IRE SCALE in Pentax meter, what if any colour films it fits? : LUSENET : Large format photography : One Thread

The IRE scale in the Pentax Digital spot meter spans 5 stops. Looking at the Density / Log exposure curves for Velvia, this film's log exposure in Lux seconds spans only 2.5 decades. There must be a correlation between the IRE scale and the log exposure but I do not know how to connect the Fuji exposure data with the IRE scale.

Question 1) For example, I would like to know where IRE zone 10, IRE zone 1 and IRE zone 3.3 (midpoint, Kodak Grey Card)fit in the Fuji densitometric curves. Question 2) I understand that the IRE scale was developed to fit reversal films used in cinematography. Does any one know of any papers that explain the technical criteria behind the IRE that explains its use?

-- Julio Fernandez (, February 27, 2002


The IRE scale is used for video, not film. The scale is calibrated to show the black and white points on a composite video signal. IRE is the Institute of Radio Engineers. You should be able to find some information with a google search on +IRE +VIDEO.

-- Dave Mueller (, February 28, 2002.

I think you're making it a lot more complicated than it is. The IRE scale, in fact, is an exposure scale created by the Institute of Radio Engineers, for use in broadcast television.

It's based on a reference of a theoretical 100% 'lambertian' reflectivity standard, with the recommended shadow luminance being limited to 10% of that maximum.
This 10:1 lighting ratio embodied in the IRE standard is too narrow to be practical outside of a TV or cine studio, and for photographic purposes that lower limit can be extended to -5.5 stops for reversal materials, and -7 stops for B&W and colour negative stock.
-5.5 stops would be 2.2% on the IRE scale, and -7 stops would be 0.8%. In all cases, the IRE 100% level is that exposure which just gives white with a hint of detail, or roughly VIII in the zone system.

The IRE scale is totally comparable with a normal photographic meter, as long as you remember that it uses a maximum white as its reference. In other words, the 18% grey reference (17.68% to be more precise) of many photographic meters indicates an exposure 2.5 stops more than the IRE 100% level, and reads (surprise, surprise!) 18% on the IRE scale.
The alternative photographic 'standard' of a 12.5% relectance grey level is exactly 3 stops away from the IRE 100% reference.

From using the IRE scale, you can easily see that a reference of 18% grey is ideal for reversal materials, and a reference of 12.5% is better suited to the wider exposure range of negatives. This is probably why there seem to be two different reference standards for photographic meters, and manufacturers can't make their minds up whether to use 18% or 13% as their reference grey. The IRE scale helps to eliminate this dilemma by allowing you to choose a reference.

-- Pete Andrews (, February 28, 2002.

Pete: Many thanks for your reply. I was hoping to get hold of 'Beyond the Zone System' in hope that it would help clarify some of the points you have made. I still have not and it is getting late in the life of the thread but I will post the questions anyway and if you do not see it I will post them in the LFF.

1) you refer to a 10:1 lighting ratio in the IRE; The IRE encompasses a 5 f/stop range, which had read as a ratio of 1:32 (2 to the 5th power.) Obviously my mistake but I still do not know where the 10:1 ratio you refer to comes from.

2)The math by which you derived -5.5 stops as being 2.2% and -7 stops as being 0.8 % on the IRE scale.

Since the two different reflectivity standards arise from the characteristics of recersal and negative material, does it not follow that meters should provide users with a choice of which to use in calculating exposure, as for example offering both two scales? Does the Calumet scale that can be pasted onto the Pentax digital spot meter reference the 12.5% reflectivity? if so, pasting it to the Pentax wipes out the IRE. The IRE I interpret, in light of what you have said, as being more closely related to reversal films, -even though a little short of its exposure range by 1/2 stop at the low end. Is this right?

-- Julio Fernandez (, March 09, 2002.

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