Understanding ISO

[David Frøhlich]

Since the total dynamic range remains unchanged, this effectively shifts the number of stops that fall above and below middle gray, and compresses or expands tones accordingly. At low ISO speeds, most of the dynamic range is below middle gray, whereas at high ISO speeds most of this dynamic range lies above middle gray.

As written on red.com, shooting at low ISO speeds increase the dynamic range below middle gray. Hence, I keep the ISO speed low when shooting dark scenes since it allows me to record more values between black and middle gray. Am I right or is there any significant error in reasoning?

 

[Brendan H. Banks]

As written on red.com, shooting at low ISO speeds increase the dynamic range below middle gray. Hence, I keep the ISO speed low when shooting dark scenes since it allows me to record more values between black and middle gray. Am I right or is there any significant error in reasoning?

You actually have that dead on. How to expose or rate the sensor is entirely a part of the artistic process in my mind so how you use it is all at your discretion. But yes you have that concept. The opposite goes with higher ISO...some folks like to rate the sensor closer to 1600 (depending on camera) to protect their highlights a bit better in bright scenes.

 

[Nick Morrison]

ISO when shooting RED (and RAW on any platform) in general can be hard to understand.

I think what's important to know is what your camera's native ISO is. That's paramount. Todo that, just check "RAW" on your touchscreen, and you'll see what your camera is REALLY recording.

Epic Dragon is 320 ISO. Period. It can look good at 800 iso, but the native ISO is still 320. Even Davinci Resolve has the camera set that way.

You can adjust your ISO as you like, but you are then adjusting your midpoint. At 800 ISO, you are getting more highlights, but shadows are on the edge. Anything beyond 800, and you may get shadow noise.

Now Weapon and DSMC2 improve that shadow noise, but the native ISO of the Dragon sensor hasn't changed. It's still 320.

Helium is different, and by all accounts the native ISO is higher - 640 or perhaps even 800.

But your ISO choice is always a method of protecting or feeding your sensor. Outdoors with tons of light, 800 ISO on Dragon makes total sense. There are very little shadows to protect, but you do have to protect highlights, so the extra headroom is great to have in camera.

But for interiors, sometimes the opposite is better. Shooting at 320 almost guarantees silky blacks (assuming you light everything well), and as long as you protect your highlights, then you're good.

So ISO is a choice. But native ISO is not. That's fixed. And always will be. The camera is a scientific instrument, and will never change. The better you know it, the wiser you can make ISO choices.

Hope this helps.

 

[David Frøhlich]

... You can adjust your ISO as you like, but you are then adjusting your midpoint. At 800 ISO, you are getting more highlights, but shadows are on the edge. Anything beyond 800, and you may get shadow noise. ...

Indeed, when shooting in dark sceneries I normally rate the ISO speed as low as possible to record more values between black and middle gray. But why are there still people who use higher ISO speeds when shooting in dark sceneries, is it just misunderstanding of the whole concept?

Red Epic-W (3200 ISO) Insanely Dark Inside A Saw Mill

 

[Ignacio Aguilar]

But why are there still people who use higher ISO speeds when shooting in dark sceneries, is it just misunderstanding of the whole concept?

Maybe yes and maybe not.

For instance, I've shot anamorphic with old lenses several times on Dragon (using the Skin-Tone Highlight OLPF, which as Nick says, makes the camera a native 320 ISO aprox) and I had to chose between using deeper stops (such as T4 or T5.6 on interiors) to try to have the best performance from the lenses, or use a low ISO rating for cleaner images.

So as much as I like to rate the camera for interior work at 250-400 ISO, when shooting anamorphic I prefer to start with noisier footage (800-1000 ISO) so I can stop down a bit more. In that situation I had to decide between optics performance (or depth of field) and noise, and I preferred to expose to a higher ISO (still getting rich exposures) and have some grain, rather than soft shots and a very shallow depth with lots of anamorphic aberrations due to poor lens performance at T2.8. This could happen too if you're lighting package is not powerful enough to shoot at a healthy stop for the set of lenses that you're using, or get the required depth of field for certain jobs (such as macro shots).

As long as you expose properly even 1280 ISO can be usable, though you'll get the best colors and richer, noiseless blacks with lower ISO and proper exposures for them.

