Native ISO…Confused ?…”So you should be”

Categories: Miscellaneous 7 Comments

native iso

What ever happened to signal to noise ratios the bar at which all video cameras were judged, we talk about native sensor ISO (International Standards Organization) these days, total confusion. Recent 4K/HD cameras give you a choice of either decibels (dBs) or ISO for gain, the problem with ISO is its far from standard.

CREW CLOSER

Back in the days of film your film speed was measured as ASA (American Standards Association), as an example you had T-MAX 100 ASA and T-MAX 400 ASA film. The T-MAX 400 provides greater under-exposure latitude that results in more detail in the shadow areas. However, this is achieved at the expense of “smokey blacks”. Are we starting to see a similarity to filming in LOG which is where all this terminology originally stems from. We are so intent in making video footage look cinematic we have built a science around it.

During the 1960s news reports were usually filmed on 16mm film rated at 400 ASA allowing a faster shutter speed and footage in lower lighting conditions, it was slightly more grainy than 100ASA film.

Todays camera manufacturer sets the cameras native ISO (equivalent to film speed)  taking a few points into account…

1. Noise and grain

2. Optimum Dynamic Range (12-14 stops)

3. Detail

The sensor is at its best when all 3 are optimised and given its “Native ISO” but that may not correspond to 0dB.

In the world of ISO…if your camera has an ISO of 200 that should be the same on all cameras but in the real world that only works for the same model and make of camera.

ISO 500

Take the Panasonic DVX-200 its native ISO is rated at 500 which is the lowest ISO on the camera therefor it also equals 0dB. For every 6dBs you double the ISO, so 6dBs = 1000 ISO and 9dBs = 1500 ISO with 12dBs = 2000 ISO which is fine for the DVX-200 but if you have a Sony PXW-FS7…

FS7 ISO 2000 copy
The Sony PXW-FS7 has a native ISO of 2000 so you can see where the first problem lies there is 1500 ISO of a difference between the Panasonic and the Sony.

My gripe is that in the film days 400ASA was a set film speed that many makes of camera could use and exposing via a light meter would be standard throughout, today you can’t apply the same rule for ISO as the sensor sensitivity differs between each camera model.

Take the two cameras above, setting both cameras to 0dB at f4 will still give you vastly different exposures to each other. You would have to set the DVX-200 to 12dBs to match the Sonys native ISO of 2000, but the noise and grain would be a lot more visible on the Panasonic.

So is 0dB the same on all cameras ? 0dB should be the quietest point in a cameras sensor giving the best possible picture quality.

Why do we use ISO at all ? The camera’s ISO setting is its sensitivity to light. The higher the ISO, the more sensitive it is. This is measured according to international standards. Each ISO setting is double the one before, if you increase the ISO from 100 to 200, you double the camera’s sensitivity.

XDCAM User “Sony’s native ISO rating for the FS7 of 2000ISO has been chosen by Sony to give a good trade off between sensitivity, noise and over/under exposure latitude.” and “If you were to use SLog2 or SLog3 with the camera in custom mode and not use the native ISO by adding gain or changing the ISO away from 2000, you will not get the full 14 stop range that the camera is capable of delivering.”

JVC LS300

Now for more confusion the Sony FS7s native ISO is 2000 but you have ISO 800, 1000, 1250 and 1600 before you hit the native 2000 ISO, most FS7 users I know prefer ISO 800 in Cine EI mode. You have to make up your own minds on ISO, some so called scholars will tell you never to go below the native ISO, thats just nonsense.

Use your eyes it all depends on the lighting conditions and what overall effect you want to achieve, the BBC prefer not to have noise levels above 6dB but how can they tell these days with cameras like the Sony FS7 !

To me the whole ISO system stinks with deliberate confusion and is far from standard, I spoke to 2 engineers and three competent cameramen today who were all confused by this topic so what chance does everyone else have.

