Neutral density filters is what you are thinking of.

What you want is a neutral density filter

ND as above.
I also use a rotating polarised filter - it is brilliant with water shots and light reflecting.

What they said...basically sunglasses for your camera.

If you have a polarising filter that'll give you two stops' worth of ND.

Ok thanks fellas

Is there just one kind of neutral density filter or do they come in different err...densities?

Depends on budget but the Lee Filter holder with a set of Hitech (graduated and non-graduated) ND filters of varying strengths is what I use.

Yup different strengths. You can get a 10 stop filter for really long exposures. Graduated filters are because the sky is generally much brighter than the ground, and effectively allow you to get more dynamic range in the picture.

Bit of a long shot but would the AF still work with a ND filter?

Yes

As above. You can get round filters that will screw in to the lens or square filters like the Lee system that will allow you to use them with different lens via an adapter bracket. The Lee system is more expensive but it is better. From what i've read avoid variable neutral density filters

Filters, + the best tripod you can afford

Bit of a long shot but would the AF still work with a ND filter?

Doesn't work for me with an 8 or 10 stopper.

As already said, in order to push out the exposure time, you need to make what the camera is seeing darker. Which is done by neutral density filters.

These basically come in 2 forms. Circular, which screw onto the end of the lens itself and match the round end of the lens. Or square. You attach a filter holder onto the end of the lens and then slide square filters into the filter holder. There are adapters which join the holder to the correct diameter of lens. That way you can have more than one filter attached the end of the lens.

I would recommend going down the route of the square + holder , because by just changing the adapter on the end to fit whatever lens you are using, you can use all of your filters across all of your lenses.

There are 2 main sizes of square filters ... I'd recommend you go for the larger one at 83mm ... that way you can attach the filter onto wider and physically larger lenses.

As a basic place to start and get them, 7dayshop.co.uk do a good selection of holders and filters in the standard sizes. Its not pro-range stuff, but its good enough to learn with and if you really want to go further, then you can buy the expensive stuff later.

Some other things to consider:

A neutral density filter will darken the whole picture. In the type of shot you posted, its quite often the case that the sky is bright and the foreground much darker. A graduated ND filter can be used to darken just one half of the picture ( such as the sky ) to prevent over-exposure of the sky and allow the darker foreground to remain visible. Using a square filter also means that you can slide the filter up and down in the holder to adjust the fade point to match the scene as required.

Another filter worth considering would be a circular polariser. They can add a lot of the type of photograph you've posted too. ( plus they are quite dark filters and act as an neutral density filter at the same time. ) I'll let you read up about polarising filters in your own time.

Thanks again!

I bought cheap and bought twice in this regard, both tripod and filters.

A good tripod will cope better with getting wet / salty at the beach, and will be a much more sturdy platform.

Good neutral density filters are neutral, cheap ones tend to have a colour-caste. In particular, the darker hi-tech and cokin ones can be very difficult to colour correct for.

Oh, and the picture up there was probably taken with a relatively short shutter speed 1-2 seconds - you don't need much in the evening, and it's surprisingly easy to over-do it!

Couple of points to add to that.

You can get 'variable ND' filters, which are basically two polarisers that you rotate to get different degrees of, uh, darkness. These will screw up AF (due to one being a linear polariser).

Some lenses rotate at the front when you focus. Something to keep in mind when looking at square filters; they'll turn along with the lens.

I used a ND10 and there is not a hope in hell of AF working. I use AF, then switch to MF, then mount the filter.

The kind of picture we are aiming for is the 'boiling sea' effect i.e. a shutter speed long enough to completely blur the waves on the rocks on a stormy day.

Have had some success with this at twilight but the results tend to be quite monochrome.

Just wanted to try and reproduce the effect during the day and hopefully get some colour into the shot as well.

TBF, if you're faffing about with filters and a tripod, it's no hassle at all to absolutely nail focussing by using live view and zooming in on your focus point. This also has the advantage of negating vibrations from 'mirror slap'

That image was likley shot either a dawn or just before senset with perhaps some of the folowing.
Grey Gaduated to bring the sky in.
Warm up to make it look inviting but this could be done afterwards.
Neutral density to slow the camera down to make the sea into a foam and timing as it's not going to be much more that a couple of seconds.
Oh and a tripod to hols the world still while your camera captures it.

AF works fine even with a 10 stop filter on my Panasonic M4/3 cameras, but not on my 5D II.

Sounds like you want a 10 stop filter. Lee Big Stopper or the B-&W ones always get recommended, I've only tried the B&W and it works well.

this sort of thing

Argh my eyes!

If the AF doesnt work. Take the filters off. Use AF to focus, flick camera into manual focus.put filters back on and take image. Bingo!

Just get it right in the first place 😉

This is pretty much out camera, with a polarised filter on.
[url= http://farm9.staticflickr.com/8180/7939310642_0ac6954f8f_b.jp g" target="_blank">http://farm9.staticflickr.com/8180/7939310642_0ac6954f8f_b.jp g"/> [/img][/url]
[url= http://www.flickr.com/photos/matt_outandabout/7939310642/ ]River Awe canoe surf[/url] by [url= http://www.flickr.com/people/matt_outandabout/ ]matt_outandabout[/url], on Flickr

Just get it right in the first place

Well, why didn't you? 😉

If you're just playing a couple of cheap ND4s and a cheap CPL might do. You can then fix you exposure / caste in your photo editor. (this is obviously not the pro approved route)

Ive had some great results with a poloriser lens on seascapes, I also like to use graduated filters as like others say on here it helps keep the colour and detail in the darker half of the image.
Another option is HDR, not something I have tried much of myself yet but I have seen some great results on images like the one above.

And if you're buying screw in filters, look on the back of the lens cap to see what size you need - it'll say something like 58mm or 67mm. Also, Hoya do a good line in affordable, effective ND screw in filters.