Re: ISO for beginners

Often, the "noise present from the get-go" is actually the "shot noise". This is the noise inherent in the quantum nature of light. Even if you had a perfect camera, that did not add any noise to the image, there would still be visible noise at very low exposures. That's just physics.

Getting back to my raindrop analogy, imagine a tray of shot glasses that you expose to the rain for a very short period. Perhaps the glasses average 5 raindrops each. Some may have 4, some 6. Perhaps a few will have 3 and a few 7. Even though the average is 5, there's a significant variation between 3 and 7. Those +/- 2 drops represents a +/- 40% variation in raindrops captured.

Now leave the glasses out for a 100 times as long. Now there's going to be an average of 500 raindrops in each glass. Some will have 499, some 501, a few at 498, and a few at 502. With such a high exposure to the rain, those +/- 2 drops represents only a +/- 0.4% difference in raindrops captured. Hardly a significant deviation.

Of course, this is a simplification, but it should get the idea across that at low exposures, the noise is in the light itself, and not merely a product of an imperfect camera.

In absolute terms, there will be more variation in the glasses left out for longer. Just like photon noise increases with exposure.

But, just like photon noise, as you point out, the signal to noise ratio will get higher for longer time out in the rain.

We shouldn't subject a beginner to this detail, but the higher the QE, the higher the photon noise SNR, all else equal.

Let's say we had a sensor with a QE of 1. Then 40000 photons would produce 40000 electrons, and we'd have 200 electrons of noise, for a SNR of 200.

Let's say we had a sensor with a QE of 1/8. Then 40000 photons would produce 5000 electrons, and we'd have 71 electrons of noise, for a SNR of 71.

The noise isn't in the light itself, but in the counting.

Thank you. That's a good suggestion. While less formal than "maps", I think "relates" is easier to understand.

Yep. I pay particular attention to when the ISO in the viewfinder bottoms out at 100 (ISO 140 is when I start thinking about adjusting the EV settings), and since I set EC to zero that generally gives me some latitude to avoid overexposing.

Why is most of this discussion in a "Beginner's Forum." ?
Do any of the posters here actually teach raw beginners?
I do, and many of them would walk away from their cameras and back to their smart phones after trying to read this thread.

I read a lot of this and… I don’t understand how the information in this thread will help me improve my photography. I know you guys are working hard at this, but it doesn’t seem to relate to photography very much.

In theory, the knowledge in this thread should help you set up your camera to produce the best possible results. I know it helped me once I learned about the role of ISO in the camera.

Yes, this thread long ago jumped the track and headed off in a different direction. It was intended to be a simple introduction to the concept of digital camera ISO. It has morphed into a discussion of various implementations, and exceptions/limits to the general rules.

Hopefully, a beginner can read the first few posts and learn enough to understand the concept behind the ISO setting.

As a general rule, as you get better at producing good results, there are diminishing returns on further improvements. At some point you may decide that the effort needed for additional incremental improvements, may not justify the resources.

For instance, if you have a 20 megapixel camera, you may be able to spend a few thousand to upgrade to a 50 megapixel camera. The 50 megapixel camera is capable of producing a higher resolution image. However, many are in the position where 20 megapixels is more than good enough for their needs, and the incremental improvement from a 50 megapixel camera doesn't justify the increased cost. Similarly, if you have no visible noise in base ISO images with exposure based on the camera's meter, then the incremental benefit of ETTR may not justify the increased workload. Someone who is getting the results they want when shooting JPEG, may not want to spend the extra time and resources to shoot raw.

Obviously, the cost/benefit analysis will differ from photographer to photographer. There are a wide variety of budgets, and a wide variety of needs.

But we all need to start somewhere. For many beginners, that start is a smart phone, where they learn composition and timing. Those using an interchangeable lens camera may want to learn about aperture in order to influence depth of field, shutter speed to influence motion blur, and exposure/ISO in order to influence how dark/light the image looks, and visible noise.

There's a lot of incorrect information about ISO out there, which if followed can lead to bad outcomes. This thread can be a helpful corrective. If you're not infected with bad information, as a first approximation set your camera to auto ISO and don't worry about it.