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08-01-2017, 07:19 AM   #1
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Square vs circular ND filter

I'm planning on investing in some ND filters and I wanted some advice.

I'm trying to decide which route to go. My main dilemma is, how many circular filters can I stack before significant vignetting occurs?

My plan was to get two ND filters, 3-stop and 6-stop. If I ever needed a 10 stop I could just stack those two to get close.

My worst case scenario would be a CPL + both ND filters on a 21mm lens

A more common scenario would the 21mm + CPL + 1 ND filter

My hesitation with the square filter set up is the additional cost.


Last edited by serothis; 08-01-2017 at 07:52 AM.
08-01-2017, 07:39 AM   #2
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Generally, I prefer round filters that thread on, and I have ND's like that. But here's a curve ball to think about. If you are doing a lot of landscape photography with horizons involved, you may want to look at square/rectangular graduated ND filters that let you slide them up/down to adjust where the transition occurs based on where the horizon is. I have the D FA 15-30, which is a great lens, but requires a very expensive filter holder setup. I went with the Fotdiox for several reasons, and one was the large rectangular graduated NDs, but we're talking about a lot of money, and I'm still twitching over how much I spent on filters (but they seem really nice and I know will be a good long term investment).

I think stacking even two screw on filters risks vignetting on the 21mm but I have not tried it personally.
08-01-2017, 07:58 AM   #3
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The grad ND filter has crossed my mind. The other thing I've considered was if I ever get a larger diameter lens the holder would be easier to adapt.
08-01-2017, 08:04 AM   #4
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You could get your filters at a larger size, then use a step-up ring(s). This will allow you to avoid vignetting. I didn't do it this way, because I invested in filters gradually, but I've read the suggestion to buy your filters at the largest size needed to cover all your lenses or intended lenses, then use step-up ring(s). It's more expensive up front possibly, since 77mm filters for example will cost more than 49mm, etc.. I got away with 49mm filters for years because I wasn't stacking.. Then even when I needed larger filters I got away with just a couple and step-up rings. Now my latest lens (Laowa 12mm) requires that I use 100mm filters so I am really paying the piper.

08-01-2017, 08:12 AM   #5
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My largest lens has a 82mm filter thread. Going larger and circular isn't saving much over the square/rectangle system.
08-01-2017, 08:22 AM   #6
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Yeah. You could still just get 58mm ND and CPL filters though and a 49-58 step-up for your immediate need and spend way less...
08-01-2017, 08:45 AM   #7
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My initial kit just had two lenses, both with 52mm threads. I bought a 52mm CPL. I have way too many lenses now, but lots of them have 52mm threads, and I still use that CPL a lot.

When I decided to add ND filters I had more lenses and more thread sizes. I opted for 67mm round filters as a reasonable compromise.

When I bought a 200/2.8 I added a 77mm CPL. Still use that a lot.

When I decided to add graduated ND filters to the kit, I invested in the Lee system. Since then I've also added medium and large format, so this is a versatile filter kit for me. I should probably sell the 67mm stuff; I haven't used it in a good while. One advantage of this kind of system vs. oversized round filters and step-up rings is that you can put a mounting ring on each lens in your bag and then quickly changes lenses, no need to unscrew adapters. Can't say I've actually done this, however, although I do have some of the caps that fit over the mounting rings.

One can easily go too far with trying to cover all the possibilities. I'm happy enough with the gradual approach I took in building a filter kit. For example, you ask, "if I ever needed a 10-stop...". Well, do you need a 10-stop? (I have one, used it some; turns out I'm less interested in those really long exposures than I used to be.) In a pinch you could use the 6-stop, take enough exposures to cover the desired amount of time, and stack them in Photoshop (or whatever). As todd says, start with your immediate needs. I'd say it's more important to have something that's convenient and easy to work with than something that will cover all the possibilities.
08-01-2017, 09:21 AM   #8
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QuoteOriginally posted by todd Quote
I've read the suggestion to buy your filters at the largest size needed to cover all your lenses or intended lenses, then use step-up ring(s).
I wonder if ND grad filters could be a problem with this approach? Using a 77mm ND grad on a 49mm thread lens, there's going to be quite a significant portion towards the borders of the filter that won't be in the light path. You'd still see a gradient, but the effect wouldn't be as strong, right?

I'm just thinking out loud, here - I have no idea whether it's a problem or not

08-01-2017, 11:28 AM - 1 Like   #9
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21mm on a K-1 (that's what you have, right?) is pretty wide. The FOV cone angle on the diagonal is just over 90. That means that for every 1 mm of filter ring thickness you add, you need at least 2 mm extra diameter.

Standard filters seem to be about 5.5 mm thick, CPLs tend to run 9 mm, and step-up rings seem to be about 3 mm.

Overall, the math suggests that for one ND and one CPL, the answer is 74 mm filters (or bigger) on a step-up ring should work fine. (assuming the lens can handle a single 49 mm filter without vignetting)

But for two ND and one CPL, the math suggests needing filters at least 85 mm in diameter (or bigger). (You might give your 82's a try because the lens may vignette less than this simple calculation suggests, does not vignette at the f-stop's you use, or vignettes in a correctable way.)

Also, you might find lower profile filters although you'll want to make sure some of them have front filter threads so you can stack.

P.S. If you use "composition adjust" then you might need even wider filters.
08-01-2017, 11:53 AM   #10
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QuoteOriginally posted by baro-nite Quote
One can easily go too far with trying to cover all the possibilities. I'm happy enough with the gradual approach I took in building a filter kit. For example, you ask, "if I ever needed a 10-stop...". Well, do you need a 10-stop? (I have one, used it some; turns out I'm less interested in those really long exposures than I used to be.) In a pinch you could use the 6-stop, take enough exposures to cover the desired amount of time, and stack them in Photoshop (or whatever). As todd says, start with your immediate needs. I'd say it's more important to have something that's convenient and easy to work with than something that will cover all the possibilities.
Well that's part of my dilemma. I don't need a 10 stop. It's not cost effective for me to buy one. My thought was IF I wanted to play around with that kind of exposure time, a stack of filters could do. But how much can I get away with before vignetting. By bigger concern is CPL + ND.

