This is a follow up to my posts yesterday in the thread “shifting to MF”:
http://www.luminous-landscape.com/forum/index.php?topic=45747.msg384166#msg384166As the thread had swung onto a completely new topic (my fault!), I am starting this new one.
So I did open a P1 support case yesterday...and here's the response. I thought I should share it here for interested parties:
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Dear Ray
Please specify which back specifically you are looking for .
In this case you have specified P30+
P30+ and P45+ is using a BG50 1.7mm
While P30 and P45 is using BG50 1.5mm
Information about BG filters can be obtained from Schotts.
Kind Regards
Phase One Support
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Great! So after I thanked the PhaseOne support guys, I went to Schott's filter page
http://www.schott.com/advanced_optics/english/our_products/filters/...but they don't currently list a BG50; BG40, BG42, BG55 yes, but no BG50. For some reason, you have to enter "BG50" into their search box to locate it.
2005 version:
http://www.schott.com/advanced_optics/english/download/bg50_data_sheet_25.11_sheet_1_and_2.pdf?highlighted_text=BG502008 version:
http://www.schott.com/advanced_optics/chinese/download/bg50_data_sheet_05.05.08.pdf?highlighted_text=BG50(There are subtle differences in transmission between the 2005 version and the 2008 version. Not sure which version applies to the PhaseOne backs).
Now these specs are for a 1mm thick filter; transmissions will be lower for the 1.5mm and 1.7mm thicknesses that PhaseOne use.
So I played with Schott's rather cool "Filter Calculator Program" - actually an Excel file that you download as a zipfile - which allows interactive plotting of the transmission for different filters and thicknesses.
Again, the BG50 filter is missing from the spreadsheet's internal database, but I selected the BG42 as the nearest proxy, and an equi-energy (flat) spectrum as the illuminant type.
(In the meantime, I've emailed Schott to ask them how to insert the BG50 filter specifications into the spreadsheet database).
First thing I noticed was that while the datasheets only give "Internal Transmittance" - which is several percent higher than actual overall Transmittance, presumably because it neglects effects at the exterior air-glass boundaries - the spreadsheet shows both Internal Ti and overall T. One should really ignore the Ti values and just look at actual "T linear". Both plots and tabulated values to single nm precision are produced on the fly - really excellent.
Second thing I noticed was that increasing the thickness from 1mm to 1.5 and 1.7mm mainly has the effect of steepening the bandpass edges and nearly killing off the fainter red/IR and blue wings. This is no surprise - it's not linear. You might instinctively think that a 1.5mm dye filter absorbs 50% more of every wavelength than a 1.0mm one does, but no...the formula is this:
T = 1 - [(1 - T1)^(t/t1)]
where T and T1 are transmission factors between 0 and 1.0 (0% - 100%), t and t1 are thicknesses in identical units (here mm), and T1 is the transmission for a reference thickness t1 while T is the transmission for some other thickness t. The datasheet gives us our reference values t1 = 1 mm and T1 = whatever the table says for a particular wavelength.
A few trial values for t = 1.5 or 1.7mm quickly shows you that at the peak of the transmission curve where T1 is high, then so is T; whereas low T1 means high (1 - T1) and the exponent then really amplifies this, leaving a really small T.
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Now I am especially interested in the astronomically vital H-alpha line (656nm), and the general IR transmission/leak beyond that.
At 1mm thickness, the BG50 is better at 656nm (~30% transmission) than the typical DSLRs measured here:
http://astrosurf.com/buil/filters/curves.htmhttp://www.beyondvisible.com/BV3a-ICF.htmlHowever, the formula above shows that this 30%, being a rather low intial T1, will be sharply dropped to
16% at the actual P-back filter thickness of 1.5mm and
13% for the P+ back filter thickness of 1.7mm.
So in reality, the H-alpha (and IR) filter transmission of the PhaseOne backs is just as awful as from stock DSLRs!
Conclusions: I used to wonder; now I know: the H-alpha (and IR) filter transmission of the PhaseOne backs is just as awful as from stock DSLRs. This is a terrible shame, as their Xpose+ long exposure ability would otherwise make them the best MFs out there for astrophotography. But MFDBs with easily user-removable filters were made until recently (Kodak ProBack & Mamiya ZD), so there is no reason why PhaseOne cannot also make their filters removable/interchangeable!