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Author Topic: Today's DSLR should have another exposure mode  (Read 27292 times)
dtrayers
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« on: May 01, 2006, 09:38:05 PM »
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I shoot dance and theater and the dynamic range of the scenes can be a challange for the meter of my 1DmkII.  I've experimented with all four metering modes and I've found that in the changing light conditions of the stage that more often than not, evaluative works the best.  I do have to dial in a bit of exposure compensation to keep the highlights from blowing out, but it can vary based on the costume of the subject and the lights.  Lots of hard light with light skin and/or light costumes need more negative exposure comp than a less contrasty scene.  Makes sense, but hard to do in practice.  At the high ISO's I shoot, I don't want to underexpose which will accentuate the noise, but I definitely don't want to overexpose.

But over the weekend as I was shooting for a local non-profit dance studio, it dawned on me that a very useful variation of the evaluative metering mode would be a 'no-clipping' option.  The camera would evaluate all the metering zones and then based on the measurements calculate the exposure such that the brightest zone would be just short of clipping, or 'exposed to the right'.

It would be a kind of "automatic" exposure compensation.  It would only really work for the evaluative mode where the camera would evaluate 21 areas and pick the one to expose on.

For digicams that offer a live histogram, the same concept could be used to calculate exposure, only instead on relying on 21 metering zones, the camera could use the actual histogram and calculate exposure such that the histogram is 'exposed to the right'.
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Peter Jon White
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« Reply #1 on: May 01, 2006, 10:15:20 PM »
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The idea has been floating around for quite a while. And I'm surprised that no company, as far as I know, has implemented it in some fashion. Of course there would have to be a way of adjusting a threshold of some sort. Some highlights you don't care about blowing. A spot of sunlight reflecting off of the chrome trim of an automobile, or sunlight on water can and should be blown, otherwise you'll have nothing but noise when you're adjusting the exposure later. But it should be possible to do. And since highlights are the key to optimum digital exposure,  the ideal metering system should be looking at highlights.
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Sheldon N
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« Reply #2 on: May 01, 2006, 10:28:08 PM »
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I think Peter has hit the nail on the head with the issue of the "threshold". The difficulty would be that the variability of that highlight you are trying to save detail in may be 5 or 10 stops.

One example I've seen is in the impementation of the Canon ETTL-II flash system. It is sensitive to reflective items in the photo, cutting flash output to try and prevent highlight details from blowing out. You can see this by taking a picture in a mirror. If the flash head is visible in the shot, the system intervenes drastically cutting output by 3-4 stops to try to save the detail in the flash head.

Using your example of a shooting dancers in a theatre, the camera would have to make an intelligent choice about whether you were trying to capture highlight detail in the dancer's costume (at 3 stops over) or if you were trying to capture detail in the spotlight you accidentally included in the frame (at 20 stops over). There may not be a consistent threshold to use, and the incorrect choice could give you an unuseable shot.

To me, the real problem is not that the highlights clip, it's that they look so bad when they do so!
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Schewe
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« Reply #3 on: May 01, 2006, 11:23:31 PM »
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The irony is that in a digital sensor, you have, in effect, a -REAL- accurate (and real expensive) light measuring device since the whole sensor is a photon counter...

The way I see it, a half push of the shutter release could lock onto both auto-focus while at the same time shooting a non-recorded sensor capture that would evaluate every pixel in the capture and choose the optimal real capture exposure. A function button could lock the exposure setting until you hit the button again to release and re-meter...

This would be particularly useful to have the camera TELL YOU the scene dynamic range (assuming the camera makers would finally get real and adopt a standard). Then you could dial in a compensation factor based upon whether you want to bias the exposure to the shadows or the highlights-in the case where the scene is beyond the dynamic range.

Unfortunately, camera makers still seem to want to put in a separate light measuring device-a meter-when the real light measuring device should be the sensor itself.

Just a thought...
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bjanes
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« Reply #4 on: May 02, 2006, 08:58:26 AM »
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The irony is that in a digital sensor, you have, in effect, a -REAL- accurate (and real expensive) light measuring device since the whole sensor is a photon counter...

The way I see it, a half push of the shutter release could lock onto both auto-focus while at the same time shooting a non-recorded sensor capture that would evaluate every pixel in the capture and choose the optimal real capture exposure. A function button could lock the exposure setting until you hit the button again to release and re-meter...

This would be particularly useful to have the camera TELL YOU the scene dynamic range (assuming the camera makers would finally get real and adopt a standard). Then you could dial in a compensation factor based upon whether you want to bias the exposure to the shadows or the highlights-in the case where the scene is beyond the dynamic range.

