# Talk:Light value

The LV 10 definition is shaky. Presumably, there is a better definition in the ISO standard for film speed. Anyone got access to this? -- Egil 06:48 Apr 14, 2003 (UTC)

The SV is shaky, too. Both are incompatible with the only good reference I can find, which is Doug Kerr's article: http://doug.kerr.home.att.net/pumpkin/#APEX Anybody else have a reference? The units specified for LV are luminance, which is usually called a BV. LV should be an illuminance, according to APEX descriptions I have seen. Dicklyon 03:58, 3 May 2006 (UTC)

++++++++++++++

Lv is not a recognized APEX symbol.

The use made of "Lv" in the article suggests that it is an indication of luminance (brightness).

The APEX symbol for brightness value (which is a logarithmic representation of luminance) is Bv.

It appears to me that the notion of "Lv" in the article may be an attempt to rehabilitate the unfortunate practice in which brightness is (inappropriately) stated in "Ev", which is of course a measure of exposure (the joint effect of aperture and shutter speed). The convention is that the "Ev" stated to describe a luminance is the Ev that would be recommended by a properly calibrated exposure meter, set for a sensitivity of ISO 100, measuring the luminance of interest.

Thus, Ev 10 would correspond to Bv 5.

Perhaps the author thinks that this convention can be rehabilitated by calling this measure (Bv+5) "Lv". (Why, I wouldn't know, as Bv is a perfectly good APEX value for luminance. But of course that way, the numbers would be the same as with the improper use of Ev!)

The equivalence stated by the author, Lv 10 = 107.74, is essentially the significance of "Ev 10" used in that inappropriate way (that is, the luminance indicated by Bv 5.) Bv 5 will correspond to 107.74 candelas per square meter if we assume the use of an appropriate value of the reflected light exposure metering constant, which influences the definition of the scale of Bv (as discussed below).

Notes:

The definition of the scale for Bv is not absolute, but is predicated on a certain value of the reflected light exposure metering constant, K. This constant is used, in connection with the standard calibration equation for reflected light exposure meters, to allow the manufacturer to follow his thoughts as to "proper exposure". The definition of Bv is predicated on a value of K in order that that the standard metering equation, expressed in APEX terms, will not need to have a term reflecting K.

Similarly, the definition of the scale for Iv is not absolute, but is predicted on a certain value of the incident light exposure metering constant, C. This constant is used, in connection with the standard calibration equation for incident light exposure meters, to allow the manufacturer to follow his thoughts as to "proper exposure". The definition of Iv is predicated on a value of C so that the standard metering equation, expressed in APEX terms, will not need to have a term reflecting C.

Doug Kerr 2006.06.24

Thanks, Doug. See if my edits capture the gist of your comments here and at APEX system. I've removed the 'disputed' tag. By the way, if you sign your comments with four squiggles (NOT octotherps), that will automaticallly expand into your username and date. Dicklyon 17:10, 25 June 2006 (UTC)

I think there still are some issues with both light value and APEX system. I have yet to see any reliable evidence that LV is anything other than an unofficial synonym for "EV at ISO 100," and, even then, what is arguably one of the more reliable sources (the Arrowin Kyoritsu catalog) specifies "LV at ISO 100" for the calibrated light sources (interestingly, the same catalog uses "EV at ISO 100" for the multi camera testers). In sum, I'm not sure the use of "LV" is any better than "EV," especially given the near-universal manufacturers' practice (right, wrong, or indifferent) of using "EV" to specify luminance ranges for metering and autofocus sensitivity.

The relationship between LV (or EV) and luminance is necessarily linked to the reflected-light meter calibration constant, so it would seem necessary to make some mention of the exposure equation (perhaps in a separate article). With the most common value of K (12.5, used by Canon, Nikon, and Sekonic),

LV 10 = 128 cd/m2

With the value of 14 (used by Minolta and Pentax),

LV 10 = 143.4 cd/m2

At approximately 1/6 exposure step, the difference admittedly is not huge, but it is a difference nonetheless, and the use of numbers without the defining formulae makes it difficult for a reader to determine what really is happening.

