Pentax Spot: 12.5% v. 18%?

[Arvee]

Scenario:

A film worthy scene, I set up the camera, get out the grey card and take a reading with the Pentax spot (V, to be precise, I like needle-swingers). While waiting for just the right light, I see Kodak's caveat on the grey card instructions, adjust exposure by this amount....

Do I:

a) use the exposure recommended by the spotmeter, or,

b) incorporate the Kodak instruction of 'opening an additional 1/2 stop' to account for 'average scene brightness,' which is now widely recognized as, and has been proven to be, 12 to 14% instead of the mythological 18%?

I might add to answer with the thought in mind that the neg will only be analyzed on a densitometer for proper density. At this time, answer will not be derived by a final printing.

Which will produce the better neg with the Pentax spot: a) or b)?


-Fred

The reason I am asking is that the Pentax already reads 1/2 stop more exposure than my Sekonic 448 Auto Studio and Luna Pro SBC looking at a grey card. With the other two I adjust the exposure, with the Pentax, no. If the differences were smaller, I wouldn't be concerned. BTW, fresh batts in all instruments.

 

[hal9000]

Since you are going to analyze the density of the negative, I assume you are testing your film speed. In that case, I would use the meter reading as is and then figure out what your personal film speed should be when placing the grey card at Zone V. Or did I misunderstand your question?

 

[Arvee]

Unfortunately, I don't own a densitometer. I used one as a matter of course when in college but haven't used one since.

My question really goes to whether or not the Pentax spot meter is calibrated to include the Kodak 1/2 stop exposure factor or is simply out of cal.

I was surprised to see the difference between the meters, which is normally .1 to .3 EV meter to meter from past experience. With a .5 variance, I immediately jumped to the conclusion that the adjustment was included in the factory calibration but I don't have any data that details how the Pentax spot meters are cal'd, hence the 12.5 v. 18% in the title.

My experience says to use the spot meter's recommendation as displayed and assume the Kodak factor is conveniently accounted for. The other meters must have their recommendations adjusted to get the density I like. I can read a neg fairly well and use the magazine method when I think my eyes/light table aren't cooperating.

The thing was that I never suspected a difference until I happened to take all the meters out and took individual readings thinking all was well and within a couple of points of each other but a 1/2 stop difference is definitely noticeable in the final product.

You know the old axiom about 'the man who owns one watch knows what time it is.....'

So does someone know how the Pentax meters are cal'd?

-Fred

 

[Alan Johnson]

Nikon probably calibrates its meters to ANSI standards and a 12% gray card would probably give correct exposure:
www.bythom.com/graycards.htm

 

[wiltw]

The ISO standard equation incorporates a Variable in the equation, which is left to the manufacturer to choose precisely what value (within a range of values). That accounts for why you will encounter differences in the reading from different brands of meters, and you might even see the difference between models of the same brand (in the case of meters in digital cameras, the 20D meter responds differently from the 40D/50D, for example). This fundamental truth layers on top of the 12% vs. 18% topic for reflected light meters. You would not only see this in reflected light meters but also incident meters.

 

[Chuck_P]

I'm not sure what density target you are hoping for, but as a ZS practictioner, a Zone V (middle gray) density of 0.65 to 0.75 is the value by which I compare the middle portion of a characteristic curve when comparing one curve to another. The low end of 0.1 at Zone I and the upper end of 1.3 at Zone VIII are easily established by testing. But for control of the middle values, different developers, dilutions, etc... are the key.

To my understanding of this discussion so far, it does not matter------what matters is that however you do it, you should do it consistently if you are not inclined to test for personal EI.

If you are going to be doing the proper testing for film speed and development times, it definitely does not matter as any adjustment for film speed will mitigate the need to be concerned about it because testing for speed does not employ the use of a gray card. It's concerned only with finding the EI that provides the level of exposure needed to achieve a 0.1 net negative density on the film. I have a Pentax V and have used it for all my testing. I carry an 18% gray card with me all the time and the times when I use it I don't make an adjustment, the half stop difference from 12% to 18% reflectance simply has not been discernible, IMO, if there even is one.

 

[Stephen Benskin]

Meters are calibrated to a given luminance and not reflectance. However, if you base the reflectance value on the exposure the meter will indicate and base it on the statistically average conditions, then it's 12.5%. I would suggest that the difference you are seeing with the meters is do to spectral sensitivity differences in the meters photo cell. Have you tried the meters under various lighting conditions?

There is a variable that manufacturers can apply, but K is used to adjust the exposure calculation to compensate for various physical factors of the meters. In other words, the K factor brings the real world results in line with the theoretical mathematical results. Spectral sensitivity is one of the biggest reasons for an adjustment. For instance, if the test target reads 297 footlamberts and the meter reads it as being brighter because the photo cell is overly sensitive to the color temperature of the test light, then a constant factor or K is added to the exposure formula to compensate for the difference.

 

[Richard T Ritter]

Best is to test and calibrate you meter to the materials and equipment you are using. The Zone I and Zone VIII test can be done in the dark room no densitometer is needed. For detail instruction on doing the test get the DVD Finely Focused Dead Link Removed Takes all the guess work and confusion out of the picture.

 

[Stephen Benskin]

There should be a correction to my previous post. The relative reflectance should be 12% and not 12.5%.

 

[Arvee]

Thanks, all, for the informative discussion.

I am happy with the performance of the meter and my interpretation of the measurements in the final prints. Being a retired engineer, I have always asked the question "why" when things don't behave as I expect them to behave.

I thought perhaps someone had the information about Pentax calibration standards on hand or could point me to them (I am almost certain I read that Pentax and Minolta cal'd to 12% and Sekonic to 14%, but I am unable to dredge up that source). Not that it really matters that much, but if I thought the Spotmeter V was cal'd at 18%, I would say it needs a trip to the shop.

Again, thanks for the help! If anyone has more to add, it would be appreciated.

-Fred

 

[Stephen Benskin]

Fred,

The ISO calibration for reflective meters equation is:

A^2/T = B*S/K

Where
A= aperture
T= shutter speed
B = luminance (297 Footlamberts)
S = film speed
K = constant factor (1.16)

For incident meters:

A^2/T = I*S/C

I = Incident light (7681 footcandles)
C = Constant (30)


(B*S/K) * pi = I*S/C

K (in footcandles) /C = average scene reflectance

1.16*pi / 30 = 0.121 or 12%

or

297*pi / 7681 = 0.121 or 12%



You can find more in three seminal papers:

Connelly, D, Calibration Levels of Films and Exposure Devices, Journal of Photographic Science, Vol 16, 1968.

Nelson, C.N., Safety Factors in Camera Exposures, Photographic Science and Engineering, vol. 4, no. 1, Jan-Feb 1960.

Stimson, Allen, An Interpretation of Current Exposure Meter Technology, Photographic Science and Engineering, Vol. 6, No. 1, Jan-Feb 1962.