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Author Topic: Exposing to the left?  (Read 6269 times)

TrumpetPower!

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Re: Exposing to the left?
« Reply #15 on: February 24, 2013, 04:18:19 PM »
Sanj, the OP, didn't mention if he was judging exposure when using M or an exposure mode. If he is using an exposure mode and is getting underexposure it is because of my answer. If he is in manual and using the "sunny 16 rule" then it is almost certainly Canon's iso calibration, they are generally overoptimistic. The dreaded DxO actually covers that difference.

Even still, the question isn't between Canon and other manufacturers, but between Canon's two top-of-the-line models. And the more general discussion here has moved to a desire to figure out if the difference is in metering, actual exposure, or post-processing (including that post-processing which is automatic and normally invisible to the user).

That is: is it the meter, the sensor, the software, or some combination of the three that differs between the 5DIII and the 1DX?

Cheers,

b&

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Re: Exposing to the left?
« Reply #15 on: February 24, 2013, 04:18:19 PM »

bdunbar79

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Re: Exposing to the left?
« Reply #16 on: February 24, 2013, 04:20:08 PM »
My point was, even if you shoot a frame filling perfectly illuminated 18% grey card the histogram will show a hump 2/3rds stop below midtone.

All cameras will Expose To The Left, it is up to us, knowing whether we are shooting a black cat in a coal mine, or a white bird in a snowstorm how to adjust that meter reading, but even for a standard grey target we need to adjust EV compensation approx 2/3rds stop positive. This exposure would need no adjustment in post.

However this is still not ETTR, to ETTR an 18% grey card you would use plus 1 2/3rds EV compensation, then lower the exposure 1 stop in post.

You are right, but not my point.  My point is very, very simple and easy to see.  It doesn't take any photographic knowledge whatsoever to observe the effect I'm discussing.  1/200s, f/5, ISO 2000 is brighter on a 5D3 than 1/200s, f/5, ISO 2000 on a 1DX, by merely changing cameras on the tripod.

Go to 1/1250s, f/5, ISO 10,000 and now the 1DX file is brighter than the 5D3 file. 

You don't need any of those fancy programs or charts to see this.  Presenting this may not tell us WHY it is doing this, but it'll at least show that it is real, which to me makes the most sense to do first, since we haven't even posted any photos at all yet that demonstrate this effect.

Once you guys see what I am talking about, then I'd love to theorize why and what is happening.  I don't want to say that it is metering or sensor, or anything yet.
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East Wind Photography

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Re: Exposing to the left?
« Reply #17 on: February 24, 2013, 04:54:44 PM »
So I think I figured this out...The 1DX has more DR than any of the other models. Therefore it makes sense to expose toward the middle of the DR range.  On 5DIII and predecessors, most of the DR is on the upper(brighter) end of the DR range.  So to minimize noise they shift the exposure slightly to the + side to force the noise toward the lower end of the DR range.

I think its because the 1DX can record more DR and therefore better to have it expose to make use of it.

Yes, 1DX does and more so than the 5D3 and I noticed this on both bodies.  For instance, everything equal, at ISO 2000 the 5D3 image will be brighter than the 1DX image.  It's well-known among most of the 1DX users I know.  You can do one of two things.  You can either shoot more to the right, or "fix" it relative to cameras you are used to using in the past.  Page 325 of the Manual, under AE Microadjustment (top explanation) will tell you how to do this.  AE Microadjustment is in 1/8 increments, and automatic metering is currently being done to 0 EV.  If you'd like it to meter relative to 1/8 EV, go ahead and up it to 1/8.  I set one of mine to 5/8 EV as a test and didn't have to ETTR much anymore.  But I'm not used to that vs. past cameras, so I put mine back at 0, and chose option 1 as simply getting used to the metering and now I just shoot at +1 to +1 2/3 EV if I can.

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Re: Exposing to the left?
« Reply #18 on: February 24, 2013, 04:56:59 PM »
Perhaps by design???  Higher ISO means less DR so they shift the exposure up slightly to make the noise drop down to the lower end?

