News and Views from Dave Wilson

Archive for December, 2009

Musings on Dynamic Range, Bits and Stops

by on Dec.28, 2009, under Computer, Photography

I got into a conversation with Guy Tal, Jim Goldstein and Pete Carr on Twitter this afternoon and it quickly became clear that it wasn’t going to work in 140 character chunks so I reckoned a blog post would be in order. This is something I’ve been meaning to write about for a while now so I’m happy I now have an excuse.

The issue under discussion related to the relationship between the number of bits used to store a High Dynamic Range image and the light levels that the image can store. An HDR image is typically thought of as being represented with 32 bits per color component (32 bits of red, 32 bits of green and 32 bits of blue for every pixel). Some of the most common HDR file formats actually use less than 32 bits per pixel but, regardless, the question comes up about what each of those bits represents and how the number of bits dictates whether an image is “High Dynamic Range” or not.

I had always assumed that the dynamic range of an image depended to some extent on the absolute maximum light level that could be recorded. On reading Christian Bloch’s rather good “HDRI Handbook” last year, I was surprised, however, to read that the dynamic range is defined instead in terms of the ratio of the number of discrete values (2**32, for example if we’re talking about a 32 bit number) a measurement can represent divided by the smallest measurable difference (the change in the signal represented by 1 least significant bit). Nowhere in this calculation does any absolute value appear – the dynamic range is a ratio of the largest and smallest values that can be represented but it says nothing about the actual quantities that those values represent.

This didn’t seem intuitive to me. Where was the reference? How would you know how bright a given pixel would be? Thinking about this for a while, however, it became clear that I was forgetting to take into account the camera as a whole rather than merely the sensor.

The camera’s sensor has a fixed, maximum signal that it can record and a certain number of bits of resolution. For example, a 12-bit sensor can differentiate between 2**12 (or 4096) different levels of red, green and blue light. At some absolute level of light, the sensor saturates and outputs its maximum value. If you increase the light level falling on the sensor above this amount either by keeping the sensor exposed to the light for longer or by increasing the brightness of the light falling on the sensor, you get no new information and the signal stays saturated.

Thinking about this from a photography point of view, however, this is exactly as you would expect and corresponds to overexposure. In this case, we close the aperture, reducing the brightness falling on the sensor, or speed up the shutter to reduce the total amount of time the sensor is exposed to the light. These changes reduce the total amount of light falling on the sensor and allow us to take another, hopefully correctly exposed image. Although the maximum light level the sensor sees has dropped and it no longer saturates, it still records 4096 different levels falling on it. The recorded dynamic range is the same but we’ve shifted the recorded values so that all of the actual image brightness levels fall within the recording capabilities of the sensor.

This is exactly as you would expect in a film camera too – if the film is overexposed, it “saturates” to opaque and you can’t store any more information (there is the complication of logarithmic vs. linear response here but let’s gloss over that for now since it’s not really relevant to this discussion). In these cases, you reduce your exposure to get the amount of light hitting the film such that you don’t saturate the medium.

Considering things this way, it is now clear (to me at least) that the absolute value of light represented by the sensor’s (or film’s) maximum output value is irrelevant. The photographer adjusts the exposure to ensure that the brightest highlight in the image is at or just below the sensor’s saturation point and, hey presto, you end up taking maximum advantage of the sensor’s dynamic range. The absolute maximum light level that will cause the sensor to saturate is, therefore, related to the sensitivity and not the dynamic range. If the sensor saturates with very little light hitting it, we end up with a high sensitivity (or high ISO) sensor that allows us to record images in lower light than one which saturates at higher light levels.

So how does this tie back in to the normal photographic system of defining exposure in terms of stops or EVs? For every bit you add to a sensor, the number of values it can represent doubles and, as a result, its theoretical dynamic range will also double (forgetting about noise which reduces this somewhat). Thinking about exposure calculations, you know that increasing your exposure by a stop also doubles the amount of light hitting the sensor. There is, therefore, a direct correlation between 1 stop and 1 bit. Reducing your exposure by a stop divides the sensor output in two or shifts the value one bit to the right. Adding a stop to your exposure does the opposite, doubling the sensor output or shifting the value one place to the left.

