Back to Notes on Photography
Honda February 9, 2005
The sharpness of the
final photographic image is mainly determined by the
resolution of the lens and the film used. In digital
photography, the film is the digital sensor.
They have to go
hand-in-hand. Your lens may form a
sharp image on the focal plane, but if the film is incapable of
capturing it, we do not have a good image. And vice versa. Both the lens
and film must be comparably good, else the lesser performing
becomes the bottleneck. In today's
world of digital photography, the lens
tends to perform much better than the digital sensor. That is, the film is limiting in the overall
resolution calculation. Any improvement on the sensor resolution,
therefore, is likely to have a direct impact on your final
With the 24 MP
digital sensor for a full-frame (35mm) camera, we have just arrived at
the same film resolution of Fijichrome Velvia. The 50 MP sensor
is a little better. The digital
sensor must go further --- to 500-1000 MP in the full-frame 35
mm format to be "on par" with the current lens resolution.
Optical resolution is the ability to resolve
resolution is defined in terms of "line pairs per millimeter",
or lp/mm for short. We are interested in determining how
many lines are discernable in the distance of 1 mm. One line
pair consists of a black line and the adjacent white line of the
equal thickness. The higher the lens resolution, the greater the
number of lines you can resolve. You need a film to capture the
image created by the lens. The higher the film resolution, the
more you can capture on the film. The final resolution of the
resultant picture that you see is the accumulative effect of the
lens resolution, the film resolution, and the resolution of
other optical/non-optical systems
involved in the entire
process. This overall resolution,
sometimes referred to as the system resolution, is given
by the following approximation.
System Resolution: Rs =
---- + ----
where Rl is the lens resolution and Rf is the film
instance, if your lens resolution is 100 lp/mm and the film
resolution is also 100 lp/mm, the overall
system resolution you get is
50 lp/mm. If you use the lens having 200 lp/mm resolution, with
the same film, your system resolution will improve to 67 lp/mm.
We often use the term "aerial
resolution" in reference to the lens resolution to emphasize the
fact that it is a resolution measure of the aerial image formed
by the lens at its focal plane. Typically a microscope is
precisely focused on the aerial image created by the lens being
tested. The lens resolution is totally
independent of the film you use. The lens aerial resolution varies widely depending on
the lens quality. To give some frame of reference, some high-end
35mm SLR camera lenses, often
associated with the lens manufactured by major camera
manufacturers for their own brand of cameras, yield 600 lp/mm or
more, at optimal conditions. The less pricey
lenses may yield
300 lp/mm. Also, zoom lenses are generally harder to design,
often involving greater number of elements, and
therefore, tend to yield lower resolution relative to their
fixed focal length counterpart (though
some latest zoom lenses
offer uncompromising optical quality
matching that of prime lenses).
Theoretical Maximum Resolution
The laws of physics impose maximum resolution
on the image formed by the lens. This is a theoretical limit
due to diffraction of light. As such, this is the absolute
maximum an ideal lens can achieve. The theoretical resolution is
dependent on the lens aperture used as well as wavelength of the
light, as shown below.
Theoretical Maximum Resolution: Rmax
1.22 x W x Feff
where W is the
wavelength (in mm) and Feff is the effective f-stop.
high magnification photomacrography, the exposure factor often
pushes the effective f-stop higher than the f-stop marked on the
lens, but in normal shooting situations, the effective f-stop is the same as the
f-stop shown on the lens.)
wavelength of light the human eyes can perceive lies around 0.0004 -
0.0007mm range from blue to red in the spectrum of visible
light. Using the mid-point of 0.00055mm (monochromatic green),
the resolution equation is reduced to :
Theoretical Maximum Resolution: Rmax
values thus derived are shown in Table I for various f-stops.
Again, this is the maximum resolution any lens can possibly
I * f-stops below f45 may be only meaningful in photomacrography
film resolution is often described for two situations: One is
for the test subject having a contrast of 1000:1, and the
other of 1.6:1. The first 1000:1 contrast represents a
high contrast situation in the testing laboratory, and the 1.6:1
is the average contrast of real-world subjects around us.
