Why can Portra 400 scans look darker than Portra 160 if ISO 400 film is faster?
Asked 8/14/2018
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I’m learning film exposure and I’ve noticed my Portra 400 photos often look darker than my Portra 160 photos. With digital, raising ISO usually makes the image preview look brighter, so I expected ISO 400 film to look brighter too. Why doesn’t that seem to happen with color negative film? Is Portra 160 actually brighter, or is something else affecting the result?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
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ISO (derived from ASA), can be approximated as the reciprocal of the shutter speed which, at f/16, produces a satisfactory exposure in bright sunlight. E.G. old Kodachrome, ASA 10, required 1/10 second exposure at f/16 in bright sunlight. Portra 160, at f/16 and 1/160 s in bright sun, should produce as dense an image as Portra 400, at f/16 and 1/400 s in bright sun.
However, Portra is a color negative film, meaning a print was made from that negative. Unless you developed the negative and you made the print from that negative yourself, you're relying on the processing lab's judgement of how the finished product should look. It's not unlikely someone thought the Portra 160 was underexposed, or Portra 4oo overexposed, and compensated in printing. To actually demonstrate which is more sensitive, you'd have to make images of identical subjects under identical lighting and have the lab process them the same.
Originally by user35542. Source · Licensed CC BY-SA 4.0
user35542
7y ago
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A higher ISO film is not supposed to produce a brighter final picture by itself. It is simply more sensitive to light, so you can use less exposure to get a similar negative density.
In bright sun, for example, Portra 160 at about 1/160 sec and f/16 should expose similarly to Portra 400 at about 1/400 sec and f/16. So if both are exposed correctly, neither should inherently be “brighter.”
Why your results differ:
- Color negative film is usually interpreted by the lab or scanner. The brightness/contrast of the final print or scan can be adjusted, so two rolls can look different even if exposure was similar.
- Film stocks have different contrast and rendering. Speed and contrast are not the same thing; a faster film can still look darker or more contrasty depending on the film design and how it was scanned/printed.
- Digital ISO behaves differently in practice. On digital cameras, raising ISO often makes the preview/JPEG look brighter, but that’s amplification/processing, not more light.
To compare fairly, look at the negatives themselves or have both films scanned/printed with consistent settings.
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