Further Experiments with the Nikon D600's Exposure

This post is a follow up to my earlier post about experiments on the Nikon D600's exposure.  In that post my observation was that the Nikon D600 appeared to have a tendency to not give importance to highlights.  For some shooters, the resulting image looks fine.  For me, I like to protect highlights and there were way too many blown highlights even with ADL (i.e. the exposure is too "aggressive").

I posted about this observation on DPReview.  Here is the thread.  I received many helpful suggestions.  I tried out the suggestions, and I will discuss the results in this post, learning a few things in the process.



METERING METHOD
One suggestion is that I ought to meter a midtone then check the highlights.  That means I find an object in the scene that is in Zone V (middle gray tone, such as grass and dark-toned skin).  I spot meter that object and use manual exposure to adjust the exposure to 0.  Alternatively, in matrix mode, I focus on that object using single point AF.  The camera will attempt to render that object as a middle gray tone.

If a middle gray object is rendered as middle gray then the exposure will indeed be accurate for that object, and every other object in the scene will also fall into the right place.  However, sometimes the scene's dynamic range is outside the range of the sensor, so to avoid losing highlights, I should also spot meter any relevant highlights in the scene to see how far above Zone V they are, and adjust exposure downward as necessary to protect the highlight.  (If I do this, I need to alter the tone curve in post-processing to bring the midtones back up.)

If this method works for you, then it is a fairly accurate method and you don't need to read any further.  In my case, I usually shoot candid portraits of my family (wife and two kids).  They don't usually stay put in one place, making this method hard for me to implement.

PERILS OF AUTO AF
In the previous experiment, I used Auto-Area AF.  That means the camera automatically selects the object to focus on.  The potential problem with this is that the camera may choose an object that is dark-toned or bright-toned and the matrix metering would be influenced by the tone of that object.  So, blindly trusting the Auto AF may result in an unexpected exposure.

However, in my case, when I use Auto AF I don't just accept whatever AF point the camera gives me.  Rather, I have an area that I want to focus on, even before I half-press the shutter.  When I half-press the shutter and the camera chooses the area to focus on, I see whether it is what I intended or not.  If it is as I intended, then I go ahead and shoot.  If it is not as I intended, I refocus.  I do that until the camera chooses the area I want, not some random area.  I find that, with practice, this method can be faster than single point AF.  I will post about that next time.

For purposes of this experiment, the fact that I used Auto AF as opposed to single point AF does not have an impact because I made sure the camera chose an AF point that would be the same as if I had chosen single point AF anyway.  It's just a different way to get to the same end result.

AF POINT POSITION
Another suggestion was that the reason the previous experiment showed exposure is because the AF point was positioned on the black Ferrari, which is of course a dark-toned subject.  It is well known that Nikon's Matrix Metering takes into account the entire scene but gives more weight to the object that is focused upon (because it is likely to be the subject).  In the case of the Ferrari, the AF point was indeed on the car, as I intended.  Here it is (the AF points for the other shots were similar).

One explanation to the overexposure is that because the Ferrari is dark-toned and the AF points were on the Ferrari, therefore the camera tried to make the Ferrari a midtone instead of a shadow.

I tried to take a shot of a scene yesterday to see how much the scene metering would be influenced my the selection of the object to be in focus.

I tried a yellow pipe in shade...

dark green bushes in shade...

a sunlit gray wall (I focused and recomposed a bit because the AF point couldn't reach it)

I chose Program exposure.  The exposure was the same in each case:  f/7.1, 1/200, ISO 100.  This was unexpected for me.  I thought the exposure would be influenced by the focal point, so I ran more tests.  This time I used a Fotodiox Trifold Reflector as a target. It has a gray card, black card, and white card (as well as silver reflectors on the other side).

I first ran the test with just the trifold reflector, but the camera couldn't focus on the black or gray sides.  I added a black leather jacket and a white undershirt.  With the AF Point on the gray card, the exposure was 1600 ISO, f/5.6, 1/30:

With the AF Point on the black leather jacket, the exposure was still 1600 ISO, f/5.6, 1/30:

However, with the AF Point on the white shirt, the exposure did change 1 stop to 1600 ISO, f/5.6, 1/60:

Looking back at these tests, I did not observe a change in exposure when the AF point was on a dark-toned object (doesn't mean it doesn't ever happen; just that I didn't observe it on these tests).  I did observe a change in exposure when the AF point was on a light-toned object.  Therefore we ought to pay attention to the AF point location because it may affect the exposure.