Expectedly, the film performs much better for the 1000:1
contrast condition than in the real world. Note that the 1000:1
contrast is only achievable in a back-lit projection setup of
the test pattern. Even a black and white test chart well lit by
a 45 degree illumination only achieves a fraction of this
contrast. Therefore, a resolution value measured at 1000:1
contrast is for comparison purposes only among different films,
and is meaningless when applied to a real world situation. Here
are some examples of film resolution. Generally, a B&W film
yields better resolution than color. A color transparency
(slide) film typically scores much better than a color print
(negative) film. Also, a slower film (lower ISO rating) tends
to be superior than a faster film in resolving power.
1000:1 1.6:1 RMS*
Fujichrome Velvia RVP 50
160 80 9
Fujichrome Velvia 100F Professional 100
Fujichrome Sensia 100 100
160 80 8
Fujichrome Provia 100F Professional 100
Kodachrome 25 / 25 Professional 25
Kodachrome 64 / 64 Professional 64
Kodachrome 200 / 200 Professional
*Diffusive RMS granularity value
Film Is Limiting
Typically, the resolution of the film available in the consumer
market today is much lower than the lens resolution. This often
makes the film the ultimate bottleneck of the photographic
process in the overall resolution
equation. This explains why an expensive lens from the camera
manufacturer does not drastically improve the picture quality of
your family picnic when viewed side-by-side against the picture
taken by a much less expensive lens.
chart below shows the overall
resolution (system resolution) for various combinations of lens
and film resolutions. If you use a typical color print film of
50 lp/mm resolution, your final resolution is 43 lp/mm with a
300 lp/mm lens. If you spend a top dollar to invest in a top
quality lens of 600 lp/mm resolution, your
combined resolution improves to 46 lp/mm, a mere 7 %
increase. Using the same 300 lp/mm lens, on the other hand, if
you change your film from 50 lp/mm to 80 lp/mm, your final
resolution jumps to 63 lp/mm - near 50 % improvement!
previously, the lens resolution of 100 lp/mm and the film
resolution of also 100 lp/mm will only yield the system
resolution of 50 lp/mm for the final image formed on the film,
as seen in the chart below.
In the world of digital photography, chemical
film is replaced by a digital sensor. If you have a 24 MP
(mega-pixel) digital camera with a full-frame sensor (24 x
36mm), the film resolution of this camera is calculated to be
around 83 lp/mm. In the days of chemical film, this is the same
resolution given by Fujichrome Velvia.
That is to say, only when we have reached a 24 MP full-frame
digital photography can we confidentially state that our digital
sensor technology has finally arrived at the same level as
the good-old Velvia in terms of film resolution.
Moving forward, a 48 MP full-frame sensor gives you about 118 lp/mm
film resolution. As
mentioned earlier, a typical lens resolution hovers in the
300-600 lp/mm range. This means there is a tremendous room for the
sensor resolution to catch up with the current lens resolution.
Just to give you a headsup, a full-frame sensor of 1000 MP will
give you around 500 lp/mm film resolution, compatible with
today's lens resolution. ,
Other Factors Affecting Image
that you got the best lens and the best digital sensor the money can buy (or at least your
budget allows for), what should you do to get the most out of
your gear? There are things that compromise the final image
quality: focus, camera shake, lens aperture, ....
(1) CAMERA SHAKE
Even a slightest
camera movement during the exposure can easily wipe out any
benefit of a good lens and a high-res sensor....
Use a fast shutter
speed to alleviate the problem. A traditional rule of thumb is
to use the shutter speed faster than 1/focal length (mm) of the
lens you are using (in 35mm photograph). But the kind of shake
we are concerned about in the context of high-resolution digital
sensors (like 40 PM and above) is so slight that this
traditional one-over-focal-length shutter speed may not be fast
stabilization technology is very reliable, powerful and
Using a tripod is
always a good idea.
Although cumbersome to use and time-consuming to set up (not to
mention having to
carry the darn thing along the way) the tripod does make a
difference in the image quality. A tripod comes in different
sizes and weight. The sturdier the better, of course, but
likely the heavier. The tripod is important because the
slightest camera movement might wipe out the image quality
difference of the expensive lens that you paid
additional $1000 for.
Mirror-lock for SLR cameras is something you can consider to
reduce a slightest camera shake in some critical situations.
(Obviously, a mirrorless camera does not have this problem.)
