Why use iso 800

[pushparaj santhosh]

Hi guys...I'm just curious about something...I see a lot of professional cinematographers using 800 iso while shooting be it heavily lit day light exteriors or dimly lit night exteriors...it is the same regardless of the camera system they use be it red cameras or Alexa...is there a specific reason for that? Thanks

[Robin R Probyn]

The camera manufacturer will give the camera a native ISO.. usually that just means the ISO setting with the best noise to DR ratio.. as judged by their engineers.. I guess 800 ISO is seen to be used alot as its the native ISO for Arri Alexa /Amira.. F55 is 1250 and F5 is 2000.. but like a film stock ,you can decide to set the camera/your light metre .. to what ever you want.. people often over expose a stop or two.. and pull down in post to get a better Signal to noise ratio.. while sacrificing a stop of highlight in the DR..if they dont need it for the scene they are shooting.. I read recently Roger Deakins just leaves it at 800 when he is shooting Alexa.. and he seems to get decent results :)

[pushparaj santhosh]

Thanks a lot buddy...but there is a bit which I cannot understand...how to do you gain a better signal to noise ratio by overexposuring and pulling it in the post...and can you explain a little bit about DR? Thanks

[Jan Tore Soerensen]

Thanks a lot buddy...but there is a bit which I cannot understand...how to do you gain a better signal to noise ratio by overexposuring and pulling it in the post...and can you explain a little bit about DR? Thanks

You don't necessarily have to overexpose. There are ND filters.

[Mark Kenfield]

Thanks a lot buddy...but there is a bit which I cannot understand...how to do you gain a better signal to noise ratio by overexposuring and pulling it in the post...and can you explain a little bit about DR? Thanks

 

By doing exactly that... overexposing and then pulling down the exposure in post. The additional exposure lifts the shadows of your image further away from the noise floor of the sensor, therefore making the image less noisey when you pull the exposure back down to where it's supposed to be.

[David Mullen ASC]

With the Alexa, you have a bit more than 14-stop of dynamic range and at 800 ISO, the amount of shadow detail and overexposed detail are evenly split, 7-stops under and 7-stops over. At lower ISO's, the number of total stops of DR don't change, but by giving the sensor more exposure, you are gaining shadow detail but losing overexposure detail. So at 400 ISO, you have 14-stops of DR but 8-stops under and 6-stops over. Plus your overall signal is cleaner. Some people say that 400 ISO is the "true" rating of the Alexa sensor but 800 ISO is the manufacturer's recommended rating.

 

So with most cameras there is a trade-off between noise and overexposure headroom. Some people are worried about one more than the other.

 

With some other cameras, the total number of stops captured vary by ISO rating so you have to pick a rating where you are OK with the noise and get a good DR.

 

With the Sony F55, the recommended rating is 1250 ISO but a lot of people set the camera to 1250 ISO but expose at 1000 ISO, unlike with the Alexa where they set the camera and their meters (if they use them) both to 800 ISO. To me, this suggests that whatever log to Rec.709 conversion that the Sony F55 is doing for the on-set monitors is not quite pumping up the whites enough so people feel they can get away with overexposing a bit more. Also, they probably want a little less noise than 1250 ISO is giving them.

 

In terms of noise, it is very simple: sensors get less noisy when they get more light. But the more you overexpose, the less overexposure headroom you have, and most digital cameras are already fighting for better overexposure headroom if they want to match the look of film negative, so it's a trade-off. Today as more and more people come from a digital background and don't remember film, I think they are worried more about noise than headroom, whereas many film shooters care more about headroom than noise.

[Michael LaVoie]

Thanks a lot buddy...but there is a bit which I cannot understand...how to do you gain a better signal to noise ratio by overexposuring and pulling it in the post...and can you explain a little bit about DR? Thanks

If you want a good explanation of this, I'd take a look at Alistair Chapmans blog XDCamuser. Or you can take an online workshop at Abel Cine

 

http://training.abelcine.com/shop/online-workshops/digital-cinema-online/dc401-online-camera-401-raw-log-and-uncompressed/

 

After that you'll probably have a better handle on the logic of rating a stock or camera above or beyond it's recommended ISO.

[Alexandros Angelopoulos Apostolos]

With the Alexa, you have a bit more than 14-stop of dynamic range and at 800 ISO, the amount of shadow detail and overexposed detail are evenly split, 7-stops under and 7-stops over. At lower ISO's, the number of total stops of DR don't change, but by giving the sensor more exposure, you are gaining shadow detail but losing overexposure detail. So at 400 ISO, you have 14-stops of DR but 8-stops under and 6-stops over. Plus your overall signal is cleaner. Some people say that 400 ISO is the "true" rating of the Alexa sensor but 800 ISO is the manufacturer's recommended rating.

