COMPARISON OF SENSOR VS FILM RESOLUTION

In a television advertisement a while ago the statement was made that, if it is on the internet, it must be true.  I was reminded of that a couple of days ago when I read an article comparing film to sensor resolution.  The writer boldly stated that most films have a resolution of at least 300 l/mm (lines per millimeter).  Of course that was overstating things by a huge margin.  I don’t mean to say that there aren't films that can attain such high resolution figures, but all films?

Resolution is generally tested by taking photographs of test targets which show a pattern of black lines in an ever decreasing size against a white background.  Eventually the lines will become so small that the film, or the lens, can no longer distinguish the black lines from the white spaces in between.  One black line and the adjacent white space are referred to as 2 l/mm or 1 lp/mm (line pair per millimeter).

Typical test target

It is a known fact, however, that evaluating resolution with test targets does not render very conclusive information.  The black lines and white spaces in between constitute a very high contrast.  This makes it substantially easier for the film (or a lens) to separate the two.  Reducing contrast by using grey lines on a white background would render substantially different results.

Another major factor influencing resolution is the grain structure of the film.  A film image is made up of silver halide clumps which show up in form of grain.  The smaller the silver halide clumps or the grain, the finer the detail that can be shown.  Faster films simply do display coarser grain which in turn lowers the resolution of a film. 

       

These three images are from a fine grain negative (Agfapan APX 25).

The first was scanned from an 8x enlargement, showing the entire negative area.

The second image was scanned from a 16x enlargement.

The third shows a cropped section of the same 16x enlargement 

Grainy film image.  The grain is so large that small detail simply cannot be shown

Photo: Góry Bialskie

      

For comparison:  Full image and cropped section from a 5 megapixel digital camera (Leica Digilux 2)

Finally, the structure of the emulsion or emulsion layers influences film resolution because in any case, light traveling through the emulsion, will scatter and thus reduce resolution as well.

Subsequently, to say that most films have a resolution of 300 l/mm is patently false.  As a matter of fact, only few commercially available films even have that high a resolution.

Researching this topic, I came across a report written by Joseph A. Schantz, Assistant Head of Research and Development Department at the Navel Photographic Center in Washington, DC.  He wrote that since 1963 the Navel Photographic Center and the Navel Air Systems Command as a matter of continuous policy have expanded efforts to upgrade 35mmphotography on a systematic basis.  The aim of this work was not only to improve the quality of documentary and reportage photography but also to improve intelligence collection capabilities of the Navy’s cameras.

According to the research done by Mr. Schantz, the best resolution obtainable with conventional, slow speed films, like the old Agfapan APX 25, is 250 – 300 l/mm compared to 550 l/mm with the Agfa High Contrast Copy Film and 600 l/mm with Kodak 5069 and 3414 film.

Kodak High Contrast Copy Film when processed in the POTA developer of Marilyn Levy (Levy, M., “Wide Latitude Photography,” Science and Eng. Vol. II Number I, January, February 1967) yield excellent high resolution negatives with adequate film speed.  The Agfa High Contrast Copy film gives a practical combination of good resolution and emulsion speed.

In addition, C. B. Neblette in his book “Photographic Lenses” clearly states: The resolving power of a lens-film combination is not fixed by the film alone, but by both the lens and the film (or sensor). Resolution is determined principally by the sharpness of the image (lens resolution).  But it is profoundly influenced by the tone producing properties of the receptor (film or sensor) and its ability to reproduce steep gradients.  For that reason resolution cannot be regarded as an exclusive property of the lens.

For the average films available today, a more modest resolution of 100 to 200 l/mm is a realistic figure, based on film speed and general properties of the film.  Black and white films generally have a higher resolution than color films.  The former Agfapan APX 25, for instance, had a resolution close to 300 l/mm while Fuji Velvia 50 is rated to resolve 160 l/mm. 