These critical situations occur when the field of view is very
narrow, as in telephoto shooting, or in high-magnification
photomcrography -- the camera vibration due to a
mirror flipping is not negligible. I have used
mirror-locking during high magnification photomacrography: With
the bellows extended all the way, the field of view is as narrow
as a telephoto shot, which makes the image sharpness highly
sensitive to camera movement.
Even a slightest
error in focusing can easily wipe out any benefit of a good lens
and a high-res sensor, so focus carefully...
Using a tripod may
help you focus better.
(AF) is a very reliable technology, especially in moving
situations. Cross-hair / phased
Ensure your AF is
changes as you stop down the aperture. This is a bad lens. If
the lens exhibits this tendency, the only remedy is to re-focus
at the aperture you intend to use. The only time I consistently
do this is when I do high magnification photomacrography. I use
Zuiko Macro 20mm F3.5 lens (Olympus) and Minolta Macro 12.5mm F2
lens, both specifically designed for use with a bellows
extension. Since the focus tends to move as I stop down, I
always set the aperture to f5.6 - f8 (no auto aperture for these
lenses) and then focus very, very carefully before firing.
There may be a
mechanical misalignment in the system that is beyond your
control (including the film flatness in film camera). The SLR camera
mirror may be misaligned. If the mirror does not return to the
precise mid-point (at 45 degree angle) between the film and the
focusing screen, however carefully you focus (manual or auto),
the film is not getting the focused image. Focusing the image on
the sensor itself (as in mirrorless cameras or using the live
view in the SLR cameras) will eliminate this possibility....
This is more of a problem for the film format larger than 35mm.
Digital photography does not have this issue...
(3) LENS APERTURE
Any lens has various
aberrations that reduce the resolution. Since some aberration
can be lessened as you close down the f-stops, there is some
optimal aperture (a sweet spot). Many
lenses tend to yield their highest resolution (at the image
center) when used at the aperture a couple of stops down from
wide open (though this could vary depending on the
lens). By stopping down one or two additional stops, you may
increase the overall image resolution of the entire frame area
due to the improvement of the resolution at the image corners (though the image center resolution may decrease). So, there is
such a thing as the "best" aperture for a given lens.
This is the aperture you should use if the resolution is the
only thing you are after -- I decide on which aperture
to use, first and foremost, to control the depth of field; the
resolution consideration is often secondary.
Also, as regard to
the f-stops, we have already discussed the theoretical
resolution limit due to diffraction of light.
(4) INCOMING LIGHT
Use the lens hood. This
is very important especially in outdoor photography. You should
always try to avoid the sun from hitting the lens. Even with
today's advanced lens coating technology, there is often a
visible image quality deterioration if a strong light hits the
lens (due to internal reflection of the strayed light). A so-called "ghost image" is one manifestation. "Flare"
over a large area of the frame is another undesirable
degradation of image.
carry a piece of black cardboard to use as a lens shade (if my
hand is not good enough) in order
to avoid the sun, especially for a wide-angle lens, because the
attached lens hood is often not enough. The only time I allow
the sun to hit my lens is when I am intentionally including the
sun in my photograph.
What Eyes Can See
The unaided eye cannot resolve more than 4
lp/mm on the print viewed at the distance of one foot. (Ref.
Ronald W. Harris. Understanding Resolution. Darkroom & Creative
Camera Techniques, pp.26-66. Mar-Apr. 1991).
One line pair
comprises one white line and one black line. So, 4 lp/mm is
equivalent to 8 dots/mm. This is what the unaided eye can
discern on the print. The table below shows how many dots are
needed on the photographic print to look sharp to the eye based
on this unaided eye's maximum resolving capability. These dots,
of course, must ultimately come from the image captured on the
film (chemical or digital).
(inches) Short-side (mm) Total Pixel Number Needed on
the Film (chemical/digital) *
5 75 mm x 0.5 M pixels
5 x 7 125 mm x 1.4 M
8 x 10 200 mm x 3.6 M
10 x 14 250 mm x 6.0 M
16 x 20 400 mm x 15 M
20 x 30 500 mm x 24 M
30 x 40 750 mm x 54 M
40 x 60 1000 mm x 96 M
* Total pixel number
is computed as (S x 8) x (S x 1.5 x 8) where S is the short side
length of the print (expressed in mm). The longer side of the
print is assumed to be 1.5 time the shorter side. If the print's
aspect ratio is closer to square than 1:1.5 as in film format,
it simply means the print is not taking the full advantage of
the film real estate, and some pixels are thrown away. Still,
for the purpose of pixel number calculation on the film, it is
appropriate to use 1.5 aspect ratio.
