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Author Topic: lower EC or ETTR?  (Read 23959 times)
BillMc
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« on: July 18, 2012, 01:12:01 PM »
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I am confused about two pieces of advice one often sees, that seem to me to be contradictory.  On the one hand, one sees advice to leave exposure compensation set on -1/3, in order to get better color.  This is especially urged on small sensor cameras, which have a tendency to blow out highlights.

On the other hand, one reads articles urging photographers to expose to the right, which I gather means that in taking a photograph, one should examine the histogram and if necessary boost exposure compensation so that the curve shifts over toward (but does not touch) the right side of the histogram.

These pieces of advice seem to be at cross purposes. Can someone enlighten my understanding?
 
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Schewe
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« Reply #1 on: July 18, 2012, 02:04:39 PM »
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On the one hand, one sees advice to leave exposure compensation set on -1/3, in order to get better color.  This is especially urged on small sensor cameras, which have a tendency to blow out highlights.

You can forget about this one...it's wrong. Assuming you are shooting raw the "color" you get is directly related to the raw processing you do. As long as you don't clip any textural highlight information, a normal or ETTR exposure is equally capable of having the color adjusted, so under exposing to get "better color" is a load of crap...
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Tony Jay
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« Reply #2 on: July 18, 2012, 03:09:09 PM »
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Bill, the advice you are getting is not contradictory because it likely relates to shooting with different purposes in mind since reducing the EC in the way that you describe relates to shooting JPEG's while ETTR techniques relates to shooting RAW files.
The key point is that, even in Lr with non-destructive parametric editing, JPEG files respond poorly to tonal editing so what you get out of the camera is the probable endpoint.
Obviously RAW files lend themselves very well to tonal manipulation so in this case ETTR just optimizes the file information for the process of tonal manipulation.

Broadly, shooting JPEGS the approach to shooting is similar to shooting colour film while shooting RAW demands a completely different approach summed up as ETTR.

Regards

Tony Jay
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BillMc
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« Reply #3 on: July 18, 2012, 04:31:58 PM »
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Thanks very much.  This helps a lot. I do shoot RAW, and so I'll use my histogram to expose to the right and adjust the color later in processing.
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luxborealis
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« Reply #4 on: August 04, 2012, 08:18:23 AM »
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Furthermore, depending on the sensor you have, you don't always need to limit yourself to the right side of the histogram. Some sensors allow you to clip by upwards of 1 stop and still recover detail in the highlights.

Check your system carefully by shooting a scene with bright highlights (e.g. brightly lit clouds) using what I call "highlight bracketing": Shot 1-not clipped; Shot 2-clipped by 1/3 stop; Shot 3-clipped by 2/3s stop; Shot 4-clipped by 1 stop; etc. Then bring them into your editing app (LR, Aperture, CaptureOne) and process them to see what detail you can recover. That way, you can learn about the boundaries of your equipment, your visualization and shooting techniques. I have been amazed at how much can be recovered in my D800e files using LR 4.1 from areas that show as "clipped" on the histogram and with the highlight clipping indicators.
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hjulenissen
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« Reply #5 on: August 04, 2012, 09:56:35 AM »
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On the one hand, one sees advice to leave exposure compensation set on -1/3, in order to get better color.  This is especially urged on small sensor cameras, which have a tendency to blow out highlights.
If your sensor clips, you will have erroneous data in the affected pixels. Clever raw developers may do a good job of concealing that error if the clipping is not too severe, but it will still be a "guess".

If your cameras auto exposure is too "hot" for your taste and processing, it might be a good advice to do some negative exposure compensation. Current cameras are not doing a good job of telling you how close to clipping any given image was.

-h
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Graystar
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« Reply #6 on: August 09, 2012, 08:27:31 PM »
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I am confused about two pieces of advice one often sees, that seem to me to be contradictory.  On the one hand, one sees advice to leave exposure compensation set on -1/3, in order to get better color.
As already said, reducing EC to get better color is wrong. 

Quote
This is especially urged on small sensor cameras, which have a tendency to blow out highlights.
It's wrong here as well.  Highlights are usually blown by the one-size-fits-all image processing of the camera.  Also, so-called "intelligent" metering modes may increase exposure to avoid underexposure.  These modes attempt to apply their own exposure compensation to the shot.  They may over or underexpose the image if they guess wrong.

