Bob, my guess is that the catalyst for this thread was a recent discussion in a different thread about the amount of noise an ISO setting contributes to a photo. As you know, the topic of noise in photos is a contentious one. Disagreements are sometimes the product of miscommunication between the participants.
To illustrate, one person might say, "High ISO images are noisier," and somebody might respond, "High ISO images are not noisier." The immediate impression one might take from this exchange, is that the two participants fundamentally disagree. However, there is a scenario in which the two people may largely be in agreement.
Suppose the underlying and unspoken context of the first comment is that it's an observation about the prominence of shot noise in a photo made with a low exposure. A high ISO was used to deliver a photo having a pleasing lightness but the underlying weak exposure results in an image displaying prominent shot noise.
Suppose the second comment is made from an underlying but unspoken context as a clarification that noise gets more prominent in low light photos but it's not the high ISO that's the primary contributor. It's shot noise that we see. Read noise decreases as ISO increases so, isn't the culprit.
In this scenario, it's possible the participants agree that shot noise is what we're seeing. But they may be misunderstanding each other. The second commenter may assume (incorrectly) that the first commenter is making an observation about the contribution of read noise to an image's appearance. The first commenter may misunderstand the response as denying that shot noise becomes more prominent as exposure (and the total light used to make a photo) decreases.
This "disagreement" arises from miscommunication; an absence of context or clarifying info in the written comments. The unfortunate outcome is an appearance of a fundamental disagreement about a topic on which the participants are broadly in agreement.
Now, I can't speak for the OP or any participants in this thread other than myself. My sense - and I may be mistaken - is that there's some miscommunication at play, here. That's why I posted a series of graphs illustrating that read noise does, in fact, tend to decrease as ISO increases.
https://www.dpreview.com/forums/post/66429775
So, when some folks talk about the common misperception that noise increases as ISO increases, those comments can be misunderstood as a denial that shot noise becomes more prominent as exposure decreases and less total light is used to make a photo. In fact, just the opposite is true. Calling out the high ISO noise misperception is an acknowledgement that photos look noisier as ISO increases. However, the reason for that noisier look is less total light being used (more prominent shot noise) and not, as some mistakenly believe, the result of increased read noise at higher ISOs.
The structure of this argument goes something like this:
- Shot noise, which is determined by the total light used to make a photo, is typically the most prominent kind of noise we see in a photo.
- Shot noise becomes more prominent as exposure (and total light used) decreases.
- While the ISO used tends to increase as exposure decreases, the resulting correlation between high ISOs and noise visibility, is not evidence of causation.
- It's not the read noise at high ISOs that makes photos look noisier. In fact, the amount of read noise is probably lower than the read noise in a well-exposed photo made at a low ISO.
- It's the reduced signal-to-noise ratio (SNR) and corresponding increase in the prominence of shot noise that we see.
- At very high ISOs, pattern noise and other artifacts can make read noise stand out and be more prominent. However, these are outlier situations and atypical of most photography.
So, yes, photos made at low ISOs typically do look best and have the highest quality. They're well-exposed with a high SNR and minimally visible shot noise. As exposure gets lower and less total light is used to make a photo, shot noise becomes more prominent and ultimately annoying. We use higher ISOs when making these photos but it's not read noise that compromises image quality. It's the more prominent shot noise that we see.
