Thursday, May 24, 2012



Most people are under the mistaken impression that the higher the megapixel count in a camera, the better the performance.  Even though it may appear to defy logic, this isn’t the case at all.  Any optical system, even a theoretical one that is made without any optical aberration or other faults has a performance limit.

With the ever increasing pixel density of digital sensors, an ever increasing number of megapixels are being put in the same space.  That brings up the question if our lenses are even capable of a level of resolution to take advantage of the sensor’s capability.

Even the most accurately made lenses have an absolute limit: the light.

The resolution of any optical system is limited by diffraction.  The light, the aperture and especially the diffraction of the light at the edges of the diaphragm constitute an absolute limit of the overall resolution.  It is impossible to achieve a resolution higher than that allowed by the lens, regardless of the resolution capabilities of the sensor or film.

To demonstrate what this means, here are several photographic examples. 

The photograph shows 99.99% pure manganese chips, oxidized in air.  For scale a cube with a one centimeter base length is included.  All photos by Heinrich Pniok.




F/16 (Image deterioration due to diffraction becomes visible)

f/22 ( Image deterioration due to diffraction definitely visible)


The cropped images show an exact area of 500 x 500 pixels of the entire image of 5616 x 3744 pixels.

Resolution capabilities of various optical systems

This brings up the question of what resolution a photography system is capable of under ideal conditions.  This is a very interesting question in view of the fact that many digital cameras are getting close or even exceed the limits of diffraction – the sensors are of a resolution level that the lenses are hardly capable of achieving.  To be clear, this has nothing to do with the quality of the lenses, it is solely because of the limits of physics.

Here is a table of these limits based on sensor size.

The lens opening or aperture is the deciding factor for the resolution capabilities of any optical system.  The larger the diameter in relation to the focal length, the larger is the theoretical resolution.  Each subsequent smaller aperture will half the visible resolution.

Unfortunately, the performance of most lenses is usually less wide open than when moderately stopped down.  The theoretical values at f/1.4 can hardly be reached in the real world.  Optimum performance usually is not reached until stopping down to a range of f/2.8 (at best) to f/5.6 or f/8.  Please note that due to diffusion within the emulsion these figures extend to f/11 with most color and black and white films.

This is especially important with smaller sensor sizes.  While a 35mm full frame sensor is capable of a theoretical resolution of 60 megapixels at f/5.6, a 2/3 sensor is reduced to just 4.4 megapixels at the same aperture.

At this point, the actual pixel size becomes important also.  As a rule of thumb, we use Aperture divided by 1.5 equals pixel size in microns or µm (Aperture/1.5=pixel size).  For example, 2 / 1,5 = 1,3; 5,6 / 1,5 = 3,7.  At f/2, all sensors with a length of 1,3 µm or more on each side are capable of resolving all the lens can deliver, but at f/5.6 the individual pixel size has to increase to a length of at least 3,7 µm to do the same.  If the individual pixel size is smaller, resulting in a seemingly higher sensor resolution, we are dealing with a so-called blind resolution which cannot be achieved because of the physical limits of the resolution of the lens.  

It must be pointed out once more that these values all are based on theoretically flawless optical systems.  Not included are negative impacts from anti-aliasing filters, signal processing (keyword Nyquist Frequency), increases in noise and the necessary interpolation of sensors with Bayer mosaic (RGB filters).

We also must not forget camera movement.  Without a tripod, these figures are reduced by another 25% at a shutter speed of 1/125 sec.  Of course this increases noticeably with longer exposure times.



This is an extreme example which shows the image deterioration quite well.  However, we must also consider the increase of depth of field with smaller apertures.  At times this might be more important and subsequently deliver an overall better image, even with the overall image deterioration associated with small apertures.  

Ultimately it is up to each individual what performance parameters we want to set for or expect from our camera equipment.  This article hopefully made it clear that megapixel resolution is not necessarily the key to overall performance of our camera equipment, that lens performance is of equal, if not even greater importance.

This brings us to Leica lenses in particular.  Since theoretical resolution is highest at the largest aperture of a lens, it makes sense to use lenses which do offer good performance at those apertures.  In this regard Leica lenses are unsurpassed.  While competitor lenses might come close in performance to their Leica equivalents at smaller apertures, their performance fall off wide open is usually noticeably greater than with their Leica counterparts.  For example, the 180mm f/3.4 Apo-Telyt R was specifically designed to offer optimum performance at maximum aperture.  There was no appreciable performance increase at smaller apertures.  Thus the lens is capable of taking full advantage of the performance increase when used wide open.  We should always evaluate a lens by its performance at ALL apertures and not only the ones that result in the best results.  It doesn’t make any sense to by a fast f/1.4 lens, for instance, if it requires to be stopped down to f/4 or f/5.6 to deliver adequate results.

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