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Photo Forum / Digital Photography / Digital Photo / May 2008crossed polarising filters to reduce light?
I want to take some long exposure photographs. To achieve this I need to greatly reduce the light entering my camera. My A630 will only go down to f8. I could use a stack of ND filters, but this is tedious, and quite expensive. A thought struck me from my school physics days. 2 polarising filters, at 90 degrees (in theory) block all light. If they planes are lined (0 degrees) up the light transmission is at its maximum. http://dl.clackamas.edu/ch106-07/optical.htm This appears to give me a nicely variable light reducing filter, albeit at a high-ish price. (2 x 25 on Hoya's uk list, against 16 for an ND4) Can any body comment of the feasibility of this? Ideally, I'd love to hear from anyone who'd used this technique successfully. BugBear >I want to take some long exposure photographs. > 2 polarising filters, at 90 degrees > (in theory) block all light. > Ideally, I'd love to hear from anyone who'd > used this technique successfully. > > BugBear This technique is commonly used by astronomers to reduce glare when viewing the moon. Meade make such a filter, but it has a fairly savage colour caste - green if I recall correctly. I suspect it may be difficult to get a set that will not introduce a strong colour caste. The same problem (only worse) will be evident with welder's goggles or solar filters (e.g. Thousand Oaks or Baader film). Having said that, crossed photo polarisers would probably be your best bet. Just make sure you buy linear polisers, not circular! :-) ...and have Photoshop ready to fix the colour afterwards. -- Jeff R. > Having said that, crossed photo polarisers would probably be your best > bet. Just make sure you buy linear polisers, not circular! :-) > > ...and have Photoshop ready to fix the colour afterwards. The polarizer nearest the camera should be circular, in order to avoid fouling up the beam splitters for AF and/or metering. The polarizer furthest away from the camera must be linear for this trick to work. Also, avoid "thin polarizers" which don't have front filter rings, for obvious reasons. For a prepackaged solution, look at a Singh-Ray Vari-ND filter.  SignatureMichael Benveniste -- mhb@murkyether.com (Clarification required) Legalize Updoc. >> Having said that, crossed photo polarisers would probably be your best >> bet. Just make sure you buy linear polisers, not circular! :-) [quoted text clipped - 9 lines] > > For a prepackaged solution, look at a Singh-Ray Vari-ND filter. That sounded interesting, right up until... >> 77mm Standard Ring Mount $340 gack. BugBear > The polarizer nearest the camera should be circular, in order to > avoid fouling up the beam splitters for AF and/or metering. > > The polarizer furthest away from the camera must be linear for > this trick to work. But surely you would then not be able to vary the light level by rotating one filter would you? Robert >> The polarizer nearest the camera should be circular, in order to >> avoid fouling up the beam splitters for AF and/or metering. [quoted text clipped - 4 lines] > But surely you would then not be able to vary the light level by > rotating one filter would you? Sure you would, the polarizing function works exactly the same on both of them. >> I want to take some long exposure photographs. > [quoted text clipped - 12 lines] > I suspect it may be difficult to get a set that will not introduce a > strong colour caste. Not a problem in the digital era, of course. > The same problem (only worse) will be evident with welder's goggles or > solar filters (e.g. Thousand Oaks or Baader film). Hmm. Not heard of those. I'm trying to go from (around) 1/125 exposure to 10 secs, which is 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1, 2, 4, 8 11 stops. Any know of a ND22 filter? > Having said that, crossed photo polarisers would probably be your best > bet. Just make sure you buy linear polisers, not circular! :-) Ooh. Thanks for the reminder! > ...and have Photoshop ready to fix the colour afterwards. Well, Gimp, or UFraw, but point taken ;-) BugBear >> I suspect it may be difficult to get a set that will not introduce a >> strong colour caste. > > Not a problem in the digital era, of course. When I put a couple of my filters in front of each other with polarisation set as such there is a very deep purple-blue caste - suspect that it is just too far gone to correct. Signaturehttp://www.petezilla.co.uk In rec.photo.equipment.35mm bugbear <bugbear@trim_papermule.co.uk_trim> wrote: > Hmm. Not heard of those. I'm trying to go from > (around) 1/125 exposure to 10 secs, which is > > 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1, 2, 4, 8 > > 