[rollei_list] Re: CdS, SPD, and Selenium

From: "jayers" <jayers@xxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Date: Sat, 30 Apr 2005 22:51:28 -0700
I have used Weston meters on and off for 50 years, and during this time =
I
have used 3 different spot meters, an SEI, which went by the wayside for =
a
Pentax digital, and a Minolta SpotF. I also have used a Minolta IIIF.
 I am of the opinion when it comes to determining exposure the metering
technique that one uses effects the outcome more than the choice of =
meters.=20
Jonathan

-----Original Message-----
From: rollei_list-bounce@xxxxxxxxxxxxx
[mailto:rollei_list-bounce@xxxxxxxxxxxxx] On Behalf Of Richard Knoppow
Sent: Saturday, April 30, 2005 8:53 PM
To: rollei_list@xxxxxxxxxxxxx
Subject: [rollei_list] Re: CdS, SPD, and Selenium


----- Original Message -----=20
From: "Marc James Small" <msmall@xxxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Saturday, April 30, 2005 3:19 PM
Subject: [rollei_list] CdS, SPD, and Selenium


> At 09:28 AM 4/30/05 -0700, Peter K. wrote:
>>Just out of curiousity, why do you use Weston meters?=20
>>Because tey are
>>batteryless? I used to use a Norwood Director, but hated=20
>>it. I went to
>>digital with a Minolta IIIf years ago and never regretted=20
>>it. Now I
>>use a Sekonic L508. Simple and easy and far more accurate=20
>>then the
>>Director ever was.
>
> Peter
>
> I am not certain as to what you mean by "accurate" or=20
> whehter your Norwood
> was out of whack.  Selenium more closely matches the=20
> response patterns of
> modern emulsions than do either CdS or SPD or even Gallium=20
> Arsenide.
> Selenium suffers from a weak response and a wide=20
> acceptance angle but, with
> allowances for these quirks, it should provide a more=20
> accurate estimate of
> the required exposure than do more "modern" light meters.
>
> Marc
>
> msmall@xxxxxxxxxxxx=3D20
> Cha robh b=3DE0s fir gun ghr=3DE0s fir!
>
> NEW FAX NUMBER:  +540-343-8505
>
   The acceptance angle depends on the design of the light=20
gathering arrangement rather than on the type of sensor. Don=20
Norwood's contribution was the hemispherical diffuser for=20
incident readings. The standard accessories for the Director=20
include a flat diffuser, which is for measuring the incident=20
level of individual light sources and a perforated grid for=20
narrow angle reflectance readings. The latter two are very=20
useful for measuring lighting ratio and actual subject=20
contrast (vs: lighting contrast). Norwood's original meter=20
is the one made by Spectra and later continued as the=20
Spectra Combi with an added low level photo sensor. The=20
current Sekonic meter is almost unchanged from the original=20
Director. If one uses it correctly it is quite accurate.
   The Weston meter has a multiple lens device over the=20
photocell which controls its directivity. The idea was to=20
include the entire picture area but Weston also designed the=20
meter to make subject contrast readings. These are the=20
reason for the extra markings on the scale. Originally, the=20
readings for some types of subjects were supposed to be=20
entered using the A or C pointer rather than the center=20
arrow (which was marked B on the original meters).
   The spectral curves shown in _Photographic Materials and=20
Processes_ indicate that "Silicon Blue" photoelectric cells=20
have very similar spectral response to Selenium. Cds has=20
greater red and less blue sensitivity. Without the built in=20
filter of the Silicon Blue cell Silicon has a curve which=20
rises with wavelength well into the infra-red.
   I've read an explanation of why films no longer carried a=20
Tungsten speed rating after sometime in the mid-1960's. It=20
attributes this to the change in meters from Selenium to Cds=20
but the spectral curves make hash of this explanation.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx=20

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[rollei_list] Re: CdS, SPD, and Selenium

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