Question on how dark CPL's are compared to ND filters.

I have a couple of cicular polarising filters - both Hoya. One, the Pro1 is darker than the other, a HD. My question is approximately how many stops of ND filter are the filters equivalent to in terms of how much light they pass.

Apologies in advance if that is a spectacularly stupid question.

They're generally considered to be equivalent to 1.5 to 2 stops of light loss.

I expect that there is variation between brands.

Find somewhere with a constant light level, center the needle in Manual screw the filter on and see how much the exposure changes?

I always considered my own CPL to be -1.5EV but never actually quantified it. I just measured B+W MRC CPL with a Minolta spotmeter that measures to 0.1EV and it measured -1.4 to -1.5EV.

This, of course, was the pure ND effect on something not illuminated by polarized reflective light. That is, no apparent effect of 'more darkening' vs. 'less darkening' as the filter is rotated.

The CPL adjusts as you "spin" it. I've seen a "guesstimate" of 1 stop at the "lightest" polarization and 2 stops at the "darkest". Of course it can vary with the individual unit, but that can give you something to work on.

I myself have successfully used a CPL to get a moderate effect, slowing down water "a bit", but not dramatically like the "artsy" stuff we see all the time!

The one on the left (The "pro) is distinctly darker than the other, the HD. Hoya make mention of 25% more light for the HD but I have no clue how this correlates to stops. Even with a 5 stop compensation on the camera it is difficult to get things dark enough to have any amount of motion blur. It is ok in low light but hopeless when the sun comes out. I understand the answer is in a X stop ND filter but I was trying to get a feel for what the CPL (Particularly the pro on the left would approximate to).

djr81 wrote in post #18199678
The one on the left (The "pro) is distinctly darker than the other, the HD. Hoya make mention of 25% more light for the HD but I have no clue how this correlates to stops. Even with a 5 stop compensation on the camera it is difficult to get things dark enough to have any amount of motion blur. It is ok in low light but hopeless when the sun comes out. I understand the answer is in a X stop ND filter but I was trying to get a feel for what the CPL (Particularly the pro on the left would approximate to).

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Actual results can vary from filter to filter, even in the same line. So, your most accurate answer is mentioned earlier; go set up on a tripod, zero out your exposure and then screw on the filter. Note how much light you lose.

Things are a bit confusing here!

First, the photos are displayed top (B&W) and bottom (color). So, which is the Pro and which is the HD?

Second, the two scenes are dramatically different, one out with the open sky, the other under a canopy of forest trees,

And third, the exposure settings for the two shots are dramatically different: 1/4 sec for the top one, 1 sec for the bottom, ISO 100 for the top, ISO 400 for the bottom.

These things mix up our ability to comment on things! However, your statement that there is 25% difference between the light from the two filters, well, the pic you showed on the first post shows that the one filter is either "dialed down" to make it darker than the second filter, or in fact it is not a CPL at all, but rather an ND filter. I'd suggest that you just do some tests -- one scene, take a shot with one filter then the same shot (same composition and exposure settings) with the second filter, having both dialed all the way to the "max" darkness/filter, and see what you come up with!

I doubt that in practice you will see a lot of difference, a 25% difference in light "flow" shouldn't be too noticeable. As you've said, you really want an ND filter (not CPL) to get a lot of the "slow water" (or slow clouds) effect. ND filters come in different "levels", you can get, for example, a 10-stop ND filter (which will really darken things down/slow things down). And, folks who use ND filters will often combine them to add to the effect!

Both were the HD filter. They were just by way of an example to show - in the case of the second shot you can get a half decent result in low(ish) light and exposure compensation on the camera. The first shot was just about ok because of the clouds cover. In direct sunlight it all just blows out.

The shots were really just holiday snaps no tripod, no nd filters just the HD CPL and camera I was carrying at the time.

I cant comment on the filter "dialling" I just picked them out of their packets and plonked them on the desk. They are both CPL's the first noticably darker than the second.