As an aside, in case you're wondering, what if instead of autofocusing, I just used manual focus override (I was using the 24-70 with M/A mode) but kept the AF point over the black or white card?  I did try that and in that case: 1) the camera did not record the AF point - as though I had used manual exposure; 2) there was no change in exposure no matter where the overriden AF point was.

TRYING OUT THE SUGGESTIONS
So anyway I had taken more shots yesterday, this time using some of the suggestions from the DPReview forum.  My subject was my son, whose complexion probably falls into Zone V and I used single AF point.  Here are three test shots where I was able to focus on a midtoned subject.  And a screen capture showing the clipped highlights.

Test shot 1:

Test shot 2:

Test shot 3:

As we can see there are still plenty of clipped highlights.  In the previous post where the subject was a black Ferrari, one hypothesis was that the exposure was weighted toward the black Ferrari, which fooled the camera into overexposure, thus the abundance of blown highlights.  In these examples, the focal point was on a midtone, but there are still many blown highlights.

The exposures above are not necessarily wrong. Like I said in the last post, if you don't postprocess at all, the images look fine (lacking in contrast because I intentionally chose the Neutral picture style, but the exposure of the subject is more or less correct).

However, it appears that the camera doesn't care much about protecting highlights.  That is a concern for shooters like me who postprocess their images and who care about highlights (some shooters don't).  For shooters like me, I would describe this kind of exposure as "aggressive."

NOT ALL IS LOST
When I looked at the images above and their clipped highlights, it seemed disappointing because of the amount of clipping, even in relevant areas like the skin (the red channel was clipped).  But what I found is that the clipping display is not accurate and that there is indeed more recoverable information than is shown by the clipping display.

Sample 1:

Clipped highlights:

But if instead of highlight recovery I use digital exposure compensation, here's what I get (in this case -1 EC).

No clipped highlights at all.

And what's strange is if I undo the digital exposure compensation, here are the clipped highlights:

Sample 2:

Clipped highlights:

Digital exposure comp -2 [barely any clipped highlights :) ]:

Undoing the digital exposure compensation:

I found out that what ViewNX2 was doing was to take the initial display of the clipped highlights and histogram from the JPEG image (!).  Here's the JPEG's clipped highlights - identical to that of the initial display of clipped highlights and histogram:

Sample 3:

Initial clipped highlights from Raw:

Clipped highlights from JPEG for comparison:

With -2EV exposure compensation applied - virtually no clipped highlights.

Exposure compensation undone.

So, after all, the D600 wasn't losing anywhere near as much info as I previously thought.  Indeed, it was great that the camera appeared to be maximizing the amount of light, short of blowing relevant highlights, i.e., exposing to the right.  This could be awesome for minimizing noise.

However, in low-contrast scenes, it appears that the histogram is not biased to the right.  In the test shot below, you can see that the camera placed the tones toward the left side of the histogram even though there was plenty of room for highlights.

To my eyes, this was close to the actual amount of light in the scene.  If the camera were exposing to the right, then the histogram would be bunched toward the right side but it is not.  So, rather than ETTR, I think the camera is attempting to get the 'correct' exposure of the subject.


In high-contrast situations, the D600's attempt to get the "correct" exposure of the subject may result in the loss of some highlights (even with Active D-Lighting).  Nonetheless, you could say that the D600's exposure is correct in the sense that the subject will be "correctly" exposed.  For shooters with that perspective, the D600's metering algorithm is perfectly fine.


AN ALTERNATIVE APPROACH


On the other hand, because I typically postprocess shots, then my preferred type of exposure is the one that maximizes the amount of information on the sensor.  For shooters like me, you need to be aware that it appears the D600 will prefer to get the "correct" exposure of the subject rather than try to fit the dynamic range of the scene within the sensor's range.  Fortunately, the D600 appears to have more highlight range than was previously apparent, and therefore it is actually able to preserve much of the highlights.

