What's surprised me is not that people are confused - I expect that because the idea does go against the grain of the 'virtual film' paradigm that digital photography has developed. It's just the level of hostility amongst some, of which chuxter is the most extreme example, and has now made it his mission to expose my 'false religion'. It's not even as if anyone says that photographers must do it this way - we just say if you know this, you can do it this way, if you like.
he seems to have misunderstood what the demonstration was about. i looked at the example and immediately said, "that's not what it shows." it pretty clearly shows why it's better to boost ISO in-camera than it is to correct it after the fact.
That's right. And the reason for that is that high ISO settings add less visible read noise to the image.
now, much of his OP is about how "technologists" (you, et al?) need to explain better. so perhaps by his own admission, he doesn't get it. and that's okay. this is pretty complicated stuff. i can't say i even get it completely after reading your posts.
Actually, I don't think it is complicated, but I do think I have difficulty getting it across. One reason is probably that I don't communicate as well as I might. Another though is that the facts get blurred amongst the hostile and devious debates which inevitably seem to go with it. Knowing that the debate is bound to come I try to phrase what I say carefully so that it isn't vulnerable to simple attacks. So I wouldn't say 'high ISO reduces noise', what I'd say is that 'high ISO reduces visible read noise'. However, this language doesn't really help people that much, and gets dismissed as technobabble. I think when I reduce the subject to its basics, it's pretty clear.
On the confusion, I think it is because it goes against some deeply ingrained myths. One is that the high noise seen in high ISO images is electronic noise due to high amplification.
but isn't that some factor in the noise? the sensor has a weaker input, and it becomes harder to tell the stray photons and other interference from the actual signal input?
In electronic terms it isn't a noticeable factor at all, contrary to what people think. The front end electronic noises are a small contributor to the overall noise, so although they do get amplified, the removal of the much bigger back end noises, which get swamped by the amplification have a much bigger effect.
The relative electronic contribution to the noise almost invariably goes
down as ISO is raised, in some cameras by a large amount. The extra visible noise you see in a low light image is almost entirely photon shot noise. You see it because the dynamic range of the display medium exceeds the dynamic range that the camera can give with that little light going into the sensor.
The other is that there is some 'correct' exposure, and that it is imperative to image quality to find that 'correct' exposure.
"quality" is purely subjective. on this site, it seems to mean "maximum sharpness, lowest noise." but clearly lighting, ambience, mood, subject, artistic composition, etc, can all be counted as factors contributing to "quality." there might well be a reason to under expose an image by four stops, and clip all your shadows.
I can't see why you'd want to do that, unless you were looking for high noise in your image. Otherwise, if you're looking for a dark image with plugged shadows, you're etter darkening ind adjusting the shadow curve in processing. In noise terms, there is
never a reason to reduce the exposure.
The hostility comes from some self proclaimed experts who have been going around promulgating those myths - they just can't stomach being wrong.
personally, i can get over being wrong about things. but i would like to understand what's actually going on.
Me too. generally it's being wrong and learning that helps one understand. Being wrong is only embarassing if you been wrong with a whole load of bravado and derision directed against those who are right.
what you're saying isn't really all that different from the standard mantra. and it is, indeed, what i do. keep the exposure itself to the maximum, expose to the right, and keep the ISO as low as possible.
If you're exposing to the right, you're keeping the ISO as high as possible with that exposure. Where I think what I'm saying is radically different is that the consequence of what I say is don't use exposure to adjust image density. That cuts right across the way most people work withb their cameras, and the way that manufacturers set up the meters.
the only times i reduce the exposure are:
- when i'm already at base ISO, and it's just too bright. ie: in the sun.
Got to then
- when i need to stop motion (and i'm willing to make the trade-off with noise, but not flash)
That, as I say, is a case when your maximum exposure is constrained by your pictorial requirements
- when i find it really hard to get the DOF or the focus i want with a lens wide-open. for instance, with my MF fast primes, i can sometimes be tricky to get accurate focus where i want it with shallow DOF, and i have to stop down slightly.
And that too.
...and that's pretty much it, i think?
That's pretty much what I do too. Except I've come round to just setting the exposure I want, set the ISO to get it to the right and shoot away (in M), just checking the meter in case I need to change the ISO. I just wish I had auto ISO (and that auto ISO's were properly designed as 'exposure priority' mode) it would make life simpler.
--
thomas
