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Thread started 27 Aug 2009 (Thursday) 15:30
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Are you Shooting HAMSTTR? - ETTR - Expose to the right

 
Curtis ­ N
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Aug 31, 2009 20:56 |  #16

jayadeff wrote in post #8555960 (external link)
The danger of exposing to the right of the histogram is that if your white balance isn't perfect, you run the risk of blowing out one color channel and not being able to recover it, even if you shoot in RAW mode.

Bogus.

Your white balance setting doesn't change the RAW data. You have photocells covered with red, green or blue filters converting photons to voltage. The camera's processor records the voltage from each photocell.

The white balance setting isn't part of the equation until the RAW data is converted to an image.


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Curtis ­ N
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Aug 31, 2009 21:02 |  #17

Naturalworldphotograph​er wrote in post #8551444 (external link)
in low light situations where you want to achieve a sufficiant shutter speed, surely, if you can afford to overexpose slightly, or expose to the right, wouldn't you be better off bringing the iso down instead - to reduce noise?

ETTR is a concept that really only applies to those situations where you have shutter speed to spare at the camera's lowest ISO setting.

In the real world of low light photography, using the ISO setting necessary for proper exposure will get you the best image quality.


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Sep 01, 2009 10:40 |  #18

Naturalworldphotograph​er wrote in post #8551444 (external link)
...

But in low light situations where you want to achieve a sufficiant shutter speed, surely, if you can afford to overexpose slightly, or expose to the right, wouldn't you be better off bringing the iso down instead - to reduce noise?...

Well, no, the point is that ETTR / HAMSTTR will give less noise than, and better color info than a "mid range" HAMS. (histogram and meter setting)


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toke ­ lahti
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Sep 02, 2009 13:03 |  #19

Daniel Browning wrote in post #8553332 (external link)
No. If you keep f-number+shutter the same, and increase ISO, it will improve the SNR. Here is an example using the 5D2. These are very raw. Not even white balance or demosaic.
  • ISO 100 f/4 1/500
  • vs ISO 1600 f/4 1/500
QUOTED IMAGE
QUOTED IMAGE

Should I understand this so that you should use highest ISO and also adjust exposure compensation to ETTR (to get best SNR)?




  
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Daniel ­ Browning
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Sep 02, 2009 13:14 |  #20

toke lahti wrote in post #8570290 (external link)
Should I understand this so that you should use highest ISO and also adjust exposure compensation to ETTR (to get best SNR)?

Yes. Increase light first, then ISO second (up to 1600).


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Curtis ­ N
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Sep 02, 2009 13:55 |  #21

toke lahti wrote in post #8570290 (external link)
Should I understand this so that you should use highest ISO and also adjust exposure compensation to ETTR (to get best SNR)?

We should clarify the concept here.

ETTR (or HAMSTTR if you prefer) is all about maximizing the signal/noise ratio. Increasing exposure in a way that actually delivers more photons to the sensor increases the signal, and therefore the ratio. (We're talking about slowing down the shutter, opening the aperture, using more flash power, etc.)

Increasing exposure by boosting the ISO does not change the signal/noise ratio. Boosting the ISO amplifies both the signal and the noise.

Read the Luminous Landscape article. Link in CDS' post #1 in this thread.

In post #15, Daniel Browning posted two screen shots, the first of which was underexposed by 4 stops and then brightened in post with RAW conversion software. What it seems to demonstrate is that the camera does a better job of amplifying low signals than RAW conversion software does. While I tend to agree with that premise, it has nothing to do with ETTR or HAMSTTR.


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backslash
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Sep 02, 2009 14:19 |  #22

Curtis N wrote in post #8570626 (external link)
In post #15, Daniel Browning posted two screen shots, the first of which was underexposed by 4 stops and then brightened in post with RAW conversion software. What it seems to demonstrate is that the camera does a better job of amplifying low signals than RAW conversion software does. While I tend to agree with that premise, it has nothing to do with ETTR or HAMSTTR.

In post #15 we see 2 raw RAW files, and you can see that the ISO 1600 has less noise in the dark area. No conversion involved yet. And the conversion will produce a cleaner ISO 1600 jpg.

The message is this:
1. ETTR with aperture and time and only if you cannot go further,
2. ITTR (ISO to the right) increase ISO

And you will see, that an image with histogram not at the right and low ISO has more noise than a higher ISO with the same exposure (aperture and time) and a histogram at the right.


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Curtis ­ N
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Sep 02, 2009 14:26 |  #23

backslash wrote in post #8570798 (external link)
In post #15 we see 2 raw RAW files, and you can see that the ISO 1600 has less noise in the dark area. No conversion involved yet. And the conversion will produce a cleaner ISO 1600 jpg.