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On the other hand, one reads articles urging photographers to expose to the right, which I gather means that in taking a photograph, one should examine the histogram and if necessary boost exposure compensation so that the curve shifts over toward (but does not touch) the right side of the histogram.

These pieces of advice seem to be at cross purposes. Can someone enlighten my understanding?

ETTR is an interesting concept, but it's very tricky.  It's only valid at your base ISO (which may not be your lowest.)  If there's any white in the scene, then you can't ETTR.  If there's any bright red in the scene, then you can only ETTR by 1/3 stop or so before getting into trouble.  I guess if you're taking shots of black cats lying on coal piles all day long, then ETTR will have value.  Otherwise, you may find it more trouble than it's worth.

ETTR works well in the studio environment, where you have control over lighting.  Then it's easy to create situations that have one stop's worth of highlight leeway.  You can overexpose by one stop and then pull exposure down in post to get less noise.

If you have a camera with an expanded ISO range, then you have ETTR built in.  All you need to do is to utilize your expanded ISO.  For example, my Nikon D90 has a base ISO of 200, and an expanded ISO of "L 1.0" (much like ISO 100, but I lose one stop of highlight space.)  I just use L 1.0 when I want to ETTR (under the right conditions, of course,)

I find that I get the best exposure by using a gray card.  Well, I don't actually use a proper gray card...I just use my white-balance reference card that I carry in my back pocket.  Through a simple "calibration" process I determined that I can meter the WB reference and set EC to +1.3 for standard exposure.  That gets the whites as bright as possible without clipping (regardless of what the camera's histogram says.)

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jeremypayne
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« Reply #7 on: August 09, 2012, 09:16:41 PM »
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If there's any white in the scene, then you can't ETTR.  If there's any bright red in the scene, then you can only ETTR by 1/3 stop or so before getting into trouble. 

Care to explain these statements?  They don't really jive with theory or my experience ...
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Graystar
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« Reply #8 on: August 09, 2012, 10:44:21 PM »
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Care to explain these statements?  They don't really jive with theory or my experience ...
Sure.

First, let's make sure we're on the same page (so to speak.)  Spot/CW metering on all cameras is calibrated to 12.7% reflectance.  That's easy to verify...just spot meter an evenly lit white wall and take a picture.  You should get a gray image with the metered area around sRGB 100, 100, 100, which is about 12.7% gray (you need a RAW converter and a "neutral" profile that doesn't apply any processing to the image.)  That gives you nearly 3 stops of highlight space above 12.7% gray (12.7%x2=25.4%x2=50.8%x2=101.6%...you can't have a signal greater than 100%, so it's just a smidge under three stops.)

So let's say you have a 12.7% gray card (Lastolite makes one) or an 18% gray card + 0.5 EC (Kodak and others) or as I have, an RM Imaging Digital Gray Card (designed and marketed as a white-balance tool) + 1.3 EC (an EC amount I determined through a "calibration" process.)  Any of these metering references will get you the same exact exposure.

So you spot meter your chosen reference, apply EC if called for, and take a picture.  This exposure level would constitute standard exposure.  Any white objects that are reflecting 90%+ of the light falling on them will be giving you a signal near saturation on the green channels.  You can use something like RawDigger to see the RAW histogram.  If you try to push the exposure by even 1/3 stop, you'll likely clip the green channels.  This is why I say that you can't really ETTR if you have anything white in your image...because standard exposure already gets your white objects near saturation.

As for red...what I've found is that when you push red a little, even though you don't clip the channel, the scaling factors will cause the red channels to clip in the RGB conversion.  So technically, the RAW channels aren't clipped, but you have to deal with the red clipping after the RGB conversion.  That can be done by messing with the scaling factors and with functions such as highlight protection.  I find that it takes a bit of work to get things to look right.

So my bottom line is that for most scenes, standard exposure gives the best exposure level.  As I said previously...if you've got the lighting under your control then you can certain create a situation where you can overexpose by a stop or so, and then ETTR.

