Why ETTR?

Started 9 months ago | Questions
Ysbrand Galama
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Why ETTR?
9 months ago

Can someone explain to me why I should expose to the right (ETTR) in low light conditions.

Let's take an example to explain what I do not understand. Let's say my camera measures F2.0, 1/50th and ISO 800 (no over- or under exposure). I want to ETTR so I set my camera to F2.0, 1/50th and ISO 1600.

Why would ISO 1600 give better results than ISO 800? I maybe exposing to the right, but also the sensor noise increases, negating the effect.

I must be missing something.

Y.

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s_grins
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

Ysbrand Galama wrote:

Can someone explain to me why I should expose to the right (ETTR) in low light conditions.

Let's take an example to explain what I do not understand. Let's say my camera measures F2.0, 1/50th and ISO 800 (no over- or under exposure). I want to ETTR so I set my camera to F2.0, 1/50th and ISO 1600.

Why would ISO 1600 give better results than ISO 800? I maybe exposing to the right, but also the sensor noise increases, negating the effect.

I must be missing something.

Y.

Why not to set camera to F 2.0  1/30  ISO 800?

BTW, I do not ETTR, or ETTL, except of just a few cases.

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AndyGM
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

ETTR principles don't involve changing the ISO setting at all. They involve shooting in RAW and changing the shutter speed and/or aperture to move the maximum exposed measure to the far right of the histogram, because that increases the signal to noise ratio, even if it means the image is essentially overexposed. You then downward adjust the exposure in post processing to reveal the headroom in the RAW file.

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Ysbrand Galama
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Re: Why ETTR?
In reply to AndyGM, 9 months ago

I still don't get it. I Can ETTR by exposing longer. But why not decrease the ISO? Yes, if I decrease the ISO, the exposure curve goes to the left. But so does the sensor noise. I don't see the benefit.

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smithling
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

To ETTR properly, you need to increase the amount of light that hits the camera sensor. This means making the path to the sensor bigger (aperture), or giving the sensor more time to collect light (slower shutter speed). More light hitting the sensor means a higher signal to noise ratio. Think of ISO as the sensor's electronic gain, which introduces noise as it increases. Sometimes an increase in ISO is needed however, given the photographers choice of shutter speed and/or aperture.

If you ETTR properly, the OOC jpeg will look overexposed. Just make sure to keep the exposure as high as you can, without blowing out important elements in your scene.

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Ysbrand Galama
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Re: Why ETTR?
In reply to smithling, 9 months ago

I am still missing something. The purpose of ETTR is to improve the signal to noise ratio, so increasing the exposure time makes sense, I understand that.

But if you want to improve the S/N-ratio, why not reduce the ISO instead of lengthening the exposure time? A lower ISO has a better S/N-ratio, so the end effect is the same.

Or, worded differently: when your lense is wide-open you have two ways to increase the S/N-ratio: increase shutter time, or reduce ISO. Why is increasing the shutter time better than reducing the ISO?

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Brigcam
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

ETTR only makes sense at base ISO. The older sensors could be pretty noisy even at base ISO. So ETTR was used to help clean up the shadows at the expense of possibly blowing out highlights.

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CobaltFire
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

ETTR is not a simple concept.

First the reason: the amount of information in each photo is tied to the brightness level. The higher the brightness, the more information (colors, detail) there is, until you exceed the DR of the sensor/camera and clip. You can test the relatively easily by manually shooting the same scene with offsets of two or three stops and looking at the file sizes (for a quick reference) and developing the photos (for a more thorough analysis). Note that the file may get SMALLER if you step up ISO, due to the loss of DR. This indicates that you've gone past the point of gaining anything.

Second is how to do it. This is not as simple as it sounds. I'm sure you are aware of the tradeoffs present in opening your lens up or slowing the shutter speed down. However, knowing if you can use ISO to slide your exposure is quite a bit more difficult. You must understand your camera and how much of an advantage you will gain by ETTR, then compare that to how much of a disadvantage stepping up the ISO will give you. As mentioned above raising ISO will cause a loss of DR. On many cameras the first stop of ISO (on my E-M5 400) is not actually that much of a loss. This is usually the "gimme" step that you know you can take. After that it's up to you to decide how much DR you can trade for more detail and color.

For example, I shoot an E-M5 and an A65. On the E-M5 I have it set to +1 Exposure Comp and stay with that down to ISO 800, with shutter speed based on subject (between 1/10 and 1/100) and f/stop based on subject. On the A65 I have the camera set to ISO 400 (no auto for P mode on that camera), f/2.8 to f/4, and let the shutter speed float. Both of these sets of numbers are from MY testing, MY expectations, and MY skillset.

Note that when I owned the GH2 I only biased 0.5 stops due to the camera offsetting RAWs more to the shadows (lots of information in the shadows, early clipping of highlights). This is another factor that must be understood and compensated for.

Once it's all said and done it's worth while for me, but it has ruined a few shots that were on the limits of my skills/equipment to capture. I don't think those pictures would have been worthwhile if they were that close, but it's possible they were.

