Bruce Girrell on wed 1 sep 99
Ivor Lewis wrote:
> Nowhere in literature I have read ... has a
> comparison been given
> which compares the size of a pixel with a silver grain.
The information is out there. As I'm sure you're aware, silver grain size
depends on the speed of the film (actually, it's the other way around), so
there is no fixed size to compare to. In addition, there is the thickness of
the emulsion layer and lens effects that affect sharpness in the final
image. Of course lens effects will affect both digital and film images.
The best information that I could come up with suggests a top practical
resolution of about 100 lines/mm or what would equate to about 2500 pixels
per inch. In actuality, the resolution will probably not reach that level.
The resolution of film could safely be put in the 1000-2000 "pixel" per inch
range.
> I have
> been out on shoots with a pro photographer who is frequently
> commissioned by a
> major Australian magazine publisher and he uses a large format
> camera and 120
> film, not 35mm.
Oddly, the resolving power of large format lenses is poorer than 35mm
lenses. The big advantage is that you don't have to enlarge the image as
much for the larger format, hence the image appears sharper.
For $Au2000 dollars it is possible to buy a
> single lens reflex
> and have a selection of excellent optics. But I would doubt if
> the same quality
> of initial image could be obtained from any of the digital
> cameras I have read
> about for the same price.
> Until grain size versus pixel size is
> sorted out I am
> being cautious about investing in a digital imaging system. Nor
> do I see the
> small lenses used in these devices emulating the quality of a
> 90mm portrait lens
> on a 35mm camera.
And there's the key - "for the same price." Digital cameras offering very
good image quality can be had for about US$10,000. These cameras do offer
the interchangeable lenses that you would expect from a professional camera.
There are more problems, though. Let's assume that film has a resolution of
1500 "pixels" per inch. A standard 35mm image (1 inch by 1.5 inches) then
would have about 3.4 million pixels. If we got a digital camera with a
resolution of 1500 pixels per inch would we match a film image? The answer
is no, not by a long shot.
The first problem is the image size. Charge coupled device (CCD) image
sensors are typically 1/3 inch square. Some are 1/2 inch square. So to
achieve the same size image, the digital image would have to be enlarged
more - and the pixels would be more visible. To achieve the same pixels per
inch in the enlargement would require 3 times as many pixels per inch for
the CCD, assuming the larger 1/2 inch square CCD. The 1/3 inch square CCD
would require 5 times as many pixels per inch.
The second problem has to do with our visual perception. The human visual
system is uncannily adept at picking out patterns and the nice, linear grid
of the pixel array of the CCD is a very simple pattern for our brains to
recognize. Film grains, on the other hand, are placed randomly within the
film emulsion. It would take roughly four times as many pixels above what we
have already calculated before the gridding would no longer be noticed. High
quality film recorders - laser devices that take a digital image and write
it back on to film - usually write at 8000 lines per inch to overcome this
effect.
It will be a while before we get a reasonably priced digital camera that
matches film quality.
>
> Now, about the value of Computer Image Enhancement Programmes. I
> have Corel
> Photo-Paint installed and tested others. I play around with it to
> crop images,
> change tonal ranges, alter colour saturation, eliminate flaws and
> other simple
> tasks. It has the capability of doing lots of other things,
> distorting shape,
> changing colour balance and adding "artistic effects" which at
> one time could
> only be achieved using complex chemical and optical techniques.
> However, it
> cannot add in what a top class photographer should be able to
> achieve at the
> click of the shutter.
Not totally true. Film is very limited in its ability to handle large ranges
of brightness. It also reacts to different colors of light in a way
different from the human visual system.
Digital enhancement programs are capable of redistributing the brightness
levels and adjusting the colors to more closely match what the photographer
saw at the time the shutter was tripped. So these "simple tasks" are doing
things that the photographer, no matter how skilled, cannot do.
> It is always limited by pixel size and
> imput, so that
> "sharpening" will never add new factual information from the
> original subject.
A flawed image will probably never be rescued. And you will never see a blur
become a recognizable face the way they show in the movies. But a photo that
is "almost great" can be saved from the trash bin by careful application of
these programs.
The Magazine Outdoor Photographer has had several articles recently showing
appropriate use of digital technology. Even such greats as Galen Rowel are
now extolling the virtues of digital image correction.
There are many advantages to digital photography. Right now resolution is
not one of them. There are many advantages to image manipulation programs.
Right now most people, even professionals, are too heavy handed with them.
Digital photography is a new technology. Give it the amount of time that
silver-based photography has had since the first daguerreotypes, then let's
do a re-evaluation.
Bruce "sometimes I just get carried away" Girrell
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