The Mystery of the Missing ISO Stop: ISO 100 to ISO 200 on Canon EOS Cameras

Asphotography enthusiasts and photographers, you might have noticed that your Canon EOS camera doesn’t have an ISO stop between ISO 100 and ISO 200. This absence of an ‘ISO 101’ or ‘ISO 112’ often leaves many wondering why. To understand the reasoning behind this, we must delve into the basics of ISO and ASA/ISO.

The Geometric System of ASA/ISO

The International Organization for Standardization (ISO) and American Society for Testing and Materials (ASTM) standardized a system for measuring film speed called ASA/ISO. This system is based on a geometric progression, where each successive value is doubled from the previous one. The formula for this progression is expressed as ASA/ISO 12 × 2^n, where n is the stop number.

For example, starting from the lowest range:

ASA/ISO 12 ASA/ISO 24 ASA/ISO 48 ASA/ISO 96 ASA/ISO 192 And so on...

This doubling pattern forms the basis for the ISO numbers, making the system mathematically consistent and straightforward for photographers to understand and use.

Film Makers and the 1/3 Stops

While the geometric progression of ASA/ISO is clear and logical for the film makers, the photographers often use a more granular system in their work. Film makers used 1/3 stops to rate their film. This means that film speeds are often expressed in numbers that do not follow the perfect doubling pattern.

Examples of such film ratings include:

ASA/ISO 6 ASA/ISO 10 ASA/ISO 25 ASA/ISO 50 ASA/ISO 64 ASA/ISO 125 ASA/ISO 160 ASA/ISO 200 ASA/ISO 320 ASA/ISO 400

This system allows photographers to make fine adjustments and achieve precise exposure without having to rely solely on doubling each time.

ISO Stops on Modern Cameras

Today, we see a more seamless progression of ISO stops on modern digital cameras. Unlike the historical progression, many modern cameras adopt a similar doubling pattern. For instance, many digital cameras feature ISO settings like:

ISO 100 ISO 200 ISO 400 ISO 800 ISO 1600 ISO 3200

However, photographers often find that between ISO 100 and ISO 200, there is no intermediary stop. This absence seems puzzling when considering the precision many photographers seek in their work. Why do manufacturers not provide an intermediate stop?

Manufacturing Decisions and Practicality

The answer lies in the practical decisions made by camera manufacturers. In many cases, it is more efficient to cater to the needs of the vast majority of photographers, who primarily adjust exposure using aperture and shutter speed. Most photographers do not require a finer degree of control over ISO settings.

Manufacturers recognize that:

Aperture and shutter speed adjustments provide a highly granular and precise method of controlling exposure. ISO is typically used to achieve the desired exposure in less ideal lighting conditions or for different film speeds. Providing intermediate stops would add unnecessary complexity to the camera without a significant benefit.

Furthermore, maintaining a clean, logical system for setting ISO speeds simplifies the user experience. This allows photographers to focus on more critical aspects of photography rather than getting bogged down in fine-tuning ISO settings.

Conclusion

In conclusion, the absence of an ISO stop between ISO 100 and ISO 200 on many cameras is a result of the practical decisions made by manufacturers. While photographers may sometimes find this omission inconvenient, it aligns with the overall goal of simplifying the camera interface for ease of use. The geometric progression of ASA/ISO provides a solid foundation for film makers and photographers alike, and the doubling pattern in modern cameras is a testament to the enduring relevance of this system.