , the images were going to be over exposed. So I put the ND filter on, dial it in to nomore than half strength, usually less( I realise with variable nd's that dialing them to max can cause problems) then adjust the shutter speed down accordingly to get the desired exposure. These shots were usually handheld, auto focus. Shutter speeds were always above 1/1000, depending on how much I had the filter dialled in so motion blur wasn't a consideration.

Afterwards at home when I saw how bad the shots were was when I set up the camera on the tripod and did some tests.
All tests were done in manual mode manually focused on a static subject using live view. Photo was first taken without the filter on at various apertures and shutter speeds, then the filter was applied and the process was repeated again, but this time also dialling in various filter strengths as well. Every time I got the same results.

I do have a polarising filter as well. Sometimes that is sufficient to get my shutter under max shutter speed while at 1.8, not always though. I think in the future I will probably just use this and dial the aperture down a fraction if needed.

If anyone can either give me tips on shooting wide open on bright days or a better way to test the filter then by all means, let me know. As I have said previously, I don't have the experience to be confident with my findings.

On a side note, if I was to upload sample photos for scrutiny, what is the best way to go about this to get the required data to you guys?. I use Lightroom 5.

Thanks for everything
Grant

Probably not the 2 best test shots, but they are consistent with everything else I have shot with nd filter on. Shot in RAW, manual mode, manual focus using tripod. ND filter was on minimum setting, but anywhere through the range and the same thing happens. No processing apart from cropping.

!st photo 1.8 1/2500 iso100
2nd photo 1.8 1/1250 iso100

OK having seen the samples the problem is almost certainly not related to technique and is in fact due to a sub-standard filter. Put simply the filter is optically sub-standard and is reducing the image sharpness - either that or it's very dirty.

I have a nice variable ND made by H&Y but I can't remember how much I paid for it. I do know that variables are more problematic than standard ones because you are putting two pieces of glass in front of your lens so they have to be of a very high standard not to cause problems.

Not sure if you got a one off bad lens or if the brand you bought are not f a high enough quality. Either way you are almost certainly going to have to get a new/better variable ND.

Thanks for that Dan.

So, should I just accept that $142 isnt enough money spent to get a decent filter, or do I approach it from the 'I may have got a faulty filter and would like a replacement?'. I purchased it online, which makes it a tad harder to deal with. How do you go about the cost vs quality balance?.

fiwi wrote in post #18174400
Thanks for that Dan.

So, should I just accept that $142 isnt enough money spent to get a decent filter, or do I approach it from the 'I may have got a faulty filter and would like a replacement?'. I purchased it online, which makes it a tad harder to deal with. How do you go about the cost vs quality balance?.

I would certainly start w/ requesting a replacement; but, be prepared that that line of filter may just not be all that great. This is one of the reasons that filters can be tricky.

I see from the first post that you have a Cokin filter, and they have never had a reputation for being the best in class filters. Of course they did invent the whole square adjustable resin filter genre, and so for many years were the only game in town as far as those were concerned. Also back in the 70's when they started to become popular we were still all shooting film, and they offered lots of cheesy effects available in camera, which for most people were the only way to do them. I still have about two dozen Cokin filters from that period, and most are completely pointless in the digital age. The polariser is linear not circular, so not ideal with AF, and of the rest only the graduated ND might be useful. The other thing about them is that usually they were of a sufficient quality back then when used with film, unfortunately digital is much more optically demanding of the lenses and filters that we put infront of them.

Alan

fiwi wrote in post #18174400
Thanks for that Dan.

So, should I just accept that $142 isnt enough money spent to get a decent filter, or do I approach it from the 'I may have got a faulty filter and would like a replacement?'. I purchased it online, which makes it a tad harder to deal with. How do you go about the cost vs quality balance?.

Think harder about whether you actually need a variable ND filter rather than one or two standard ND filters. The truth is, it's pretty tough maintaining quality even in a standard ND filter when the density gets down to 6-10 stops. But at any price range, you'll get better standard filters than variable filters.