 

So, David, what happens at ISO 1600, which is one full stop above ISO 800? How much stops of dynamic range do you have for shadows and how much for highlights?

 

In terms of noise, it is very simple: sensors get less noisy when they get more light. But the more you overexpose, the less overexposure headroom you have, and most digital cameras are already fighting for better overexposure headroom if they want to match the look of film negative, so it's a trade-off. Today as more and more people come from a digital background and don't remember film, I think they are worried more about noise than headroom, whereas many film shooters care more about headroom than noise.

 

But, wait: I thought noise depended on the ISO number. Higher ISO, more noise. But you say that at a certain ISO level with more light I will have less noise? I guess it makes sense.

 

What does “most digital cameras are already fighting for better overexposure headroom if they want to match the look of film negative” mean?

[Adam Frisch FSF]

I've stopped using fixed ISO. I now regularly shoot the ISO as low as I can, as it makes for a cleaner image, and I like the look a lot better there. But I will vary it if it's easier than switching an ND. 800 is noisy. No point for that on a bright day lit exterior.

[Tyler Purcell]

I agree with Adam. I always shoot at the proper ISO for the given situation and then add filtration if needed. At the same time however, I've found it important to under expose slightly, just enough to protect your highlights. The noise at 800 ISO is unnecessary if you're shooting bright daylight scenes.

[David Mullen ASC]

 

 

So, David, what happens at ISO 1600, which is one full stop above ISO 800? How much stops of dynamic range do you have for shadows and how much for highlights?

 

 

But, wait: I thought noise depended on the ISO number. Higher ISO, more noise. But you say that at a certain ISO level with more light I will have less noise? I guess it makes sense.

 

 

 

 

 

I said that switching from 800 to 400 ISO loses 1-stop in the highlights and gains 1-stop in the shadows, so what do you think happens when you go the opposite direction?

 

You have to forget about the ISO setting to some degree and imagine the amount of light reaching the sensor in terms of how it affects noise. More light = less noise. Setting the camera to 800 ISO and overexposing by 1-stop versus setting the camera to 400 ISO and exposing normally actually means the same amount of light is reaching the sensor.

 

 

What does “most digital cameras are already fighting for better overexposure headroom if they want to match the look of film negative” mean?

 

Seems self-explanatory... film negative has a lot of overexposure latitude compared to underexpose latitude, the opposite of digital sensors generally.

[David Mullen ASC]

I have mixed feelings about lowering the ISO in bright daylight situations -- the lower noise does feel like switching to a finer-grained slower film stock, but on the other hand, there tends to be a lot of bright highlights in day situations where 800 ISO has an easier time dealing with those than 400 ISO. Of course, the Alexa has such a nice overexposure range that even at 400 ISO, you aren't seeing a lot of clipping.

 

I shot most of "90 Minutes in Heaven" in 2K ProRes 4444 at 500 ISO because I wanted a slightly cleaner frame since I wasn't recording raw and was concerned about ProRes compression... but in the end, we add a film grain pass to the image and it ended up looking more like I had shot it at 800 to 1000 ISO anyway, makes me wonder if it was worth using the lower ISO setting. But in general, I liked the look of 500 ISO on the Alexa.

[Stuart Brereton]

I must admit that I don't generally notice any difference in noise with the Alexa at 400 or 800 iso under most circumstances. Certainly, when shooting day exterior, I'd rather have the extra stop of highlights as there generally aren't that many deep shadows anyway.

[rob spence]

"I have mixed feelings about lowering the ISO in bright daylight situations -- the lower noise does feel like switching to a finer-grained slower film stock, but on the other hand, there tends to be a lot of bright highlights in day situations where 800 ISO has an easier time dealing with those than 400 ISO. "

 

David, is this because sensors behave like film stocks...the lower the film asa , the higher the contrast.?

[Robin R Probyn]

You don't necessarily have to overexpose. There are ND filters.

 

 

Hi Jan.. In this case you are "over exposing" on purpose.. to get a "cleaner" image when corrected in post.. i.e. simply letting more light fall onto your sensor..

[Alexandros Angelopoulos Apostolos]

Thanks a lot buddy...

 

Up to this point I thought Robin was a woman. :ph34r:

[pushparaj santhosh]

Thanks a lot for your replies...but I don't understand one bit...why do we need to use ND filters to cut down the exposure when you can always lower the light levels by cutting down the aperture...is it for the simple reason for working with a bigger aperture or is there a specific reason for working with a bigger opening? Thanks...and one more thing...what is the use of shooting with higher resolutions when you're going to anyways release ur footage in a lower resolution...what are the advantages of shooting with a higher resolution say 6k...

[Miguel Angel]

Well,

 

ND filters to cut down the exposure is something that you can or cannot do in film or in digital both.

 

However, many cinematographers nowadays like working with the lenses wide open for aesthetic reasons so ND filters are a necessity when you happen to have T64 on a bright day and need to take that reading down to T1.9 or T1.3.

