[Leica] Re: Filter transmission measurements

["Rob Studdert" <audiob@ozemail.com.au>]

On 17 Nov 2000, at 22:26, Erwin Puts wrote:

> Tina wrote in part:
> "In room light, without the UV filter in front of the meter I get an f
> number of 2.89.  With the UV
> filter, I get an f number of 2.86.  Not much difference, it's true, but
> enough to matter when you are taking pictures in the dark!"
> 
> Let us first present some general principles. Most consumer measuring
> instruments have an accuracy within 5% as does the mechanical tolerance of
> the aperture stop. Restricting ourselves to the Minolta exposure meter, we
> should allow for a tolerance of 5%.
> The calibration of the Minolta meter is for the 2800 Kelvin range.(tungsten
> light) The B&W data sheets show that for the red part of the spectrum (around
> 600 nanometers) the UV filter has a transmission efficiency of 97 to 98%.
> Theortically then the transmission loss of about 2% would be within the 5%
> margin of tolerance and thus be not detectible with reliability. Further to be
> sure one should do a series of readings to see if there is statistical validity.
> I did some tests myself with a UV filter and noted In tungsten light and without
> the UV filter, my Minolta Autometer V gave a consistent reading of 2.8 + 9/10
> with 10 readings.With the filter on the measuring dome and without my hands
> around the filter (to avoid shadows from my hand) I noticed a drop of only 1/10.

I too tested some filters using a tripod mounted Minolta Spotmeter F pointed 
at a sheet of paper illuminated via tungsten/halide lamp providing a reading 
without filtration of f4,0 0/10 for ISO100 @ 1/60thS (in 10 measurements 9 
read f4,0 0/10 and one read f2,8 9/10).

I placed the filter under test directly in front of the lens and measured several 
times to obtain the following results:

Leitz UVa f2,8 9/10
Hoya UV(0) f2,8 9/10
Hoya UV(0) HMC 2,8 9/10
Hoya Skylight(1A) f2,8 8/10
Hoya Skylight(1B) f2,8 9/10
Hoya 81B f2,8 7/10
Hoya 82B f2,8 5/10
Hoya NDx4 f2,0 0/10
Pentax SMC Skylight f2,8 9/10

I don't use filters in front of my Leica glass unless it is absolutely required 
but in any case I don't believe (given the tests above) that adding quality UV 
filters will make a visible difference in exposure value given that they don't 
contribute to mechanical vignetting (the apparent scene illumination could 
conceivably rise through flare though :-)

PS I got stung by a mighty flare artifact the other day shooting people around 
a blazing bonfire using the Summilux 35 ASPH wide open with hood and no 
filter, not pleased, it's times like this that I appreciate my CZ35f1.4

Cheers,

Rob Studdert
HURSTVILLE AUSTRALIA
Tel +61-2-9554-4110
Fax +61-2-9554-9259
UTC(GMT)  +10 Hours
audiob@ozemail.com.au
http://www.ozemail.com.au/~distudio/publications.html