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Using image transformation to stack pictures shot handheld without a tripod.
High dynamic range scenes challenge virtually all cameras, no matter if CCD, CMOS or Analog (Film). Even the capability of the human eye to capture very high dynamic range is limited. We cannot see details in the shade of a tunnel, next to a bright burning lamp or a tunnel opening. Cameras are even more limited than the human eye, but we can make all those details visible with special techniques.
One of these techniques, recording several images with different exposure levels and blending them on the computer, is relatively easy. It requires that the images overlap perfectly, which usually necessitates a tripod during the shot.
Using a tripod to capture images takes time and is cumbersome. Many casual photographers got used to the idea of lightweight cameras that fit in their pocket and carrying a tripod sounds unappealing. Even professional photographers do not like the idea of carrying a tripod in difficult terrain. I am going to show you a method to blend a series of handheld exposures by perfectly aligning the source images.
I almost always use a tripod but some places prohibit using them. Careful planning and good preparation is essential to obtaining good results.
Taking all images in rapid succession minimizes the chance for the scene to change, e.g. cloud movement, and helps maintaining the same position.
Foreground elements introduce parallax errors that are hard to compensate. Avoid them if you can!
Take Images in Aperture Priority Mode (Av), if you plan to blend exposures! This keeps the depth of field between the images the same. Lock your focus so that the focal points will not change between the shots!
Ensure that the exposure time of the overexposed image is short enough for handheld operation! Simply set your camera to overexpose, choose your aperture and check out the exposure time. Advanced photographers are probably familiar with the exposure time scales. Adding one stop will reduce the exposure times accordingly.
I usually just set my camera to bracket the exposure and fire off 3 exposures in rapid succession, trying to keep the camera as steady as possible.
Even with all these precautions, the images will not align perfectly and need to be aligned through transformation.
The tools of choice for image transformation are the legendary Panorama tools. They are the most powerful free tools available. Numerous programs have been developed to serve as a graphical front end. I am using PTAssembler (shareware) for this article, but other programs like PTGui (shareware) or Hugin (freeware) work equally well.
Resourceful programmers and photo enthusiasts developed these tools to create panoramas. I am using the features of those programs slightly out of context for my purposes.
Download and install the fully functional PTAssembler trial version. It contains all the necessary Panorama Tools libraries.
We also need the Autopano Plugin (freeware) to simplify our work. Download and unzip Autopano. Then go to File -> Preferences in PTAssembler and select the location of the Autopano executable file (autopano.exe). Now we are ready to work.
In PTAssembler simply click on “Add” and select your source images. PTAssembler will quickly check them and then display the images in its window.
Then select all images (shift + click) and click on Autopano Selected.
Autopano will now calculate the control points for your images. Those are the points of convergence between the images. The images will then be warped and transformed so that all control points fit together and everything is aligned perfectly.
Now click on “Step2” and select the following parameters:
We don’t want PTAssembler to try to match the brightness of all images.
Hopefully you don’t need this step. Here you can manually select control points, which can be very useful in case Autopano cannot determine the correct control points.
Select the image with nominal exposure in the first window (it will serve as reference image)! The reference image will not be rotated or tilted. All other images will be transformed to fit the angle of this image. In a panorama this should usually be an image in the middle along the horizon. In our case any image will do.
Then click Auto Optimize and let Panorama Tools optimize your control points.
Don’t change the FOV setting, it has been calculated from your images!
Set Feather to 0 (always)!
Under “Output File Format” you need to select “Multiple img TIFF (TIFF_m)”. You can also chose a layered PSD file if you are planning to post process (blend) in Photoshop.
Now click the “create” button and let the Panotools do their magic.
The final output will be multiple files that fit exactly on top of each other. Some cropping will be required though.
If you are unsuccessful with this procedure, the most likely cause is bad control point data. I usually try to add control points manually and let the optimizer do its job fine-tuning them. Some images do require a little bit more attention then others, where everything works automatically.
The Tiff files written by PTAssembler do not contain an embedded color profile. I usually open them in Photoshop and assign the correct color profile (Edit -> Assign Profile -> ProPhotoRGB (sRGB/Adobe RGB)).
Next time you cannot use your tripod, don’t hesitate and take multiple images anyway. You may be able to combine them into a single high dynamic range shot.
This was a Reader Submitted Tutorial
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