Incident & Reflective Light "Readings" In Outdoor Photography?
I would appreicate someone explaining why indicdent and not reflective light readings are used in the BTZS. Also, how are accurate incident light readings made of non-accessible objects?
In short, when photographing out doors, what are the theortical and practicle advantages of using incident light readings in place of reflective light readings?
I would recommend contacting Fred Newman or Dennis Kibbe at the View Camera Store. They have a newsletter that deals with the BTZS method of exposure and some of the back issues cover using the incident meter.
BTZS is based on incident metering because measuring ambient light is not subjective but an absolute. Basing an exposure on a reflected reading always involves some degree of subjectivity.
Originally Posted by Flauvius
One can not actually take an ambient meter reading of non-accessible objects so you must assume that the light falling on it is of the same quality as the light falling on an accessible object in a similar lighting situation. This usually works well enough but not always. If in doubt I will take a reflected reading on non-accessible objects as a check on the assumption.
The basic advantage of using the incident system in photographing outdoors is that it is less prone to misinterpretation than reflected readings. The result is that it is virtually impossible to make a serious mistake in exposure which ensures a higher percentage of negatives that have received optimum exposure, at least from a technical point of view.
The disadvantage of the incident system is that it does not lend itself quite as well to interpretative metering as the zone system, at least on the surface. But people who understand BTZS find ways to creatively apply the system.
Most people who use BTZS came to it after first using the zone system and tend to use both systems depending on circumstances. That is true in my case. I used the zone system for years before learning BTZS and don't hesitate to take a reflected reading if I think it will help make a better negative. But for the great majority of situations the incident system is kind of a no brainer because the results are so consistent and reliable.
Last edited by sanking; 09-18-2004 at 03:38 PM. Click to view previous post history.
Minor White proposed the use of an incident reading technique in the Zone System Manual (Chapter 8: "Short Cut Exposure-Development Calculations") that is very similar to Phil Davis' technique. Minor White's method used the palm of your hand instead of a white dome, but it's all the same really.
Reflective metering, and spot metering in particular, is prone to error. Examples: Your meter may not have the same spectral sensitivity as your film, especially in the infrared. This can affect shadow readings dramatically (the meter picks up IR in the shadows that your film does not see). Spot meters are not flare-free, and measuring small areas of shadow surrounded by lighter areas may not be accurate or even precise.
Incident metering introduces consistency. Whether you meter from your hand or through a white dome or flat, any error caused by the difference between the spectral response of your film and the spectral response of the meter is systematically eliminated or at least dramatically reduced.
The leap of faith is in the way that the incident technique relies upon real objects having a limited range of reflectance. This assumption should be remembered when you are metering, and allowances made.
I guess that the required craft skill is in the optimum placement of the incident meter to take the highlight and shadow readings. I think that Phil Davis gives excellent examples of this in BTZS.
As Sandy says, I suspect that most of us use our judgement between the two methods. I work as a DoP/cinematographer, and it is normal for us to carry three meters - incident, spot and colour - and for all to be used.
Thanks to everyone who has replied to my incident / reflective BTZS light metering question.
From Helen's reply, am I correct to infer - that with mental disclipine and artful exposure selection - incident and reflective light meter readings should produce the same negative densities? In this regard, it is assumed that film, and their developers, times, and concentrations are identical.
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