Table - How many
pixels are minimally needed on the image in order to produce a
print of various DPIs - provided no pixels are lost during the
printing (from the image to print).
(dots-per-inch) 100 200 300 400
500 600 700 800
Print Size (inch)
3 x 5
5 x 7
8 x 10
10 x 14
16 x 20
20 x 30
30 x 45
40 x 60
Angular resolution of the eye
Viewing distance, to
see the whole picture - not scanning
Max print size from
various format film, resolution at the film
Transfer Function (MTF)
now, we have been discussing the subject of resolution as it
relates to the image sharpness. However, the true measure of
sharpness involves image contrast as well as resolution. In
reality, you can have a lens with an excellent resolution but a
poor contrast, and vice versa. In either case, the final picture
sharpness is compromised. Resolution and contrast must go
A graph of contrast transfer (i.e., how well the subject's
contrast is retained in the image) for a given spatial frequency
(in line pairs/mm) is called the Modulation Transfer Function or
MTF. This measures the image/subject contrast transfer in the
normalized range of 0.0 to 1.0, with 1.0 being 100% transfer.
Often, the values of contrast transfer for a given spatial
frequency are plotted for different locations on the film away
from the image center. MTF graphs assume radial symmetry of the
lens. MTF graphs are also created for various lens apertures,
often for lens wide open, and for some stopped-down apertures.
To be useful, the test target chart (comprising parallel lines)
should be rotated at various angles to see the effect of
aberrations. Typically, for simplicity, only two angles are
used. One with the target lines parallel to the outward line
from the image center, called sagittal (radial) lines, and the other
tangent to the outward line, called meridional (tangential/concentric/circular) lines. MTF graphs
are generally prepared for white light, but can be measured for
a specific monochromatic light as well.
MTF is useful to
describe the quality of the lens accurately. Unfortunately, not
all manufacturers provide MTF data for their lenses. Unlike
resolution testing, MTF data are not readily derived outside of
testing laboratories. To truly describe a given lens, many MTF
graphs are needed, depending on lens apertures, spatial
frequencies, and the light used. It is even possible to measure
MTF for differing subject distances. Such is the case, MTF data
from different sources cannot be directly compared without
noting the conditions under which MTF data are derived.
How To Read MTF
To help interprete
the MTF data, sample MTF graphs are given below for a fictitious
50mm f2.8 lens for 35mm format camera. The upper graph shows MTF
data at lens wide open at f2.8. The lower graph shows MTF data
for the same lens at f8. Each graph shows MTF curves for three
different spatial frequencies: 10 lp/mm, 20 lp/mm, and 40 lp/mm.
For each spatial frequency, a pair of MTF curves is given, one
for the sagittal target lines (green) and one for the tangential
target lines (red). Contrast always drops off as the spatial
frequency increases. Recall from the discussion of theoretical
maximum resolution that the f-stop limits the maximum
resolution. For a given f-stop, contrast drops to zero at that
spatial frequency. Of three pairs of curves on each graph, the
MTF values for the lower spatial frequency (10 lp/mm in this
example) are indicative of the overall contrast of the lens. The
higher the curve throughout the distance range to the corner of
the image (toward right in the graph), the better contrast the
lens exhibits. The MTF values for the higher spatial frequency
(40 lp/mm here) are more indicative of the resolution of the
lens. Comparing the two MTF graphs below, both contrast and
resolution are seen to move up as the lens aperture was stopped
down to f8. General improvements toward the image corner are
provides MTF data for various lenses at the end of each page.
DIFFERENCE DUE TO SENSOR SIZE
Given the same MP resolution
(say, 20 MP), the larger sensor gives the better final result.
That is, a full-frame sensor is better than the APS-C format if
both have the same 20 MP.
1. The higher the MP, the
more pronounced the difference becomes. For instance, at 10 MP,
the full-frame is better than the APS-C format only by 5-15%. But at 500 MP, the
full-frame is twice as good (100% improvement). Recall, some 15
years ago, that Nikon kept the APS-C format for their flagship D2 camera (around 10 MP) exactly
for this reason.
