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Author Topic: Underexposure or Overexposure?  (Read 19755 times)
jeffoldbean
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« Reply #20 on: January 06, 2008, 04:57:21 PM »
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Check out Michael's other tutorial on "Expose (to the) Right): http://www.luminous-landscape.com/tutorial...ose-right.shtml .
[a href=\"index.php?act=findpost&pid=93748\"][{POST_SNAPBACK}][/a]

Eric oldbean,
     in MR's expose to the right essay.
Could you please explain, what "The First f stop, which contains the brightest tones" is.
IE, with what is its reference point.
  Surely we do not have to shoot wide open all the time.

jeffoldbean.
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Jonathan Wienke
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« Reply #21 on: January 06, 2008, 05:02:22 PM »
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Could you please explain, what "The First f stop, which contains the brightest tones" is.
IE, with what is its reference point.

What Michael is referring to is the first stop's worth of tonal values immediately below the clipping point, and has nothing to do with aperture. Go back and read Michael's article again, and this article as well.
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eronald
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« Reply #22 on: January 06, 2008, 06:38:40 PM »
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I'd ask "Expose with or without insurance" ?

If you're a street shooter you should underexpose by one stop, or else risk a lost shot if your meter got fooled.

Edmund
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jjj
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« Reply #23 on: January 06, 2008, 07:31:30 PM »
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If you're a street shooter you should underexpose by one stop, or else risk a lost shot if your meter got fooled.[a href=\"index.php?act=findpost&pid=165525\"][{POST_SNAPBACK}][/a]
Not quite sure what you mean by that Edmund.  
If I am street shooting I simply set the expusure manually and then there are no worries about the meter being fooled.
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John Sheehy
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« Reply #24 on: January 06, 2008, 08:04:43 PM »
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Not quite sure what you mean by that Edmund.   
If I am street shooting I simply set the expusure manually and then there are no worries about the meter being fooled.
[a href=\"index.php?act=findpost&pid=165531\"][{POST_SNAPBACK}][/a]

So, you set it for something in the sun; then, all of a sudden, you have a small window of opportunity to shoot something in the shade. Your manual setting may result in gross under-exposure, far more than 1 stop.  The gamble Edmund speaks of may under-expose a bit at times, but never as much as a fixed manual can.

It's all a matter of what you have time to pay attention to in the type of shooting you're doing.  Opportunities don't always sit around and wait for you to meter and turn dials.
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Steven Draper
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« Reply #25 on: January 07, 2008, 06:18:53 AM »
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Hi

This is the principle as to  how I set exposure and BOTH comments are correct.

IMHO Digital capture is about data collection and I see exposure as really a question about how to capture as much data as possible to best fulfil my final image requirements. I ask a number of exposure based questions when thinking about getting the data to establish what the critical issues are, what the compromises may be, do they provide creative alternatives etc.

In many of my subjects I must have data available from the white clouds in the sky for my final image as I hate blown skies. In crude terms for this kind of shot I set my camera up such that the clouds are not quite clipping, and then adjust exposure by another +1/3 or 2/3 (if feeling brave)

Often the subject is technically under exposed by upto a stop but I have all the highlights and can generate a pretty decent image. To process I have to recover the highlights by moving the ev correction and then add a fairly scary curve to the shadows, or produce multi exposure derivative files and either blend the layers or use something like photomatix to create a starting point TIFF.
 
If there are NO white clouds then in theory I can set the camera up the same, find the clipping point and and a possibly another 1/3 to 2/3. (Diff cameras and raw capture combinations will have diff values - WB should be set as accurately as possible, although by adjusting exposure in processing I find it will often require revisiting) This time the subject is normally overexposed by up to a stop and I drag the whole image back. In this case shadow detail is better. However we are normally working with less overall light so it may be that my desired combination of ISO, shutter speed, aperture etc are reached before a right exposure is achieved and I may consider any further moving the histogram to the right will not improve the data because of the need to make compromises to ISO, or aperture setting.

I haven't done tests here, but some people have determined that in limiting light conditions overall data is better by accepting an underexposed image and then pushing in processing rather than increasing ISO at image capture.

I carried out an experiment last year and the histogram clip point plus 2/3 over exposed at capture and corrected had slightly more detail in the shadows. Wish I'd kept the files now! It wasn't so much that it would be worth it for everyone, but it was enough to convince me to keep doing it.

I would imagine that there is as to limit to how far one should take a fairly narrow tonal range, but I have had some very good results from data that initially produced a very nearly bright screen!!!