However, if I truly want to fit the scene's range into the sensor's range, a more conservative method would be to just avoid blown highlights, and let the rest of the tones fall where they may.


Here is a shot with the D600's automatic exposure:

Here are the clipped highlights after -2EV exposure compensation applied in ViewNX2:

Here is another shot of the same scene.  This one had -1.3EV dialed in at the time of shooting.

Here are the clipped highlights - without any digital exposure compensation applied.

With further exposure compensation in post, all but the brightest specular highlights were recovered, i.e., the scene's range was able to fit in the sensor's range.

I could even reduce the exposure compensation adjustment and still retain every relevant detail.  This is at -1.5 exposure comp:

Using this method in a low-contrast scene, the image would appear overexposed but that is fine because the sensor was exposed to the maximum amount of light possible.  Bringing down the exposure would result in a lower-noise image than a normally-exposed image.  In a high-contrast scene, this method may make the subject look underexposed but the D600 appears to have exceptional shadow recovery abilities, therefore bringing the exposure back up would not be difficult for a D600 image.  I would also prefer to have a little more noise than to lose highlights that would otherwise be within the sensor's range.

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Experiments on Nikon D600 Exposure

UPDATE:  I posted about this on DPReview, got some helpful responses suggesting that there are issues with my shooting method.  That DPReview thread is here if you want to check it out.  I have done follow up experiments that are inconclusive as to the advice given in that thread.  The D600 still appears to overexpose.  I will do a followup post about it.

UPDATE 2: 2nd set of experiments here

I've shot with a number of cameras and they all tend to have different exposure tendencies. For example the Pentax K100D and Fuji S5 attempt to avoid blowing highlights, often producing straight-out-of-the-camera (SOOC) images that appear underexposed.  The Nikon D80 doesn't seem to be very concerned with highlights and produces SOOC images where the subject is correctly exposed but the image often has blown highlights such as in the background. Then there are cameras such as the Nikon D70 and Nikon D3 which in my opinion have a balanced exposure (taking into account the limits of the sensor's dynamic range).

In this post, I took some test shots with the D600 to evaluate whether the D600 has a tendency to overexpose or underexpose. All shots were taken in raw and processed in View NX2.



TEST SHOT 1:
I took a shot of some backlit foliage below an overpass. It was sunny and bright outside. In aperture priority mode at 100 ISO, the camera chose an exposure of f/4, 1/100. Given how sunny it was I would have expected a much higher shutter speed, perhaps 1/800 or 1/1600 (it was quite sunny). Anyhow this is the resulting image, and its histogram.

In ViewNX2, I applied a highlight recovery of 100 to see how much I could avoid from blowing out. This screenshot shows the clipped highlights AFTER the highlight recovery.

As you can see the sky is totally blown. Sometimes that really can't be avoided especially with older digital sensors. But if you look at the histogram you'll see that in this case none of the shadows were clipped at all, so there was room to accommodate at least some of the highlights. This seems weird - almost as if the camera was exposing to protect the shadows rather than the highlights. That is the right thing to do when using negative film, but with digital it's quite the opposite - you ought to protect the highlights because digital sensors have tremendous latitude in the shadows but limited range in the highlights. When highlights are clipped they disappear suddenly, as though a hole were cut out of the picture (there's no gradient, unlike with negative film).

TEST SHOT 2:
I took a shot of a tree and some more foliage, under similarly sunny conditions. Again I used aperture priority. I was at 100 ISO, f/4.  The camera chose a shutter speed of 1/160. This was the result and
its histogram:

Look carefully and you can see the very tall spike at the highlight end. Uh oh. And here are the clipped highlights after 100 highlight recovery.

Well, at least the D600 is consistent. It looks like the camera again decided to just forget about the sky. As with the previous test shot, no shadows were clipped.

TEST SHOT 3:
I took a shot of this black Ferrari. It was in open shade, specifically in a covered parking lot but facing the sunlight just a few feet away. I used aperture priority ISO 100, f/4. The camera chose a shutter speed of 1/3 secs. (WTH?) Here is the result.

