Resolution and Tonality in 35mm Film


The 400 megapixel Camera


Film photographs and digital images are produced in completely different ways using very different technologies.  So it is no surprise that the end results do not look the same between the two.  When comparing the same scene from both a photograph and a digital image, the photograph will usually be described as having depth, texture and "richness" to it.  The digital image will often be described as flat, artificial or stagey; as if the objects in the image were just flat, cardboard cutouts of the real things.  This difference is due to the ability of the film to record much finer detail, as well as the ability to record subtle variation in this detail.  These qualities are referred to as resolution and tonality.

There is no way to directly compare film photographs with digital images when looking at the level of very fine detail.  Digital images are made up of a grid of little squares called pixels.  These pixels are all the same size, and are lined up in precise rows and columns.  Each individual pixel on a Full Frame sensor is relatively large, being right around the threshold of what a person can see with the naked eye. (A Full Frame sensor is the same size as a 35mm frame - 24mm x 36mm)  On the other hand, the details of film photographs are produced at the level of molecules and atoms.  These arrange themselves into natural patterns called grain.  So the photograph becomes the result of the interplay of patterns combining with patterns. 

Despite these fundamental differences, digital camera manufacturers and other pundits have been putting out various figures for the "megapixel equivalent" for 35mm film for several years.  These numbers can vary widely, but they seem to have settled now somewhere between 10 and 20 megapixels.  These numbers are usually illustrated with low quality scans of a questionable photograph, which typically focus on a straight edge of a building or something similar.  This is because the programs that generate digital images are made to recognize and accentuate the edges of objects.  The real quality of depth in a picture depends on how well it can reproduce the three dimensional textures of surfaces.

The scans below were taken from a 35mm transparency (slide) of Kodak E100VS.  It is important to note that this film is by no means the best product for sharpness or fine grain.  In fact, VS is one of the worst professional films for these measurements, which makes the detail it is able to record all the more amazing.  The picture was shot with an Olympus OM-1 and a Zuiko 28mm, f2 lens.  The scans were done on a Plustek 7200i.  This is an extremely high resolution film scanner, able to resolve up to 7200 dpi.  7200 dpi works out to be just about 70 megapixels for a full frame, 35mm slide.  It's interesting to note Plustek does not make digital cameras, unlike many other scanner manufacturers.


This is the full frame scan of the slide.  Note the holly tree in the center of the picture.


There is a branch on the lower left side of the tree.  There is a cluster of berries on this branch.


There is a berry in the lower left side of this cluster.


Scanned at 7200 dpi, the shape of the berry is clearly shown in the subtle variations of color and brightness.

Keep in mind that this is a resolution of about 70 megapixels!  And the film obviously has even more detail than this scan resolution can pick up, perhaps twice as much.  It certainly has at least 100 megapixels of REAL image information.


Once the scan is resampled we can get an indication of what it looks like on the film.  Note that the progressive shadow across the surface clearly shows the berry's shape and outline.


Here is another perspective on the detail. This is a microphotograph of the bunch of berries. Unlike the scan, this is a direct view of what's on the film, as seen thru a microscope.   This shows how much information is on the film, that even a high resolution scan misses.

The subtle colors and lighting across the berries are clearly visible.

So now I'm sure you're wondering, "OK, I can see the 100 megapixels.  So where do the 400 megapixels come from?"  One of the little secrets of digital cameras is that their ability to record color is four times less than the published pixel count.  This is because the sensors use something called the Bayer interpolation in order to determine colors.  This requires four pixels (two green, one red and one blue) to determine the complete color information for the smallest possible area.  So a 12 megapixel digital camera can only determine 3 megapixels of true color information.  By the same token, 100 megapixels of true color information would equate to a 400 megapixel camera.