At equivalent settings and in the same exact situation, the 5D3 files are brighter than the 1DX files.

It'd be nice to know if the raw data in the files differ, or if the differences happen elsewhere.

Anybody with both cameras and suitable software (dcraw, Rawnalize, Raw Photo Processor, etc.) and a couple spare moments care to chime in?

And, just to be sure, this means ignoring what the meter says and setting the exact same lens / aperture / shutter / ISO on both cameras with the lighting held constant....

Cheers,

b&

Yes, very interesting thing.  I was hoping you would know exactly why this could be, but it's very strange.  Also though, this is the clincher:  Push to ISO 10,000 on both cameras, and now the reverse is true.  The 1DX file is brighter.  I'm interested again in this and I'd like to do some test shots before I draw conclusions.  I'm going to do them tonight and unfortunately I'll have to convert to jpg, but I'll post them and won't process them.  Thanks.

TrumpetPower!

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Re: Exposing to the left?
« Reply #19 on: February 24, 2013, 05:55:39 PM »
If the cameras are leapfrogging each other in exposure when used on the same settings the answer is the iso gain amplification used.

Probably...but we still don't know if the difference is in the raw data or if it's getting introduced elsewhere in the pipeline.

And we won't know that until we can examine raw files from each camera in the same scene with the same settings.

Just to be clear, these guesses all y'all're making are reasonable ones. But they're still guesses. Only examining the raw files can answer for certain.

b&

bdunbar79

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Re: Exposing to the left?
« Reply #20 on: February 24, 2013, 06:09:52 PM »
TrumpetPower, yes I can get you RAW files, no problem.
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Re: Exposing to the left?
« Reply #21 on: February 24, 2013, 07:38:46 PM »
Exposure compensation has always been a confusing subject to me so, I totally ignore the subject by using only manual mode via live view information.
Realizing that the camera's LCD monitor is a jpeg rendition of your image, I adjusted/neutralized all my "picture style" settings to the base line>
then insured that my histogram, and blinkie notification if available, was always present in live view mode>make your SS, f/stop, ISO settings contingent
 upon that histogram's position (you want it as close to the right edge as possible without going past that edge, this is called "exposing to the right").

It's then a simple matter to "plug-in" any 2 of the 3 image parameters, then move that histogram by using the third adjustment parameter and,
it guarantees proper exposure and completely negates all concerns about 18% gray camera readings, exposure conpensation and whatnot.

It continues to baffle me why this method is not being more widely used as it's really, necessary in my case, a no-brainer.

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Re: Exposing to the left?
« Reply #21 on: February 24, 2013, 07:38:46 PM »

bdunbar79

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Re: Exposing to the left?
« Reply #22 on: February 24, 2013, 08:47:01 PM »
Exactly, I want to also eliminate software.  If I upload both in Camera RAW or both in Lightroom, even though unlikely, I'd like to eliminate how they render based upon software.  I will shoot some shots and if you'd like RAW files, just PM me.  Thanks. 
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TheBadger

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Re: Exposing to the left?
« Reply #23 on: February 24, 2013, 09:07:56 PM »
Digital exposure is a complex topic, surrounded by lots of confusion and clouded by obfuscation.

The sensor records light in a linear fashion. For each pixel, for each exposure, a number is reported from 0 to 16,384 (typically; there are other possibilities). Each pixel has either a red, green, or blue filter in front of it and so each pixel records (mostly) only red, green, or blue photons. (The filters relatively wide, mimicking the sensitivity of the nerves in our eyes. A very few blue photons will make it through the green filter and even fewer through the red filter, and vice-versa. And blue-green photons will make it through both the blue and green filters, but not as many as blue will make it through blue and green through green.)

Exactly how many photons it takes to result in a pixel recording, say, the number 1126, will vary from sensor to sensor, but exactly twice as photons will cause the sensor to record 2252.

