ISO

[work in progress]

Topic: what are the best ISOs in Canon DSLR?

Most people believe that 160, 320, 640 and so on (the so-called native ISOs) are the best choice. Not everybody agrees though. So, let's break it down.

Reading through the 'net, I've noticed two theories:

1. Multiples of ISO 160 are native. Advocates: [1]...
2. Multiples of 160 are digitally pulled by 1/3-stop (from 200, 400 etc), so they have 1/3-stop less dynamic range. Advocates: [2]...

My theory: the answer is somewhere in the middle.

Background: Magic Lantern can alter the amount of digital ISO gain (either push or pull). In the image processing pipeline, this happens before the data above some preset level (let's call this white level) is clipped.

Push/pull? What's that?

Well.. it's just darkening or brightening the image digitally. If you push by 1 EV, you brighten the image so that you get the same thing as if you increase the exposure by 1 EV in the camera.

To do this, you need to:

1. either have access to raw data (and multiply raw data by 2 before developing it - for 1 EV);
2. or, in 8-bit space, know the response curve (and shift the image by 1 EV along that curve).

Some facts

• ISO 160 has more highlight detail than ISO 200 (all other parameters being equal). This invalidates theory #2.
• ISO 160, ISO 200 and ISO 250 are identical in RAW. Proof: try it yourself with dcraw. This means that all of them are obtained by different digital processing of the same RAW data. In all 3 cases, the analog circuitry is configured at the same parameters. Some (or all) of these ISOs are either pushed or pulled digitally.
• At all ISOs, white is 255. This might suggest that ISO 200 may be digitally pushed from 160. Not true, see below.
• ISO 160x have a harsher highlight rolloff than ISO 100x. Proof: [response curve plots]. This is probably to mask the digital pull effect.
• Canon's ISO 160 has less highlight details that ML's ISO 200 digitally pulled by 1/3 EV (more exactly, 3/8 EV). Midtones and highlights are identical. Proof: [comparison images].

What I believe

• Full-stop ISOs are done by throwing away a small amount of useful highlight data. Proof: [images]
• The true ISOs is somewhere in-between 160x and 200x, close to dxomark measurements.
• Because of this, ISO 200 is digitally pushed, and ISO 160 is digitally pulled.

OK, so, from all this mess, let's try to answer some questions.

Are ISO 160 multiples the best to use?

NO. They have harsh highlight rolloff and intentionally clipped details in highlights. I have no idea why.

Are ISO 100 multiples the best to use?

NO. While they have smooth highlight rolloff, they are digitally pushed by roughly 0.2 stops. This means, these 0.2 stops, which contain data with the best SNR possible, are simply thrown away.

Then, what is the best ISO?

To the best of my knowledge, the best ISOs are the ones available in recent Magic Lantern versions (April 2012 or later), obtained from ISO 100 multiples adjusted with a small amount of negative digital gain:

• ISO 85, 175, 350, 700, 1400, 2800 - best for Neutral -4 and other low-contrast styles.
• ISO 80, 160, 320, 640, 1250, 2500 - good for a wide range of situations.
• ISO 70, 140, 280, 560, 1100, 2200 - best for high-contrast styles.

To enable them, set DIGIC ISO gain (in ISO submenu) at -0.2/-0.3/-0.5 EV and dial your ISO from ML menu or shortcut keys.

What should I use for high-contrast scenes? Should I try highlight tone priority (HTP)?

YES. Strongly recommended for high-contrast scenes. Combine it with a slight negative DIGIC ISO gain.

Why? Because of its place in the image processing pipeline. HTP is applied before 8-bit conversion and picture styles. So, when Flaat12 can no longer get any more shadow detail, HTP does the trick.

Keep in mind that ISO 200 with HTP is actually ISO 100 with shadows and midtones lifted. In movie mode, it has 1 extra stop of dynamic range (if you expose to the right, you get 1 extra stop of shadow detail).

What about noise? Negative gain in ML will reduce it.

[proof: screenshot].

If HTP and negative gain are not enough, try HDR video.

[thinking to place Dod3032's video with the houses]

What about low-contrast scenes?

Low-contrast scene: a scene with a lot of unused "space" in the histogram. [histogram screenshot]

1. Choose a high-contrast picture style.
2. Expose to the right. This will maximize SNR => minimal noise.
3. Center the histogram (or adjust the brightness as close to the final look as possible) with a strong negative digital ISO gain.

Is ISO 6400 digitally pushed?

YES. It's digitally pushed by 1 stop from ISO 3200. Guillermo Luijk arrived to the same conclusion.

But this does not mean you should not use it. The reason is the 8-bit codec. Same for higher ISOs.

Avoid it in photo mode.

Pink highlights: I've set DIGIC ISO gain at -1EV or lower, and now I'm getting strange colors. What's that?

You are getting something similar to the output of dcraw -H1. This is data above the white level, which is normally clipped by Canon JPEG engine. It may contain roughly 1 extra stop of correct luma data, but with incorrect colors. If you can guess the colors, or if your highlights are white, you have the recipe of roughly 1 extra stop of dynamic range (or less noise) in movie mode.

ML digital ISO pulling happens before Canon clips the raw data to white.

This happens due to the internal workings of white balance. WB is performed by multiplying the raw R, G and B channels with some constants. The values for Canon 350D, which are very close to the ones used in current Canon DSLRs, are [3]:

• Default (D65 lamp): multipliers 2.395443 1.000000 1.253807
• Tungsten: multipliers 1.392498 1.000000 2.375114
• Daylight: multipliers 2.132483 1.000000 1.480864
• Fluorescent: multipliers 1.783446 1.000000 1.997113
• Shade: multipliers 2.531894 1.000000 1.223749
• Flash: multipliers 2.429833 1.000000 1.284593
• Cloudy: multipliers 2.336605 1.000000 1.334642

So, for white highlights, worst case of extra DR is 1 EV under Fluorescent lighting, and best case is 1.34 EV under Shade. Caveat: you need to know how to postprocess.

Practical advice

How to set ISO in MOVIE mode

• Golden rule 1. Do not underexpose.
• Rule 2. Do not overexpose.
• Rule 3. Use a little negative digital gain.

How to set ISO in PHOTO mode

• Golden rule 1. Shoot RAW.
• Rule 2. Use full-stop ISO, at most 1600.
• Rule 3. Do not overexpose.
• Rule 4. Do not underexpose.