Check out that histogram. On the highlight side, a bunch of pixels are crowding apartment number 255,255,255. Meanwhile on the shadow side, it's pretty lonely. Let's check out the clipped highlights.

Whoooaaa! Really? That's just crazy and pretty hard to justify.  Applying 100 highlight recovery, let's see how much we can get back.

That's still a pretty big chunk to give up, in my opinion. I tried turning Active D-Lighting on. Active D-Lighting is supposed to preserve highlights by underexposing slightly, then brings up the underexposed midtones and shadows by tweaking the tone curve.  (Canon's version is called Highlight Tone Priority). The adjustment to the tone curve doesn't matter to raw shooters like me because we prefer to apply our own adjustments, but ADL is useful for influencing the automatic exposure to avoid blowing highlights.

I turned ADL on Auto. The shutter speed was still astonishingly slow so I bumped ISO to 1600 (note: this reduces dynamic range). Even at 1600 ISO the shutter speed was just 1/60. That seemed crazy to me --
it's not like it was night time... :-? Anyway, I took a shot. Here's the result.

And here are the clipped highlights.

Much improved but still, lots of clipping on the red and green channels (yellow is the intersection of red and green). With highlight recovery there were even fewer clipped portions:

The thing is, I don't like to use too much highlight recovery because at some point the image starts to look unnatural. I wanted to see if I could coax the D600 into preserving all highlights. I turned ADL to
Extra High. Here is the result.

And here are the clipped highlights with ADL at Extra High:

Still showing many clipped highlights.  Yes they are mostly just specular highlights but still, I expected that there would be none with ADL at Extra High.  Now with an older sensor I might have said, perhaps the dynamic range of the scene was just wider than what the sensor can handle.  But if you look at the shadow side of the histogram, it looks like there's still room to back off the exposure.  Anyway, here's the same shot, with highlight protection at 100:

Finally, in an effort to really stamp out any blown highlights, I tried just dialing down exposure compensation by -1.3 EV.  Here's the shot.

And here are the clipped highlights:

Even with no highlight recovery there are barely any clipped highlights.  Yes the image appears underexposed judging by the grayness of the white van, but I know I am going to adjust that in post-processing.  This is just the starting point for me, and I don't want to have any cutout holes in my canvas.

OBSERVATIONS
My hypothesis from these test shots is that the D600 chooses an exposure that attempts to get a correct exposure of the subject, without necessarily looking at the rest of the frame. In thinking about this, I guess it might be ok if you have no plans whatsoever to postprocess the shot, and all you want is a SOOC image that looks as good as possible without any editing.  I mean if you were to judge the Ferrari shots above, the -1.3EV shot looks underexposed, and the default shot arguably looks better if you don't look closely at the van's lost highlights.

For someone who processes his images as part of his normal workflow, this algorithm is not ideal. For me, the SOOC image is almost never the endpoint. It's just the beginning and I like an image that has as much available relevant information as possible.  I'm not going to try to capture EVERYthing.  I mean if the sun was in the shot I'm not planning to dial down the exposure til I see sunspots. :)  But generally, I would like to keep the details for everything that is relevant to the shot then I can adjust them to my preference during postprocessing.

What makes this tragic in the case of the D600 is that from earlier experiments, it appears to have tremendous shadow recovery abilities.  So, even if a shot is underexposed, it appears the D600 should be capable of correcting that underexposure without too much drama.  The D600's exposure behavior doesn't take advantage of that capability.

One factor to consider is that ViewNX2 is not that great compared to Lightroom 4. It's possible that there's actually recoverable detail in those areas in Lightroom 4 even though you can't recover them in ViewNX2, in which case the D600's exposure might be ok.

For now, though, the D600's exposure seems too aggressive for me.  I will tone it down by using the extra high or high ADL setting and I will be watching that histogram closely.

And if any of you readers have suggestions or corrections to my technique please chime in through the comments.  Or email me at info AT betterfamilyphotos.com.   Meanwhile I'll be working on part 2 of the review.

RELATED POSTS:
Nikon D600 Hands-On Review Part 1
Choosing the Exposure for Dynamic Range

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