Unfortunately, camera makers still seem to want to put in a separate light measuring device-a meter-when the real light measuring device should be the sensor itself.

Just a thought...
[a href=\"index.php?act=findpost&pid=64240\"][{POST_SNAPBACK}][/a]

Jeff's suggestion makes a lot of sense, but of course it has not been implemented.

However, some progress is being made. I'm not familiar with Canon metering, but the Nikons have a CCD for metering that contains 1005 cells (alternating RGB) arranged in 15 rows and 67 columns covering virtually the entire frame. This sampling is not as extensive as Jeff suggests, but can be done in real time without pre-exposure. It should give the camera a good indication of the dynamic range and color content of the scene and probably could be put to better use for pre-exposure evaluation by the user and in evaluative metering.

One constant complaint of D70 users was "underexposure"--i.e. the camera refused to expose to the right in a high dynamic range scene. Apologists said this was to protect the highlights, but it made absolutely no sense to have images with the brightest stop of the histogram completely unpopulated. It was quite frustrating for advocates of ETTR.  I suspect the exposure algorithm was adapted from their film cameras and not really appropriate for digital. However, the D200 in its matrix evaluative mode is much better--one has to override the camera much less often in order to expose to the right. For once, Nikon has listened to its users.
« Last Edit: May 02, 2006, 09:00:43 AM by bjanes » Logged
Tim Gray
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« Reply #5 on: May 02, 2006, 10:40:24 AM »
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Live histograms are likely not that far off (my recollection is that it's already implemented on at least 1 dslr).

I suspect it would be fairly easy to dial in a % permissible clipping, either highlights or shadows.

As an interim step, I'd like more accurate RAW based histograms (even if only after the shot).  I often find that my brackets are not required and even if the histogram showed clipping at the time of the shot in the RAW image there is actually no clipping.
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sergio
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« Reply #6 on: May 02, 2006, 11:22:45 AM »
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It can help to know your sensor range. My 1DsMII for example goes up to + 2 2/3 stop and can recover 1 more stop (most of the time) in ACR making my usable range to + 3 2/3 of a given tone spot metered to normal exposure.

I carry a spot meter because of the inaccuracy of the histogram. In many shot it won't matter much, but in high contrast scenes you just need the last bit of usable range you can get to expose correctly to the right, to have nice clean shadows.
« Last Edit: May 02, 2006, 11:23:36 AM by sergio » Logged

bjanes
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« Reply #7 on: May 02, 2006, 12:24:43 PM »
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It can help to know your sensor range. My 1DsMII for example goes up to + 2 2/3 stop and can recover 1 more stop (most of the time) in ACR making my usable range to + 3 2/3 of a given tone spot metered to normal exposure.

I carry a spot meter because of the inaccuracy of the histogram. In many shot it won't matter much, but in high contrast scenes you just need the last bit of usable range you can get to expose correctly to the right, to have nice clean shadows.
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I agree with Tim Grey (above) that a raw based histogram would be useful, and it should not be white balanced. Sergio is taking great pains to expose to the right, but with both Canon and Nikon digital SRLs with daylight exposure, the output of the red and blue channels is considerably (about 1 stop) less than the green, and this is not shown on the camera histogram. During conversion, a multiplier is applied to the blue and red channels to equalize them with the green.

The green channel histogram may be exposed properly to the right, but the blue and red fall short of the right. One way to balance the channels and gain better dynamic range is to use a cc100M filter over the lens to hold back the green light and equalize the channels. One then performs white balance with the filter in place and the blue and red multipliers then approach unity. One can gain up to 1 stop of DR with this technique. Here is an example of an unfiltered daylight exposure for the D200:

[attachment=533:attachment]

For some images showing this effect with the D200 and EOS 1D Mark II see this thread in the Adobe Camera Raw forum:

[a href=\"http://www.adobeforums.com/cgi-bin/webx?14@@.3bb6a85c.3bbf0e1d/10]http://www.adobeforums.com/cgi-bin/webx?14...85c.3bbf0e1d/10[/url]

I originally learned of this technique from a post on the DPReview forum by Julia Borg.
« Last Edit: May 02, 2006, 12:31:58 PM by bjanes » Logged
Ben Rubinstein
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« Reply #8 on: May 02, 2006, 01:32:12 PM »
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in high contrast situations you can easily hold the highlights from clipping while horribly underexposing your subject. Backlighting would be a good example and probably pretty relevent to your stage shooting. If you have a scene with a lot of white the camera would hold the whites and but the midtones would underexpose. The trick with digital is to decide where you want the highlights, sometimes you have to blow highlights for a good exposure. An ETTR metering mode would cause underexposure in too many situations.