 

[Björn Benckert]

Indeed, when shooting in dark sceneries I normally rate the ISO speed as low as possible to record more values between black and middle gray. But why are there still people who use higher ISO speeds when shooting in dark sceneries, is it just misunderstanding of the whole concept?

Red Epic-W (3200 ISO) Insanely Dark Inside A Saw Mill

ISO is post digital gain, nothing else. High ISO = more noise. Low ISO = Less noise. Rate the camera from what its actually captures and strive to use a large part of the sensors DR in all 3 channels, then you get a clean image.

 

[Aaron Lochert]

Indeed, when shooting in dark sceneries I normally rate the ISO speed as low as possible to record more values between black and middle gray. But why are there still people who use higher ISO speeds when shooting in dark sceneries, is it just misunderstanding of the whole concept?

Red Epic-W (3200 ISO) Insanely Dark Inside A Saw Mill

There's a fundamental difference between "rating" a sensor for a certain ISO, and just "shooting" at an ISO.

Rating sort of implies that you have the ability add/remove light for a proper exposure at a given ISO. So for these low key scenarios, it implies that you're able to walk around set with a light meter set at 320 or something like that and set up all your lights for an exposure at that ISO.

The guys in this video had no choice but to shoot 3200 because they were already at the limits of their lenses and lighting capabilities. So there's really only two choices here, neither of which are ideal: shoot high ISO and accept the noise or shoot a lower ISO and have an image that is just far too dark. And remember, since it is raw, it's no different from shooting low ISO and boosting it in post. More ISO = more boost = more noise & smoother highlight roll off.

If you think of ISO in these cameras like the curves adjustment that it is, then it all clicks into place really easily. This is an overly simplified demonstration of what is going on, so take this with a grain of salt, but it helps me think about ISOs on RED and other raw cameras and maybe it can help you, too:

As you can see, when you're in a higher ISO the highlights flatten out as they approach true white, but you can also see that the shadow range is steeper as it approaches true black. This means that your highlights are smoother and your shadows are not. The opposite thing happens in low ISO: shadows roll off as they approach black, but the highlights ramp up dramatically and the appearance of clipping will be much more obvious. Couple this phenomenon with that because a high ISO is brightening the whole image, it's revealing noise where a low ISO is squashing it and hiding it.

You can see why your logic of using low ISO in low key lighting makes sense for smoother shadows.

 

[Michael J Brennan]

ISO originated for film.
Signal to noise ratio originated for electronic kit and is the core STANDARDISED measurement by manufacturers for video cameras.

Just saying..

Mike Brennan

 

[Brendan H. Banks]

There's a fundamental difference between "rating" a sensor for a certain ISO, and just "shooting" at an ISO.

Rating sort of implies that you have the ability add/remove light for a proper exposure at a given ISO. So for these low key scenarios, it implies that you're able to walk around set with a light meter set at 320 or something like that and set up all your lights for an exposure at that ISO.

The guys in this video had no choice but to shoot 3200 because they were already at the limits of their lenses and lighting capabilities. So there's really only two choices here, neither of which are ideal: shoot high ISO and accept the noise or shoot a lower ISO and have an image that is just far too dark. And remember, since it is raw, it's no different from shooting low ISO and boosting it in post. More ISO = more boost = more noise & smoother highlight roll off.

If you think of ISO in these cameras like the curves adjustment that it is, then it all clicks into place really easily. This is an overly simplified demonstration of what is going on, so take this with a grain of salt, but it helps me think about ISOs on RED and other raw cameras and maybe it can help you, too:

As you can see, when you're in a higher ISO the highlights flatten out as they approach true white, but you can also see that the shadow range is steeper as it approaches true black. This means that your highlights are smoother and your shadows are not. The opposite thing happens in low ISO: shadows roll off as they approach black, but the highlights ramp up dramatically and the appearance of clipping will be much more obvious. Couple this phenomenon with that because a high ISO is brightening the whole image, it's revealing noise where a low ISO is squashing it and hiding it.

You can see why your logic of using low ISO in low key lighting makes sense for smoother shadows.

THIS IS SO HELPFUL. I totally understand that this is an oversimplification but the visual curves help demonstrate the concept. Thanks!