Regards
Steven Draper
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Ray
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« Reply #26 on: January 07, 2008, 07:22:38 AM »
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There are a couple of effective approaches here. The simplest is to bracket exposure. However, there can be a disadvantage if timing is critical and capture of the precise moment is desired. It might be safer to change the default order of the exposures so the 'underexposed' shot is taken first.

Another approach which I found quite precise, but a little time-consuming and not ideal for the quick shot, is to focus the camera's spot meter on the brightest part of the scene, white clouds in a landscape or a white napkin in a restaurant, camera in manual mode. Take a note of the shutter speed through the viewfinder, after adjusting the exposure with dial so the needle in the viewfinder is in the centre, whilst still looking through the viewfinder.

Then increase exposure by 4 stops. You should get a full ETTR. The figure of 4 (or is it 3. I haven't tried this for ages) might vary amongst different models of cameras. You'll have to experiment.
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John Sheehy
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« Reply #27 on: January 07, 2008, 07:58:58 AM »
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I haven't done tests here, but some people have determined that in limiting light conditions overall data is better by accepting an underexposed image and then pushing in processing rather than increasing ISO at image capture.

There are basically three classes of camera in this regard; cameras that perform markedly better at higher ISOs than with under-exposure (all Canon DSLRs except the original 1D, plus the Nikon D3 and possibly a few other cameras), cameras that perform slightly better at higher ISO (only because there is less noise from the ADC; this includes most DSLRs and some P&S cameras), and cameras that botch high-ISO up and are worse at high ISO than with under--exposure (usually P&S cameras with tight budgets).  This is with RAW, of course, and is mainly relevant in regards to shadow areas.  Highlight areas are affected more by absolute exposure than ISO settings per se.

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I carried out an experiment last year and the histogram clip point plus 2/3 over exposed at capture and corrected had slightly more detail in the shadows. Wish I'd kept the files now! It wasn't so much that it would be worth it for everyone, but it was enough to convince me to keep doing it.

I would imagine that there is as to limit to how far one should take a fairly narrow tonal range, but I have had some very good results from data that initially produced a very nearly bright screen!!!
[a href=\"index.php?act=findpost&pid=165594\"][{POST_SNAPBACK}][/a]

You can take any exposure as high as you want, and bring it back down.  That is like shooting at a lower ISO, only the RAW data is actually better.  IOW, if you have a scene that is black writing on a gray wall, and shoot at ISO 100 with +3 EC in RAW, and pull the gray back to grey in the converter, you are actually shooting at ISO 12, with better quality than if the camera actually had an ISO 12.
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01af
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« Reply #28 on: January 07, 2008, 08:01:19 AM »
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Michael Reichmann, in his "Understanding DSLR Workflow" article on this site, states: "Digital has a remarkable ability to extract detail from the shadows. When shooting digital, always err (if you have to) on the side of underexposure."

whereas Bruce Fraser---another highly respected digital processing expert---states (in Real World Camera Raw): "correct exposure in the digital realm means keeping the highlights as close to blowing out, without actually doing so, as possible ... it's better to err on the side of slight overexposure".

So who is right?[a href=\"index.php?act=findpost&pid=93725\"][{POST_SNAPBACK}][/a]
Both are right.

The ostensible contradiction only comes from different angles of view ... and from exactly what is the term 'overexposure' supposed to mean. Bruce was looking at the topic from a technical point of view, and 'overexposure' in his context means overexposure of the JPEG image that comes from the capture parallel to the raw file (or from the raw file via standard settings in the raw converter). When the JPEG is (slightly) overexposed then the raw file often still will have some headroom to work with. Michael is looking at the topic from a practical, hands-on point of view. In real life, underexposed images can be saved through proper processing much easier and more often than overexposed ones.

The critical point is, 'as close to blowing out without actually doing so.' If you go just a quarter of an f-stop above the blow-out limit then the image is damaged beyond salvation. If you stay two or even three full f-stops below the limit then the image usually will still be fine.

In any case, Expose To The Right (or expose for the highlights, as we used to say in analog days) is the correct approach. The problem is, how far to the right is far enough? Often, photographers are pushing the exposure too far to the right without noticing, simply because the tiny histogram display on the camera's screen is not accurate enough. The closer you try to approach the the theoretical ideal the more likely you will eventually blow your exposure---and thus, your work.

