stephen_benskin

Mark,

 

Not that I have seen. Each manufacturer is required to state the K factor for their meters. Just check the owners manual. Most state a K = 1.16 (depending on the unit of measurement). This equates to a 12% equivalent. Manufacturers are part of the determination of the ISO/ANSI standards. It's for the benefit of all. Why would they not follow them?

 

Exposure is based on the following formula:

 

A^2/T = B*S/K

 

A = Aperture - f/16

 

T = Shutter Speed - 1/125

 

B = luminance = 297 footlamberts

 

S = Film Speed - 125

 

K = Constant - 1.16

 

To balance the equation, K must equal 1.16.

 

Reflected light meters use:

 

A^2/T = I*S/C

 

A = Aperture - f/16

 

T = Shutter Speed - 1/125

 

I = Illuminance - 7680 footcandles

 

S = Film Speed - 125

 

C = Constant - 30

 

Because both of the right hands of the two formulas equal the same left hand side, they also equal each other.

 

B*S/K = I*S/C

 

Equivalent reflectance can be found either by converting B to footcandles and dividing it by I, or dividing K (converted to footcandles) by C.

 

B*pi/I = Reflectance or K*pi / C = Refectance

Meters are actually calibrated to a specific luminance (297 footlamberts), but the equivalent reflectance is 12% according to ISO/ANSI specs. The standards allow for some variation, but no where near 18%.

An EI of 160 for TMX sounds a bit high. Maybe you should shoot some pictures and see if you like the results. You might also want to comfirm the degree of development of the speed test.

Kevin,

 

I didn't notice that there are messages from two different Kevins. Sorry about the mix-up. However, on the subject of film base, based on Tone Reproduction Theory, film-base plus fog does not factor into the contrast of the film or the reproduction curve. That's why everything is figured above film-base plus fog. It's just not part of the equation.

 

As to your second response, I can only conclude you misunderstood my post. It's a very involved subject and it was only touch on here. Any miscommunication on my part, I hope, is understandable and forgivable.

Xtol doesn't seem to give really good speeds with any of the T-Max films. TXZ has an EFS of around 800 in Xtol. T-Max RS developer is designed to achieve better speeds with TMZ. I remember getting something around an EFS of 1600 when pushing the film to EI 6400. That's only two stops under.

The reason 0.10 above film base plus fog is used is because the ISO standard uses this point to determine film speed, and that is for all B&W formats. The problem with testing for Zone I using a camera based on stopping down 4 stops from the meter reading is that the meter calibration point is located 1.0 log-H units above speed point. That's only 3 1/3 stops. You stop down 4 stops and you will be over rating the film. The reason why many people like this under rating (over exposing)is because the meter calibration point reads an effective 12% Reflectance and not 18%. That's a 1/2 stop difference. So if you are basing everything on an assumed Zone V of 18%, you are underexposing everyting by 1/2 stop. Meters where once calibrated to a different effective reflectance, but's questionable if it was ever 18%.

 

Why is the difference between the meter calibration point and film speed only a 3 1/3 and not a 4 stop difference? Flare. Speed point is determined under sensitometric conditions which do not contain flare. So, the standard has to compensate for this discrepancy. Statistically, shadows fall about 4 1/3 stops down from the meter calibration point(1.28 log H units to be exact. The average flare is one stop. In practice, this brings the film density of the shadow back up to around 0.10 above FB+f.

 

I have to disagree with Kevin about his long toe, short toe explanation. He has it backwards. Long toes allow for more variation in exposure because variations in exposure don't have as large of an effect on the density. That's why "pushing" films are generally long toed. Kevin also said something about base fog having an influence on the paper grade. It doesn't. Film base adds to printing time and can have an affect on grain, but not contrast.

Kodak refers to TXP as a studio film. Studio lighting has less flare than exterior. Supposedly, the shadow reproduction will be similar between the two films when used under their respective conditions. An interesting side note: The effect of increased or decreased exposure on the density ranges of long toed films tends to be greater than with a more linear curve as many have noted with TXP.

According to Jack Dunn in "Exposure Manual", the safety factor

was reduced to about 2.5 in the 1955/57 revision of the standard.

According to C.N. Nelson in "Safety Factors in Camera Exposures",

Photographic Science and Engineering 1960, the safety was somewhere

between 2 and 4, but says, "It is a remarkable fact that the exact

size of the safety factor has not been definitely known." While I

stated it could have been 4 prior to the reduction to 2 or more

accurately 2.5, then to the present value of 1.2, I might have been

confusing it with the average flare factor of 4.0 in the 40s, however,

there is an indication that the safety factor may have been larger

than 2.5 prior to 1955/57. How much is anyones quess according to

Nelson. But from a paper by Loyd A. Jones, "The Brightness Scale of

Exterior Scenes and the Computation of Correct Photographic Exposure,"

Journal of the Optical Society of America 1941, Jones talks about a

safety factor of 2.5. So, there seems to be some difference of

opinion between some of the great photographic theoreticians.