Look closer. The first image was shot at ISO 100 and cranked up 4 stops with his software. That's why it looks like crap.

The comparison shown is digital amplification of RAW data (via software) vs. in-camera analog amplification of voltage (via higher ISO setting). It appears to show that the camera does a better job. I tend to agree.

It's still unrelated to ETTR principles.


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Daniel ­ Browning
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Sep 02, 2009 14:31 |  #24

Curtis N wrote in post #8570626 (external link)
Increasing exposure by boosting the ISO does not change the signal/noise ratio.

I kindly disagree. It does not change signal, but it does change noise (specifically, read noise).

Curtis N wrote in post #8570626 (external link)
Boosting the ISO amplifies both the signal and the noise.

It amplifies the noise *less* than it amplifies the signal. (Actually, what's happening is that the late-chain read noise sources such as the ADU are contributing less to the read noise because they are fed with higher amplification).

Curtis N wrote in post #8570626 (external link)
In post #15, Daniel Browning posted two screen shots, the first of which was underexposed by 4 stops and then brightened in post with RAW conversion software. What it seems to demonstrate is that the camera does a better job of amplifying low signals than RAW conversion software does.

I only brightened it for a visual demonstration, it's not necessary to prove the point. The ISO 1600 has better SNR when you measure the raw file directly (without any brightening), as demonstrated below.

Furthermore, it works the other way too:

  • Convert the ISO 100 shot to TIFF with no brightness modifications.
  • Darken the ISO 1600 shot by four stops, so that it matches the darkness of the ISO 100 shot, then export TIFF.
  • Post process both TIFFs with the *exact same* settings.
  • The ISO 1600 has much less noise (higher SNR).
If you don't believe me, I would be happy to demonstrate it for you.

The average RGB level of blue pixels in the ISO 100 shot is 58.6. Std Dev is 6.67. SNR = 58.6/6.67 = 8.8:1. Another way to state SNR is in stops. log_2(8.8 ) = 3 stops.

The average RGB level of blue pixels in the ISO 1600 shot is 55.6. Std Dev is 2.87. 55.6/2.87 = 19.37:1 SNR. log_2(19.37) = 4.3 stops. So the SNR on the ISO 1600 shot is 1.3 stops better (4.3 - 3 = 1) than the ISO 100 shot.

That's using the RGB values, though. For a second illustration, I'll use other test shot that I posted.

Here is the first image. It's the tree shot at ISO 100 with no brightening applied:
IMAGE: http://thebrownings.name/images/2009-07-29-iso-compare/rawnalyze_iso_100_tree_0ev.png
IMAGE LINK: http://thebrownings.na​me …lyze_iso_100_tr​ee_0ev.png  (external link) .

And here is the second image, it's the tree shot at ISO 1600, but darkened with -4 EV to match the darkness of the ISO 100 shot:

IMAGE: http://thebrownings.name/images/2009-07-29-iso-compare/rawnalyze_iso_1600_tree_-4ev.png
IMAGE LINK: http://thebrownings.na​me …ze_iso_1600_tre​e_-4ev.png  (external link)

If we look at the RGB values again, we see that both shots have average values between 2-6 out of 255. That's obviously way too dark to see anything on a monitor. So instead, let's look at the actual raw values, before they are mapped for display on a monitor. First, note that the "adj" checkbox is clicked. This maps the raw levels to 0-255 in a linear fashion (no gamma curve). It doesn't change anything about the actual results.

ISO 100 average 8-bit raw level in red pixels: 12. red StdDev = 7.36.
ISO 1600 average 8-bit raw level in red pixels: 180, red StdDev = 49.8.

Now we convert calculate the SNR from the ratio between signal (average level) to noise (std dev):

ISO 100 red: 12/7.36 = 1.63:1 SNR. log_2(1.63) = 0.7 stops SNR.
ISO 1600 red: 180/49.8 = 3.61 SNR. log_2(3.61) = 1.85 stops SNR.

So the ISO 1600 shot has raw values with 1.15 stops higher SNR (1.85-0.7). But when the pixels are clipped to black on the monitor, the difference is hidden. The reason they are clipped to black is because it's using just a standard sRGB gamma curve. If instead one were to use a gamma curve and tone curve that preserved all the shadows (in both images), as well as highlights, then one would see the difference without having to turn up the brightness. (Rawnalyze doesn't have such a curve, though.)

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Curtis ­ N
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Sep 02, 2009 14:57 |  #25

Daniel,

You're comparing RAW files coming out of a camera. And you're showing that the cameras does a great job of minimizing noise at high ISO. We don't disagree on that.

