----- Original Message ----- 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? >>Because tey are >>batteryless? I used to use a Norwood Director, but hated >>it. I went to >>digital with a Minolta IIIf years ago and never regretted >>it. Now I >>use a Sekonic L508. Simple and easy and far more accurate >>then the >>Director ever was. > > Peter > > I am not certain as to what you mean by "accurate" or > whehter your Norwood > was out of whack. Selenium more closely matches the > response patterns of > modern emulsions than do either CdS or SPD or even Gallium > Arsenide. > Selenium suffers from a weak response and a wide > acceptance angle but, with > allowances for these quirks, it should provide a more > accurate estimate of > the required exposure than do more "modern" light meters. > > Marc > > msmall@xxxxxxxxxxxx=20 > Cha robh b=E0s fir gun ghr=E0s fir! > > NEW FAX NUMBER: +540-343-8505 > The acceptance angle depends on the design of the light gathering arrangement rather than on the type of sensor. Don Norwood's contribution was the hemispherical diffuser for incident readings. The standard accessories for the Director include a flat diffuser, which is for measuring the incident level of individual light sources and a perforated grid for narrow angle reflectance readings. The latter two are very useful for measuring lighting ratio and actual subject contrast (vs: lighting contrast). Norwood's original meter is the one made by Spectra and later continued as the Spectra Combi with an added low level photo sensor. The current Sekonic meter is almost unchanged from the original Director. If one uses it correctly it is quite accurate. The Weston meter has a multiple lens device over the photocell which controls its directivity. The idea was to include the entire picture area but Weston also designed the meter to make subject contrast readings. These are the reason for the extra markings on the scale. Originally, the readings for some types of subjects were supposed to be entered using the A or C pointer rather than the center arrow (which was marked B on the original meters). The spectral curves shown in _Photographic Materials and Processes_ indicate that "Silicon Blue" photoelectric cells have very similar spectral response to Selenium. Cds has greater red and less blue sensitivity. Without the built in filter of the Silicon Blue cell Silicon has a curve which rises with wavelength well into the infra-red. I've read an explanation of why films no longer carried a Tungsten speed rating after sometime in the mid-1960's. It attributes this to the change in meters from Selenium to Cds but the spectral curves make hash of this explanation. --- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx --- Rollei List - Post to rollei_list@xxxxxxxxxxxxx - Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' in the subject field OR by logging into www.freelists.org - Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with 'unsubscribe' in the subject field OR by logging into www.freelists.org - Online, searchable archives are available at //www.freelists.org/archives/rollei_list