 

Even if you wanted to shoot with the T-stop almost closed at T64 / 800ASA you wouldn't be able to do so as most of the lenses have a T22 T-stop as the closest one so you still would need ND filters.

 

There is also the physics part of the story which says that by shooting with a lens with the T stop almost closed (T16, T22) you start seeing aberrations, sometimes even the iris blades in old lenses and everything looks really sharp among other things, which are usually not a good thing.

 

By the way, the next short-film I'm shooting I will be shooting it at either 1600ASA or 3200ASA on the Alexa and with anamorphic lenses all in daylight! :D

 

Just for a change, you know!

Have a good day!

 

Have a good day.

[pushparaj santhosh]

Thanks a lot Miguel :)...and what about the resolution part...can you help me with that

[Tyler Purcell]

Thanks a lot for your replies...but I don't understand one bit...why do we need to use ND filters to cut down the exposure when you can always lower the light levels by cutting down the aperture.

Well as Miguel said, you can't close the aperture that much. Most digital cinema cameras are so sensitive even at their lowest ISO, it's hard not to use filtration.

 

The other issue is depth of field. The more you close the lens, the flatter the image. Cinematographers in general, kinda like shallow depth of field. This is what separates our subjects from the backgrounds and foregrounds. This is why a lot of cinematographers will tend to shoot at a certain stop for the whole show and compensate for light differences with ISO and filtration.

 

As Miguel pointed out as well, most glass doesn't like to be all the way open or all the way closed. Glass in general is best used in the meat of the stop range. This in conjunction with keeping the aperture more on the open side for depth of field reasons, tends to push people towards T4 through T8 in a lot of cases.

 

The reason why any of this is an issue is because unlike film cameras where you change the stock to get different native ISO's, digital cameras have a built-in native ISO that you can't change. The more light you let into the lens OVER it's native ISO, the less dynamic range you'll have. That extra light saturation, over-drives the imager and as a consequence, the highlight detail is lost. This isn't such a problem on cameras with a native 800 ISO, working at lets say 400. But it's a huge problem with cameras at a native 1600 or more, like some of the lower-end Sony cameras. The imager really looses it's ability to deal with highlights when you lower the ISO to a normal working outdoor ISO without several stops of filtration, which is not only expensive, but kind of a waste.

 

In terms of your resolution question... I mean, the more resolution you have, the more you can manipulate in post. For instance, if you shoot something and you want to push in on the subject, you CAN do that with a 6k camera delivering in 4k, you've got lots of room. Plus, the rate technology is going, some people think we'll have 8k televisions and theaters soon, in that case we should be shooting everything today in 8k, to protect our assets for the future. Unfortunately, that's not what's happening at all. Most everything is shot in 4k and finished in 2k (1080p) for delivery. One funny side note, my friend works at one of the top content delivery/archival houses and he said 90% of his work today is still 1080p, including features. Which is really sad to think of, we went from shooting 35mm film for most features, which is arguably 4.5k when the original negative is scanned, to basically finishing everything in 2k or worse. The only reason they do it is because 2k is cheaper all the way though the process. Less storage space needed, visual effects take less time to render, you don't need super powerful computers to compile/composite your work either. This trend will eventually change, but for the time being, it's what most people do.

 

The interesting part is, so far digital distribution has not succeeded the best quality of film and I doubt it will in my lifetime.

[David Mullen ASC]

In direct mid-day sunlight on a clear day, the exposure is normally f/16 at 50 ASA -- so you can't stop down the lens enough if your camera or film stock is much faster than 100 ASA. ND's are essential filters for daytime work. It's just that some people aren't comfortable with using the extreme ND's necessary to shoot at 800 ASA at a wider aperture (in order to get less depth of field.) You start to get color cast problems and infrared issues when using filters that are that dense.

[Alexandros Angelopoulos Apostolos]

The more light you let into the lens OVER it's native ISO, the less dynamic range you'll have. That extra light saturation, over-drives the imager and as a consequence, the highlight detail is lost. This isn't such a problem on cameras with a native 800 ISO, working at lets say 400. But it's a huge problem with cameras at a native 1600 or more, like some of the lower-end Sony cameras. The imager really looses it's ability to deal with highlights when you lower the ISO to a normal working outdoor ISO without several stops of filtration, which is not only expensive, but kind of a waste.

 

But isn’t this the opposite of what David said above? :blink: If I understood him correctly, though I’m interpolating a bit further from what he precisely said, the number of stops of dynamic range doesn’t change at different ISOs. What changes is the distribution of those stops for highlights and for shadows.

Then I didn’t get the part where you say that you drive the sensor “over” its native ISO (that sentence can easily be interpreted to mean that a lens has an ISO) if you use a lower ISO. Doesn’t lower ISO mean less sensitivity to light?!

