Overexpose in harsh lighting?

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I thought I would take advantage of the great expertise in this forum to ask what might be a dumb question. I was shooting in an urban environment with another photographer on a very bright and harsh day. He said in that situation he overexposes his 400 film by one stop and underdevelops. I understand the underdeveloping part but I'm not sure of why you would overexpose in this situation (assuming the meter and your judgement are correct).

-- Andy Laycock (agl@intergate.bc.ca), September 28, 1998


It has been said "there are no dumb questions, only dumb answers", so here comes one.

He does this because he is a zone system fanatic. Or, at least, he knows some of the underlying principles.

His subject is contrasty. He knows that a normal exposure and development will yield a negative that is also contrasty, and will be difficult to print. So he plans on developing less. This will reduce the contrast on the negative: subject highlights that would have been a dense black will be, say, a dark grey. But it also affects, to a lesser extent, the other end of the characteristic curve: shadows that would have been light grey will be slightly lighter, or totally clear. By giving the extra exposure, he knows that he can raise the tone of the shadows back where they would have been with normal development.

The opposite also applies: increasing development ("pushing" the film) gives increased contrast as well as requiring less exposure (increasing the EI).

Of course, he has read Adams and BTZS, and has done extensive testing, and knows how to adjust EI (Exposure Index) and development to give a required CI (Contrast Index), while retaining Zone 0 at density 0.1 above film-base-plus-fog.

On the other hand, perhaps he just knows a couple of rules of thumb.

-- Alan Gibson (gibson.al@mail.dec.com), September 28, 1998.

Thanks for the reply Alan. It makes sense for the shadow detail but what will it do to the highlights? We were shooting around alot of light coloured concrete and buildings and I was worried that the detail in the highlights would be blown out by overexposure.

-- Andy Laycock (agl@intergate.bc.ca), September 28, 1998.

Andy, the basic "rule of thumb" Alan mentions, and the one you may wish to remember, is: Expose for the low values, Develop for the high values. This is the basic concept of the zone system.

Exposure/density in the low values/zones is determined by the chosen film EI. Choose a lower EI and you have increased exposure/density in the low values/zones; choose a higher EI and you have decreased exposure/density in the low values/zones. Density in the low values/zones is determined more by initial exposure than by subsequent film development.

Density in the high values/zones is determined more by development times/temperature/dilution than by the EI chosen. Develop for longer times/at higher temps/in stronger dilutions and you increase density in the higher values/zones at....and this is key!....a greater rate than in the lower values. Develop less, and the opposite is the case.


-- Sergio Ortega (s.ortega@worldnet.att.net), September 29, 1998.

Sergio puts it well. Although the "overexposed" highlights will be denser, this will be more than compensated for by the "underdevelopment".

-- Alan Gibson (gibson.al@mail.dec.com), September 29, 1998.

The zone system rule in a nutshell is (as mentioned) "expose for the shadows, develop for the hilights".

The _reason_ for this is (with negative film) you don't want to underexpose, and you don't want to overdevelop. So if you've added exposure to get shadow detail, if the hilights are very bright, you need to reduce development to not overdevelop those areas and block up all off the hilight areas.

These are the ideas behind the zone system. Now you can pile on all of the calibration and testing to very precisely set your film speeds and development times...

-- mike rosenlof (mike_rosenlof@yahoo.com), October 01, 1998.

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