I find it strange that Nelson didn't remember Jones' statement in

his seminal paper. Nelson worked if not with Jones directly, then

with H.R. Condit who worked directly with Jones (Condit did the actual

experiments and later took over for Jones when Jones retired).

Was Dunn referring to a change in 1955/57 or was he remembering

the confirmation of the 2.5 factor in Nelson's paper? In any case, in

1960, the factor did changed from 2.5 to 1.2 and film speeds did

double. It appears that the safety factor of the ASA standard in the

1940s is in some question.

There was also an approximate two stop safety factor incorporatated

into the ASA standard of 1943. It was later reduced and then almost

eliminated in 1960. So, a 200 speed film in the forties would be a

400 or 800 speed film today without any changes to the film. It's

hard to do an apple to apple comparison between films then and now.

 

Before the ASA standard in the 40s, there were a number of different

methods of determining film speed.

I don't know about your results with Neopan 400, but what are you

rating the 1600 at? In some developers, its effective film speed is

about 600.

Larger negatives would account for the minimal grain, and the lens

type and the increased amount of flare from the old uncoated lenses

could account for some of the tones. But let's not forget the paper

types used. There was a portrait paper made that is said to have no

straight-line portion. This would give compressed softer tones. My

guess is the look you are describing is probably the result from a

variety of factors.

Just as a point of information. The times given by the manufacturers

is for diffusion enlargers / contract printing. If you are using a

condenser enlarger, the times will be shorter. Always keep tight

check on the dev. temperature and keep your agitation consistant.

These factors will have a big influence on your results.

 

Different film/developer combinations will influence the rate of

development of the film differently. The rate of development is also

known as development velocity. For example, a one stop change of

contrast with a film in one developer maybe a 10% change in time and

the same film in a different developer could require a 100% change in

time. So, always be careful about taking processing advice without

doing some tests.

This is completely off topic, but Tri-X and Tri-X Professional are two

totally seperate emulsions. Tri-X Professional is a long toed film

while Tri-X is an average towed film. I feel this causes a great deal

of misunderstanding among photographers.

Normal processing is based on the statistically average 7 1/3 stop

lumuninance range. Walter is simply saying that part of the this

range comes from the difference between the illuminance of the subject

in the sunlight and skylight and the subject in the shade (only

skylight). This difference is approximately two stops. That would

make the average subject range without shade approximately five stops.

This is correct.

 

Another response claims a subject can't have a reflection of more then

100%. This is also true, but it can have a reflectance greater than

100% and it is relectance that photography uses. The reason it can be

greater than 100% is that the surface used in reflectance is a

theorectically perfect diffuser or Lambertian surface. This is

calculated as (illuminance * 1/reflection density)/pi.

 

Steve

I forgot to mention the average lumanance range of 7 1/3 stops is only

the range of the subject. The illumance range at the film plane is

one stop less because of flare. If you are planing to test the range

by shooting a gray or white card, there will be little flare. In

order to avoid a misinterpretation of your development time, use only

a six stop exposure range when shooting a card. The third of a stop

isn't enough of a difference to worry about.

 

The approximate seven stop subject luminance range equates to the

seven stop difference between Zone I and Zone VIII.

 

Steve

Another problem is that there are appears to be two scales: one for

making the negative and one for the print. From a sensitometric

perspective, the average scene has 7 1/3 stop luminance range. When

you use the ANSI ISOR range of the paper of .04 above paper base plus

fog and 90% of D-max as the aim for processing, you get 0.04 as Zone

VIII and the 90% point as Zone I. The remaining 10% toward D-max is

the difference between Zone I and Zone 0 and the difference between

0.04 and paper white or near paper white is for Zone VIII and Zone IX.

 

You can also argue that the print Zones are not equally spaced log-H

values but spaced visually like the Munsell system. The Munsell

system utilizes ten steps that are visually equal. The psychophysical

perception of the human eye tends to compress the tones of lower

values even if they are equally spaced. So to appear equally spaced,

the differences between the darker values must be greater than the

differences between the lighter values.

 

I personally believe the missing Zone X print tone comes the Ansel's

less than perfect understanding of sensitometry.

 

Steve

Howard,

 

The film really isn't new. It is just coated using a new

facility. This can change the way the film responds to development.

It also may not. The "new" TMY may not respond to the particular

developer you are using while the "new" TMX will.

 

Stephen Benskin