There just isn't enough DR in the highlights with digital to always capture a high contrast scene in one frame which is why HDR gives such great noise, it uses the ETTR for each level of the brightness so you have the maximum information, not only in the highlights but in the shadows too. ETTR is better for the shadows than not doing so, and far better for low contrast scenes, but it is only part of the technique needed to capture the full range of a scene with good quality. At a certain level of contrast ranges you need more than one exposure or you need to work out and understand what highlights you don't mind sacrificing for cleaner midtones.
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DiaAzul
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« Reply #9 on: May 02, 2006, 01:50:15 PM »
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Unfortunately, camera makers still seem to want to put in a separate light measuring device-a meter-when the real light measuring device should be the sensor itself.

[a href=\"index.php?act=findpost&pid=64240\"][{POST_SNAPBACK}][/a]

You are always going to have a two sensor (metering and capture) solution whilst manufacturers retain the current capture sensor design. It is just not feasible to have a live capture type metering with, say, Canon's CMOS sensor design. This is part of the reason for having the two sensors separate. I also suspect that the metering sensor has a greater dynamic range compared with the capture sensor to provide the 0-20ev range that is required to accurately meter a scene. If you do go for an all in one design then the capture sensor would need to have a 0ev to 20 ev dynamic range which is not yet economically realistic, but in 2-5 years may be the panacea that all photographers are looking for.

This doesn't preclude, though, metering sensors with larger numbers of pixels (like Nikon's) and the ability through that type of sensor to provide a live histogram, preferably as an overlay in the viewfinder. However, this may ultimately come down to market demand balanced up against the costs of implementing such a solution.

My point and squirt (Panasonic FX-01) provides live histogram, which is useful given its propensity to clip highlights - though as reviews have pointed out the dynamic range is not great and the sensor is noisier than most would prefer. Having a similar feature on my DSLR would be a welcome addition to existing metering modes - though in reality I can cope quite well with the tools I have at the moment provided a certain amount of learning experience is factored into adjustments and RAW is used to provide additional latitude in the post processing.
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jd1566
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« Reply #10 on: May 05, 2006, 10:50:03 AM »
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dtrayers, excellent suggestion.  Why don't you contact Chuck Westfall of Canon USA about it.  He seems to be their ear to the ground on user suggestions.  He writes a monthy article/response to users'questions and suggestions, and is generally a good source of information if you are having problems with something Canon.

http://dirckhalstead.org/issue0604/westfall.html

At the bottom of the article there is a link to his e-mail.
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Anon E. Mouse
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« Reply #11 on: May 06, 2006, 02:48:16 AM »
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I think the answer is what the answer has always been. The photographer needs to learn how to expose correctly for the equipment and the situation. Machines cannot make subjective decisions which is what good exposures are based on in complex situations. No silver bullet here, I am afraid.

I am not sure a RAW histogram has any practical value as it would be difficult to evaluate unless you convert it first (which is what RAW processing previews do anyway). Also displaying RAW data is not possible either - which is why a thumbnail file must be created so you can preview the image on the camera. Even if you could display the RAW data, it would be hard to evaluate the image as it is not easy to equate it to the converted data (and forget about white balance).
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Jonathan Wienke
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« Reply #12 on: May 07, 2006, 05:43:05 PM »
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A RAW histogram would be trivially simple to implement. You wouldn't need to do any Bayer interpolation, white balance adjustments, or any color processing whatsoever. Simply create a bar graph with 32 segments from left to right (32 vertical bars going left to right) with each bar representing 1/3 stop of exposure. The rightmost bar should be red, and its height should represent the number of clipped pixels in the RAW data. The next two bars should be yellow, and their heights represent pixels that are within 1/3 stop of clipping, and between 2/3 and 1/3 of a stop from the clip value, respectively. The remaining bars should represent successively decreasing 1/3-stop exposure intervals, with the leftmost bar or three colored yellow to indicate the possibility of increased noise levels in those exposure levels. A simple lookup table in the camera firmware would suffice to indicate which bar any given RAW value should be assigned, to make the 1/3 stop per bar paradigm work properly. This would require far less firmware programming than the current converted RAW-to-JPEG bastardgram, work perfectly with any sensor regardless of whether it was Bayer-pattern or monochrome, and indicate to the user the exact exposure adjustment necessary to achieve ideal exposure with a single "polaroid" exposure test shot. What's not to like?
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bjanes
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« Reply #13 on: May 07, 2006, 09:40:22 PM »
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A RAW histogram would be trivially simple to implement. You wouldn't need to do any Bayer interpolation, white balance adjustments, or any color processing whatsoever.