Using the bit shifting idea, we can, therefore, get some idea of how much dynamic range an HDR image may have based upon how we recorded it. I typically use 3 images bracketed 2 stops apart when shooting HDRs. If the original scene contains very bright highlights or lots of dark shadow areas, I will use more brackets but 3 is usually enough. My Nikon D90 has a 12 bit sensor and, at low ISO values, has close to .12 bits or 4096 levels of dynamic range. Adding +2 stops to the exposure has the effect of multiplying the sensor output values by 2**2 or 4 or shifting them 2 bits to the left. Taking away 2 stops has the opposite effect, dividing the output values by 4 or shifting them 2 bits to the right. When I combine all three exposures, -2 stops, 0 stops, +2 stops, into an HDR image, therefore, the maximum dynamic range I could possibly record would be 12 + 2 + 2 bits or 16 bits of information. Even though I may save this image in a 32 bit file format, I’m not actually storing pixel values that extend from the lowest to the highest possible value in the file format. Regardless of the fact that I’m not using the full dynamic range of the recording system (the 32 bit pixel component representation), I still have 16 bits of information per colour component which is twice the number of bits I would have had if I had saved in JPEG and 4 bits (or 16 times) more than I would have got from a single RAW file from the camera. The dynamic range of my image is something like 65536:1 versus 256:1 for JPEG or 4096:1 for my 12 bit RAW format.

Confused? If so, leave a comment and I’ll try to clarify this a bit!

8 Comments :, , , , more...

Best of 2009

by on Dec.26, 2009, under Photography

As 2009 draws to a close, I thought it would be fun to follow last year’s example and produce a post containing my favourite images of the year. These are in no particular order since I like them all. I’ve included links to each image on my photoblog, ImageKind or ArtMuse just in case anyone feels moved to purchase a print 🙂

Overall, it’s been a good year in pretty much every respect. You can read about the non-photographic stuff in the annual Christmas newsletter but, camera-wise, I’ve added 9076 images to my Lightroom catalog (which is just over 24 images per day, a figure that I find amazing given how little time I think I spend actually taking pictures) exhibited in several galleries and shot images in Scotland, England and New York as well as in and around Austin. Anyway, here are my picks for 2009:

Chevron Tower, Houston, Texas


Chevron Tower, Houston, TX

This black-and-white HDR was taken early on a Sunday morning in March during a family weekend visit to Houston. The streets were deserted so I managed to get right into the middle of the road with the tripod as low as I could make it to get this shot of the building framed by the circular walkway above the junction. I knew I loved the composition but was extremely happy to find the cirrus clouds when I processed the image since these were not visible to me when I took it.

1915 Farmer, Johnson City, Texas


1915 Farmer

This is the earliest image in the set since it was taken on January 3rd during a photo trip I took to Johnson City with 4 or 5 local photography friends. The Sauer-Beckmann Living History Farm at LBJ Ranch was a dream location for me and I ended up with many shots I am happy with. I shot mostly HDR that day but this image was a single frame grabbed as the guide/smith/farmer looked out of the smithy building. The furnace was lit and the sun’s rays shining through the smoke were fabulous. In post processing, I wanted to bring out the rays as much as I could and ended up with this almost-monochrome, high contrast version of the image.

Waiting on Congress Bridge, Austin, Texas


Waiting on Congress Bridge

Frequently, I go out to try to shoot something and end up coming home with something else entirely. This is a great example – I spent an evening downtown with 3 fellow photographers from work and we intended shooting pictures of the bats emerging from underneath Congress Avenue Bridge. In the end, the bats left early for some reason but I ended up with one of my favourite shots of the year. I love the minimalist composition and the graduated colour in this image. The colour is pretty much as it was on the evening – I warmed the image very slightly – and the only real retouching I did here was to remove some electrical cables that were visible underneath the bridge.

