Understanding Resolution - Do You Need More Pixels? in Photography
May 05, 2009, 06:44 PM
by Felisia A
Understanding Resolution - Do You Need More Pixels?
limitation comes from the lens resolution. There''s a point where more
megapixels simply becomes a burden without bringing any perceivable
benefit at all. I personally believe with 35mm, it is probably around
20 megapixels. Of course, there is not a clear cut off line, but a
With 35mm full frame, the first critical point of megapixel
arrives at 4M. I will show you why below. Before reaching that point,
any increase of number of megapixel resulting a quite strict one-to-one
increase of resolution. For example, 2M quite literally has twice as
much much resolution as 1M. After that, this will start to degrade. For
the matter of better communication, consider a coefficient called ratio
between the perceivable resolution increase and the pixel number
increase. This ratio is practically 1/1 before the number of pixels is
4M, but starts to fall off gradually after that. My estimate is that it
would fall to less than 0.5 when the number of pixels reaches 10M, and
probably less than 0.1 at 20M. I''m pretty certain that this ratio
approaches 0 by 40M. What
that means is that after 10M, the perceivable resolution increase one
receives from every 1M increase of megapixels is less than .5M, and
after 20M, the perceived resolution increase for every 1M is less than
.1M. As a practical matter, after 20M, the more megapixels simply
become "the Emperor''s new cloth" which really is not there to be seen. It just makes some people happy. And I''m talking about full frame 35mm. For APS sensors, the number is even smaller.
All that is because the inherent limitation of the lens resolution.
Take 35mm full frame. I''ll use approximation to simplify the
explanation. Suppose there are N columns (or rows) of photosites for
every 1mm. The total number of pixels of a full frame 35mm sensor is
then (Nx36)x(Nx24), which is roughly 1000N^2 (pixels). Now, good 35mm
lenses usually have excellent MTF at 30 line pairs per millimeter.
Especially at the center of the frame, a good 35mm lens can have 80% or
above MTF at 30 line pairs per millimeter. So it is reasonable to take
30 line pairs per millimeter as a baseline. A sensor that matches this
resolution would have N=2x30 (two photosites for each line pair), and
correspondingly the total number of pixels of 1000xN^2 = 1000x60^2,
which is approximately 4M (or slightly less). This tells you two sides
of the story. First, on a 35mm full frame sensor, before the number of
pixels reaches 4M, most lenses have enough resolving power to take full
advantage of the more pixels. Second, when the number of pixels goes
beyond 4M, most lenses will not be able to fully take advantage of the
increase of the pixels.
That of course doesn''t mean that after 4M, the increase of the
pixel numbers will have no benefit at all. They still do, but will not
have the full benefit due to the lens resolution limitation.
For example, suppose you have a lens that has an MTF value of
50% (or MTF50) at 50 line pairs/mm (meaning that at this resolution,
the lens delivers a perceivable contrast of only 50% of the full
contrast, i.e., losing 50% of contrast). A sensor that matches this
resolution would have N=2x50=100, and correspondingly a total number of
pixels of 1000xN^2 = 1000x100^2 = 10M. That means, with this lens, a
10M sensor is able to represent each line pair with two columns/rows of
photosites, but with a loss of 50% of contrast at such resolution. It
could be said that at this level, you are only getting 50% of the
perceivable resolution out of the increased number of pixels.
Now, those who are familiar with lens tests will tell you that
a 35mm lens with an MFT50 at 50 line pairs/mm is a very good lens. For
35mm full frame, this is about 1200 lp/picture height.
With a 20M sensor, you need a lens that has an MTF50 with 70
lp/mm or an equivalent of 1700 lp/picture height for full frame. This
is already surpassing the optimal resolution (center of the frame with
a stopped down aperture) of even the best prime lenses. And we are
talking about MTF with a 50% loss of contrast here.
Overall, even with a full frame, going from
10M to 20M has less than 50% increase in perceivable resolved pixels
even with the best lenses. You''re certainly not doubling the resolved
pixels there. Of course, even an increase of 50% can be important for
some people who (1) actually need a greater resolution; and (2)
actually have possession to the top lenses and use them. On the other
hand, with the same technology, going from 10M to 20M will lose at
least one full stop of the ISO performance. I have no
problem with people''s preferences. I just have a problem with people
making choices without the necessary information and understanding.
Still, one could make a rational (albeit discounted) argument
for 20M for a full frame sensor. But beyond that, it certainly looks
like an Emperor''s new clothes to me, unless they suddenly come up with
drastically better optics, which is extremely unlikely.
Besides, unless you want to make prints larger than 30 inches,
you really don''t benefit from more than 20M anyway even if you had a
lens that could resolve more. And you''re not getting those large files
as a freebie. They impose a heavy tax on your computer system.
Dinogroups™ is powered by VEELABS. Please also visit
DinoMarket - Tempat jual beli, pasang iklan dan buat toko online gratis.
DinoBar - The stylish online shopping for gifts and flowers