To make film resolution more understandable in this comparison, let’s refer to the smallest detail a film can show as pixels.  On a standard 24 x 36mm 35mm frame, a film with a resolution of 100 l/mm would render a total of over 8,6 million pixels.  That increases to over 19,4 million pixels with a film resolution of 150 l/mm and over 34,5 million pixels with a 200 l/mm resolution.

Of course a 35mm negative or transparency is of little use just by itself.  Today transparencies generally are scanned and then further processed digitally.  Does anyone still use a slide projector?  Many film users still make their own enlargements, mostly from black and white negatives, or the negatives are scanned for further processing.  Regardless how films are used, any further processing will have an image degrading effect, based on the slide projector, enlarger or scanner used and by their respective quality.

For more details on this topic go to LEICA Barnack Berek Blog article “LEICA LENSES – WHAT GIVES THEM THEIR OUTSTANDING QUALITY.” 

How does this compare to digital sensors?  Top level full frame (24 x 35mm) cameras currently have resolution levels of approximately 25 to 35 megapixels, with higher pixel counts to be expected in the near future.  The general belief is that the higher the pixel count, the better the image quality.  However, there is a lot more to that than meets the eye.  The new CMOS sensor in the Leica M (Typ 240) has definite advantages over conventional CMOS sensors.  For a more detailed description of digital sensors and some of the major differences, got to LEICA Barnack and Berek Blog article “THE PIXEL RACE - DOES IT REALLY MAKE SENSE?” 

Section of a typical sensor

 Image courtesy of  Red Dot Forum

Conventional CMOS sensor with deep pixel wells and flat microlenses

Leica CMOS sensor with very shallow pixel wells and tall micro lenses, allowing for larger pixel area

Unlike film, digital sensors will render the same contrast level up to the finest detail.  This has the result that the finest detail becomes less visible. A color image is made up out of RGB (red, green, blue) image elements.  With the exception of the hardly ever used Foveon sensors, digital sensors can record only in black and white.  In order to obtain a color image, the light passes through an array of red, green and blue filters, the Bayer filter grid.  This means that the total number of pixels in a sensor are exposed to either red, green or blue light only.  To form a color image, the information obtained from the sensor is then processed by interpolation in the camera or by raw conversion software.  It takes the pixels of each color, and assigns all colors to each pixel.  With other words, the software will take a red pixel, for instance, and assign theoretical green and blue values as well to form a complete color image.  As good as these types of software have become, there are certain losses involved.

With lower quality cameras these losses can be as much as 50 percent of the resolution.  The only exception to this is the Leica M Monochrome.  Here the Bayer filter and interpolation software is eliminated to record just black and white images.  See LEICA Barnack and Berek Blog article “MONOCHROME SENSOR - WHAT IS THE DIFFERENCE.”  The result is an unsurpassed image quality and tonal range. 

In the final analysis, just as there are definite performance differences among films, there are also considerable differences among sensors.  CCD sensors used to be the choice of most camera manufacturers.  These have been widely replaced by CMOS sensors.  In this respect, Leica is no different.  Many people are under the mistaken impression that the CCD sensor in the Leica M9 delivers superior results than the current CMOS sensor in the Leica M (Typ 240)  A recent comparison test by David Farkas of the Leica Store Miami thoroughly debunked that.  He took this very subject to task in a three part series in the Red Dot Forum (www.reddotforum.com).  See LEICA Barnack Berek Blog article “LEICA M (TYP 240) VS LEICA M9”.

The debate of which is better, film or digital sensors, cannot be answered with any certainty because the large differences among films and sensors.  What can be said is that both film and sensors are capable of delivering very high quality images.  In many cases they do exceed the requirements of the photographer since extreme cropping or enlarging is necessary to even reveal the limits of their capabilities.  Thus it is more a matter of personal choice than effective differences pointing to one or the other medium as being superior.

As for myself, I used to spend many hours in my lab developing films and printing with a Leitz Focomat V35 as well as medium and large format enlargers.  Having switched to digital, I don’t miss analog photography at all, and I can say with certainty that I am not compromising the overall quality of my work by having done so.

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