Further, prior to that number getting recorded, different amounts of electronic amplification gets applied to the signal before recording. The number of photons that causes the camera to record 2252 at ISO 100 will cause it to record 4505 at ISO 200, and 9008 at ISO 400.

Just as turning up the volume knob on your stereo causes more and more distortion, so does the electronic amplification of the camera's ISO setting. What in a perfect world would result in recording 9008 at ISO 400 might result in 8996 in one pixel and 9012 in a pixel next to it, both of which received the exact same number of photons.

Now, let's assume you're in a perfectly-controlled studio environment with a flat subject and even illumination. And, you're photographing a scene that includes a light trap (a hollow black-lined box with a small hole at the top), an 18% gray card, and a piece of Teflon thread tape (which reflects 99.9% of the light that hits it).

Your picture, if perfectly exposed, would record 0 for the light trap, 2^14 * 0.18 = 2949 for the gray card, and 16384 for the tape.

But.

Remember those filters? They really mess things up.

First, "white" light (which doesn't exist, but never mind) is a mixture of light of all frequencies, but not in equal proportions. Second, for very good technical reasons, your camera has as many green-sensitive pixels as it does red and blue pixels combined.

So, the perfect exposure now becomes one in which the brightest channel (which is virtually always the green channel) has those values indicated above and the other two channels fall where they do. Typically in daylight, the blue channel will be about 2/3 stop underexposed relative to green, and red will be just over a stop underexposed relative to green. Those ratios shift depending on the light source, and figuring out exactly what those ratios are is what white balance is all about.

So, let's say that we nailed the in-camera exposure and the gray card came out at 2949 in the green channel. The red channel might have come out at 1475, and the blue at 1946. Presumably, the light trap still came out at 0 for all three channels, and the thread tape would have come in at R=8192 G=16384 B=10912.

To get your perfectly white balanced perfect exposure, you'd double all the red values, leave the green values alone, and multiply all the blue values by 3/2 -- after which the red, green, and blue values for the three objects under discussion (all of which are different shades of gray) would be the same.

But.

What about the ball bearing in the scene? Much of it is just mirroring the rest of the scene, but there's that specular highlight that's reflecting the light source, and that part is a lot brighter than the thread tape. In our photo, all the parts of the ball bearing that are exactly as bright as the thread tape get recorded as R=8192 G=16384 B=10912, but the parts brighter...well, green can't get any brighter, so it stays at 16384, but let's say that R=10912 and B=14549. The camera is telling us that it's a lot more magenta than it really is. And an even brighter spot (but not the brightest spot) has all three channels maxed out at 16384. We're back to white, but it's telling us that this is the same white as the thread tape, which it clearly isn't. And what about this other part of the scene where we've turned up the brightness on our lighting?

...and that brings us to how digital exposure actually works in the real world.

Specifically, all camera meters on the market actually tell you to underexpose the scene. My 5DIII, for example, underexposes by about 2/3 stop. The raw processing software (both in-camera and Lightroom, etc.) then applies an equivalent amount of digital overexposure to compensate. You could make a spreadsheet of all the values recorded by the sensor, multiply by (roughly) 3/2, then multiply red and blue by whatever you need for white balance, and the numbers would (basically) match the numbers coming out of the JPEG.

What that does is give you an extra 2/3 of a stop of headroom for your highlights. It also means that something more sophisticated than a simple linear multiplication can preserve more visible detail in those highlights.

There's no such thing as a free lunch, of course. The worst noise is always in the shadows, and applying 2/3 stop of digital boost will make those shadows that much noisier. All things considered, though, those shadows are awfully clean these days, so the tradeoff of having a bit more headroom for highlights is well worth a bit of invisible noise in shadows.

There are some additional caveats. Virtually all RAW processors (including the one in the camera that makes JPEGs) apply all sorts of other modifications to the data before you see the image. Most significant is the gamma adjustment; the sensor records in linear gamma, but the rest of your workflow is set up for gamma 2.2. Linear gamma images displayed without proper adjustment look very dark and contrasty. Next most significant is color profiling, which is yet another can of worms. And then there's an s-curve generally applied for contrast and "pop," there's almost always some sort of "secret sauce" (the picture style) to give a certain "look," and then there's all the knobs that you can fiddle with yourself (contrast, saturation, shadow boost, highlight recovery, and the rest). The end result is a very complex mathematical transform applied to the data.

