Aeb

[Frans]

My first try at using Auto Exposure Bracketing (AEB).
Below low is the result of combining three photo take in raw format from my Canon 350D into one. The combined image has an increased dynamic range of at least 2 F stops or 4 times over each of the three originals. I tried using one of the free programs that does this automatically, Qtpfsgui, but was not able to get the results that I wanted with out lots of fiddling. I ended up using a Gimp to combine the three images. I believe that there is also a plugin for Photo Shop to automate the process.

First image is the three photos combined.
Second is -1 F stop.
Third is normal light setting.
Last is +1 F stop.

[drg]

The generic term for this type of combination of photos is HDR or High Dynamic Range photography. Yes, Adobe Photoshop CS2 has an automated HDR capability as well as some other programs .

They all work better with more images spread across as wide as possible range of exposures.

You've got the idea here, now just increase it. Exposure for highlights and for shadows usually should by themselves be 'bad' photos. They will be blown out in the exposures for shadows and will have underexposed areas in the images for highlights.

I 've found for Adobe Photoshop and CS2 HDR at least 5 images over a +/- 3 stop range is required to really make the tool work well. The aperture kept constant at say f8 and varying the shutter speed seems to work the best. That way the DOF doesn't change and cause edge effects during the combination process.

You have a good start here. Look forward to seeing more.

[MB1]

As an experiment I guess it is a start but there is no center of interest so it kind of fails as an image.

[Frans]

Hi Drg,
Thanks for your detailed reply and your explanation on HDR. There are a couple of items that you mentioned that you may be able to help me on. I have been using the AEB on my camera for the main reason that I won’t get any movement between shots. I have found that it is very time consuming trying to align multiple frames. Taking one shot and readjusting the shutter speed will inevitably cause repositioning of the camera on the tripod. I have thought about lugging a laptop around to take remotes shots but I think this is not very practical. The canon remote controls I have for both the 350D and the 5D don’t have does facilities. You may have some better suggestions? The Cannon 5D does have an extra stop of bracketing on either side that I have tried. In most cases +/- 2 stop seems to be too much and the images look washed out. It seems to me, but like to be corrected, that the lowest and highest light readings need to be taken before choosing the range for AEB.

The method I used to create the above photo was based on ref 1 with some modification. The modifications were UFraw to do some fine adjustments. CR2 images have 12 bits to play around with instead of 8. And then curves in Gimp to do the fit the light levels I wanted in the frames. I have yet to try this on CS2. I expect it being 16 bit should provide better results than Gimp. Although according to reference 2 PhotoShop isn’t considered to have fine enough resolution to manipulate true HDR files and can’t save these files (3).

At the end of the day all that I am wanting is the overcome the inability of the camera senor to be able the capture what the eye sees. I didn’t particularly want to go back to the old days carrying a suitcase of Graded ND filter around. I still think Graded ND filter still have there place as AEB is only useful for static scenes.

References
(1) http://www.schwen.de/wiki/Exposure_blending
http://www.openexr.com/about.html
http://www.flickr.com/groups/hdr/
http://www.lightspacewater.net/Tutor...toPano2/paper/
http://qtpfsgui.sourceforge.net/
http://www.luminous-landscape.com/tutorials/hdr.shtml
http://www.luminous-landscape.com/tu...blending.shtml
http://gl.ict.usc.edu/HDRShop/
(2)http://wiki.panotools.org/HDR_Software_overview
http://www.halley.cc/photo/multexp/gimp.multexp.5.html
http://turtle.as.arizona.edu/jdsmith/exposure_blend.php
(3)Photoshop CS2 help

[drg]

Frans,

HDR is for static scenes that have tremedous range due to variations in light. The usual application is landscapes and naturally lit exteriors with light ranging from shadows to blown hightlights such as reflections. The Graduated Neutral Density filter or a polarizer to shift the intesity is still valuable with non stationary subjects.

A topic that does not get the play that it should is the difference in Dynamic Range performance of sensors. It is extremely important to test/calibrate the range of the individual camera to see where and how it performs across its response range. They are not linear by any means! Some will perform with greater precision or granularity in response on one end of the spectrum than others. In camera processing can affect as well how the will produce varying exposure, evaluated traditionally or with a historgram. This too will alter the effectiveness of HDR composition.

Photography Review has had several posts in various forums on success/failure of HDR which you might want to search for and read through several to compare expereinces.

The Luminous Landscape references you list are quite valuable and outline a quite effective method within its acknowledged limits.

It seems every author who has published a book on Digital Photography, or a professional guide, or is writing on a particular product has of late weighed in on this topic. Some use a tonal mapping or density combination variation, and they all have limits to their applicability and ultimate success.

Some of the algorithms used to map panorama combinations have a limited ability to 'match' exposure variations, because they are built around assumptions of similarity and thus generally fail in the more extreme range.

HDR cameras/software, such a Better Light, look for the widest possible extremes to work with. Not just two or three stops. And they shoot in very small increments such as 1/3 or 1/4 stop or less. It is easier to combine these small increments 'cleanly' than a full whompin' stop at a time!

There are a couple of more comments/suggestions.

First, with a camera mounted on a tripod, you should be able to change the shutter speed without moving the camera enough to create any problem. By using the Exposure Bracketing and shifting the base/center shutter speed only once beyond the bracketing range, you should be able to quickly get six exposures. Many small increments work better than less larger ones. Or you can reset the shutter speed several times in small increments and use a smaller bracket and get 9 or more bracketed exposures.

Secondly, another technique which is often overlooked is to use a graduated neutral density filter in conjuntion with the AEB. Using a Cokin type in front of the lens, the filter can be easily removed or inserted and another set of exposures made with the same setting and again, six exposures without making any shutter adjustments.

Here the key to success is choosing the proper density of filter to maximize the difference in stops between the two sets of exposures. The problem is by doing this many of the HDR automation routines will not work because they will see the same shutter speeds. Thus you will have strip the EXIF data (such as saving for web in Adobe PS)

I wouldn't worry much if I were you about bit depth beyond 12-bits unless you want to print at very large scale with the super inks. If so I'll add more to this post.

We are going to see much of this done in camera shortly. Fuji in the new D200 based S5 is reportedly going to be doing a lot more with camera options for dynamic range. It will be available later this year and we may get the first peeks at what is really in one companies mind.

Effective tonal mapping already exists in Face Detection technology which when fully implemented can pick a particular face out of a crowd at 200+yards with the right equipment.


If I did not address something you wanted let me know!!

[Frans]

Thanks again for your posting drg,

I can see some of the problems you have mentioned that I face in my Cannon 5D. Considering that most of what you have mentioned can be carried out with a small change in the camera’s software.
I suppose if they gave us all that is known to make a good photo there would be very little motivation for us to up grade to the next model.
One area I have found the 12bit to be very effective is when working with curves. The 1024 levels per colours dramatically reduces the effect of steps between the shades or hue in. I believe that some photo editors can simulate the same effect using 8bit per colour manipulation. The comment you made regarding the response curve of the camera’s sensor I have been aware of for sometime. A few years ago I was involved in sending analogue video signals down fibre optics fibres. I had to compensate for the transducers non nonlinearity. Again the effect on the camera seems to be much more critical as I was read on:
“ http://www.clarkvision.com/imagedeta...Sensor%20Sizes ”.
I have heard that Fuji have had a four cell sensor for some time to increase dynamic range on the green light, but I know little more than that.

Thanks again for you information on bracketing. I will try using 6 or more shots over +/- 3 F stop shortly as I need to take a photo of white water which I have always found difficult in do on a clear sunny day.
Frans