I have some simple tips as follows…

  1. Use your eyes, can you see grain in your picture, make sure you have a good monitor and a decent viewfinder.
  2. Use -dBs if available to avoid using the ND filter as even ND filters can introduce their own noise. (Its down to how perfect they are made and are they dust free). TIP. When changing lenses make sure you have the ND filter out (clear position) to avoid dust getting onto the ND filter.
  3. By using 0dB you may affect the full dynamic range of the camera i.e. 12 stops rather than 14 that in my opinion is a sacrifice I prefer to make to have noise free footage, remember the full Dynamic Range (DR) is only used in LOG mode once again something a lot of us do not bother with in day to day filming.
  4. Most professional video cameras give you the option for dBs or ISO, personally I am happier to use dBs to control gain.
  5. Remember dBs and ISO give you the same result its only useful to use ISO if you need to get all of your cameras dynamic range by sticking to its native ISO.

Zero

 

One of my readers David Heath has some interesting input…

“You have to go right back to basics – to sensor level – to really understand where the term “base ISO” comes from, and why it’s so important.

Start from total blackness, then gradually increase the light level – at some point the sensor will give a just discernible output. Carry on increasing light level and the output will correspondingly increase until it reaches a saturation point – no matter how much more light is input, the output won’t increase any more. It follows that the difference between these min/max points is the basic dynamic range of the camera, normally expressed logarithmically in stops.

Note this is AT SENSOR LEVEL, and is independent of such things as camera gain etc. Hence there is ONLY one grey scale range that can give this, so taking absolute brightness, f stop etc into account the camera can only have one ISO value where this is true. This defines the “base ISO”.

All “ISO value” really means is that for a given illumination, at a given shutter speed, it tells you what to set the f stop (more correctly, T-stop) to for correct exposure.

It’s not really true to say “0dB should be the quietest point in a cameras sensor giving the best possible picture quality”. It normally corresponds to MAX DR, which is not necessarily the quietest in terms of S/N. As example, on cameras before the complications of log etc it was typical to have a position for negative gain – which would give a better noise performance than 0dB. But at the expense of dynamic range, normally highlight handling, which is why the amount was typically restricted to only -3dB, or maybe -6dB on pro 2/3″.

C300

So yes – the quote from XDCAM user is perfectly valid. By rating it at ISO 2000, you can then expect to set aperture etc to get THE BEST RESULTS THE CAMERA IS CAPABLE OF. (At least overall – I agree with the way the XDCAM user quote refers to best “trade off”.) It may not be a surprise that rating it at a higher ISO won’t give such good results – we’re all familiar that such implies gain and likely more noise – but the same is true if you try rating it at a LOWER ISO. In the case of the FS7, ISO 2000 produces the OPTIMUM result.

And if Panasonic say the base ISO for the DVX200 is ISO 500, then the same is true there. It then becomes worthwhile querying why the FS7 is then 2 stops better than the DVX200. Most obvious is sheer sensor size, but as s35 is only about twice as large as 16:9 4/3 (not 4x), that only accounts for about 1 stop in itself. I can only presume the other stop is due to Panasonic using a higher than 4K pixel count sensor (hence smaller than optimum photosites), and Sony exploiting their back-illumination technologies.

It does mean that assuming the same T-stop, the FS7 has a 2 stop advantage over the DVX200, and the FS5 (base ISO 3200) is nearer 3 stops!

And sorry, but your point 3 above is incorrect. You can expect 0dB to be where you get the BEST dynamic range. Using a log setting won’t automatically give you a higher dynamic range (contrary to a lot of belief) – what it WILL do is give better post flexibility as to how the basic camera DR is mapped to output. If you don’t use log, then the high DR gets mapped to a lower one (suitable for the eyes more limited range) in a set fashion, according to gamma curve, knee, slope and such factors.”

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7 comments on this post

  1. Phill Dixon says:

    Very Confusing

    db is probably best,

    Life is too short!!