What's not to like?
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I was thinking along the same lines. What Jonathin says is quite true, but I'm not certain how useful raw histograms would be.

On the Adobe Camera Raw forum I once suggested that a raw histogram would be useful, but Bruce Fraser said that it would not be very helpful because, with linear data, all of the data are to the left of the histogram. I wasn't convinced until I did some testing.

Here is a shot of a red flower in which the red channel appears blown as shown in the ACR preview with sRGB:

[attachment=548:attachment]

However, with ProPhotoRGB the red channel is no longer blown:

[attachment=549:attachment]

Here is the raw histogram from the DCRaw conversion:

[attachment=550:attachment]

The raw histogram shows that no channel is blown, but is indeed hard to interpret just like Bruce said it would be. Comments are welcome.
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BernardLanguillier
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« Reply #14 on: May 09, 2006, 04:08:33 AM »
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Even if we keep the current metering devices of the cameras, it would still be highly valuable to have some form of automatic expose to the right double exposure:

- first exposure is done based on camera metering device, in a way as to fully capture the non specular highlights,
- second exposure is over-exposed enough compared to the first one so as to get non clipped non specular highlights (based on true RAW histogram data) while fully using the available DR.

The problem is that

- companies like Canon and Nikon just won't release half cooked cumbersome functions, even if they were potentially very useful to knowledgeable users...
- the resulting images would be over-exposed, and would require the typical expossed the right images post-treatment -> many users would complain that the thing doesn't work well,
- ...

Cheers,
Bernard
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John Sheehy
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« Reply #15 on: May 22, 2006, 10:04:23 PM »
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The raw histogram shows that no channel is blown, but is indeed hard to interpret just like Bruce said it would be. Comments are welcome.

What is hard about it?  The last half is one stop, the quarter to the left of that is another stop; the eight to the left of that is another; the histogram could have dark lines for stops, and thinner lines for 1/3 stops.

That shot should have been taken with 1.3 stops more exposure with almost no clipping, and 2 stops with light clipping, even if it took a higher ISO to do so.

The problem, however, is that most RAW converters don't understand exposing to the right.  Setting the Exposure slider in ACR to -2, for example, does not necessarily divide the RAW data by 4, as you'd expect.  It may do that to the midtones, but the highlights might be divided by quite less, distorting the transfer curve of RAW to RGB.
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Serge Cashman
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« Reply #16 on: May 22, 2006, 10:36:12 PM »
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Somehow I get an impression you guys are talking about the exposure to the LEFT. "Just make sure highlights are not blown (an unrealistic demand in most cases, especially in the low light situations) and push everything else to the left".

Exposing to the right is supposed to take advantage of the fact that most of the bits in a RAW file are dedicated to the highlights... Which is unfortunate cause sensors don't really capture that many details in highlights in the first place, no matter how many bits are dedicated to them.

The idea of ETTR is to squeese as many details as possible into shadows by moving everything else to the right as far as possible, not to push the shadows out of the picture based on highlights (by moving everything else to the left).
« Last Edit: May 22, 2006, 10:50:39 PM by Serge Cashman » Logged
dlashier
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« Reply #17 on: May 22, 2006, 10:37:35 PM »
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The problem, however, is that most RAW converters don't understand exposing to the right.  Setting the Exposure slider in ACR to -2, for example, does not necessarily divide the RAW data by 4, as you'd expect.  It may do that to the midtones, but the highlights might be divided by quite less, distorting the transfer curve of RAW to RGB.

C1 does this quite well, at least with the 1 stop EC's I measured.. IIRC I did a similar test with ACR and it also did ok.

- DL
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Serge Cashman
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« Reply #18 on: May 22, 2006, 10:53:54 PM »
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As far as live histogram - you either need two separate sensors or not an SLR. Both of which exist already.
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John Sheehy
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« Reply #19 on: May 23, 2006, 07:31:34 AM »
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Exposing to the right is supposed to take advantage of the fact that most of the bits in a RAW file are dedicated to the highlights... Which is unfortunate cause sensors don't really capture that many details in highlights in the first place, no matter how many bits are dedicated to them.

What are you basing that on?

The sensors capture highlight the best, right up to the saturation point at the lowest ISO, or RAW clipping point if that is lower, and at the higher ISOs.  Perhaps you are defining highlights as the areas that sometimes clip?  The whole point is getting the highewst exposure possible without clipping desired highlights.  Clipping them is not "exposing to the right" properly.
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