January 2009 Digital Group Meeting - Sensors & HDR

Meeting Notes March 2009 to 2018.
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spb
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Joined: Tue Mar 25, 2008 7:04 pm

January 2009 Digital Group Meeting - Sensors & HDR

Post by spb » Fri Jan 16, 2009 10:22 pm

NEWS

If you are thinking of buying a compact camera there have been a series of reviews of compacts on the excellent dpReview site. These have been grouped into Superzooms, Enthusiast, Premium, Ultra Compact and Budget cameras. See the first five items under the Reviews/Previews topic. Many other individual cameras and a small but increasing number of lenses have also been tested to great depth - always worth a look.

I have mentioned the excellent free Layer Mask tool for Photoshop Elements several times. In response to a query from a local photographer the author has confirmed that it continues to work in Elements 7. See link

The recommended Audio Visual programs in the ProShow range (eg ProShow Gold) have all been updated to Version 4. This is a significant update and includes many useful new features. It will be free to anyone who purchased the program in the last 12 months.

SENSOR SIZE

Arguably, the trend to increase the pixel count of cameras is no longer driven by picture quality - it is driven by marketing. This is especially true of compact cameras where picture quality seems to be a minor concern of the designers.

Sensor size dictates lens and body design and camera cost. As a consequence the designers usually do not have the freedom to increase sensor size so they have to squeeze in the extra pixels by making each photodetector smaller. Although the technology is improving all the time, there is usually a net loss in the areas of dynamic range and noise as the pixel count increases. These undesirable characteristics are all-too evident in compact cameras - burnt-out highlights, blocked-in shadows and coloured speckles in low-light situations and at high ISO settings.

Digital SLRs have the luxury of larger bodies and lenses and more space for the sensor. As a result they exhibit greater dynamic range and in the latest full-frame models spectacular low-light performance. On the down side, large sensors can be incredibly expensive - sometimes up to half the cost of the entire camera.

Allowing for cropping, an A4 inkjet print at maximum quality requires about 4M Pixels. An A3 print needs about 8M Pixels. It is therefore hard to see the justification in compact cameras appearing with up to 15M Pixels, particularly as all of these pixels have to be stored and processed to little or no purpose.

When opting for a camera with a small sensor there are some other characteristics to take note of; greater depth of field at a given aperture, the focal length multiplier factor and the aspect ratio - often 4:3. These may be seen as positive or negative depending upon your particular type of photography.

The conclusion is that we may choose to be wary when the salesman tries to persuade us to upgrade our camera because it's got a bigger 'number'.

HIGH DYNAMIC RANGE (HDR)

The human eye has a dynamic range of about 14 stops. Film and digital cameras are about 8 stops and prints about 6 stops. Consequently we are constantly trying and usually failing, to squeeze both bright highlights and deep shadows into our images.

Until cameras are designed which can cope with the full dynamic range (and eventually they will be) we have to get around the problem. At the moment the state of the art is to take multiple exposures and combine them with software. Although Photoshop CS can do this (CS3: File>Automate>Merge to HDR), the current darling of the HDR community is Photomatix Pro software .

Although a single Raw file can be used, best results are achieved by taking a series of exposure-bracketed shots each differing by two stops either side of a normal exposure. A tripod gives optimum results but hand-holding with a fast automatic exposure bracketing setting on the camera can prove acceptable. Aperture priority mode should be set to keep a constant depth of field.

The resulting set of images is processed through the software, firstly to combine them into a single image containing a high dynamic range and secondly to compress this (called tone mapping) to produce a viewable, printable image. The results can be quite dramatic with saturated colours and remarkable levels of contrast. Many find such images too artificial but it is equally possible to choose settings which are more realistic and subtle. Google 'HDR images' for many examples.

STEVE SMITH

Thanks to Steve who brought along an example of his powerful monochrome work and showed how it was created, largely through innumerable layer masks each meticulously controlling the tonal range of a small carefully selected area. It is not unusual for Steve to spend two days on a single image before deciding if it was all worthwhile or not!

NEXT MEETING

Thursday 5th February. This meeting will be hosted by Peter Jones.

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