Spot meter & ZS vs. BTZS & incident light
The most recent shot I posted to the standard gallery was of some metal tanks in full sun. With a spot meter, I was able to determine the highlight values and shadow values. How would an incident meter function in this situation for use with BTZS? Pointing it at the lens in full sunlight would completely miss the reflections from metallic surfaces, would it not?
This is still somewhat of a problem for me to understand. How does an incident meter deal with specular reflections? Do you just turn around and point it at the reflection of the light? If so, this is reflected light, not incident light. If not, how do you measure a bright reflection with an incident meter? This is an ongoing issue I have with trying to understand exposure and development using BTZS concepts.
By specular reflections I assume that you are addressing small regions of highlights that exceed the scale of the film, the paper or both.
Originally Posted by noseoil
Typically specular highlights will be relatively small in relation to the more important tonal representations. For that reason I would not concern myself with specular highlights whether that would be with a spot or an incident meter. The reason that I would not concern myself with these is that they would tend to skew the development indicated.
In BTZS metering the meter dome would be pointed toward the camera lens from both the sunlit and shaded positions. The incident reading from any of these positions would be representative of a five stop scene brightness range. If the scene brightness ratio exceeds seven then the development is reduced. If the scene brightness ratio were less then seven then the development is expanded.
If you are speaking of larger areas of reflective nature in which detail is desired then the scale of the materials will not be exceeded if the incident metering is done in the manner that I have described.
AFAIK it doesn't deal with reflections. Since very bright reflection usually become blank in print you don't have to deal with them ?
Originally Posted by noseoil
I hope this does not turn into a one is better than the other.
I looked at your print and I would say that those really bright areas were not specular. That was just where the sun was hitting the tanks so you could meter from just that point. I always assumed specular meant the sun shining off Chrome. Seems the spot meter did a fine job.
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For that kind of situation I would take my lower(shade) reading as usually but for the high reading I would turn the dome of the meter slightly towards the bright metal and this would show a higher SBR and in turn would reduce dev. to make sure the high values weren't blocked. I sometimes do this if there is a bright value in the scene, such as clouds or white water. Hope this make sense.
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I don't see how incident metering can be used for metering specular reflections of light sources, or for light emitting objects/surfaces for that matter - both very similar situations. There are times when you want detail in such circumstances: backlit snow and aluminium-painted rooftops come to mind immediately, in addition to the sort of subject in noseoil's picture.
The use of an incident meter to determine correct exposure relies on the assumption that the illuminated surfaces in the picture have a reflectance factor that lies within a certain range of values. The reflectance factor in the lobe of specular reflection can exceed that range, and so experience and judgement, or a reflected-light meter, come in handy.
BTZS Incident system
The BTZS incident system is very easy to use. It is based on the fact that from the blackest black to the whitest white that can be created with paint is approx. 41/2 to five stops difference in reflectivity when evenly lit. Film speeds are tested as outlined in the BTZS system. A shadow reading in the most important area is taken at 2 times the tested speed. The dfference in stops between highlite and shadow areas determines the developing time. Specular reflections are ignored since it is assumed that they should print white. The system assume that all tones from white to black are present in both highlighted and shadow areas. If they are not present then you can either accept that fact or change exposure/development time to compensate. It is an easily applied system that does not require a great deal of practice. To make the system work properly, one must preform some sensimetric tests...readings with a desitometer are required. Charting is also required but they can be hand drafted. One must consult the BTSZ manual since the subject can not be fully covered here.
I think the pros and cons, at least for me, is that on does not specifically know which zone a part of the subject will end up on with the incident system thereby making visulization more difficult. Problems that a spotmeter can cause...such as sensitivity differences by color, flare problems and judgement regarding readingsof distant items, IR light sensitivity etc ...are eliminated. In the best world one would use both systems. The testing is the same and the charting requirements similar.
Thank you all for your comments and input. I'm working on the BTZS text as a method of understanding light better and consequently, exposure and development. Based on the above comments, I guess my trouble with understanding the BTZS concept, in this instance, stems from the lack of a uniform scale of values which fit the "normal range" for a shot. I added a variable which is not a natural part of most scenes, shiny metal.
Granted, this is an extreme example of reflections and perhaps my lack of proper wording is to blame for my use of the term specular reflections. I consider specular reflections to be the light reflected from the sun (or any light source) off of an object which has a reflective surface such as chrome, a mirror or shiny metal, for example. I suppose a highly reflective paint could also qualify if the finish is polished enough.
If I had exposed the shot I took while ignoring the reflections completely, the tanks would have been completely white over a large portion of their surfaces. I chose to reduce development and add exposure in order to try to balance the amount of light reflecting from the tanks and make it seem less blinding, while still retaining the feeling of reflected light. I use this tank farm as a test scene, as usually there isn't as much harsh light and contrast to deal with in the natural world.
So it seems that I should retain a balance between both worlds. The ZS and BTZS both have their place, I just need to keep plugging away at the learning end of things and continue shooting. Again, thank you all for your answers. tim
'...a specular reflection is of the same intensity as the source...'
Um, Jay, I hope that you don't mind me offering a different interpretation:
Most materials reflect some of the light striking them in a specular manner and some in a diffuse manner. Few materials are totally specular or totally diffuse (there's a name for totally diffuse like 'La...ian' - Laplace, Lagrange, Lambert? - I forget: I'll have to look it up). Further: the specular part of the reflection often occurs in a lobe (ie a 'single ray' of incident light is reflected into a narrow cone of specular reflection. So when you see the reflection of the sun on most surfaces you can see it over a small range of angles, and thus it is visible as a bright spot that is larger than the one you would see from a mirror.
All that means that there is no need for the specular reflection to have anything like the same intensity as the source.
It's a lot easier to explain with a little intensity vs angle polar diagram, so I'll draw one and scan one in if you like.
'so an incident reading in direct sunlight should be the same as a spot reading of a specular reflection'
Oops, that doesn't follow. An incident reading in full sun is, and should be, very different from a spot reading of the sun*! The fact that a spot reading of the specular reflection is nowhere near a spot reading of the sun (unless you are reading the reflection of the sun off a mirror) is just an example of the specular reflection not being the same intensity as the source.
*not that one would ever contemplate doing such a thing.
Last edited by Helen B; 01-17-2005 at 12:11 PM. Click to view previous post history.
A matter of definition
My idea of a specular highlight is direct sunlight hitting polished chrome or other metals or glass and it is intended to be printed paper white in the print. The zone system or BTZS incident methods are intended to cover the range of tones from jet black in shadow to titianium white in direct sunlight. This is more than adequate for a full range print going from maximum black to paper base white. Another good example would be the intense scintillations on sidelit or backlit water that are in direct sunlight. If you tried to print them other than a textureless white with any system I believe it would be very difficult to do and would end up with a very strange looking photo. What would one expect to do with something that reads zone 27 for a brilliant, intense specular highlight(s) for example when you have skin tone that you wish to have at say zone 6 1/2?.