05-07-2015 08:11 PM
Hi, I'm new to DSLR's and was wondering if the ISO rating on DSLR Cameras is exactly the same ISO value as for film in 35mm/120?
In other words, if you were using manual mode, could you use the exposure rating chart on an old analogue flash gun and it would match up to the settings on your camera, set for the corresponding ISO?
I'm guessing the answer is that they are the same, but thought it was worth checking - many thanks
06-06-2015 09:47 AM - edited 06-06-2015 09:48 AM
06-12-2015 12:27 PM
They are not is the simple answer.
Digital cameras, ISO sensitivity is a measure of the camera's ability to capture light. Almost the same as film ISO or ASA numbers. Digital cameras convert the light that falls on the image sensor into electrical signals for processing. Here is where ISO and film ISO have nothing in common. ISO sensitivity is raised by amplifying the signal. Doubling ISO sensitivity doubles the electrical signal, halving the amount of light that needs to fall on the camera sensor to for optimal exposure. When ISO sensitivity is raised from ISO 100 to ISO 200 and the aperture is left unchanged, the same exposure will be achieved with a shutter speed twice as fast.
What this really means is you are trying to compare film 'grain' to an electric signal. You can not! You can increase the signal to the amplifer circuits in the camera untill they are overloaded. IE, noise or grain, if you will. It is actually no different with your stero. If you increase the volume enough the sound will be loud but will be distorted. Right? Same, same. If I decrease the volume at some point you won't be able to hear it at all. It might not be distorted but you still can't hear it. Right? Same, same.
In digital the only thing that remained were the numbers because most film guys would be comfortable with them.
06-17-2015 10:41 AM
I suppose the question has two answers.
By the math... they are the same. ISO 100 is ISO 100 and it doesn't matter whether it's digital vs. film. If you use an incident meter to take a reading, then the exposure it gives you is correct and the meter doesn't care if you use digital vs. film.
BUT... whether your film or digital sensor actually gives you the ISO performance that you selected is a different question. The answer is that it doesn't always give you what you'd expect. Film has an issue called "reciprocity failure".
The exposure you gather depends on the intensity of light coming through the lens, and the amount of time you allow that light to continue to come through the lens. If you double the amount of time, you should collect double the light. The issue with film is that it's more sensitive to the initial amount of light but becomes less sensitive as you continue to keep the shutter open longer. Digital sensors typically don't have this same "reciprocity failure" issue and I wouldn't focus on just this single issue. It's really just an example.
You should be able to trust your ISO settings and if the ISO is inaccurate, it'll tend to occur at the extreme limits of the sensors performance. If you're not shooting at the edge of the sensor performance then you probably don't need to worry about it too much. (Even the reciprocity failure issue is something that tended to only be noticed when film photographers were pushing an exposure well beyond what would be typical.)
On a digital camera, use your cameras's histogram to determine if you are getting a good exposure. If you aren't familiar with the histogram, we can provide links to articles that explain how it works. It's an excellent way to visualize whether your shot was either under-exposed or over-exposed by looking at the distribution of tonality of light recorded by the sensor.