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Author Topic: Phase One MFDB filter responses: Revealed at last!  (Read 2497 times)

ondebanks

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Phase One MFDB filter responses: Revealed at last!
« on: August 27, 2010, 11:43:54 am »

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#msg384166
As 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=BG50
2008 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.htm
http://www.beyondvisible.com/BV3a-ICF.html

However, 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!

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Kumar

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Re: Phase One MFDB filter responses: Revealed at last!
« Reply #1 on: August 27, 2010, 11:14:37 pm »

I believe the early Phase One single-shot backs had no filters in front of the sensor, and you had to use a filter in front of the lens, like scanning backs.

Kumar
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gazwas

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Re: Phase One MFDB filter responses: Revealed at last!
« Reply #2 on: August 28, 2010, 06:46:16 am »

I believe the early Phase One single-shot backs had no filters in front of the sensor, and you had to use a filter in front of the lens, like scanning backs.

Kumar

Yep, my first Phase One digital back, the original Lightphase had no filter on the chip but a filter on the lens. From memory it was a 6MP back I think....
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trying to think of something meaningful........ Err?

Alexey.Danilchenko

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Re: Phase One MFDB filter responses: Revealed at last!
« Reply #3 on: November 14, 2017, 05:30:23 am »

Resurrecting the old thread - I need the IR filter details for Phase One P20+ and P25+ backs. If anybody knows the details please share them here.

Tried to follow ondebanks example and ask Phase One support but they proved quite ridiculous. Apparently these days I have to obtain special agreement and sign NDA and this information is not available to measly mortals like me. They even claim that what was posted here was a breach of their security protocol. Full quote of communications with their support.

Quote

14-11-2017 10:57
Supporter
 
Hi
Someone has breached protocol and data is also some what wrong.
But use the P30/P45 data and you will be fine.

Kind Regards
Phase One support

14-11-2017 10:49
Me

Since when the filter or sensor information require NDA????
The P45 details were freely given and the sensor spec and response curves are available online from Kodak for P+ back series.
Just as a confirmation - the details from P45+ were shared here http://forum.luminous-landscape.com/index.php?topic=45889.0 with the directly cited P1 support response that did not require any NDA?

14-11-2017 10:43
Supporter
 
Hi

We do not give you this details without specil agreement and signed NDA.
If you do have details for the P45+ then it is a 0.6mm thinner filter instead of what is on the P45+
This I believe I can say without signed NDA.

Kind Regards
Phase One Support

14-11-2017 08:48
Me
 
This is not what I asked for.
Could you please provide the details about filters I asked for?
You provided similar details in the past for P45+ so I don't see the problem.

14-11-2017 07:02
Supporter
 
Hi

We can convert the P25 for this by replacing filter to a clear glass.
This is always done in a clean room and on request we can refocus it for IR wavelenght.
Contact your local Phase One partner for service request

https://www.phaseone.com/en/about-us/partner-locator.aspx

We strongly advice agains doing anything like this on your own.

Kind Regards

Phase One support
13-11-2017 17:11
Me
 
Hi

I am trying to convert P20+ and P25+ backs that I have to use replaceable IR filters to allow for IR/UV shooting (used in astrophotography and generally). Sensor response curves are available through Kodak sensor specification for those backs so I would like to request the specifications for IR filters used by Phase One in those two digital back models please: the thickness, size, refractive index, transmission curves please.

Thank you

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