11 stops. Any know of a ND22 filter? You can get most of the way there with an ND 3.0 filter. <http://www.bhphoto.com/?O=productlist&A=details&Q=&sku=8110&is=REG> They used to have filters up to 6.0 listed, but it seems the catalogue is getting smaller. Peter. Signaturepirwin@ktb.net > In rec.photo.equipment.35mm bugbear <bugbear@trim_papermule.co.uk_trim> wrote: >> Hmm. Not heard of those. I'm trying to go from [quoted text clipped - 7 lines] > > <http://www.bhphoto.com/?O=productlist&A=details&Q=&sku=8110&is=REG> Surely that's only 1 1/2 stops worth? BugBear >> In rec.photo.equipment.35mm bugbear >>> Hmm. Not heard of those. I'm trying to go from [quoted text clipped - 11 lines] > > BugBear As I recall, neutral density filters are based on a logarithmic base 10 scale, so that an ND1=1/10th of light transmitted through it, ND2=1/100, ND3=1/1000, and so forth. Just calculating in my head, it looks like the difference between 8 seconds and 1/125 second would be 1024, right at the strength of an ND3. You do well to double-check this before you buy, but if I'm correct, one ND3 should be much less expensive than two polarizers. Allen In rec.photo.digital bugbear <bugbear@trim_papermule.co.uk_trim> wrote: >> In rec.photo.equipment.35mm bugbear <bugbear@trim_papermule.co.uk_trim> wrote: >>> Hmm. Not heard of those. I'm trying to go from [quoted text clipped - 9 lines] > > Surely that's only 1 1/2 stops worth? No, density 3.0 cuts out 99.9% of the light - 10 stops. 0.3 - 1 stop 0.6 - 2 stops 0.9 - 3 stops Peter. Signaturepirwin@ktb.net / > In rec.photo.digital bugbear <bugbear@trim_papermule.co.uk_trim> wrote: >>> In rec.photo.equipment.35mm bugbear <bugbear@trim_papermule.co.uk_trim> wrote: [quoted text clipped - 14 lines] > 0.6 - 2 stops > 0.9 - 3 stops Hmm. I think there's more than one scale of units being used: Here's a Hoya "ND4" filter http://www.jessopsbusiness.com/Product/?Prod=18837 Hoya HMC ND4 Filter 72mm Filter - Factors & F-stops Filter Factor = 4 f-stop = 2 I'm now (officially) confused. BugBear In rec.photo.equipment.35mm bugbear <bugbear@trim_papermule.co.uk_trim> wrote: >> No, density 3.0 cuts out 99.9% of the light - 10 stops. >> [quoted text clipped - 8 lines] > > http://www.jessopsbusiness.com/Product/?Prod=18837 That is a 4x filter which has a density of 0.6. It would be fine to call it a 4 times ND filter, a 4x ND filter or a two stop ND filter, but Hoya is using bad terminology if they call it an ND4 filter. Density is a well defined term in photography and B+W, Heliopan, Kodak, Tiffen and Lee get it right. Peter. Signaturepirwin@ktb.net >> Here's a Hoya "ND4" filter > [quoted text clipped - 4 lines] > filter. Density is a well defined term in photography > and B+W, Heliopan, Kodak, Tiffen and Lee get it right. Here is a Web page that explains the difference between the Hoya and other manufacturers' nomenclature. http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-ND.html -- Don >>> Here's a Hoya "ND4" filter >> That is a 4x filter which has a density of 0.6. [quoted text clipped - 7 lines] > the Hoya and other manufacturers' nomenclature. > http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-ND.html Excellent info - thanks for that. I will add that Jessops in the UK follow Hoya's nomenclature. BugBear ["Followup-To:" header set to rec.photo.digital.] >> Here is a Web page that explains the difference between >> the Hoya and other manufacturers' nomenclature. [quoted text clipped - 4 lines] > I will add that Jessops in the UK follow > Hoya's nomenclature. I can see the headline in next week's AP: "Jessops staff know no photography. Shock!" Signaturesavvo orig. invib. man >> I will add that Jessops in the UK follow >> Hoya's nomenclature. > >I can see the headline in next week's AP: "Jessops staff know no >photography. Shock!" More accurate apparently is "Nikon staff know no photography. Shock!" >http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/filter/filter-ND.html It does show that Hoya do at least differentiate between their own nomenclature and industry standard, using 4x 8x etc. Nikon however, having long been the Microsoft of the photographic world, ride roughshod over existing convention with their own, totally wrong, nomenclature eg. from the page linked above: "The two ND filters come with Nikon's Coolpack are of ND4 and ND8 type, which means they reduce two and three stops, respectively." The first part of that quote is confirmed by the photos - Nikon do indeed call these filters ND4 and ND8! They might be "4x ND" and "8x ND", as per Hoya's nomenclature but they certainly aren't ND4 and ND8 by industry standard nomenclature, despite how Nikon labels them. SignatureKennedy Yes, Socrates himself is particularly missed; A lovely little thinker, but a bugger