Ill have a play about and see if I can come up with some sort of an equivalent number as suggested. I just thought someone may have alreay done something similar.

This is all probably heading toward me buying an ND filter at some point.

With regard to the combination do you have one screw on and a second plate type out in front?

djr81 wrote in post #18199755
Both were the HD filter. They were just by way of an example to show - in the case of the second shot you can get a half decent result in low(ish) light and exposure compensation on the camera. The first shot was just about ok because of the clouds cover. In direct sunlight it all just blows out.

That's what you would expect. The CPL gives a max of about 2 stops of "dimming", which is not so handy in the bright daylight!

Have you read the info/instructions on using the CPL? I don't have those two models, but as far as I know CPLs as a whole have the rotating/dialing design -- you rotate the ring and the effect shifts as you do.

The difference between the two in your shot would be because the darker one had been rotated for more of a max effect.

Yeah, if you are after that stuff, then the ND is the way to go.

An old "tradition" is to use a Graduated Neutral Density (GND) filter that will darken down a light sky so that darker things in the foreground will get a better exposure. You can in fact get good results from that approach, although in these days of shooting digital Raw, with the Raw files/data having a greater dynamic range that is adjustable, and also with the High Dynamic Range (HDR) approach, well, GND filters are not as popular as they once were.

I'm not sure about what you are asking here: a combination of more than one CPL filters, or a combination of more than one ND filters, or???

tonylong wrote in post #18199788
That's what you would expect. The CPL gives a max of about 2 stops of "dimming", which is not so handy in the bright daylight!

Have you read the info/instructions on using the CPL? I don't have those two models, but as far as I know CPLs as a whole have the rotating/dialing design -- you rotate the ring and the effect shifts as you do.

The difference between the two in your shot would be because the darker one had been rotated for more of a max effect.

Yeah I get that and use them as instructed (Chromosomal challenges relating to reading instructions notwithstanding) The two CPL's from Hoya allow distinctly different amount of light through for the same setting. So maxed out on both the pro is much darker than the HD.

This may help:
http://www.hoyafilter.​com …/hdfilters/hdfi​ltercirpl/

tonylong wrote in post #18199788
I'm not sure about what you are asking here: a combination of more than one CPL filters, or a combination of more than one ND filters, or???

I was thinking of a polariser to remove reflections from the water and then an ND to darken things up some more as the CPL is not dark enough on its own.

So if you are going for the longest exposure possible, to blur the water, why are you using ISO 400? Using ISO 100 would have got you all the way to four seconds, without the need for any extra ND filters. I would have thought that 4s would have given a really good effect considering the rate of flow.

Alan

BigAl007 wrote in post #18200443
So if you are going for the longest exposure possible, to blur the water, why are you using ISO 400? Using ISO 100 would have got you all the way to four seconds, without the need for any extra ND filters. I would have thought that 4s would have given a really good effect considering the rate of flow.

Alan

Wouldn't that have led to overexposure, too bright highlights?

tonylong wrote in post #18200576
Wouldn't that have led to overexposure, too bright highlights?

He was shooting 1s @ ISO 400; so ISO 200 gets him 2s, and ISO 100 gets him 4s, since you have to double the exposure time every time you halve the ISO value. It's simply two stops down in ISO and two stops longer in shutter time, effective exposure remains the same.

Alan

BigAl007 wrote in post #18200443
So if you are going for the longest exposure possible, to blur the water, why are you using ISO 400? Using ISO 100 would have got you all the way to four seconds, without the need for any extra ND filters. I would have thought that 4s would have given a really good effect considering the rate of flow.

Alan

tonylong wrote in post #18200576
Wouldn't that have led to overexposure, too bright highlights?

BigAl007 wrote in post #18201216
He was shooting 1s @ ISO 400; so ISO 200 gets him 2s, and ISO 100 gets him 4s, since you have to double the exposure time every time you halve the ISO value. It's simply two stops down in ISO and two stops longer in shutter time, effective exposure remains the same.

Alan

OOPS! I think I must have been groggy!