I think the problem that faces most people is that of being able to set standard exposure reliably.  In just about any scene that's static enough to implement ETTR, you should be able to use a gray card to set standard exposure.  But you do need to have a card with a good quality matte surface...otherwise the reading will be all over the place.
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ckimmerle
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« Reply #9 on: August 11, 2012, 07:25:10 PM »
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I think the problem that faces most people is that of being able to set standard exposure reliably.  In just about any scene that's static enough to implement ETTR, you should be able to use a gray card to set standard exposure.  But you do need to have a card with a good quality matte surface...otherwise the reading will be all over the place.


There is no such thing as "standard exposure". Not only will each scene have its special exposure considerations, but different photographers will meter each scene slightly differently, depending on how they visualize the resulting print. As for an 18% gray card, they pretty much became useless with the advent of the histogram which, unlike an 18% gray card, actually provides you with useful/vital exposure data.

I'm not sure where you are getting your information, but much of it is simply not correct.
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Chuck Kimmerle
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« Reply #10 on: August 11, 2012, 08:22:49 PM »
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There is no such thing as "standard exposure". Not only will each scene have its special exposure considerations, but different photographers will meter each scene slightly differently, depending on how they visualize the resulting print. As for an 18% gray card, they pretty much became useless with the advent of the histogram which, unlike an 18% gray card, actually provides you with useful/vital exposure data.

I'm not sure where you are getting your information, but much of it is simply not correct.

Actually, all my information is correct.  Standard exposure is what you get from ISO metering standards.  If there were no standards, you couldn't use a handheld meter to set your camera.  Standard does not mean "correct".  It simply means "based on a standard."

You're talking about the preferred exposure for the scene, which is completely subjective.  However, your notion of exposure is out of place in this thread because we're discussing ETTR.  And the rules for setting exposure via ETTR are short and sweet...maximize your exposure so that the brightest highlights you care about are near saturation.  So someone taking a picture of a black cat sleeping on a pile of coal is going to overexpose his image by 5 stops.  In post, the photographer will apply -5 EC (or as much as he likes) to restore the look of the scene. That's ETTR.

I personally find the histogram to be useless.  It's too small to indicate certain important clipped highlights, such as the whites of the eyes, and it doesn't tell you if those overexposed areas are specular in nature (in which case you should probably leave them be.)  I find the "blinking" function to be far more useful.  For scene with a wide contrast range, the gray card tends to give me the maximum exposure possible.
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jeremypayne
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« Reply #11 on: August 11, 2012, 10:28:56 PM »
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Actually, all my information is correct.

So for every possible image there is only one exact place to put "the grey card" and one and only one exposure that meets "ISO standards"?
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ckimmerle
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« Reply #12 on: August 11, 2012, 10:31:23 PM »
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However, your notion of exposure is out of place in this thread because we're discussing ETTR.

Umm....you were the one who started talking about gray cards and standard exposure (neither of which was relevant to the OP). I was only responding.

And the rules for setting exposure via ETTR are short and sweet...maximize your exposure so that the brightest highlights you care about are near saturation.

Actually, that is incorrect. When using ETTR, you should only increase exposure enough to ensure that there is relevant detail in the important shadow areas (minimize noise). That might mean increasing exposure only one stop. Pushing the histogram so far than you're near clipping is simply bad practice as you're possibly sacrificing tonal range in the highlights. ETTR is all about increasing the SNR in the shadows. That's all does.

I personally find the histogram to be useless.  It's too small to indicate certain important clipped highlights, such as the whites of the eyes, and it doesn't tell you if those overexposed areas are specular in nature....I find the "blinking" function to be far more useful

Sure, the histogram doesn't plot every exposure level on the image, but it offers a helluva lot of information for the photographer. Most importantly, it displays placement and amount of the brightest and darkest areas of the image. Also, the slope of the end points, for instance, tells a lot about the tonal ranges of shadow and highlight regions. As the camera uses the jpeg preview to calculate the histogram, worrying about specular highlights is a waste of time as the resulting image, especially if RAW, will have a much greater overall tonal range than the camera histogram indicates. As for the blinking function, it uses the same data set as the histogram, so offers nothing new.
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« Reply #13 on: August 11, 2012, 11:16:48 PM »
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So for every possible image there is only one exact place to put "the grey card" and one and only one exposure that meets "ISO standards"?
No.  For whatever you meter, you get standard exposure.  If you meter a white wall, you'll get standard exposure and the wall will come out gray.  If you meter a black wall, you'll get standard exposure and the wall will come out gray.  That's how reflective metering works.  The meter starts with a presumption that the world is gray...12.7% gray, to be exact.  It then computes exposure so that the metered area comes out a gray of 12.7%.

