ND filters explained for novices

There have been several threads lately about ND filters, largely following the series of superb landscapes posted. Plenty of advice about which manufacturer is best and why, but is there anywhere someone can point first time ND filter users like me at.

I'd like to take one with me travelling next week, but don't want to buy loads of different ones just yet until I have a go and see the results. I understand how they work and how many stops a given filter will reduce, but don't know what effect this would have in practice.

I was pretty poor at equations at school, but presumably there must be some equation that would let me work out the effect of the filter?

I was out early this morning in the mist. It was just getting light with the sun not up, but I still couldn't expose for longer than around half a second, even at f36.

How do I work out what exposure time I would have had with a given ND filter attached?

Ideally I would like to take just one with me, but don't know which would be most useful. Either that or get a Cokin set, but the reviews on those are variable. I can't justify a Lee set or similar at this stage.

The variable ones look convenient, but in my experience you usually pay for that in quality.

I should add that I am not all settled yet about which lenses I will end up keeping. I have a DA 18-250 and FA 50 for this trip, but that could easily change afterwards!

Any help appreciated.

I think it is something like this... When talking about 'stops' in respect of shutter speed, each movement of one-stop is effectively achieved by halving or doubling the shutter speed in question... the camera will select shutter speeds along a continuum with the fastest being 1/8000th of a second (or something like that) all the way up to 30 seconds, or longer when 'self timed' using bulb mode.

Therefore the sequence of shutter speeds which represent changes of one stop is: 1/8000; 1/4000; 1/2000; 1/1000; 1/500; 1/250; 1/125; 1/60; 1/30; 1/15; 1/8; 1/4; 1/2; 1 second; 2 sec; 4 sec; 8 sec; 15sec; 30 sec.... i.e doubling the exposure time to get the next in the sequence (or halving if going the other way). (The actual speeds selectable on the camera will probably be more than this as it probably allows for variations of a third of a stop or something like that).

Therefore if you had a shutter speed without filter of 1/4000, and then you used a '4 stop' ND filter, assuming all other aspects of exposure stayed the same (same ISO and same f-stop) then the new shutter speed would be 1/250th (four stops along from 1/4000th). In you example, where you had a half second exposure time at f36, then a 4 stop filter would give 8 secs I think. A 'bigstopper' (a ten stop filter I think) would give an exposure of several minutes and you would need to use bulb mode and a timer - but the effects could be magical because in that time the clouds would have moved and they would be rendered as such in the final image.

The camera meter, as it works through the lens, will indicate the appropriate shutter speed when the filter is attached, although it will only go up to 30 secs I think.

I have a couple of ND filters, including a big stopper. I, like you, wasn't sure about what to get and didn't want to splash out straight away, so I got a couple of screw in Hoya 49mm ND filters from amazon which were pretty cheap, and are fine for use with the DA ltds I have as they all have a 49 mm thread. That would also be the case for your FA50 but I don't know about the 18-250... I too have been interested in the variable ones but never actually seen one 'in practice....

Hope that helps

Hi Matt

Thanks very much. That helps a lot. I should have been able to work it out for myself, really, but it's a little confusing (like the f-stop scale)

Unfortunately it means that I almost certainly need more than one, which makes the Cokin set or similar a better buy, especially if there is a possibility of me changing lenses before things settle down.

The 18-250 has a 62mm thread.

Thanks again - much appreciated.

what lens are you using that stops down to f36?

My DA 18-250 at 100mm.

I took plenty of shots this morning using the smallest apertures that I could (still let in too much light!). It's f36 maximum up to about 100mm, f40 from around 100-180mm and f45 from around 180mm to 250mm.

I wasn't using a wideangle because I was concentrating on a couple of old oaks with the mist swirling round the base and couldn't get closer without trampling the crop.

I think he's missing the decimal point.

f3.6 (3.5?) up to 100mm, f4.0 to 180mm and f4.5 for the rest sound much more likely in this context.

Myles

I might be relatively inexperienced with DSLR photography, but am old enough to have used film SLRs in the past. Not so old though that I can't tell the difference between f4.5 and f45

Pretty unlikely to misread f4.5 for f45 at 180mm and above when the maximum aperture of the lens is f6.3 at that length.

Not sure why this is a big deal, but I'll happily post a photo with the exif data if my feeble brain and eyesight can cope.

Venerable Myles need to do his homework , particularly when trying to correct someone who might have quite a bit of experience in the field

The DA 18-250 mm f/3.5-6.3 has a minimum Aperture of 45

Many zoom lenses have variable maximum apertures, but don't forget that all the other apertures change as well, including the minimum. I have seen f/38 registering on my screen from a Pentax lens before now. Can't remember which one offhand.

Even the aged Pentax SMC-A 400mm F5.6 goes down to F45.

My apologies for not concentrating when I probably should have been in bed! I certainly meant no offense.

Must pay attention.

Must try harder.

Best,

Myles

No offence taken

I wasn't sure if I was getting a duff reading on the K-r or not as there seemed to be surprise at the aperture sizes.

As I have no idea whether or not I'll make a lot of use of a ND filter I'm going to get one of the 7 day shop variable ones. The reviews are mixed, but it's not a lot of money and should show me what sort of stepping down I'm most likely to need and how I will use it.

I'll be able to get something better then without wasting lots of money trying various filters out first.

I'll post a proper review of it after I get home as reviews seem to be lacking, as well as some photos if they aren't too embarrassing.

Any updates on the 7dayshop ones?

http://www.7dayshop.com/catalog/default.php?filterwords=variable+ND&filterwordsa...

Still a mixed bag is it?

I bought the 7 dayshop variable. As I didn't know what I was doing the first attempts were dire and its been in it's box ever since . It's very easy to get a dark band where you don't want one and black corners (using the 62mm on a pentax 18-135mm). I'm sure with a bit of practice and reading up on the use of ND's it will be ok.
I fully expect to win the lottery this weekend and have my order all ready for a full set of Lee filters

+1

I've struggled with it as well to be honest, but probably haven't used it enough for a fair trial. Biggest problem for me was that it doesn't seem to exclude enough light to make it useful, but that might be me as well.

I'll try to use it over the next couple of weeks and report back.