Smartphone uses in archaeology and heritage applications.

Episode 2:  High Dynamic Range (HDR) imaging with a smartphone.

Optimising the scene captured in a photograph is a skill of balancing a number of factors including exposure, aperture, shutter speed, film or sensor speed and composition. One scenario that occurs regularly in photography is that where the scene to be recorded has a very high dynamic range. For example where the foreground is in shade but with glaring sunlight in the distance or perhaps a bright window in the centre of a dark internal wall. Even with the best professional camera equipment choosing the optimum settings to capture this type of scene is always a trade off and clarity in all areas of the image is often not possible.

High Dynamic Range or ‘HDR’ imaging is a strategy for overcoming this problem by blending two or more images. The process has become a standard tool in the professional digital photographers toolbox, however the origins of the technique can be traced back to the very earliest photographers. For example there are records of the famous photographer Samuel Fry combining several negatives into one image in the 1860’s to achieve his desired landscape and sky photography style.

With the modern digital HDR process the photographer sets up the camera on a steady tripod and then uses a setting available on professional quality DSLR cameras to achieve bracketed shots. These shots take a series of photographs, normally three, in quick succession. The camera varies the exposure of the three shots. Back at the computer, the three images are then opened in professional photograph manipulation software and are combined into a single image using the best parts of each photograph. This HDR process is fairly streamlined and an experienced professional photographer can produce HDR images at a good speed.  HDR imaging is the technique that achieves those kind of images of internal areas with that certain ‘wow’ factor or images of landscapes and buildings with superb detailing in all fields of the image.

For day to day survey recording often it is not appropriate to commission a professional photographer. In the course of taking some general photographic records the surveyor will often face the issue of a scene with high dynamic range. Experimenting with HDR imaging is probably not practical since the process requires a degree of experience and also does not produce instant results as normal digital photography does.

However we have discovered a recent development in the smartphone software. The advantage of a smartphone is that it is a camera device and mini computer all in one package. The cameras on modern smartphones may not have the quality lenses of other digital cameras but the processing within the smartphone can be used to compensate for this. A number of instant HDR software apps have evolved for smartphones. The user simply holds the smartphone by hand pointing at the high dynamic range scene and uses the software app to take a series of simulated bracketed shots. The app then instantly blends them into one image and even adjusts them to remove the ghosting affect of the scenes being not quite aligned due to hand movement. The processors in smartphones are surprisingly powerful and can achieve some very impressive results in just a few seconds. The HDR images will never be of a quality of professional HDR imaging using a DSLR however the important advantage is that the results are instant and automatic.

Here’s an example we shot recently to test an HDR smartphone app. For this test we used an Android app called Pro HDR Camera costing around £1.20 although many similar apps are available. The subject is the glorious Albert Memorial in Hyde Park London. We chose a horrible angle to shoot at looking up into the ornate ceiling with the sun right behind the statue, the light glaring off the gold surfaces and the foreground steps in the shadow of the monument. This would have been a very unforgiving angle to shoot from in normal circumstances.

First the automatic best balance exposure single shot. The smartphone has done a pretty good job but has struggled to balance the brightness of the sunlight with the shaded foreground and as a result has underexposed many of the foreground surfaces:

Then the over exposed shot that captures the detail of the internal ceiling:

Then the under exposed shot that captures the detailing of the sky and glinting surfaces but completely looses the detail of the Parnassus frieze:

Finally the HDR blended shot. It must be stressed that this is the original image blended in a few seconds at the time of shooting. The image has been downloaded straight from the smartphone and not edited in any photograph editing software:

The final HDR image is certainly not perfect but there is a great balance of foreground and background and all of the essential detailing can be seen. Around the outside of the image you can see the banding effect of the slight displacement of the three scenes due to hand movement. It is quite remarkable that the app can resolve this misalignment and still blend the images. There is a small amount of ghosting in the clouds and around some of the brighter objects but the clarity is sufficient to establish the detailing of all of the visible surfaces.

The important result from this test though is that the smartphone HDR image is certainly better than an image that would have been obtained in the same situation using a point and shoot digital camera. The image took perhaps a total of a minute to capture so there was no significant delay. If this was a simple photographic record to support a survey then it would have been quite reasonable to perhaps shoot the three sides of the memorial in good lighting conditions using a point and shoot digital camera and then capture the fourth side into the sun using a smartphone HDR technique.