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 is not necessarily the case. 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 physical properties of 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/2.8
F/4
F/8
F/16 (Image deterioration due
to diffraction becomes visible)
f/22 ( Image deterioration
due to diffraction definitely visible)
f/32
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.
f/2.8
f/32
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. Other examples are the Summilux f/1.4 lenses
and the Noctilux f/0.95. These lenses,
are 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 buy 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.
Leica 180mm f/3.4 Apo Telyt-R and 50mm f/0.95 Noctilux
While these are compelling reasons
for considering Leica cameras and lenses, their sensors do set themselves apart
from the competition also. Instead of
participating in the pixel race, Leica and their sensor manufacturer have done
a remarkable job of optimizing sensor performance.
Compared to their
competitors, both the Leica M and S line of cameras seem to be lagging
behind. As far as total pixel count
goes, that is definitely correct.
However, that does not at all translate into lesser performance and
capabilities. The Leica sensors differ
in several respects, all of which are designed to optimize performance. Besides, what is really gained by a higher
pixel count?
Take the Leica S sensor for
instance. The difference between the
37.5 MP sensor of the Leica and a 50 MP sensor is only 10 – 15% as far as the increase in linear resolution is
concerned. Is that really enough of an
increase compared to the other advantages of the Leica MAX_24MP CMOS Sensor as
used in the Leica M? Let’s take a closer
look.
It is no secret that
individual pixel size does make a difference.
The larger the individual pixels, the better they will perform. To increase resolution, either the pixels need
to be made smaller to fit into a certain sensor size, or the sensor size needs
to be increased.
Regardless of the number of
pixels on a sensor, not the entire surface area of each individual pixel is
light sensitive. The pixels need to be
supported by a substructure and each individual pixel needs to be connected to
the system by small wires. Canon, for
instance uses wires with a size of 0.35 micron.
Sony is definitely better with a size of 0.18 micron. Leica by far exceeds that with a size of 0.09
microns. As a matter of fact, when
determining the specifications for the sensor, Leica demanded that the
structure sizes be kept as small as technically possible.
One goal was to keep the
non-sensitive areas of each pixel as small as possible. If less of the surface of the sensor is taken
up by supporting electronics overhead, then more surface area can be used to
collect incoming light. This results in greater dynamic range and a higher initial
sensitivity.
The surface of a sensor. If the non-sensitive areas can be made as small as possible, more surface area is gained to collect light. Image courtesy of
Red Dot Forum
Another means to increase
sensor performance is to make it as thin as possible. The Leica CMOS sensor in the Leica M is the
thinnest ever developed. Each sensor
contains several layers. By making each
of them as thin as possible, the end result is a significant increase in
performance because more of the incoming light can actually reach the
photodiodes, the individual pixels.
Another advantage was gained
by using copper for the connecting material instead of aluminum, which is the
common choice because the process of using copper is substantially more
complex. Copper has a substantially lower
electrical resistance than Aluminum, meaning that conducting layers with half
the thickness could be used. In
addition, to minimize thickness, instead of having four metal layers for the
conductors typically employed on CMOS sensors, only two were necessary on the
MAX CMOS chip.
An advantage was also gained
by using a different design for the microlens covering the entire sensor. Instead of using the common flat lenses, Leica went to an elongated, parabolic design. That has the advantage that more of the incoming
light will be redirected to the individual pixel areas and, especially at the
corners of the sensor, there will be no noticeable vignetting.
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
This can easily be
verified. Several other camera
manufacturers allow the use of Leica M lenses on their cameras. Take the Sony A7r for instance. Mount a Leica 18mm Super Elmar-M ASPH on the
Sony and take a few test shots. Then
compare those to the results obtained from a Leica M. The difference is more than obvious. But it doesn't even require an extreme wide
angle lens like that. Take a 35mm
Summilux ASPH, and you will find similar difference. The Sony sensor just isn’t capable of handling
non-retrofocus lenses anywhere near as well as the Leica M does.
Therefore, when it comes to
selecting a high performance camera system, one has to decide if a gain of two
or three pixels per inch in linear print resolution is more important than the long list
of advantages gained with the Leica MAX CMOS sensor.
Some information in this post was first used in an article about the Leica S systm in Red Dot Forum
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