Sunday, 24 February 2008

while HDRI is all the rage, I'd like to show a view from the negative

I'd like to discuss a new digital process called HDR a little bit and at the same time perhaps explore a part of digital photography people seem to have forgotten about ... film.

Yes, I said film then! You see the face of digital photography is changing so fast that its easy to forget that before there were digital cameras digital photography was really all about scanning film. Its only a few years ago when most people didn't even know what a digital camera was, and barely 10 slim years ago when 1 megapixel cameras were the 'gosh' at photokina. So to me digital is not only digital capture, but the whole process.

People who've only recently moved into digital photography can be found asking about if there are even ways to turn 'film' into digital images on forums like almost every day. Most people in photography are not so enthusiastic as to have been doing their own scanning of film, and it wasn't until digital cameras started becoming popular that digital photography started to boom. From this perspective its no surprise that many digital camera photographers (even if they were film camera photographers before) have no clear idea how film and digital compare.

Well, I guess that everyone now "knows" that digital cameras are better than film cameras. While this may be so in terms of resolution its definitely not in terms of capturing the brightness levels in a scene. This is where HDR comes in.

What's HDR? Well its High Dynamic Range photography, and is the newest development sweeping around in digital photography and allows photographers to capture more of the range of the scene than before to make a print that more or less looks just like what we've been used to in the last hundred or so years of photography.

Say what? "Ohh must be some kind of old fart who thinks film is better than digital" will be what most people think first up. But consider images like this taken by A Adams well over 50 years ago.

Image:Adams The Tetons and the Snake River.jpgEven on this poor www version of this print, there is detail in the shadows, detail in the sunlight clouds as well as spectral highlights around the river. You can see an enormous range of scene brightness here represented on the page.

One of the weaknesses of digital capture so far is its lack of ability to record a scene with a high brightness range. HDR is a technique to give digital photographers the ability to capture images like the one by Adams here.

Basically HDR (at the moment) requires photographers to take multiple images of the same scene (withoug moving the camera) so that you can then composite these in software, and produce an image which has as much scene brightness range as you can possibly desire. From this, you can then produce an image which compresses the brightness ranges into what can fit onto a paper print, just like what Adams did in the image above back in the 1940's.

Its actually quite time consuming and requires attention to detail to capture an then process multiple images of a scene to produce a single HDRI image. Just as stitching images together can get bigger images than your camera can make, overlaying and tone mapping multiple images can give you a bigger range of scene brightness. Its something most people are perhaps not willing to do, and so Christian Bloch in his book "The HDRI hanbook" has also presented ideas for futures developments in cameras to allow for better capture. One such idea presented in this book is "non linear sensors" or "logarithmic response" sensors.

Friends, wait no more! They are already here! These sensors capture scene brightness ranges over double what most digital camera sensor capture now. Better yet, they are providing image storage as part of their purchase price. No more swearing when your Hard Disk dies! You probably don't need to upgrade your camera, but you'll need to buy a special interface device to transfer the data to your computer (think of it as kind of like a external USB optical drive). Fuji, Kodak, Ilford and many other companies already making this right now.

Yep, its film

Taking pictures of life's scenery is really what most consumer digital cameras are for (as I think the professionals taking images for catalogs and brochures may just use different gear sometimes).

Since 1997 the digital cameras have improved, with every year seeming to get better and better. I've had excellent results from my digital cameras since my first decent digital (a Nikon coolpix 950) that I got in 2000.

With scene's like this one, the brightness range fits neatly into what the sensor can cope with. When you take your picture look at the 'preview' on the back, and most cameras will show you a smaller version of the picture with the "washed out parts" blinking as well as a histogram of the image.

The histogram of the above image looks like this. In case you don't know much about histograms have a look here, but basically a histogram shows a graph of number of pixels which correspond to a particular value. In this case there are a lot of dark blue ones (which is where that peak is) and not much in the very very black, or in the very very white.

This is pretty much a 'good exposure' for a digital camera. No blinking white bits on that image.

But it isn't always so, often there is just too much in the scene for the digital sensor to cope with. This is called "dynamic range" and sites like dpreview are starting to take this more carefully into consideration in their testing. Have a read of this review for their latest version of that.

Digital capture systems (digital cameras) have their limits in how much they can capture in highlight. This is often balanced however by how well they capture the darker range. By carefully exposing to avoid this wash out in the highlights (called clipping) you can often bring up the shadows enough to make the image still look amazingly good. There are of course limits.

These limits are what I want to discuss here. Unlike most film VS digital camera tests where outright resolution has been examined I'd like to show that negative film is your ticket to single click HDR.

I thought I'd take an impossibly difficult image, a lamp in my room with the bare bulb visible.

This image is the linear 'raw' scan of the film. I've included the histogram of the image inserted into the image. You can see that the blackest thing in the image is not at the blackest of the data, and that the whitest thing is not at the whitest of the image.

This means that there is no loss of tonality in the image, and no clipping.

I then took the same scene with a Canon 10D digital camera. From these images you can see that exposure did not avoid clipping. Interestingly I exposed the film image 2EV's brighter than the canon meaning I should have blown it away.

Two things are interesting from this:
  • the histograms are remarkably simmilar
  • the 10D image nearly looses the shadows but definately looses the highlights.
The image of the exposure for the 10D is also a "RAW" image, and so its expectable that some correction would be needed to get it to its optimal.

The same is true also for the negative. Looking back up at the image from the negative, far more shadow detail is apparent than in the the RAW image from the digital, what's going on here?

Well, this is where film has already achieved the "future" of digital sensors, the response of the film to light is already "non linear" as the HDR people are hoping for in digital.
So after playing with levels and black points to adjust these two images to be reasonably alike I'll present them below


shows delicate rendering of the mesh behind the tassels, but notice how the bulb just blows out the scene, and the tassels infront of it even suffer.


manages to hold the tonal ranges, and still not wipe out the tassels. Something about it just seems to 'glow bright' when you look at it.

So if you're photographing a high scene brightness and you're after something which captures effectively HDR in a single click, perhaps negative is worth a look? Certainly it will require some learning of the tool, but I think you'll see some advantages in negative still there :-)


I decided to have a go at this with some HDRI software (Photomatix) and assembled an image from 5 Canon RAW (.crw) images spaced 2EV apart. This is the assembled image, and has excellent rendering of tones even better shadow detail than the Film above.

But its not all perfect here, as looking at the details it seems that there are some left over artifacts from the "higher" EV images which are composted together.

So, it would seem that one will need to spend some significant time post processing this to get it near to what I could with the negative.

I feel that this means that while HDRI is a significant development, and shows the potentials for when digital can in one go capture an image. Until then (or if you simply don't want to spend the money on expensive digital gear) just keep using negative

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