Hope that all helps!

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GeorgianBay1939
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

Ysbrand Galama wrote:

Can someone explain to me why I should expose to the right (ETTR) in low light conditions.

Let's take an example to explain what I do not understand. Let's say my camera measures F2.0, 1/50th and ISO 800 (no over- or under exposure). I want to ETTR so I set my camera to F2.0, 1/50th and ISO 1600.

Why would ISO 1600 give better results than ISO 800? I maybe exposing to the right, but also the sensor noise increases, negating the effect.

I must be missing something.

Y.

It seems that you are using ISO as an EXPOSURE variable, which it isn't.   (You are not alone!)

To better understand the mechanics of ETTR read Gollywop's excellent two page article on Exposure vs Brightening.

With a mirrorless camera proper ETTR (using blinkies/live view histogram as your primary exposure aid and your meter as a secondary) helps a lot when shooting wide luminance range subjects in RAW.

ETTR is NOT meant for JPEG use.

Gollywop will help you a lot!

Tom

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DonSC
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

Because the higher S/N ratio more than makes up for the higher noise at the higher ISO.

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smithling
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

Ysbrand Galama wrote:

I am still missing something. The purpose of ETTR is to improve the signal to noise ratio, so increasing the exposure time makes sense, I understand that.

But if you want to improve the S/N-ratio, why not reduce the ISO instead of lengthening the exposure time? A lower ISO has a better S/N-ratio, so the end effect is the same.

That is true for cameras at certain higher ISOs. For example, on my Lumix G3, if I need to conserve shutter speed, I can underexpose using ISO 1600 (keeping a faster shutter speed) and then brighten the photo later on in post, and there will be the same noise as if I matched the camera's suggestion to use ISO 3200. Anyway, the S/N remains the same in that case for that camera. The camera is "iso-less" from 1600 on.

However, if I had ETTR by slowing the shutter instead, I would have much less noise. You can reduce the ISO, but you haven't increased the signal. Your just telling the camera to underexpose.

Or, worded differently: when your lense is wide-open you have two ways to increase the S/N-ratio: increase shutter time, or reduce ISO. Why is increasing the shutter time better than reducing the ISO?

If you decrease the ISO so much that you have underexposed, you will have to brighten the photo with your raw processor (lightroom et al.) anyway to get get the required brightness. By doing so, you are increasing the noise without increasing the signal (try it). Conversely, if you ETTR, you have to darken the photo later on, which does not introduce any noise.

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GodSpeaks
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Read this article
In reply to Ysbrand Galama, 9 months ago

From the Luminous Landscape site...  ETTR

and this...  Exposure

Hope that helps.

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PurpleFringe
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Re: Read this article
In reply to GodSpeaks, 9 months ago

GodSpeaks wrote:

From the Luminous Landscape site... ETTR

and this... Exposure

Hope that helps.

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And this article

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smithling
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Re: Why ETTR?
In reply to GeorgianBay1939, 9 months ago

GeorgianBay1939 wrote:

Ysbrand Galama wrote:

Can someone explain to me why I should expose to the right (ETTR) in low light conditions.

Let's take an example to explain what I do not understand. Let's say my camera measures F2.0, 1/50th and ISO 800 (no over- or under exposure). I want to ETTR so I set my camera to F2.0, 1/50th and ISO 1600.

Why would ISO 1600 give better results than ISO 800? I maybe exposing to the right, but also the sensor noise increases, negating the effect.

I must be missing something.

Y.

It seems that you are using ISO as an EXPOSURE variable, which it isn't. (You are not alone!)

This is correct.

To better understand the mechanics of ETTR read Gollywop's excellent two page article on Exposure vs Brightening.

With a mirrorless camera proper ETTR (using blinkies/live view histogram as your primary exposure aid and your meter as a secondary) helps a lot when shooting wide luminance range subjects in RAW.

Just a reminder that those blinkies/histogram apply to the resultant JPEG only. No camera on the market AFAIK feature a raw sensor histogram or overexposure display.

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Ysbrand Galama
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Re: Why ETTR?
In reply to smithling, 9 months ago

Well... I don't know. I read a lot of technical description, but the only answer that really makes sense to me is the shortest one, by DonSC. I have a feeling it is no more complicated than that.

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rare wolf
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

Well... I don't know. I read a lot of technical description, but the only answer that really makes sense to me is the shortest one, by DonSC. I have a feeling it is no more complicated than that.

Exactly ... with ETTR you get less noise in the shadows but the result is generally over-exposed, and possibly with blown highlights. Then use raw development to remedy the over exposure and highlights.

Presumably you are already working with lower ISOs for maximizing your dynamic range.

HTH
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Paul De Bra
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It's a matter of making use of ALL the signal the sensor captures.
In reply to Ysbrand Galama, 9 months ago

When you lower shutter speed or open up the aperture a bit (but staying at the same ISO to not increase sensor noise!) it's important to not clip highlights but to use as much of the signal the sensor can capture. So the idea is to use all 12 bits or 4096 intensity levels the sensor can capture.