For instance, if you shoot something and you want to push in on the subject, you CAN do that with a 6k camera delivering in 4k, you've got lots of room.

 

What does “pushing in on the subject” mean? :)

[Alexandros Angelopoulos Apostolos]

Also, they probably want a little less noise than 1250 ISO is giving them.

 

So, ultimately, higher ISO = more noise and not

 

In terms of noise, it is very simple: sensors get less noisy when they get more light.

 

I guess reading about still photography, I always thought that you use higher ISOs in low-light situations and that the image is noisier when the ISO is higher not because of lower amount of light, but because of high ISO. Come to think of it, I even think that when they do a review of a camera, they shoot in not that lower a light and the image at high ISO levels is way noisier, though the noise gets lower with every new camera model.

 

It somehow really messed with a lot of what I thought I knew when you said that low ISO in daylight situations is a no-no, because the highlights will get blown out, when up to this point I thought the opposite would happen: the camera would be less sensitive to light and will not overblow the highlights. Perhaps it’s a still-camera thing and film cameras work differently. :ph34r: Though that shouldn’t be the case.

[Tyler Purcell]

But isn’t this the opposite of what David said above? :blink: If I understood him correctly, though I’m interpolating a bit further from what he precisely said, the number of stops of dynamic range doesn’t change at different ISOs. What changes is the distribution of those stops for highlights and for shadows.

Think of it a different way.

 

Have you ever had a dimmer on lights before? You know, a knob or switch that you can vary which increases or decreases the brightness of light?

 

An imager works in a very similar way. The dimming light switch is called a potentiometer and on an imager it's called a photosensor.

 

So imagine your dimming light switch is in the middle, you have range to go up to full bright and down to full darkness. The middle would be considered your base ISO. It has a lot of headroom at the top and lots of room at the bottom.

 

Now, imagine if you turned that dimming light switch almost to the top, what happens? Well, you don't have anymore room to go right? You have hit the peak of the switch and it's the same thing with a photosensor. If you over-saturate it with light, you are loosing your dynamic range. You've just lost that rage from the middle of the switch to the top and bottom. You've raised the black levels considerably AND over-driven the imager to the point near distortion. This is why on some cameras, you actually get color shift with lower ISO's and bright sunlight cinematography.

 

Is that a good enough analogy to explain how it works?

 

Then I didn’t get the part where you say that you drive the sensor “over” its native ISO (that sentence can easily be interpreted to mean that a lens has an ISO) if you use a lower ISO. Doesn’t lower ISO mean less sensitivity to light?!

With digital, there is only one ISO per camera. To get different ISO's, the electronics change the way they process the data coming from the imager.

 

Again, use the dimming light switch analogy from above. If you shoot 6000 ISO with an imager which is designed to work at 800, you have the dimming light switch almost all the way at the bottom, make sense? So if you wanted to see in a room with the lights all the way down, it would be really difficult wouldn't it? The way they compensate for that is by taking the little bit of data it's receiving and forcing it to create an image. Since it's an analog process in many ways, noise is introduced into the image, which is why high ISO has noise.

 

ISO has nothing to do with a lens, it's all the imager and processor. All the lens and filtration do, is decrease the amount of light going into the camera. You don't really NEED an iris in a lens if you use filters and electronic compensation in the camera to make up for the light coming in. In fact, the whole premise of an Iris didn't come until much later in the age of photography.

 

What does “pushing in on the subject” mean? :)

Zooming in digitally in post production.

[David Mullen ASC]

Tyler is not quite correct in terms of overexposure reducing dynamic range -- overexposure definitely reduces headroom, but in the case of the ARRI Alexa, the range is the same at different ISO's within reason (a few stops one way or the other, I'm not talking about massive over and underexposure). But not all cameras process the signal the same way as the Alexa and some do have different dynamic ranges at different ISO settings.

 

I've noticed, for example, with Canon still cameras (and perhaps their cinema cameras too) that at high ISO's, the image looks a bit "crushed", more contrasty, as if Canon is hiding noise and loss of shadow detail by increasing the blacks as you go higher in ISO.

 

Also keep in mind that not every real world subject has a wide range of information both in shadows and highlights, so even though in theory using a low ISO rating on the Alexa just trades highlight information for more shadow information, in some day exterior situations, the shadows are not that dark so as you expose more, you lose highlight detail but you've already captured all the shadow information out there, so at that point in increasing exposure, you are just losing dynamic range. This is where looking at a histogram would be informative because a real world scene has a range of luminance information that may be biased towards the brights or towards the shadows.

 

Some still camera websites show DR tests at different ISO ratings so you can see how the range is affected, if at all. And for some, the range is affected when recording JPEG but not when recording raw, because of how the camera is processing the signal to make the JPEG look better (i.e. hiding noise problems.)