If I'm buying a single ND filter, which stop is most versatile?

It really depends of the kind of pictures you want to take. Look at "What are the uses of Neutral Density (ND) filters?" for information about potential ND filter uses. You can also take a look at "What are neutral density filters and how do I use them to create long exposures in daylight?" to know more about those filters.

A direct way to know the ND filter you need would be to take a correctly exposed picture of the scene you want to capture (shutter speed "A") and then estimate the shutter speed you would like to use (shutter speed "B"). Now compute the ratio B/A and it will give you the f-stop reduction needed. If A = 1/50 sec and B = 10 sec, B/A gives 500 : you need a ND512 ND filter (9-stop reduction, because 2^9 = 512).

In my case, the first ND filter I bought was a "ND1000" because I wanted to take exposure of a few seconds in broad daylight. My estimate of a regular shutter speed was 1/250, so it becomes about 4 sec with a ND1000. Of course you can moderate it using different aperture and/or ISO.

Anyway, a rough estimate of the factor by which you want to multiply your shutter speed should be enough. You can also google pictures taken in Hawaii with ND filter.

ND filters ratings can be tricky. Take a look at Wikipedia (https://en.wikipedia.org/wiki/Neutral_density_filter#ND_filter_ratings) to understand the different ones.

As a side note, I would like to add that a ND filter with a high f-stop rating might prevent you from using the AF (and you won't be seeing much in the viewfinder).

As you already pointed out, cheap variable ND filter are to be avoided.

It is not nice to answer a question with a question, but my answer would be... What do you need?

Here is another answer I made for a simmilar question. There you can analize how many stops you need to decrese. Which filter would I use for daytime lightning long exposure?

You need to think of the situation you have, vs the situation you need prior to make a decision on what to buy.

If you had to purchase only one ND filter, what stop would you get?

Ever since I saw the comparison on various ND filters from Dave I'd get the Tiffen variable ND filter.

Yes, variable ND filter. Here's why:

1. You say this is a once in a lifetime experience. That means getting the shot becomes priority #1. You do not know what situation you will be facing there. Limiting yourself to a single ND filter will be very limiting. Would you bring a single prime lens? A fixed shutter-speed camera? Or a single type of film (only one ISO setting)? Certainly not! Don't do it with your ND filter either.
2. With lack of experience, a variable ND setting will improve your results. Whenever you pick the wrong ND setting, you have a second chance, a third one and so on.
3. It's not that much more expensive. The 77mm 2-8 stop is £100 on amazon uk at the moment and from your comment it looks like you are willing to spend £70.

I'm not affiliated with Tiffen, please do your research and choose another one if more appropriate. Whatever manufacturer it'd be, I'd definitely get a variable ND filter in your situation.

You mention you need an ND filter. There are different reasons for needing an ND filter, and this will affect what strength you'll need:

• Shooting wide aperture in daylight.

  For example, you want to go f/1.4 in bright daylight but that would push you past your camera's shutter speed limit (say 1/2000s).

  In this case 2 or 3 stops is fine, as either will probably give the headroom you need to reach the widest aperture, and there is little disadvantage to going 3 stops even if 2 stops would also have worked.

• Shooting long exposures as a special effect.

  For example, you want 1 second or longer exposures and it's not the middle of a dark night.

  In this case you're probably going to want the 10 stop ND a lot of the time. 2 or 3 stops won't be enough.

• Shooting video with a reasonable shutter speed (effective shutter angle)

  For example, you want to be able to shoot 24fps with a fairly standard 135 to 180 degree effective shutter angle (1/50s or 1/60s) in a range of conditions including daylight, where normally you wouldn't be able to shoot that slow.

  In this case the ND filter you need varies depending on light. 10 stops will often be too much, but sometimes 3 stops won't be enough.

For my own needs, 3 stops is the most useful of those strengths, assuming you get only one and you don't get vari-ND.

Just a note about vari-ND: these are built from two polarising filters which rotate relative to each other in order to block more or less light. The side effect of these is that it also applies a polarising filter to the image so you get the same artifacts associated with polarising filters like patterns in vehicular glass, LCD screens, or gradients and color changes in the sky.