QuoteOriginally posted by photoptimist Quote
21mm on a K-1 (that's what you have, right?) is pretty wide. The FOV cone angle on the diagonal is just over 90. That means that for every 1 mm of filter ring thickness you add, you need at least 2 mm extra diameter.

Standard filters seem to be about 5.5 mm thick, CPLs tend to run 9 mm, and step-up rings seem to be about 3 mm.

Overall, the math suggests that for one ND and one CPL, the answer is 74 mm filters (or bigger) on a step-up ring should work fine. (assuming the lens can handle a single 49 mm filter without vignetting)

But for two ND and one CPL, the math suggests needing filters at least 85 mm in diameter (or bigger). (You might give your 82's a try because the lens may vignette less than this simple calculation suggests, does not vignette at the f-stop's you use, or vignettes in a correctable way.)

Also, you might find lower profile filters although you'll want to make sure some of them have front filter threads so you can stack.

P.S. If you use "composition adjust" then you might need even wider filters.
It's a 21mm lens with a 82mm filter thread on a k-1.
08-01-2017, 01:26 PM   #11
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QuoteOriginally posted by serothis Quote
Well that's part of my dilemma. I don't need a 10 stop. It's not cost effective for me to buy one. My thought was IF I wanted to play around with that kind of exposure time, a stack of filters could do. But how much can I get away with before vignetting. By bigger concern is CPL + ND.



It's a 21mm lens with a 82mm filter thread on a k-1.
Do you see yourself getting something like the D FA 15-30 or some other ultra wide lens that might have a curved front element and not accept a screw in filter? I'm a failed LBA recovery addict, and while I can't think of what else I'll buy with a curved lens - already have a fish eye for the K3II..... - I have a hunch there will be yet another one, and I'll try to leverage the Fotodiox system I already own. Look closely at the details on the different kits and what's offered. Take note of what is multi-coated and what isn't. I'm not trying to push them, but just for reference, part of what drew me to the Fotodiox solution was being able to mount their round CPL and use a square ND filter simultaneously. Note that they have two flavors of CPL - one multi-coated, one not. From the research I did, the multi-coated one performs much better, but is twice the price. I bought their multi-coated CPL knowing I only wanted to do this once. So, I'd just spend as much time as you can muddling through the what-if scenarios for as much future proofing as possible, Check out the different things offered from each of you the vendors on your short list - how many square filters can they use at once? What thicknesses? What ranges do they offer and are the prices worth it? Is there even a CPL available? Multi-coated? Other lenses you can buy a mount for? Yeah, lotta time can get sucked up pretty quickly, but figuring it out is kinda fun too - good luck! Also, Break Through Technology should be checked out, and I don't know the status of it, but they had a combination CPL and ND filter in the works, and I think they did sizing up to the 100mm range, so that would be something to check out too. I looked at them for the 15-30, but they didn't have anything that worked. Hope that changes at some point. One last gotcha to look for is combination pricing if you go for a square/circular combo kit. With the Fotodiox, you could buy the base round kit for about $100, and if you wanted to add the square part later, it would be another $100, but if you bought their combo round and square starter, it was only about $122 - prices from memory, but you get the idea - much cheaper to get all the key pieces up front than buying after the fact bit by bit - gotcha!
08-01-2017, 02:06 PM - 1 Like   #12
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QuoteOriginally posted by BigMackCam Quote
no idea
Not sure but I'd bet you're right. I bought one circular ND grad filter at 67mm for my FA20 but only used it once earlier this year. Might take it for spin again now on a 49mm lens out of the curiosity that you've provoked...

QuoteOriginally posted by serothis Quote
It's a 21mm lens with a 82mm filter thread on a k-1.
Well there I go making assumptions that you were talking about a DA21 on crop sensor haha.. nevermind me! At 82mm already though I'd probably be taking the plunge into $quare filters if I were you.
08-01-2017, 02:23 PM   #13
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QuoteOriginally posted by todd Quote
Not sure but I'd bet you're right. I bought one circular ND grad filter at 67mm for my FA20 but only used it once earlier this year. Might take it for spin again now on a 49mm lens out of the curiosity that you've provoked...
If you do get round to testing that, I'd be very interested to know your findings. It's unlikely I'll get a chance to test this myself in the near future (too many other projects in the wings )... and, come to think of it, I may not even have a thread-mount ND grad. I have them in square format, but I don't think I have thread-mount...

EDIT: ... and now I'm thinking it's exactly the same problem with the larger square-format grads. I only bought mine to use with a couple of specific lenses, but I'm guessing they're of limited use on my lenses with much-smaller-diameter threads. Hmmm...
08-01-2017, 03:30 PM   #14
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I bought a 52mm Cokin X variable ND filter for $135. 1-10 stops in a single filter. It's pretty cool, I haven't used it to the fullest extent.

Having said that, I also have a Cokin square filter set, and being able to stack a few filters without vignetting is nice. especially in the case of landscape, where there is usually a big contrast between eart and sky and an ND filter adjusted to the horizon line is the best option.
08-01-2017, 04:31 PM   #15
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With circular filters, if you plan to stack them on wide angle lenses (<24mm), I'd suggest going with the "Slim Mount" versions of whatever brands you're looking at.

With square filters they're easier to rotate, especially when you have to use hard stop/soft grad. filters when doing landscapes.
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