With APEX system, there are some minor differences with the "official" source (ASA PH2.5-1960, which proposed the system) on terminology and values. An additional minor complication is introduced by the partial resurrection of APEX in the EXIF 2.2 standard, with yet again slight differences in terminology and values. If we accept "light value," there are at least three different systems that use confusingly similar terminology. Adding to the confusion is the use of the long-deprecated symbols E, B, and I for exposure, luminance, and illuminance in APEX and EXIF (the current ISO standards use the currently preferred SI symbols H, L, and E).

I'll try to put something together and see what people think. A hint at my thoughts can be found at http://www.lfphoto.info/articles/conrad-meter-cal.pdf (PDF), although it doesn't directly address any of the issues here. My contact info is available on this page.

JeffConrad 00:43, 17 July 2006 (UTC)

Jeff, I look forward to your revisions. This topic remains a bit confusing, as you say. Dicklyon 05:00, 17 July 2006 (UTC)

Looks as if my login timed out before I saved my edit ...

The topic probably will remain confusing.

After further thought, I am convinced that "light value" has been given so many different meanings that it essentially is meaningless. Accordingly, I think the best approach for an entry is simply to list the known meanings without implying that any one of them is correct. With this approach, I think it is inappropriate to list the "light values" of various subjects.

In particular, I have never seen the common recent usage of

LV = EV at ISO 100

cite a credible source; it's an urban legend that many people have blindly copied. I am guessing that the first meaning that I list (an arbitrary value indicated by meters such as Weston) was the origin of the term, though I'm not sure this ever can be substantiated—perhaps some Weston enthusiast will emerge with the answer. This usage was discussed in the 1948 edition of Ansel Adams's The Negative. Sidney Ray seems to imply this usage, and I think he's a reliable source. Stroebel and Zakia certainly also are credible, but because their usage is in conflict with the original APEX documentation, I'm inclined to defer to Ray. Had APEX been accepted, and luminance value retained a single meaning, it might have been a useful quantity.

I am not convinced that there is anything improper about the use of EV at ISO 100 to indicate luminance (except, perhaps, for the failure to state the calibration constants $K$ and $C$). Although such usage was not described in APEX, the concept of exposure value predates APEX by several years—EV scales were included on some Hasselblad C lenses in the late 1950s, and ASA PH2.5-1960 makes note of this. It's possible that the EV concept may predate even this. As I had mentioned, the practice of giving luminance ranges for metering and autofocus iin EV at ISO 100 is almost universal among photographic equipment manufacturers and the meaning is reasonably unambiguous, so using a new, unofficial, term is likely to do nothing but confuse.

I could find no reference for the "traditional definition," other than obvious copies of the original entry. The definition is further suspect because it is numerical rather than symbolic, when a defining equation would have been simple to provide, as APEX did for all of the logarithmic quantities. The definition is more than shaky; consequently, I removed it.

JeffConrad 07:32, 19 July 2006 (UTC)

Alas, further examination of Zakia and Stroebel (1993) and Stroebel et. al. (2000) would seem to compel restoring the illuminance interpretation of "light value." This is not an endorsement (which NPOV would prohibit anyway), but simply a recognition that the term has been used with that meaning by highly respected authors. It could be argued that it is reasonable for "light value" to be used in the context of either luminance or illuminance, because as noted in the 22 August edit, APEX luminance value and incident-light value are numerically equal, so the same camera settings should result from either interpretation. It is possible that "light value" was chosen for its reflected/incident neutrality. However, I cannot cite any authoritative source that has specifically discussed this issue. Thanks to Dick Lyon for reviewing Zakia and Stroebel. JeffConrad 23:06, 22 August 2006 (UTC)

## Edits of 26 January 2007

Although code readability is desirable, I think it's secondary to readability of the rendered article. With sans-serif type, there is little difference in appearance between HTML and ASCII quotes, but ASCII quotes look hideous if the article is printed using a serif typeface. WP guidelines allow either style; using HTML quotes provides acceptable rendering independent of typeface.

Linking luminance value to luminance and incident light value to illuminance is somewhat misleading, because the 'value' terms are the logarithms of the physical quantities. I'll concede, however, that there is little likelihood of (or need for) articles on luminance value and incident light value, so I've pointed these links to the APEX article where the definitions are given. I agree with Fru1tbat that the previous linking was excessive for an article of this length, so I've removed the redundant links.

JeffConrad 02:19, 26 January 2007 (UTC)