Sunday, January 23, 2011

Too Good to Be True?

If you came here by way my flickr D7000 13.5 Stops!! image you have seen that I added (12 Stops Maybe?) to the title. So what is the true scoop?  Bottom line--Old Scrib messed up a bit. About half way messed up.

The part about being able to pull out a very noisy image  even when you shoot 13 stops underexposure is true. I just didn't prove that using the screen shot I posted. Those images were, as I originally suspected and talked about in the last post, a little too good to be true. 

Here is my take number two on the Dynamic Range question.  How do I know this experiment is more correct?  If I had halved  my exposure one more time for the last post I would have discovered  that the CMOS sensor and A/D in the D7000 doesn't mess around. When it drops over the no data to digitize cliff it doesn't produce a very noisy black image.  Instead the image stays 0,0,0 no matter how much I tried to brighten it.

Picking the exposure where the brightest part of the image is just about to bloom and then counting down stops is subject to experiment error.  Counting up from total black is better. And since I did it both ways this time I'm more sure of my exposures.

The image below is 9 stops under exposed.  A corner of the white paper by the rack started to blink using ACR and a 4 second exposure. With a 1/4000 exposure  that image was totally black. The D7000 was set to take 14bit uncompressed RAW. The IS0 was set to 100, the only real ISO of the camera since any other setting only amplifies the ISO 100 image.
Cloudy, winter afternoon lighting.  The drapes on the windows were 80% closed.  When I spot metered the scene the strip of wall had 1/4 the lighting of the paper, the books in the closet had 1/16 the lighting.

The image is a bit noisy but remember this image was taken with 1/500 of the light needed for a "correct" exposure.  And despite the noise, it has enough color information for my WB eyedropper to do its thing and show accurate colors.

Why start off by showing a 9 stop underexposed image?  "Nine point two stops". That's the dynamic range number dpreview claims in their review of the D7000. 

Sorry dpreview folks, I beg to differ. My D7000 has much more DR than 9.2 stops. If you don't believe me take a 9 stop underexposed picture with your D7000 .

Since I am taking on the most visited photography review site on the web I'll start off by asking a question. In photography what does dynamic range  mean?

A whole bunch of different things I discovered. I spent a good part of a night in various forums, reading the true facts, half true facts, and down right silly facts that came up ever time  a newbie asked a question about dynamic range. It was sort of like asking about 'freedom and the American way." in the political forums. Folks who hang out in Internet forums are not afraid to defend their opinions--be they be right, wrong or maybe.

My first hit in a google search got me this curve. It is the film characteristic curve from an X-ray education site.  Since doctors study negatives not prints, the white to black scale is reversed from what we would see in a print.  With a print the light area labeled Base + Fog Density determines what is visible in the shadows just as the dark Shoulder does for the highlights

Back in the 19th century when a photographer took off his lens cap and counted his pulse beats to set his exposure. Mr. Hurter and Mr. Driffield of D&H curve fame (the old name for this curve) decided to work out a scientific explanation of exposure. At first they expected that if they doubled the light on their film they would double the darkening, but soon discovered things were more complicated. Film exposure is not linear. It is logarithmic.

The x axis is the f-stops. The y axis is the optical density.  Place a negative in an enlarger and at density 1 a tenth of the light hits the print paper. Similarly  at density 2 it's a hundredth and at density 3 a thousandth of the light.  

The straight part of the curve shows the useful exposure or dynamic range of the film. The roll off at bottom is because the film stock isn't completely transparent and because of the chemical action of the developer on unexposed film.  The roll off at top is because at density 3 the developer stops precipitating silver onto the negative. 

With film dynamic range is set by real chemical and physical limitations. Every film type has its own characteristic curve and dynamic range. Back in the film days, dynamic range disputes were a bit boring. The best anyone could fight over was how much of the curve  film manufactures should use when they calculated numbers for their spec sheets

Here is a set of curves from dpreview's D7000 'dynamic range' widget. If it looks familiar it should.  Relative exposure on the x axis, 0-255 display values--log corrected by the monitor's gamma setting-- on the y axis. The high tech version of the D&H film characteristic curve.

So what is wrong?  If  Mr. Hurter and Mr. Driffield borrowed Mr Wells's time machine and took a time trip to Best Buy in search of a digital camera for their experiments they would discover their original guess about a linear curve was right.  Double the light on a pixel, double the photo electrons and the A/D output in the RAW image. At the bottom you run out of A/D levels or in cheaper cameras signal to noise, At the other end, the pixels saturate and everything in the neighborhood goes pure white,   If your camera curve isn't linear in the middle, your camera is broken.

So why the roll off? For many reasons, starting with how the human eye/brain system works and ending with what we expect to see in a photograph (think of how odd a wildly processed HDR images looks) camera manufactures added tone curves to their jpg firmware. They do their best to mimic how a film photograph looks.

With a cheap P&S you live with the tone curve the firmware engineer gave you. Pay a few more bucks and you start having choices, vivid over normal for instance. After paying a bunch of bucks for a D7000 I ended up with more tone curve combinations than I'm every going to test.While some of the in camera jpg manipulations look neat, I am a dye in the wool RAW photographer.  I make up my tone curves as I go along. That's what RAW is all about.

The dpreview widget for the D7000, labeled "Dynamic Range", graphs how 20 different tone curves and ISO settings changes a grey scale image. No more, no less. Moreover, this only effects jpgs straight out of the camera. 

So if the idea of ever using a RAW converter makes you cringe,  the 9.2 stops or less numbers may apply. But if your thing is, like mine, low light flash-less candid photograph, there is a lot more dynamic range to be had.

The 1/500 second, ISO100, f13 exposure 11 stops underexposed image.  The WB eyedropper didn't work well so I ran the saturation slider down to zero.  I'm going to go with this image, and say the with proper post processing you can pull 11 stops underexposed detail out of the deepest shadow in an image.

But to prove a point this is the 1/2000 sec 13 stop underexposed image. Sure there isn't much detail left but what did you expect. As for the color cast, I played with adjusting the channel histograms and this is the closest to white I got before I became bored.

Coming up. 

A  week from today is all goes as planned, I should be photographing dancers during the 2011 Celebrate Youth festival. Since I'll be in the same auditoriums I worked in last year, a real world dynamic range comparison between my D7000 and D60 sounds like a fun blog post.


  1. Good stuff, man, I really enjoy this "gearhead" stuff. The art can't happen till you get these details down. ;)

    1. This comment has been removed by the author.

    2. Thanks. Somehow I missed your comment but to cliche 'better late than never.'
      As for 'gearhead stuff' writing that out in a long blog makes me work out the details in my own head.


  2. The y axis is not corrected. It's the digital value of the pixel. If it was corrected, the shadows would be totally straight. If you put a gamma correction curve, such as the ITU709, sRGB ones, you will get the same curve in the shadows, but linear highlights.

    Dpreview measure the actual visible dynamic range on the image. A good looking image is set to about 8 stops. The camera could be 14 or any number. Negative has very high dynamic range, but you see it printed in very low dynamic range paper, or low dynamic range slides, or low dynamic range release prints if it's a movie.

  3. If you examine an accurate grey scale in decent light you will discover your eye has a dynamic range of about 6.5 stops. In poor light the dynamic range will drop to about 4 stops. It is a easily movable target, something the less photographically useful CIECAM02 sliders were designed to compensate for.

    The definition I used in this post is much harder to move--what is lowest illumination falling on the sensor that produces a recognizable image. Which for my D7000 is about 11 stops down. So while I don't disagree with your comment, I think we are talking about two different effects that happen to share the same name. Which unfortunate is not that unusual for digital photograph.