Tire Sculpture, Austin, Texas


Tire Sculpture, Austin

I’m including this image as a representative of the shots I took during the 2009 Scott Kelby World Wide Photo Walk in July. This was a really enjoyable morning during which I got to meet a lot of new photographers and had a lot of fun taking a lot of pictures. This particular image won the Austin competition and ended up 7th in the world-wide “People’s Choice” voting which made me rather proud given that the event attracted somewhere between 35,000 and 40,000 photographers in 900+ cities.

Saturn V Second Stage, Houston, Texas


Saturn V, Second Stage

This image and the one following were taken about 10 minutes apart during a tour of Johnson Space Center in Houston during a Cub Scout camp (yes, we got to camp in the Space Center!) back in September. I had been hoping to get these shots while there and lugged a tripod around just in case I would be allowed to use it. In the end no-one questioned why I was carrying all this gear while shepherding a bunch of 9 year-olds and I managed to get several good images in the 20 minutes or so we had at the Saturn V. This image actually took a lot longer to capture than the one which follows since I had to wait quite some time for the field of view to be clear of boys climbing on the guardrail right in front of me!

F-1 Engines – Saturn V, Houston, Texas


F-1 Engines, Saturn V

This image probably ties as my favourite of the year (with the Chevron tower image above). I was amazed at the intricacy of the construction of the F-1 engine nozzles. As well as channeling the rocket thrust, they are also heat exchangers and are covered in fine plumbing work. The insides also contain a lot more fine texture that I had expected. To capture and highlight this, I turned the image into a monochrome and tweaked it to accentuate the tones inside the nozzles. I also rotated and cropped it to create a more dramatic effect. If you are interested to compare this with the original colour HDR version, you can find it here.

Flatiron Building, New York


Flatiron Building, New York

I’ve heard seasoned professional photographers state that the difference between an amateur photographer and a professional is that amateurs get good shots by “happy accidents” whereas professionals know what they want before they start and make the image by design. I actually think that this has nothing at all to do with professional/amateur status but it does define the difference between a good photographer and a less good one (I’m feeling too charitable to say “bad” here but you get the idea).

So what does this have to do with this image?

In late October, Nikki and I took a trip to New York during which I had 2 full days to shoot in Manhattan. I love wandering around cities and was especially looking forward to New York since I had not been there for about 12 years and had not done any serious photography there for over 20 (last time I was just out of university and shooting a lot of Fujichrome 100 and Ilford FP4 on a couple of Minolta X-700s). I was, therefore, very excited to go back with digital gear and the prospect of capturing some nice architectural HDRs. Right at the top of the list was this particular image.

Although I spent most of the time wandering, the shot at the top of my “must take” list was an image of the Flatiron building showing architectural detail. I had seen lots of wide-angle shots of the building but very few close ups. I planned to shoot this with a particular lens from a particular position, I went there, shot the frames and produced the image I had planned to get.

Several other New York shots I took also fall into this category (Lower Manhattan panorama from Brooklyn, dusk skyline from the Rockefeller Center, Brooklyn Bridge supports, Empire State building) but this is the one I am most happy with.

Annabel


Annabel

One of my New Year resolutions for 2009 was to take more pictures with people in them. I’ve been practicing hard, mostly on my family and have definitely improved in this area. I’m getting better at using flash and have actually taken some portraits that I’m pretty happy with (by my standards). This picture of my young niece is one that I am especially proud of. It was taken with a 50mm f1.4 lens at a very wide aperture, the idea being to get her eyes super-sharp and everything else heading out of focus. Lighting was with a single flash pointed directly upwards to bounce off the white ceiling of the room. I processed it to give a high-key result which highlights her eyes and hair.

SECC Walkway, Glasgow


SECC Walkway, Glasgow

This walkway stretches from “Exhibition Centre” station across to the Scottish Exhibition Center complex. I love the strong graphical elements in the composition and was really fortunate to be walking behind someone wearing a blue jacket giving me red, green and blue in the image. I took lots of photographs in Glasgow that morning, many of which are favourites, but this is my choice for the “Best of 2009” set.