when he's pissed. Python Philosophers (replace 'nospam' with 'kennedym' when replying) > This technique is commonly used by astronomers to reduce glare when viewing > the moon. Meade make such a filter, but it has a fairly savage colour > caste - green if I recall correctly. Filters meant for astronomy often have strange colors like this. They are designed to block the orange light from the sodium lamps often used for street lighting. > I want to take some long exposure photographs. > 2 polarising filters, at 90 degrees > (in theory) block all light. "In theory" is the operative word. Yes, it sorta works, but my experience is that when you get down to serious reduction of light, there is almost always a very strong blue/violet cast - so strong as to be very difficult to white balance out. Admittedly, I have not used the really high quality polarisers, so your mileage may vary. So I bit the bullet and bought a Hoya NDX400, which is a 9 (?IIRC) stop ND... Haven't used it much yet, but the day will come (O:. No colour cast problem, only two additional glass/air surfaces to reduce quality instead of 4, and a few stops better attenuation than my two (cheap) polarisers managed.. If you do go ahead, let us know how it goes. > [I need v. dark] ND filters > A thought struck me from my school physics > days. 2 polarising filters, at 90 degrees > (in theory) block all light. Works. With the following caveats: 1) The effect can vary with different brands of polarizers. I have a linear Tiffen that is sort-of worthless at extinction but perfectly good at sky-darkening etc.. 2) At maximum extinction there is a profound violet color shift. Violet as in deep-deep blue + UV, not violet as in magenta. Kiss off any reds, yellows and greens. 3) The darkening effect is less with off-axis light rays. The center is OK but the fringes don't darken as much. 4) The effect isn't uniform. Polaroid-style polarizers are pretty sloppy affairs and you may find bright and dark patches at maximum extinction. You can use either linear or circular polarizers. If you use a linear + circular combination then screw the linear into the circular. If you use two circulars then you need to mount them head-to-head with a double-male-threaded ring in between. Other circular combinations make for interesting effects: Two stacked circulars, or a circular screwed into a linear, make a variable warming/cooling ND filter. Two circulars mounted tail-to-tail with a double-female ring make a rather strange multi-color deep ND filter. The reason for the funny color effects with circular polarizers is the "wave plate" that is used to do the circular bit. Known as a "1/4 wave plate" it only works perfectly at one wavelength. 535nm or so green is used for the wave plates in circular polarizers. http://en.wikipedia.org/wiki/Wave_plate For more color effects try inserting various transparent things between two crossed polarizers. SignatureNicholas O. Lindan, Cleveland, Ohio Darkroom Automation: F-Stop Timers, Enlarging Meters http://www.darkroomautomation.com/index2.htm n o lindan at ix dot netcom dot com >> [I need v. dark] ND filters >> A thought struck me from my school physics >> days. 2 polarising filters, at 90 degrees >> (in theory) block all light. > > Works. With the following caveats: (lots of good info) Wow! Thanks for all that; above and beyond what I need to know, but interesting in its own right. BugBear >>> [I need v. dark] ND filters >>> A thought struck me from my school physics [quoted text clipped - 8 lines] > and beyond what I need to know, > but interesting in its own right. Remember too that a polarizer typically drops light around 2 stops on its own, so two polarizers stacked is going to get you down 4 stops right off the hop. In rec.photo.digital Matt Ion <soundy106@gmail.com> wrote: >>>> [I need v. dark] ND filters >>>> A thought struck me from my school physics [quoted text clipped - 8 lines] >> and beyond what I need to know, >> but interesting in its own right. > Remember too that a polarizer typically drops light around 2 stops on > its own, so two polarizers stacked is going to get you down 4 stops > right off the hop. No, because a polarising filter will have no effect on light already polarised in its direction of filtering. SignatureChris Malcolm cam@infirmatics.ed.ac.uk DoD #205 IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK [http://www.dai.ed.ac.uk/homes/cam/] I can't comment on the polarizing filters, but couldn't you just put a piece of cardboard with a hole punched in it over your lens? > I can't comment on the polarizing filters, but couldn't you just put a > piece of cardboard with a hole punched in it over your lens? Might introduce vignetting? BugBear |
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