  2. mark says:

    well put. Finally some light on the topic! Noise and low light is about to become one of the major tricks the manufacturers use to sell more expensive models. The difference between a USD2K camera and a 5K camera will be limited, but the difference between a 2K camera and a 35K camera will be substantial. For the old segment (us, event shooters) used to spend 5-7K (US) per video source will not make such a huge difference because we can compensate with a slower shutter speed and the lens wide open (the footage will stink but at least we can sell it). The problem becomes serious when you have to perform at a minimum shutter speed (like in sports, artificial lights). By then you’ll find out that if you have to go live then you can’t use anything below 35K(US). But even if you can edit the noise will be significant.
    One thing is shooting in manual a reception in low light at 1/60th shutter and a totally different thing will be shooting at 1/120 minimum lens wide open.
    Anyway in terms of ISO (ASA) and aperture the vast majority of the venues for sports or stage set the lighting to 800ISO , lens 2.8 (35mm reference, basically full frame).
    Back to the manufacturers: the values are pre-set in the factory, based on the final cost of the product.
    The noise trick was first unveiled by the Nikon D90. The stunning quality made the camcorders (all of them) pale in comparison overnight, but the second thing was the intentional manipulation of the noise. So in one night the D90 was able to destroy decades of tricks and marketing lies.

    Period.

  3. David Doré says:

    Confusing indeed. When I started about 55 years ago film stock speeds (ASA) were colloquially known as Weston… after the legendary light meter. I remember that Ektachrome VNF 7450 was 125 ASA and VNF 7250 was 400 ASA. Both these reversal stocks were very low contrast for television purposes. A much better reversal stock was, I think, Ektachrome 7251 at 400 ASA. However, in the last days of film I preferred Fujicolor stock, both reversal and negative.

    I do wonder if we have become over-obsessed with technology in this digital world. Just use your eyes and tell the story.

  4. Ron Evans says:

    Mark : Of course it is all based on product pricing and positioning. Why wouldn’t it be. If manufacturers gave everything away for $500 they would go out of business and we would have no cameras to use !!! As a hobby I can afford to spend about $5000 every few years. At this price point I have had a Sony VX3 Hi8, Sony FX1 HDV and now Sony NX5U and FDR-AX1. Big improvements in functionality and performance at roughly the same cost. Will any of these compete with a $12,000 FS7 kit. Of course not and why should they. You get what you pay for and to me that is correct. Technology improves what one gets at this price point every year. One has to make a choice for the functions needed, budget and uses of the camera.

    Back to gain/ISO. This too is marketing. Don’t believe the numbers in the LCD one has to test. ” Look how great this is at 0 db ” unfortunately is not usable above 9 db because of grain !!!! As an example my NX5U gain goes from -6db so is 0db really +6db on the sensor !!! Then I may understand why 12db is starting to become unusable from grain if it is really 18db !!! On the other hand my FDR-AX100 has less grain and a sharper image than the NX5U at 27db !!! Newer technology does get into the product line at less cost too.

  5. mark says:

    Ron: yes of course. I just wanted to put it there in case somebody comes out with more BS about the size of the sensor or the shutter being global and stuff like that. It’s all in the processing, and set in the factory.
    The AX100 is less “dark” than the X70. But they all are made to be “dark”.
    it will be like you buy a car and then notice that they put a washer to make the shoes of the brakes push a little to slow down the car to don’t bother a more expensive model with the same engine of yours.
    Back to the video they lift everything where the money really is: broadcast , live, sports and music.
    no washers for that segment there, with cameras costing a gazillion and more , but ehy… no washers.

    and the difference between dslr’s and camcorders in terms of luminosity and fake luminosity (or lack of it) is still huge.

    One more thing : this “battle” of luminosity and noise reduction “in camera” has been a big deal in stills just few years ago. Canon and Sony are the major players in the sensor production and processing. So they know.. believe it .. of course they know. Nikon D4s sports a sony sensor LOL .

  6. Jim says:

    0dB is where the electronics of the camera add no additional gain. On the FS7 that is at a point whether the sensitivity equates to 2000 ISO. For the Panasonic DVX-200 that point equates to 500 ISO.

    0dB and how sensitive a camera is at that point is specific to the camera. ISO is compariable across cameras.

    (The system works pretty well, except for manufacturers competing for the highest ISO rating and DR rating without detailing the signal to noise ratio at that point).

  7. Assistance please: I have the LS GY300 with v3 firmware. I use J-log. I assume dB at 0 is best for DR? They mention an 800% increase. What does that mean? What is the DR of this camera in J-log?

    Thank you
    Vince

    HDW : Will get back to you

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