Another note: remember that 2 stops is ND4, 3 stops is ND8, and 10 stops is ND1000.

First, your environment: my own experience traveling tells me that there's quite a bit more light closer to the equator. I'm going on memory here, but I feel like there's typically 1+ more stops of light in Florida than in Connecticut. It's hard to know what equipment to bring when you aren't already there, but I suspect you'll find Hawaii has quite a bit more light than the UK. That may impact your choice about what filter to get.

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Second: I think digital has changed needs (at least my needs) for a ND filter. With film, switching to a faster film was undesirable because you so often had to give up that film's characteristics. So, having an 2-stop ND4 (as well as a bunch of other options) made sense: above all else, I don't want to adjust ISO, so finding a variety of filters to let me get the exposure I want is the only way to get the results I want.

Digital is different -- I can change ISO at whim and get what I want with minimal differences. A 10-stop ND filter can easily prove to reduce the light too much. But by bumping the ISO up (while still using the filter) you are able to get the exposure desired. The big compromise is deciding if that 10-stop range would take you into an unacceptably high ISO to get the correct exposure.

Think about that for a minute: a 10-stop ND filter combined with ISO flexibility then means that your camera range is now [10 stops below it's base or low value] to [base or low value] all the way up to the camera's [high range value]. More clearly, a Nikon D800 has an extended range of ISO 50 to 25,600. Add in a 10-stop ND filter and adjustable ISO and you can hit a range of ISO 0.046875 to 25,600. To reach an effective ISO 25 (one stop below base level) you would need to use the 10-stop filter and ISO 25,600, which is likely going to be a poor photo because of the level of noise introduced! Adjusting shutter speed and aperture may be an option to maintain a lower ISO, but that won't always be true.

A 10-stop filter is a huge jump. A 2-stop filter is going to serve a very narrow range and won't be stretching ISO to also hit 1-stop below base. A 3-stop filter is, I think, more useful because the capability is greater but compromise is still equally small.

One more thought: a polarizer typically takes about 2 stops. It's clearly not a replacement for a ND filter, but I have used it for that purpose on more than a few occasions.

I carry with me a 3 stop ND, 10 stop ND, and a CPL(~1-2 stop filter factor). That combination has suited me very well without feeling like I've left out anything.

Since you will be shooting landscapes, I'd bring a CPL and consider bringing no other filters except perhaps a set of grad NDs.

My personal experience with ND filters isn't huge, but I have 2 of them and they are both Hoya (both great) and one is 9 stop and the other is 10 stop. Let me be clear about somethings here:

1 - adding glass to your lens will make it worse, so don't use filters unless you really need to.

2 - you don't need ND filters to shoot sunsets. The sunset will naturally reduce the light for you. Just wait one more minute or two to take your picture.

3 - If u really need a ND filter (eg: shooting water in daylight) you need a proper ND filter. I had a 9 stop and then bought a 10. If you are taking a tripod with you (and you need one if you are going to take 1 sec. pictures) you can take a longer exposure if needed. what you can't do is reduce light if u bought a 3 stop ND and then you realise you really needed a bigger one.

So my advice is go big. Either 9 or 10 stop the cheapest of them (when i bought my 10 stop it was cheaper then the 9 stop).

Have a nice trip.

I have shot sunrise and sunset at Baltic Sea few days ago. Now I know I would need NDx30-60 filter to allow shooting silk water after sunrise and I think it is the most universal filter.

If you want to remove people on sunny day I'd recommend something bigger.

I have a 3-stop, 6-stop, and 10-stop. The 10-stop was the first one I bought and I was very excited to be able to take long exposures with it. While it is very useful in certain situations, I rarely use it. If I were starting over and just buying one I would go with the 3-stop.

My experience:

For image, 1000ND filter will give you satisfying silky effect of water fall during day time.

For filming, 8ND or 16ND are normally for filming. You will see DJI drones' filter sets almost always include these two.