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Neutral Density Graduate Filters

Guest Brucec

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This started with Clive's thread at http://www.thenorfolkbroads.net/phpBB3/viewtopic.php?f=17&t=3352&start=20

I posted a red herring question about ND grads, followed by this explanation:

As to ND grads, it means neutral density graduated filters. Have a look at http://www.leefilters.com/camera/products/finder/ref:C475674155E58E/ They are needed because if you were to take separate meter readings for the sky and the foreground in a landscape, you would usually (unless the sky is blue) need different exposures. If you expose for the foreground the sky will often be washed out, or even pure white. If you were to expose for the sky the foreground (the land) would be too dark. Modern matrix metering and wider dynamic range sensors do a good job of averaging everything out, but ultimately they can't overcome the laws of physics - either the foreground or the sky will have a less than optimum exposure.

By using a neutral density graduated filter, you hold back the exposure for the sky by the correct number of stops to make the image look the way you saw it. Some people think they are used to darken the sky and make it more dramatic, but that is not the case - they are to take account of the fact that although the human eye makes the sky and foreground balance, the film or CCD ruthlessly records the difference, which is often 2, 3 or more stops. The main exception is when you have blue skies - which you have in quite a few of your pictures. A blue sky will often balance with the foreground (and in fact a short cut way to get a good overall reading for a landscape image is to meter directly from a blue sky). That's why a lot of your images have nicely balanced exposures without the use of grads, whereas the W. Somerton one shows blown highlights in the sky. It probably needed 3 stops less exposure for the sky than for the land. The quick way to do it is with a grad. The longer way is to make 2 or 3 exposures of the same scene, on a tripod (so that you get exactly the same image in the frame), with the exposure a few stops apart, and then merge them in Photoshop, or create what is called an HDR (high dynamic range) image.

Plesbit then posted this question:

Question for Bruce though, would an ND grad work well in the above compositions? I had assumed they were best suited to a fairly level horizon. In most of Clive's pics the horizons are far from level with many masts and uneven tree formations - indeed many of the masts actually go clear out of the top of the frame. Would it be possible to use ND grads in these circumstances without impacting the colour and shades of the masts etc?

This is my reply, having taken it off Clive's original thread (which is for his photos).

Good question. You have to use a certain amount of care, but it is surprising how rarely you can see evidence on the finished image of having used a grad.

First, I wouldn't normally use a grad with a blue sky, which Clive has in a lot of his compositions, including the "above" ones that you referred to. In those cases if I filtered the sky at all it would be with a polariser rather than a grad.

Second, you can usually position the transition area so that it falls in the right place - e.g if you've got uneven mountains on the skyline. This is an example: http://www.brucecairns.com/photo1297768.html In this case I used a hard grad on the top of the image (in fact I also had another grad coming up from the bottom at an angle to control the highlights in the loch, but that is another story). You can see a shadow on the left hand hill, but I can't tell whether that is a cloud shadow or evidence of the grad - frankly it's more likely to be the former. I could have used a soft grad (i.e one with a soft transition between the darkened area and the clear part of the filter), in which case the transition line would have been perhaps too subtle, leaving a brighter area where the transition occurred.

My original question to Clive about grads arose from his West Somerton image with a very unusual sky, which has a very bright patch of apparently pure white (i.e what look like blown highlights). As an appropriate example, therefore, here is one of mine of West Somerton, shot using a 2 stop soft grad for the sky. http://www.brucecairns.com/photo135050.html. In this case the grad was positioned so that the transition area came down over the buildings on the left, but sufficiently subtly so that they were not darkened significantly.

Of course your basic premise is absolutely right - there are situations in which the grad will darken an object which cuts into the sky area. Here is an example: http://www.brucecairns.com/photo228454.html - as you can see, the tree is darkened from the horizon line upwards. Interestingly the folly, which was also covered by the grad, does not appear dark. In this case I had 5 stops of grad (a 3 stop and a 2 stop) down to the horizon, because there were some very bright patches in the sky which would have been blown otherwise. I suspect I used one hard grad and one soft to achieve the 5 stops, and positioned the soft grad to even out the transition line as bet as I could.

If you had a light coloured mast cutting into the sky and you gradded the sky heavily, then yes it would darken the mast by however many stops of grad you are using. The question is, would it show unless someone was looking for it?

Here's a final one, gradded at an angle across the sky area, and cutting into the tree. Can you tell? http://www.brucecairns.com/photo134715.html


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  • 2 weeks later...

As I have done little in the way of Landscapes in any serious way ND Grads have not really been on my Radar but having seen this thread and done a bit of Googling I can see how useful they are to ensure correct exposure of the foreground which without would be overexposed.

Come on Bruce a bit on Polarising Filters now please :clap

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Unlike the rather nice filter sets Bruce has which remind me of the gel filters we had on the FX lights in my school hall (I did the lighting for school plays), the ones I've generally seen in use are circular threaded filters. Indeed I have two myself, both Hoya UV filters - I've tried them both on a couple of times but as yet I still cannot see any difference between images taken with them and without them.

So what are they for? :?

Is it just that they are only useful in given scenes and I have not actually been doing any of my test shots in those given scenes? One of them fits on the Minolta 24-105mm I was shooting with on the photography ramble but I seldom use it because I'm unsure of the benefits.

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I have used them in Photographing Fish, Big Un's caught across most of Europe in a previous life :-D

I found great benefit in them reducing glare from the subject and surrounding Water in addition to increasing the richness of both the subject, sky and foliage.

It did make a significant difference particularly where 'fill in' flash was also needed, but like you I would be interested to hear how and when Bruce uses them in Landscapes.

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I've heard the argument about protecting lenses. But I have two queries (this relates to my circular UV filter rather than the polarising ones the discussion has moved on to) - firstly, I've heard that you lose a stop of light, which is something I mostly prefer to avoid and secondly, my normal walk around lens is a Minolta 24-105mm which is presently retailing at around £400. Doesn't seem to make sense to then chuck a cheap filter on the front.

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As you'd expect there are two schools of thought on whether to use a UV filter to protect lenses. Photo mags and (not surprisingly, since they sell the filters) camera shops have suggested for years that a UV filter is kept on each lens for protection. Yes, you will lose some light, I'd estimate slightly less than one stop (the meter will compensate), which may well be unhelpful. The argument is that the filter has little effect on the image, therefore the protection is worthwhile and will maintain the value of the lens.

On the downside, the filter will increase the risk of flare, potentially reduce contrast, and increase the chances of vignetting on wide angle lenses when you add more useful filters to the lens. It will also reduce distance haze, which you sometimes want in your image - and you may wonder why you can't record it, forgetting your permanent UV filter!

In terms of protection, if you drop your lens the filter may well break, and broken glass all over your front element isn't a great idea. A lens hood is probably better protection. If you keep your front elements clean, there's no need for a filter.

Most importantly, you are introducing another piece of glass to your lens, which has its elements very carefully designed for optimum performance - by definition you are going to degrade that performance. The question is, is the degradation something you can live with in favour of the protection? I don't know any landscape pro who would use a filter in this way, and most would advise their workshop students to remove them immediately! But then pros are very careful will the front and rear elements of their lenses and devote plenty of time to keeping them clean and safe.

You pays your money ...

New polariser thread to follow.


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