But the ETTR technique as explained by Michael Reichmann of Luminous Landscape involves increasing the S/N ratio by delivering more photons to the sensor and making the most of the upper end of the camera's dynamic range. It's an important concept to understand, with inherent advantages and inherent risks.

And respectully, I think your contributions to this thread, however valid and insightful, involve an entirely different concept and won't help folks understand ETTR as Reichmann presented it.


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Sep 02, 2009 15:01 |  #26

The message is this:
1. ETTR with aperture and time and only if you cannot go further (because of motion blur or DOF),
2. ITTR (ISO to the right) increase ISO
== HAMSTRR


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Daniel ­ Browning
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Sep 02, 2009 15:13 |  #27

Curtis N wrote in post #8571091 (external link)
And respectully, I think your contributions to this thread, however valid and insightful, involve an entirely different concept and won't help folks understand ETTR as Reichmann presented it.

Thanks, Curtis. First, I agree that understanding ETTR is valuable, but I don't agree that Michael's presentation of it is a good one. He mistakenly emphasizes the increased utilization of raw values and implies that it somehow improves the image. It has nothing to do with that: the increase in photons *alone* is responsible for the entire benefit of ETTR. Of course, everyone makes mistakes, but we shouldn't continue holding it up as accurate six years later after it has been repeatedly disproven.

Second, this thread is not just about ETTR, but HAMSTTR too. The whole point of HAMSTTR is to add ISO to ETTR, therefore it's on-topic to discuss the benefits of higher ISO (when there is headroom to spare). I agree that ETTR is more important, which is why I emphasize that it must come first.

Kind regards,


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Sep 02, 2009 15:38 |  #28

Curtis, you might want to get some background here by checking the thread referenced in the original post.

Of course, the basic exposure model is to maximize the amount of light hitting the sensor (without blowing highlights) -- if you are shooting Raw and can expose to the right while keeping your aperture and shutter speed at acceptable levels, considering your shooting needs, with a low ISO, that gives you the best S/N ratio.

The eye-opener of that thread to me comes to play when you can't adjust your shutter speed and aperture to ETTR. We have all heard that a high ISO increases noise, but the actual finding is that raising the ISO by full stops to achieve ETTR (or, rather, HAMSTTR) actually decreases overall noise compared to the same shutter speed and aperture (light captured) captured at a lower ISO. The in-camera ISO amplifier actually produces less noise at ISO 1600 than at ISO 100. At least that is true for the cameras that have been tested.

So, the point is that if you have a shot that shows a medium level of exposure at ISO 100 and you are at the optimum settings for shutter speed and aperture, and you are shooting Raw, you will actually get a cleaner image if you boost the ISO by a full stop or two (depending on your highlights) and then lowering what you need to in post processing.

I saw this pretty dramatically when I shot an ISO 1600 shot, getting a well-exposed shot (when amplified by the ISO) then taking the same shot using the same shutter speed and aperture but at ISO 100, and then trying to boost the shot (by four stops) in PP. It was ugly!


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Sep 02, 2009 15:58 |  #29

Curtis, when you get a chance, read the POTN thread from last month, also linked in my initial post.
The fact seems to be that in the 6 years since MR's LL thread, there is more to it than just using exposure/light to get your ETTR..

Using ISO to push the meter and histogram right is also beneficial.

This IS part of "HAMSTTR" .. but it wasn't part of ETTR, (officially) thus the difference.
With HAMSTTR we are saying every tool you have at your disposal to get the Meter And Histogram To The Right,. including ISO.

It seems to work... really.

Curtis N wrote in post #8570626 (external link)
We should clarify the concept here.

ETTR (or HAMSTTR if you prefer) is all about maximizing the signal/noise ratio. Increasing exposure in a way that actually delivers more photons to the sensor increases the signal, and therefore the ratio. (We're talking about slowing down the shutter, opening the aperture, using more flash power, etc.)

Increasing exposure by boosting the ISO does not change the signal/noise ratio. Boosting the ISO amplifies both the signal and the noise.

Read the Luminous Landscape article. Link in CDS' post #1 in this thread.

In post #15, Daniel Browning posted two screen shots, the first of which was underexposed by 4 stops and then brightened in post with RAW conversion software. What it seems to demonstrate is that the camera does a better job of amplifying low signals than RAW conversion software does. While I tend to agree with that premise, it has nothing to do with ETTR or HAMSTTR.


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Sep 02, 2009 15:58 |  #30

RDKirk wrote in post #8551559 (external link)
I figure you're joking here.

cyberdyne dont joke, ever :) Im a hamsttr my self!!


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Are you Shooting HAMSTTR? - ETTR - Expose to the right
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