News and Views from Dave Wilson

Tag: HDR

Vote Request

by on Nov.19, 2010, under Photography

I have a picture in the running for the Grand Prize in Unified Color’s HDR photo contest. If you are so inclined, I would be delighted if you gave it your vote. The image is a monochrome HDR of the Chevron Tower in Houston and you can find the voting page here. Voting requires that you register on the site and you can vote once per email address registered.

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Looking for a Videographer

by on Nov.13, 2010, under Photography

A group of HDR photographers I am involved with on Facebook is trying to pull together a charity project and we’re looking for a skilled videographer who would be interested in helping us out. The plan is to produce a DVD containing examples of our work along with hints and tips on improving your HDR work and we need someone who can cut the whole thing together nicely and add a soundtrack. If you are experienced at that kind of work and interested in helping out, please leave a comment or drop me a line via the contact form.

People involved so far include (in no particular order) Scott Wyden Kivowitz, Oliver Fluck, Scott Frederick, Trey Ratcliff, Jacob Lucas, Pat O’Brien, Mike Olbinski, Phil Cohen, James Howe, Brian Matiash, Bob Lussier, David Nightingale, Jeff Revell, CJ Kern and Jacques Gudé.

We’ve not finalised on a charity that will benefit from any proceeds yet but the current front runner deals with providing clean water supplies to third-world villages. I’ll post more news as the project progresses.

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A Different Kind of Reshoot

by on Nov.01, 2010, under Photography

In the past, I’ve mentioned that I don’t mind reshooting old subjects if I missed something the first time or if I wasn’t completely happy with the technical execution – depth of fields wasn’t quite what I wanted or the framing could have been every so slightly better – but recently I’ve found myself doing a different type of reshooting. Since upgrading my camera and lenses this year, and since being so impressed with the quality improvement I’m getting with the new equipment and some changes in my processing workflow, I’ve been reshooting to get higher quality versions of existing photos. The difference in sharpness between the old Sigma 10-20mm and the Nikon 14-24mm is very noticeable and the 1 stop bracketing I’m now doing for my HDRs is definitely allowing me to create cleaner images that should print as large as I would ever want. Previously, I pretty much assumed that people would not want prints larger than 36″x24″ but I’ve handled orders for two 50″ prints in the last couple of months so I guess larger sizes are in demand after all.

I have several examples from last weekend’s Space Center Houston trip where I reshot exactly the same composition and I’ll post these later. For the time being, here’s an example where I was reshooting for both quality and composition. I love the detail in the new shot (on top) and the fact that it’s less obviously a mock-up since you can’t see quite as many plexiglass panels and “audio tour” reference numbers. Talking of size, I sent a quote to a customer in France a few months who wanted to use the bottom image printed 15 feet wide to cover an office wall. I wonder how the new version would look at that size? You can click through either image and access the original file to take a closer look.

Shuttle Cockpit, Space Center Houston

Shuttle Cockpit, Space Center Houston

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Photography Power Lunch

by on Sep.03, 2010, under Photography

I just wanted to give a shout out to some local HDR photographer buddies after a very enjoyable lunch at Chuy’s on Barton Springs Road yesterday. Austin has a thriving community of HDR enthusiasts and we manage to get together reasonably often to shoot (though I think it’s true to say that we’ve never managed to get all the “core crowd” together in one place yet – maybe this year?). Yesterday, though, it was purely social as Jim Nix invited us to get together for a lunch and geekfest. It was a great success and I’m sure we’ll do it again soon.

Yesterday’s attendees were:

You will find these guys and many others in my newly revamped blogroll over on the photoblog. By the way, if you’re new here, you may not realise that I post different material here and on the photoblog. The photoblog is strictly for image posting but I tend to include random articles and news here which I don’t have over there. Just to make life really difficult for myself, I also post different pictures to Flickr too so you may also want to take a look over there every now and then.

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HDR Presentation – March 18th in San Marcos

by on Mar.09, 2010, under Photography

I’m honoured to have been invited to speak at the Hill Country Photography Club in San Marcos at 7pm on March 18th. I’ll be talking about HDR, starting with a very small amount of theory then moving on to workflow and a show-and-tell of some of my favourite HDR images. You can find the meeting agenda here.

I’ve not visited the Hill Country club before due to a previous Thursday evening commitment and the fact that it’s a long drive from my work but I’m very much looking forward to meeting some more of the folks whose work I have been following from afar for several years. Take a look at their site and, assuming you live in the Austin/San Marcos area, I’m willing to bet that you recognise quite a few of the images in their gallery already. If you fly through ABIA, you are certain to have seen several.

The club is happy for visitors to attend so please come along and join the fun if you are in the area.

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The Austin HDR Mafia

by on Jan.21, 2010, under Photography

Austin Skyline

Practically everyone who reads this blog has heard of Austin photographer Trey Ratcliff, owner of the Stuck In Customs blog and author of the much publicised new book “A World In HDR“. What you may not know, however, is that there is quite a collection of other excellent HDR photographers in Austin. Here are a few of the guys that I enjoy shooting with and whose blogs (if they have a blog) I read regularly. In case anyone gets miffed, these are listed in alphabetical order by last name. I’ve linked to both their Flickr profiles and their blogs or Flickr streams.

If you are a fellow Austin HDR enthusiast, leave a comment with your contact details below.

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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!

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