Re: Coloured filters and EIR

[Clive Warren <Clive.Warren@xxxxxxxxxxxxxx>]

At 5:26 pm +0100 30/6/00, Andy Finney wrote:

>Hi,
>
>Chatting filters over a drink recently Spearmint and I realised that
>with ostensibly the same colour filter we got completely different
>results with IR Ektachrome. I use Hoya and he uses Cokin.
>
>This would explain why we get such diversity of results with this
>stuff. The infrared response of the coloured filters must differ
>significantly. One for Cocam I think? Any thoughts Clive?
>
>Cheers
>
>Andy
>www.invisiblelight.co.uk
>www.atsf.co.uk/ilight/

Hi Andy,

Yes many thoughts :-)

This may read a little like a marketing plug so please delete this message
and read no further if you may be irritated by the contents :-)

I first became aware of the dubious manufacturing tolerances of some filter
manufacturers years ago when using a Cokin polariser and finding a blue
cast on trannies. When you pay a lot of money for a piece of glass with a
reputable name you don't expect colour casts. Then started comparing
different colour and warm-up filters and was amazed by the differences
between those from different manufacturers claiming the same same
specifications.

The same is probably true for opaque infrared filters.  It is most
certainly true for grad. filters - I have seen some "top quality" brand
name examples of so called neutral density grad. filters which have
terrible colour casts!

One of the reasons I supply a range of filters (at CoCam) is that I found a
manufacturer willing to supply filters which conform strictly to Wratten
specifications.  So, for example, a Wratten #12 filter is always a #12 and
will do what is expected; a neutral density filter is colour neutral.  I
was shown a live spectral analysis of a range of filters selected by myself
at random in the manufacturer's factory before deciding to go ahead and
have filters made for CoCam.


Clearly it is cheaper to manufacture filters which have a wide tolerance of
acceptability as the rejection rate is lower. At CoCam I have tried to
balance price and quality to give peops value for money and results they
expect.


All the best,

             Clive   http://www.cocam.co.uk
                     Photographic Services, Filters and Equipment, Infrared FAQ


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