2. The difference is bigger
for the lenses with lower resolution (100 lp/mm) than the lenses
with higher resolution (600 lp/mm). If the lens resolution were
"infinite", there would be no difference in the final picture
quality (resolution) between the 20 MP full-frame camera and the 20 MP APS-C format camera. Of course, that is an impossibility
in physics. But in reality, better lenses will make the sensor
size difference less important.
We are on the subject of
picture sharpness: How sharp the picture looks, on print, for
Resolution vs. Contrast
Resolution is one aspect of
sharpness. In other words, resolution alone does not determine the
picture sharpness; we must consider contract also. You may have
a lens with good resolution but poor contract, and the result is
a poor photograph. The resolution and contrast must go
That said, we are assuming a
lens with a reasonable contrast here, so, for the purpose of our
discussion below (for simplicity), resolution is a main,
determining factor for sharpness.
The final picture sharpness
is an accumulated result of many factors involved in the complex
sequence of photographic process. But among them, the two main
factors are: the lens resolution and the
film resolution (digital sensor MP in the digital world).
Film vs. Lens
Typically, the film is
bottle neck), since the lens resolution is around 300-600 lp/mm
and the current film (sensor) resolution is nowhere near that
So our discussion below focuses on the digital sensor resolution.
Loss of Sharpness
Once you get your camera and
lens, you must know how to use your gear. The sharpness of
your photograph is deteriorated mainly by two factors: camera
blur and poor focus. A tiny amount of camera blur and/or
focusing error will easily wipe out the resolution advantage of
a high MP (mega pixel) camera and a professional quality lens
that you paid an extra five grand for.
Use blur to include focus
error for simplicity....
Blur Detection Sensitivity
The way to think of a high MP
camera is this: The higher the MP of your camera, the more
sensitive it is to detect a small blur in the picture.
A 40 MP camera will detect a
small blur in the picture a 20 MP camera cannot detect. You do
that by viewing the picture 100% pix-to-pix on the monitor.
Probably the movement of the image half the amount of pix-to-pix
distance can be detected at 100% viewing. But that's not a
normal way of appreciating a picture.
So, let us make a print to
truly appreciate and enjoy the picture. For this print test, we
are using a picture
that does not have any blur or poor focus (based on 100% viewing test) here.
If you keep making a larger and larger print, you eventually
come to a point (print size) where the picture is no longer
sharp. This is the print size-limit of this MP camera. If another picture you took with the same gear is
not as sharp at this same print size, it is due to camera blur
and/or poor focus that you introduced - you must improve the way you shoot.
What Went Wrong
* Shutter Speed - the shutter speed you used was too slow. The
conventional "one over the focal length" shutter speed may not
be good enough for a high MP cameras.
* Image Stabilizer - Modern cameras have very powerful
stabilizer. Take advantage of these.
* Tripod - Good, old tripod is the ultimate solution.
* Manual Focus - Maybe helpful to use a tripod for focus
* Auto Focus - Fine-tune your AF system. Focus at the sensor is
If the print is smaller than
the limit of that MP, the print may look sharp even with some
blur. What that means is that at that print size, the amount of
blur that exits is not detectable.
- Decide the print size you
- Determine MP required
- Know how carefully you must shoot (shutter speed / focus)
20 MP (DX)
20 MP (FX)
40 MP (FX)
For a given MP camera (and the
lens) you use, there is a conceptual maximum print size you can
produce without losing sharpness. This is the print size limit of your
But to achieve this limit,
you must exercise a certain degree of care when shooting your
photograph. As mentioned earlier, there are two major causes
that can reduce the picture sharpness: camera blur and poor
For the purpose of discussion
below, we talk about the camera blur, and use the shutter speed
"as an example" of a care that you can exercise to control the
mount of blur when shooting.
The bigger the print you
desire (within the limit of your photographic gear), the more
care you must exercise to achieve the goal (in our example, a
faster shutter speed to prevent the blur).
This shows the resolution, as
expressed in line pairs per millimeter (lp/mm), for three sensor
size formats (full-frame, APS-C, and four-thirds) for a range of
given camera MP (mega-pixel), from 2 MP to 64000 MP.
Total number of dots
in final print for APS-C (DX) and full-frame (FX) sensor for
various MP cameras
TOTAL NUMBER OF DOTS IN YOUR
The following shows the total number of dots (based on the
system resolution) on your photograph for various
Mega Pixel cameras of different formats...