-- Olaf
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John Sheehy
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« Reply #29 on: January 07, 2008, 08:10:46 AM »
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Eric oldbean,
     in MR's expose to the right essay.
Could you please explain, what "The First f stop, which contains the brightest tones" is.
IE, with what is its reference point.
  Surely we do not have to shoot wide open all the time.
[a href=\"index.php?act=findpost&pid=165514\"][{POST_SNAPBACK}][/a]

The higher in the range of RAW levels you record your subject, the higher the signal-to-noise ratio is at every tonal level in the image.  Obviously, you are the artist and/or technician, and should decide what aperture and shutter speed you want, but subordinate to that, within the range of freedom you can allow, you can also improve S:N by increasing the exposure (avoiding clipping desired highlights, of course).  With some cameras, you get a significant decrease in shadow noise by going to a higher ISO while maintaining the same absolute exposure, and exposing more to the right.
« Last Edit: January 07, 2008, 08:11:33 AM by John Sheehy » Logged
digitaldog
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« Reply #30 on: January 07, 2008, 08:58:22 AM »
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Does Bruce's position mean "expose to the right?'
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Yes. And there IS such a thing as correct exposure, which is what ETTR is intending to accomplish.

[a href=\"http://www.digitalphotopro.com/tech/exposing-for-raw.html]http://www.digitalphotopro.com/tech/exposing-for-raw.html[/url]
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Andrew Rodney
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jerryrock
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« Reply #31 on: January 07, 2008, 11:01:05 AM »
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Exposure bracketing would seem to satisfy all concerned.
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Gerald J Skrocki
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« Reply #32 on: January 07, 2008, 11:05:57 AM »
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Exposure bracketing would seem to satisfy all concerned.
[a href=\"index.php?act=findpost&pid=165643\"][{POST_SNAPBACK}][/a]

Yes, unless you're shooting anything that moves....

Bracketing portraits ain't going fly.
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Andrew Rodney
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eronald
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« Reply #33 on: January 07, 2008, 11:13:05 AM »
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The higher in the range of RAW levels you record your subject, the higher the signal-to-noise ratio is at every tonal level in the image.  Obviously, you are the artist and/or technician, and should decide what aperture and shutter speed you want, but subordinate to that, within the range of freedom you can allow, you can also improve S:N by increasing the exposure (avoiding clipping desired highlights, of course).  With some cameras, you get a significant decrease in shadow noise by going to a higher ISO while maintaining the same absolute exposure, and exposing more to the right.
[a href=\"index.php?act=findpost&pid=165608\"][{POST_SNAPBACK}][/a]

John,

 how does one characterize these aspects eg S:N vs ISO ?

Edmund
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bjanes
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« Reply #34 on: January 07, 2008, 12:14:28 PM »
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Both are right.

The ostensible contradiction only comes from different angles of view ... and from exactly what is the term 'overexposure' supposed to mean. Bruce was looking at the topic from a technical point of view, and 'overexposure' in his context means overexposure of the JPEG image that comes from the capture parallel to the raw file (or from the raw file via standard settings in the raw converter). When the JPEG is (slightly) overexposed then the raw file often still will have some headroom to work with. Michael is looking at the topic from a practical, hands-on point of view. In real life, underexposed images can be saved through proper processing much easier and more often than overexposed ones.

The critical point is, 'as close to blowing out without actually doing so.' If you go just a quarter of an f-stop above the blow-out limit then the image is damaged beyond salvation. If you stay two or even three full f-stops below the limit then the image usually will still be fine.

[{POST_SNAPBACK}][/a]

Shooting fully to the right requires base ISO so that the electron wells of the sensor are filled, collecting the maximum number of photons and giving the best signal to noise ratios at all levels. If you are shooting at an ISO higher than base, you are essentially underexposing and boosting the amplification of the signal. For example, if the base ISO of your camera is 100 and you expose at ISO 1600 you are basically underexposing by 4 f/stops and compensating for this by increasing the amplification of the signal. This is usually done by setting the camera to a higher ISO, but alternatively, one can do this in the raw converter. Because read noise is higher at low camera ISOs it is better to use the ISO route, at least up to [a href=\"http://www.clarkvision.com/imagedetail/digital.sensor.performance.summary/]Unity Gain[/url]. There is little point in increasing ISO above unity gain, since you lose headroom protection from overexposure but collect little if any more information.

Digital cameras have no shoulder on the characteristic curve, and clip abruptly at sensor saturation of overflow of the ADC. You can recover 0.4 to 1 stop of blown highlights with highlight recovery, but beyond that there is a complete loss of highlight detail. Furthermore, highlight recovery is less than perfect and may involve color shifts.

On the other hand, a camera with large pixels and low read noise (e.g. Nikon D3 or Canon 1DMIII) can produce quite acceptable pictures at 4 stops underexposure as defined above. Overexpose by 4 stops and you will lose 3 or more stops of highlight detail and the results will most likely not be acceptable. However, you will get good shadow detail.