The result of ETTR is lower noise, not just because of using all the values but also because of the non-linear behavior of the sensor.

Clearly the downside of ETTR is that it is more likely that highlights will be clipped. If you take a few shots at slightly higher and lower exposure you can find the best exposed image where highlights are just not clipped. Near the top of what the sensor can capture the behavior is not linear but a good RAW converter will compensate for that.

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Ontario Gone
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Re: Why ETTR?
In reply to Ysbrand Galama, 9 months ago

Heya Ysbrand, some of what has been posted is right, most of it wrong, i will try to summarize and clarify and hopefully this will help.

First we need to clarify how we are ETTR. Technically ISO is not part of exposure, but for this conversation, we will pretend it is. One can either adjust ISO higher to result in overexposure, or adjust SS/aperture. Lets cover both scenarios.

ADJUSTING SS F2.8  SS 1/1000  ISO-800: So in this first scenario, we can change the SS to 1/500 and get a +1, perhaps clipping some highlights, then pull it back down in post. This is the idea. This doesn't help though, because i could simply shoot at 1/500 at ISO-400 and achieve the same result with no need for PP. Same noise, same SS/aperture. If the scene is capable of a 1/500 SS to begin with, it makes no sense to overexpose when you don't have to, may as well just lower ISO to begin with. In fact, some cameras that apply unwanted NR like Pentax are better off shooting at 1600 ISO or less even if it's ETTL, to avoid this NR. The camera will never do a better job than good PP software.

ADJUSTING ISO F2.8  SS 1/1000  ISO-800: This is the scenario that the idea is really based on. The reason is, some sensors have a lower read noise % the higher the ISO setting is set to. As gain increases, read noise is actually less. This thinking however is flawed because of two reasons. First, this only applies to certain sensors, mostly Canon sensors, which are the minority nowadays. Second, read noise comprises such a small % of the visible noise in a photo it is barely worth paying attention to. Low read noise is much more an issue for long exposures for star watching, which is one reason canon is so good for this kind of photography. For most photography, 1/2 sec to 1/8000 SS, it is not noticeable.

So, to explain, lets change that ISO-800 to ISO-1600 to overexpose the image +1. We then pull the shot back down to 0 in post, and supposedly have less noise. Remember though, if you were shooting a sensor like in Canon cameras, the difference would be there but would be so small you wouldn't notice. In newer non Canon sensors, such as the ones Sony makes for just about everybody else, they don't behave this way. The read noise stays almost exactly the same regardless of the ISO setting. This means your ISO setting in camera does exactly the same thing that pulling or pushing in post does: it changes the amplification of the signal. Amplifying a signal darkens or brightens both signal and noise, so there is no advantage to any ISO setting over another in terms of read noise. The noise will be determined by the amount of light hitting the sensor, which has nothing to do with ISO.

Some believe (myself included), it is actually better to ETTL to preserve highlights and pull up later. If you go too far banding can show up so you gotta be careful, but i usually keep my shots underexposed by 1/2 stop or so just in case (always shooting raw). I hope this helps, there is a legitimate theory behind ETTR but the difference is negligible and it is only for a small % of today's sensors. If anybody out there can prove me wrong i would be happy to see a comparison shot.

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JeanPierre Martel
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Re: Why ETTR?
In reply to CobaltFire, 9 months ago

CobaltFire wrote:

On the E-M5 I have it set to +1 Exposure Comp and stay with that down to ISO 800, with shutter speed based on subject (between 1/10 and 1/100) and f/stop based on subject.

On my e-m5, I usually set the aperture (according to the DOF that I want) and let the camera decide the rest. If I overexpose, I never know what the camera will do: boost ISO or higher exposure time (at the risk of a blurred image).

If I understand correctly, in order to ETTR, I have to set the max ISO at a lower point (from its present value, 6400, to 800 for example) and set a +1 Exposure correction. Is that right?

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texinwien
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You're still missing something
In reply to Ysbrand Galama, 9 months ago

Ysbrand Galama wrote:

I am still missing something. The purpose of ETTR is to improve the signal to noise ratio, so increasing the exposure time makes sense, I understand that.

But if you want to improve the S/N-ratio, why not reduce the ISO instead of lengthening the exposure time? A lower ISO has a better S/N-ratio

No, it absolutely does not. ISO is not a part of the exposure equation. Shutter speed, aperture and scene brightness are the only three variables in the exposure equation. If you hold those three variables constant and lower ISO, you will not have an improved SNR.

so the end effect is the same.

Or, worded differently: when your lense is wide-open you have two ways to increase the S/N-ratio: increase shutter time, or reduce ISO.

Wrong. The two ways to increase SNR in your scenario are to increase shutter time or increase scene brightness (e.g. with a flash). Reducing ISO won't increase SNR.

Why is increasing the shutter time better than reducing the ISO?

Because increasing shutter time raises the amount of exposure while reducing ISO does not.

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