17 Comments more...

Christmas Cards (or not)

by on Dec.23, 2009, under Photography

I failed miserably this year and ended up not sending any Christmas cards at all. Believe me, it’s not that I don’t love all my friends, it’s just that things were pretty busy and something had to slip. Hopefully folks will forgive me and not remove me from the list immediately.

I’m actually not a huge fan of receiving Christmas cards as such. I do, however, eagerly await the newsletters that many of my friends include. Being so far away and having so little contact with my British friends, it’s great to have a summary of what they have been up to over the last year.

For the last few years, I’ve been writing a Christmas newsletter in the form of a web page. I thought about just posting it onto the blog this year but, for consistency (I’m building up quite a nice set of matching URLs), I’ve kept it as a static HTML page again this year. Take a look if you are interested in a summary of the Wilson/Loftin ongoings for the last 12 months.

It’s time to go and have some egg nog (or something a bit stronger) before bed. I hope everyone on my unused Christmas card list had a great 2009 and has a wonderful Christmas this year and a happy, safe and prosperous 2010!

Leave a Comment more...

PhotoNetCast #37

by on Dec.12, 2009, under Photography

I just realised that I forgot to mention that PhotoNetCast #37 is now out. I was invited to be a guest on the show again and we discussed some of the basics of landscape photography.

You can pick up the podcast on iTunes or listen to it directly on the PhotoNetCast site. Apologies if I happen to sound groggy or more incoherent than usual – the participants are in 4 different time zones and I drew the short straw and ended up taking part between 12am and 1:30am local time.

Leave a Comment :, , , more...

Book Release Party at the Driskill Hotel

by on Dec.12, 2009, under Photography

Anyone who has searched the web for reat travel photography photoblogs has almost-certainly bumped into StuckInCustoms, the rather outstanding blog of local Austin photographer Trey Ratcliff. Trey’s images and writing have inspired many people, myself included, to try their hand at a technique known as High Dynamic Range (or HDR) photography and many of the images you see on this blog and my photoblog were created using this technique.

So what has this to do with an arty picture of the Driskill Hotel Christmas tree, you are probably asking?

On Wednesday evening this week, the Driskill was the venue for the launch party for Trey’s first photography book, “A World In HDR” and many local HDR aficionados gathered there to congratulate Trey on the publication and spend some time enjoying an evening of photo-geekery.

As part of the festivities, the Driskill was also holding a photography contest with the best image shot between 6pm and 8pm that evening and submitted by Monday evening being awarded a $500 gift certificate and used for next year’s hotel Christmas publicity. You can see the entries over here on Facebook.

Despite the fact that this was a rather HDR-centric gathering, one of the images that I like best from the evening is this one which was created by zooming during a long exposure. I did, however, take lots of HDR images inside the hotel and you can find these among the collection in the Facebook album linked above.

1 Comment more...

Photography Showcase on ArtMuse

by on Dec.10, 2009, under Photography

I was delighted recently to be invited to submit images for a showcase on gallery site ArtMuse. This new site offers exclusive, limited edition prints of images from selected artists and I have 6 images on sale just now. They span 25 years of photography (though, admittedly, all but one were taken in the last 4 years) and cover a variety of styles and subjects. Prices start at $25 and all prints are archival quality (as you would expect).

You can find the showcase here and I’ve added the images below – click on each for a larger version then head over to ArtMuse and buy lots of prints as Christmas presents :-).


Cowboy Boots (Exhibition Crop)

Agua Through The Archway Saturn V, Second Stage

Hamilton Pool Aug 2007 HDR Rain Stopped Play

Texas State Capitol Rotunda

Leave a Comment more...

Punjabi Jingle Bells

by on Dec.06, 2009, under Miscellaneous

With Christmas preparations in full swing, I thought you may find this little video (a Nickelodeon promo, apparently) amusing. The kids loved it. Thanks to Luigi for the pointer.



Leave a Comment more...

Looking for something?

Use the form below to search the site:

Still not finding what you're looking for? Drop a comment on a post or contact us so we can take care of it!