And because that transform is so complex, that leads to the most unfortunate factor of all. Applying post-exposure digital exposure adjustments anywhere other than to the initial RAW recording of the data is going to amplify and otherwise interact with all those other adjustments, and generally result in chaotically unpredictable behavior. Modest changes generally don't have much of a visible effect, but the results with more dramatic changes can be...well, more dramatic.

I don't know if Camera Raw / Lightroom apply their exposure adjustments (with that top slider) before or after all the rest of the calculations. I would hope they're smart enough to do it before, but I've long since given up on ACR for color-critical work. I know for certain, for obvious reasons, that Photoshop itself is going to do anything like that after everything else has been done.

So, my recommendation for a general-purpose workflow is to target your exposures to the same as what the camera's meter is set to; that's almost guaranteed to be the best compromise between preserving highlights and reducing shadow noise. That's not to say that you should blindly trust the camera's meter, of course; meters can easily be fooled. Rather, get to know how your meter works with a perfectly-lit gray card and try to achieve that level of exposure. How to do that in practice is the usual challenge of the photographer...perhaps you'll use an external meter calibrated to your camera's meter, perhaps you'll use Adams-style spot metering, perhaps you'll gauge the histogram, perhaps you'll judge it from the preview image, whatever. But the point is to learn how to get the camera to expose the scene the same way its meter would under ideal metering circumstances.

If you can do that, you'll get the best compromise of preserving highlights and reducing shadow noise that your camera is likely capable of, and you won't have to deal as much with all the mathematical funkiness that goes on behind the scenes with digital development. Your images will be basically right straight out of the camera.

In scenes where you're still blowing highlights that you care about and have excessive noise in the shadows, you should first strive to fix the light by any and all traditional photographic means -- wait for the Golden Hour or add fill flash or use scrims or reflectors or whatever. If that won't work, either HDR of some form or a larger format camera is your ticket.

With a particularly convoluted workflow in controlled environments, you can more intelligently apply the "ETTR" concepts...but, even then, you're generally best off adjusting exposure as I described above so that the green channel is right on the 1.0 gamma graph. How to do that is much more involved than is reasonable for this forum...it involves analyzing ICC profiles built from a linear UNIWB development of the RAW image, and software (such as Raw Photo Processor) that lets you specify the channel multipliers yourself....

Cheers,

b&

P.S. If you were to shoot, for example, a ColorChecker exposed exactly according to your camera's meter reading of a gray card, and it doesn't look almost identical to an idealized ColorChecker such as you can find at Bruce Lindbloom's site, then you've got a problem somewhere with your camera's meter. But if both your photo of the ColorChecker and Bruce's look too dark, then it's much more likely that your monitor's brightness is off, or that something other than the camera is to blame. b&

Very nice comment TrumpetPower! I learned a lot today!
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bdunbar79

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Re: Exposing to the left?
« Reply #24 on: February 24, 2013, 09:47:22 PM »
I'm getting my cameras out and getting ready to do the experiment.  Any suggestions or anything you guys want me to do?  Thanks.  TrumpetPower, where shall I send you RAW files?  PM address?  Thanks.
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TrumpetPower!

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Re: Exposing to the left?
« Reply #25 on: February 24, 2013, 09:55:24 PM »
I'm getting my cameras out and getting ready to do the experiment.  Any suggestions or anything you guys want me to do?  Thanks.  TrumpetPower, where shall I send you RAW files?  PM address?  Thanks.

The biggest thing I'd want, if possible, is a ColorChecker Passport included in the shot (ideally flat and square to the camera, but a bit of perspective is okay).