Obviously, the meter is wrong on the white and black walls.  That's why it's "standard" exposure, and not "correct" exposure.

If you meter a 12.7% gray reference, then your exposure of white objects will be near saturation and anything illuminated by that same light source will appear in the photograph as it did in the live view.  If your scene has two light sources, like many outdoor scenes with part of the scene lit by the sun and part in open shade, then each area of illumination would require its own "standard" exposure.  There's no correct exposure for such scenes...you simply have to decide what's important and make a decision, and adjust exposure as you see fit.

Standard exposure is a tool...not a destination.
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Graystar
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« Reply #14 on: August 11, 2012, 11:29:19 PM »
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Umm....you were the one who started talking about gray cards and standard exposure (neither of which was relevant to the OP)

That's because I was responding to a specific question posed by a poster other than the OP.

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I was only responding.

And so was I.  And my response was in context to a question relating to ETTR.

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Actually, that is incorrect. When using ETTR, you should only increase exposure enough to ensure that there is relevant detail in the important shadow areas (minimize noise). That might mean increasing exposure only one stop. Pushing the histogram so far than you're near clipping is simply bad practice as you're possibly sacrificing tonal range in the highlights. ETTR is all about increasing the SNR in the shadows. That's all does.

That's not right.  You don't sacrifice tonal range in the highlights by pushing exposure to near saturation.  Exposure on digital sensors is linear.  As long as you don't clip the highlights you'll capture the full tonal range.

Quote
Sure, the histogram doesn't plot every exposure level on the image, but it offers a helluva lot of information for the photographer. Most importantly, it displays placement and amount of the brightest and darkest areas of the image. Also, the slope of the end points, for instance, tells a lot about the tonal ranges of shadow and highlight regions. As the camera uses the jpeg preview to calculate the histogram, worrying about specular highlights is a waste of time as the resulting image, especially if RAW, will have a much greater overall tonal range than the camera histogram indicates. As for the blinking function, it uses the same data set as the histogram, so offers nothing new.

The blinkies tell you where the clipping is.  You'll see the whites of a person's eyes blinking...but you won't see the clipping of such a small area in the histogram.  And I can see the tonal ranges with my eyes...I don't need a histogram to show them to me.
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deejjjaaaa
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« Reply #15 on: August 12, 2012, 08:40:35 AM »
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Exposure on digital sensors is linear. 

http://forums.dpreview.com/forums/readflat.asp?forum=1041&message=42236844

"...However the question is of colour linearity, that is - if we apply a matrix transform to two shots of the same scene with exposure difference, we will have different L* obviously; but do we have same a/b? In other words, what is the deltaE2000 between those two shots after the linear exposure compensation in the raw converter? One even does not need to have an accurate colour profile to check it - just any matrix will give an idea being assigned to a binned raw data..."

so did you test yours ? provided that your raw converter indeed does linear EC
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Graystar
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« Reply #16 on: August 12, 2012, 08:26:55 PM »
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http://forums.dpreview.com/forums/readflat.asp?forum=1041&message=42236844

"...However the question is of colour linearity, that is - if we apply a matrix transform to two shots of the same scene with exposure difference, we will have different L* obviously; but do we have same a/b? In other words, what is the deltaE2000 between those two shots after the linear exposure compensation in the raw converter? One even does not need to have an accurate colour profile to check it - just any matrix will give an idea being assigned to a binned raw data..."

so did you test yours ? provided that your raw converter indeed does linear EC

An ETTR discussion on an m4/3 forum??  Tell them to just buy bigger sensors!  Tongue

Yes, I've tested it many times.  I've attached an example I just created...