As Andrew stated, we should be striving for proper exposure. In an imperfect world, slight overexposure can be tolerated, but gross overexposure will cause major data loss. If you have a good camera, gross underexposure will give more noise and less dynamic range, but you will at least have an image. Since signal:noise is proportional to the square root of the number of electrons, doubling of exposure will improve the S:N by a factor of 1.4, not 2, and this improvement may not be worth the risk of data  loss if ETTR is carried to far.

Some argue that a main rationale of ETTR is to make use of the 2048 levels in the brightest f/stop of a 12 but linear file, which contains a total of 4096 levels. However, the eye can distinguish 70 levels per f/stop at maximum, so one can afford to lose a few of these levels without incurring posterization. Posterization in the shadows could occur, but usually noise is the limiting factor there.
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Tim Lookingbill
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« Reply #35 on: January 07, 2008, 02:02:48 PM »
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What is rarely mentioned in ETTR discussions like this is the ability to determine dynamic range at the time of capture. If it's beyond the sensor's capabilities it makes ETTR or any exposure technique pointless.

And frankly I'm not even sure what DR really means after experimenting with bracketed RAW shots taken on my Pentax K100D. A midday outdoor scene with the sun directly overhead creates quite a bit of bounced light for backlit objects in the foreground compared to taking the same shot with the sun just over the rooftops meaning there's less light. I can get more detail in the shadows with very little noise without blowing out highlites with the midday shot than I could with the one with less light or maybe contrasty should be a better way to describe the scene.

I can't understand how a scene with less light can create a dynamic range that overpowers the sensors. Something is screwey with the way sensors record light that doesn't fit within our notion of dynamic range.

I've examined dpreview's backlit Stouffer grayramps but it never seems to translate for me how you expect to retrieve more detail with less light as in a sunset than with more light at midday and from this determine the DR capabilities of any given sensor.
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Panopeeper
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« Reply #36 on: January 07, 2008, 02:18:12 PM »
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Shooting fully to the right requires base ISO so that the electron wells of the sensor are filled, collecting the maximum number of photons and giving the best signal to noise ratios at all levels

This is correct only

1. if it is possible to expose to the right using the base ISO; but what about insufficient light, moving subjects hand-held shooting, the necessity to work with small aperture?

2. if the base ISO offers the best DR, which is not always the case.

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This is usually done by setting the camera to a higher ISO, but alternatively, one can do this in the raw converter. Because read noise is higher at low camera ISOs it is better to use the ISO route

Higher ISO is always better than lower ISO with the same exposure, adjusted in the raw processing, at least up to some ISO limit (and even later, it is never worse than increasing the brightness in raw processing).

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This is woodoo science from a certain level, for the inaccuracy in the data used in the calculation is approaching 100%.

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Some argue that a main rationale of ETTR is to make use of the 2048 levels in the brightest f/stop of a 12 but linear file, which contains a total of 4096 levels. However, the eye can distinguish 70 levels per f/stop at maximum, so one can afford to lose a few of these levels without incurring posterization

Those, who keep only the 2048 levels in their eyes are partially blind.

However, there is another side of the issue: utilizing some of those 2048 levels means at the same time increasing the number of levels at the low end.

What worth is an additional "low noise" stop, which contains only eight levels?

Using only the first 2048 levels means, that the seventh stop will consists of only FOUR levels; going to the right increases this to eight levels. So, what about the 70 levels?
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Gabor
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« Reply #37 on: January 07, 2008, 02:31:29 PM »
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I've examined dpreview's backlit Stouffer grayramps but it never seems to translate for me how you expect to retrieve more detail with less light as in a sunset than with more light at midday and from this determine the DR capabilities of any given sensor.

DPReview's DR measurement concentrates on the "smoothness" of the strips, totally ignoring the details retained on that level (which, of course, can not be measured with the Stouffer wedge).

There is a long discussion of this subject in this thread
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Gabor
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« Reply #38 on: January 07, 2008, 02:40:55 PM »
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Those, who keep only the 2048 levels in their eyes are partially blind.

However, there is another side of the issue: utilizing some of those 2048 levels means at the same time increasing the number of levels at the low end.

Exactly! Its about getting as many of the few levels in the last stop (shadows) possible.

And what the eye can theoretically see seems rather moot when we're talking about encoding data for a computer to process.
« Last Edit: January 07, 2008, 02:41:22 PM by digitaldog » Logged

Andrew Rodney
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« Reply #39 on: January 07, 2008, 03:41:50 PM »
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And what the eye can theoretically see seems rather moot when we're talking about encoding data for a computer to process.

This is an additional aspect: an image with 70 levels in the higher stops would not withstand any post processing. Details would be rendered unrecognizable due to interpolation, rounding.
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Gabor
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