Unless you're using flash, it'd also be nice to know what the meters think you should be shooting at. (I'm assuming, of course, that you'll use the same actual settings in full manual mode for both regardless of meter readings.)

I think you should be okay emailing them to me at ben@trumpetpower.com. It certainly won't hurt anything...I just don't know if it'll make it through.

Otherwise, if you have access to any sort of file sharing service (iDisk used to work for me before Apple shut it down), let me know the link....

Have fun!

b&

bdunbar79

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Re: Exposing to the left?
« Reply #26 on: February 24, 2013, 10:00:47 PM »
I'm getting my cameras out and getting ready to do the experiment.  Any suggestions or anything you guys want me to do?  Thanks.  TrumpetPower, where shall I send you RAW files?  PM address?  Thanks.

The biggest thing I'd want, if possible, is a ColorChecker Passport included in the shot (ideally flat and square to the camera, but a bit of perspective is okay).

Unless you're using flash, it'd also be nice to know what the meters think you should be shooting at. (I'm assuming, of course, that you'll use the same actual settings in full manual mode for both regardless of meter readings.)

I think you should be okay emailing them to me at ben@trumpetpower.com. It certainly won't hurt anything...I just don't know if it'll make it through.

Otherwise, if you have access to any sort of file sharing service (iDisk used to work for me before Apple shut it down), let me know the link....

Have fun!

b&

Got it.  I'd also like to do some ambient outdoor light shots tomorrow too.  I'll do what I can with the RAW files.  If they get too big I can get semi-accurate RAW results by using mRAW or sRAW, or crop the exact same, althought now we're dealing with different pixels (which I guess we would be anyways) but I'd rather not have any processing at all whatsoever.  Thanks.
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bdunbar79

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Re: Exposing to the left?
« Reply #27 on: February 24, 2013, 10:02:25 PM »
I'll write down what each camera thinks I should be shooting.  For instance, if I begin with the 1DX, and I purposefully set it to 0EV, I'll use the same metering and settings and see what the EC is on the 5D3, if any. 
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Re: Exposing to the left?
« Reply #27 on: February 24, 2013, 10:02:25 PM »

Quasimodo

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Re: Exposing to the left?
« Reply #28 on: February 25, 2013, 01:05:32 AM »
This thread and its like is one of the reasons why I keep coming back here. I am learning tons.

I have a question though (as mentioned in some previous posts here where you explain that you switched cameras and they expose differently). Did you use the same lens? I would assume that while the camerasensor, metering system, DR and so forth on one side, the reflective properties of the material (i.e. the degree to which the subject you shoot sends back light), and the lens which is also a medium that light have to pass through would alter the process?
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TrumpetPower!

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Re: Exposing to the left?
« Reply #29 on: February 25, 2013, 08:30:08 AM »
I'll do what I can with the RAW files.  If they get too big I can get semi-accurate RAW results by using mRAW or sRAW, or crop the exact same, althought now we're dealing with different pixels (which I guess we would be anyways) but I'd rather not have any processing at all whatsoever.  Thanks.

I could probably work with mRAW or sRAW, but I don't know how much onboard processing is done to the original data before writing out the files.

Once you've cropped, you've already done all the white balancing and demosaicing and what-not, so it's very much no longer raw.

I'm sure we'll figure out a way to transfer the raw files, so don't sweat about alternatives.

This thread and its like is one of the reasons why I keep coming back here. I am learning tons.

I have a question though (as mentioned in some previous posts here where you explain that you switched cameras and they expose differently). Did you use the same lens? I would assume that while the camerasensor, metering system, DR and so forth on one side, the reflective properties of the material (i.e. the degree to which the subject you shoot sends back light), and the lens which is also a medium that light have to pass through would alter the process?

This is a very good point. It's why lenses are calibrated in T-stops rather than F-stops over in the cinema world.

Ideal would be to use a lens with a tripod collar. Leave the lens mounted to the tripod and swap bodies.

Cheers,

b&

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Re: Exposing to the left?
« Reply #29 on: February 25, 2013, 08:30:08 AM »