They're obviously different...they must be, as one was created with only 1/4th the light of the other.  There's more noise, and it's most noticeable in the black patch.  Still, the colors look fairly similar.
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stamper
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« Reply #17 on: August 13, 2012, 03:58:13 AM »
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The blinkies tell you where the clipping is.  You'll see the whites of a person's eyes blinking...but you won't see the clipping of such a small area in the histogram.  And I can see the tonal ranges with my eyes...I don't need a histogram to show them to me.
[/quote]

The problem with the blinkies is that you don't know where the camera manufacturers have set the limit? I think it is wrong to assume it is 255 255 255 or 100%. I believe it is a lower value but they won't tell. Are you serious when you say that you are concerned about the lightness values in a small area such as a person's eyes?
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hjulenissen
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« Reply #18 on: August 13, 2012, 04:17:10 AM »
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I must confess that I fail to see why "18%" or "12.7%" would ever have to be mentioned in a discussion about digital cameras. Film cameras might be another story, but I don't know much about those.

A digital camera sensor seems to be fairly well modelled (in this context) as a linear ADC with some additive noise-floor. Sure there is more to it, but when it comes to selecting exposure, you will capture the most information about the scene (provided that exposure time and aperture allows) if the brightest pixels of interest are exposed close to the saturation limit, thus maximizing SNR for every sensel without clipping. If the scene has a wider dynamic range than the sensor can reliably capture, one might want to clip e.g. highlights. In that case, it becomes a trade-off between "bright pixel error" (clipping) and "dark pixel error" (noise). The same methology can be used for this case if you are able to define the brightest pixel of interest.

This leads to the conclusion that ETTR (or as digital audio engineers have known for some time: record using hot levels, but not too hot - HLNTH) is a sound principle for digital camera capture if you want to maximize information captured. A relevant question then might be: are you (as a photographer) primarily interested in capturing the largest possible amount of information, or in "making the best images"? I have little doubt that "ETTR" will maximize the information about the scene, but that may do you little good if fiddling with the histogram means that the bird has left the scene, or if your raw editor makes it cumbersome to compensate exposure predictably (for a pleasing rendering exposure). Purchasing a new Sony-sensor-based camera might provide a similar "DR headroom" as fiddling with ETTR on an older camera.

-h
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bjanes
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« Reply #19 on: August 13, 2012, 09:38:36 AM »
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Furthermore, depending on the sensor you have, you don't always need to limit yourself to the right side of the histogram. Some sensors allow you to clip by upwards of 1 stop and still recover detail in the highlights.

Check your system carefully by shooting a scene with bright highlights (e.g. brightly lit clouds) using what I call "highlight bracketing": Shot 1-not clipped; Shot 2-clipped by 1/3 stop; Shot 3-clipped by 2/3s stop; Shot 4-clipped by 1 stop; etc. Then bring them into your editing app (LR, Aperture, CaptureOne) and process them to see what detail you can recover. That way, you can learn about the boundaries of your equipment, your visualization and shooting techniques. I have been amazed at how much can be recovered in my D800e files using LR 4.1 from areas that show as "clipped" on the histogram and with the highlight clipping indicators.

Good suggestions, but I would not rely on lightroom 4.1 or ACR 7.1 with PV2012 to judge highlight clipping, since they incorporate automatic highlight recovery and also have a baseline offset exposure of +1/3 EV. The former mitigates highlight clipping, while the latter aggravates it. It is advisable to look directly at the raw files with a program such as Rawdigger.

I performed such a test on my 800e using  Stouffer step wedge. The steps are in 0.3 EV increments.

Image 1 at 1/13 sec is the optimal ETTR exposure. The highlights are nearly 0.3 EV from clipping, but the exposure at 1/10 second shows clipping. Looking at the camera histogram shows for the 1/13 s exposure shows clipping and the blinking highlights indicates clipping of the brightest two steps. The histogram is conservative and the blinking highlights are even more conservative. These indicators are useful, but one can allow exposures that appear slightly blown and rely on highlight recovery to correct the situation. The camera has excellent noise characteristics and ETTR is less important than with some cameras, and one can not go far wrong just relying on the camera histogram and blinking highlights.

Rsw histogram for 1/13 s exposure:


Raw histogram for the 1/15 s exposure:


Camera histogram for the 1/13 s exposure:


Blinking highlights for the 1/13 s exposure:


Regards,

Bill
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