Dealing with High ISO images and noise.

Cem_Usakligil said:

Hi folks,

I am really starting to enjoy my new 5D Mk II. I was in Rotterdam this evening and I took a few shots using ISO 6400 and ISO 12800. To my surprise, they are very usable. Take the following picture for instance. This was a reflection of a building in the canal.

Canon 5D Mk II, EF 70-200L f2.8 IS, f3.2, 1/25, ISO 12800, handheld. C&C is welcome.

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Very nice. This is one of my favorite types of photography. In NYC, we don't have much in the way of canals (the few that exist are either inaccessible due to private property, or are out in the open with nothing to reflect), so I have to settle for puddles after the rain (after kicking out the cigarette butts and comedy club brochures).

As for the high ISO, it seems that noise issues tend to be greatest when you have extensive areas that are flat and dark, but not quite near black. The converters tend to pull those up and give the zones full color saturation, emphasizing the noise. When you have an image that is either high key or high contrast, it's not really much of an issue, as the blacks are rendered close to black, and the SNR in the brighter areas is not particularly offensive.

However, if you're shooting RAW, nothing over ISO 3200 is actually using any extra real sensitivity; an under-exposed 3200 has everything 25600 has, plus 4 stops of extra highlights, with smaller files to boot. These are a big trade-in for accurate EC and FEC, and a bright review image and bright embedded JPEG.

Canon is just so slow and conservative with ideas; they could have implemented all ISOs above 3200 (or any user-set ISO level) with HTP automatically, with zero compromise, and many gains (smaller RAWs, more headroom, and accurate EC and FEC).

I get far more noise when I go over 6400. I'm wondering why.

Rachel Foster said:

I get far more noise when I go over 6400. I'm wondering why.

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Rachel

In part it may be because downsizing can be used to mask noise - if you blur then resample and then sharpen you should remove a lot of visible noise - and also because it's really the siganl to noise ratio that matters. In Cem's picture the light parts are well exposed and the remainder is basically black. The black is easy to force down by moving the black point above the noise threshhold (try moving the black point in ACR from 5 to 25 say on a high iso image. This is fine if you don't need the lower tones to have detail) and noise can be amazingly well controlled if ou get enough exposure to the light tones, so avoiding having to lighten the tones and the noise together, or needing to stretch the contrast which also make noise moreapparent.

Just a few thoghts.

Mike

Rachel Foster said:

I get far more noise when I go over 6400. I'm wondering why.

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That's because when you set the camera to a higher ISO, it calls for less real sensor exposure, so it collects less photons, but the high ISO circuitry itself is not adding more noise at higher ISOs. The camera actually *adds* the most noise, relative to a fixed, weak exposure, at ISO 100. When you look at it in the context of shooting with the same bias in all ISOs, however, the higher ISO will always be noisier, but only because there is less exposure on the sensor, not because the high ISO forces the camera to add more noise!

John Sheehy said:

That's because when you set the camera to a higher ISO, it calls for less real sensor exposure, so it collects less photons, but the high ISO circuitry itself is not adding more noise at higher ISOs. The camera actually *adds* the most noise, relative to a fixed, weak exposure, at ISO 100. When you look at it in the context of shooting with the same bias in all ISOs, however, the higher ISO will always be noisier, but only because there is less exposure on the sensor, not because the high ISO forces the camera to add more noise!

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John, Mike or Bart or anyone else with thoughts on this,

How about if one can say, I don't need blacks below 25 can one make any better judgements on choosing a high ISO versus underexposing at a lower ISO?

Or what if we now say our result will be delivered in B&W, what then?

Asher

Asher Kelman said:

John, Mike or Bart or anyone else with thoughts on this,

How about if one can say, I don't need blacks below 25 can one make any better judgements on choosing a high ISO versus underexposing at a lower ISO?

Or what if we now say our result will be delivered in B&W, what then?

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The only disadvantage in RAW with under-exposing the lower ISO is that the read noise is increased, relative to exposure. The shot noise is only dependent on the Av and Tv settings in the given light. The more you stick to high key images, the less important it is to avoid the extra read noise of using low ISOs, but for ISO 100 on the 5D2, to push to 1600 or more, you will start seeing read noise even in the midtones. Better to push high ISOs from 400 or 800.

For B&W, the noise never manifests as chroma, so it is far more tolerable.

Very helpful info, thanks!

In a less technical term ... I have found that there is more noise on an underexposed ISO 800 image than a properly exposed ISO 1600 image.

I found that proper exposure goes a long way in noise reduction.

Gary

PS- Cem I really like that reflection image ... try flipping it 180 degrees.
G

Gary Ayala said:

In a less technical term ... I have found that there is more noise on an underexposed ISO 800 image than a properly exposed ISO 1600 image.

I found that proper exposure goes a long way in noise reduction.

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That's the apparent correlation with some ranges of ISO on some cameras, but the real reason is not the relative "exposure", but the electronics. ISO 400 at -1 EC is the same real sensor exposure as ISO 1600 with +1 EC. The difference is that the clipping point is lower and more likely to clip your subject's highlights with ISO 1600, but ISO 1600's electronic gain also adds less noise, relative to the fixed sensor signal.

With some cameras, there is no difference at all in the RAW data shooting at ISO 400 and -1 EC, vs ISO 1600 and +1 EC. With yet others, there is a difference, but only in the amount of highlight headroom.

With JPEGs, the "normal" exposure is almost always better, because JPEG renders generally show no respect for deep shadows, and severely posterize the shadows and/or clip them away, so they are not as pushable as RAWs.

Not much to add really. The more light hits the sensor (i.e. the greater exposure through longer time or more open aperture in any given conditions) the more signal to mask the noise added by the camera at any given setting.

Following Asher's comment, if you are shooting in lvery ow contrast conditions and expose to the right you may find that you do not have much or any signal below 25 say (in ACR), or if you are shooting in low light at iso 3200 and get sufficient exposure to allow all shadows below 25 to go black then you will remove a lot of visible noise (as well as increasing global contrast). Of course setting the balck point to 25 may lead to too much contrast although you could probably do something with levels and curves to fix this if you work in 16 bit mode.

I'm afraid that I'm still so amazed at what is possible with the my cameras that I don't worry too much these days - we have an abundance of riches.

Mike