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The
Image Quality
The image quality is the first thing to consider
when you are choosing a camera. Of course, there are also other very
important features, as FPS, sensor size, autofocus, etc, but a good
image quality is essential - would you buy a super fast, sturdy and
advanced camera if it had a crappy image quality? I guess the answer
is no. When I choose a camera, I want both features and image
quality: for this reason, I pay much attention to resolution, noise
and dynamic range.
The resolution
The resolution is an often misunderstood topic: the
megapixels are not resolution. The number of
megapixels is the number of photosites that compose the
sensor; usually, every photosite corresponds to a pixel in the
finished image. Generally, an higher number of pixels correspond to
higher resolution, but it is not always true, since the structure of
the sensor, the anti-aliasing filter, the demosaicing algorithms and
other factors influences the resolution.
The Sigma SD14 is a
good example of the fact that megapixel aren't resolution:
even though it gives "just" 4.66 megapixel photos,
it has the same resolution of some 8-10 megapixel cameras as
Canon 30D the Nikon D80. It may sound odd, but the reason
is quite simple: the Nikon D80 and the Canon 30D have a
one-layer Bayer sensor, while the Sigma SD14 has a three-layer
Foveon sensor, that "packs" more resolution in a
minor numer of pixels.
To measure precisely the resolution, it is necessary
to make a test using the ISO (International Standard Organization)
chart, to determine the number of lines that the camera can resolve.
Luckily, you don't have to do the test by yourself: there are
various excellent websites that test cameras and publish online the
results. The most complete and reliable website is Dpreview.
Every Dpreview's review list the
actual resolution of the camera, measured in LPH (Lines per Picture
Height), both horizontal and vertically, and it shows some crops
from the resolution chart. |
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Nowadays, the majority of SLR cameras have enough
resolution even for very large prints. Even cheap cameras as the Canon
400D or the Nikon D80 have enough
resolution for razor sharp, detailed 40x60 centimeters prints. Since
it is rare to print larger than that, you might wonder if super high resolution
cameras as the Canon 5D or the Canon 1Ds MkIII are truly necessary.
In my opinion, the answer is surely yes: more resolution is useful
for large prints, but it also allows more substantial crops.
Of course, to achieve the maximum resolution of the
camera you need to use good lenses and excellent techniques. Other
than that, there are also other variables, as the noise, that
determine the maximum prints size, for example, a camera might have
a very high resolution but a strong noise; the resulting images will
have a grainy, unpleasant look, even if they are very detailed.
Remember that interpolation doesn't increase resolution.
If you upsize the file with interpolation, you get just a larger
image and bigger file sieze, but you don't increase the detail.
Interpolation is useful only to avoid pixelization when you make
very large prints; in this case, you have to interpolate the photo
to a large size, to avoid showins the pixels in the print.
The noise
The sensor of every digital camera has
a fixed sensitivity, called "base sensitivity", that
correspond to the lowest sensitivity available through the standard
ISO range. The majority of SLR cameras have a base sensitivity of
ISO 100, even though few cameras have a base sensitivity of ISO 200.
To obtain higher sensitivities, the camera amplifies the signal
registered by the sensor; the unavoidable small errors and
imprecision that occurs during the amplification create a grain-like
disturbance, the noise.
The amount of noise is proportional to
ISO sensitivity - high ISO values have more noise than low ISO
values. That said, the amount of noise depends also by the camera.
Until some time ago, Canon was by far the best choice for those who
need low noise; nowadays, both Nikon and Canon offers DSLR with
extremely low noise, as the excellent Canon 1D Mark III or the Nikon
D3 (that has an ISO 200-25,600 range!)
If you want to judge the noise of a
camera, I'd recommend to download some full size samples from
reliable websites, as Dpreview. Avoid studio tests, in particular
those that try to rate the noise and image quality with graphs and
numbers - they are useless or even misleading. Many cameras use
noise-reduction algorithms that gives very noise-free images even at
high ISO, but they kills the subject detail. In studio test these
cameras might get good rates, but as soon as you give a look to
"real world" photos you will realize the difference
between tests and reality: the cameras that use excessive noise
reduction gives "plastic", artificial-looking photos. The
Canon cameras, instead, needs much less noise reduction since their
sensors give a cleaner output, and in practice they gives better
images, in particular at high ISO.
You might wonder
how much important it is to have low noise. If you photograph
mainly landscapes, the noise doesn't matter - in landscape
photography, there is no need to raise ISO sensitivity, since
the camera is almost always mounted on the tripod. Almost all
SLR cameras have very little noise at ISO 100 so, in this
field, the resolution is more important than noise. If you
photograph wildlife, instead, it is very important to choose a
camera with low noise: you need fast shutter speed the avoid
the blur due to camera shake and to subject movements, so you
will often use ISO 400 or higher. When I photograph wildlife,
I almost always use ISO 400 (or even 800): it is difficult to
predict the subject and you need high ISO if you want to be
alway ready to capture amazing action-stopping image. For this
kind of photography, I wouldn't hesitate to prefer a camera
that has low noise to one that has high resolution - of
course, if I can have both low noise and high resolution it is
even better, that's the reason why I use the 40D.
Other than choosing a camera with
low noise, there are some techniques to get better results at
high ISO. First, pay attention to the exposure: if you have brighten up with Photoshop an
underexposed photo you increase noticeably the noise. For
example, if you take a
|
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| Canon
20D with Canon 300 f/2.8 L IS, 1/2000 f/2.8 iso 800. Low
noise is important for action and wildlife photography. |
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| photo at ISO 200
underexposed by two stops, it is possible to recovered the
right exposure with Photoshop, but you get the same noise of
ISO 800 (or even more). Whenever possibile, I try to get the
brigtest possible image, of course without blowing the
highlight.
Even with the best cameras and best techniques you still
get a little of noise at ISO 400 and higher: to improve image
quality, I often use the Reduce Noise function and the Layer
Mask of Photoshop, to apply a slight noise reduction on the
subject and a stronger reduction on the background. |
The
dynamic range
The dynamic range determines the
maximum difference of brightness between lights and shadows
that can be handled correctly by the sensor. If a camera has a
DR of 6 stops, for example, it is able to expose correctly the
scenes where the difference of brightness between the darkest
area and the lightest area up to 6 stops. If the brightness
range of the scene exceed the dynamic range, the highlights or
the shadows will lose detail. Of course, a wide dynamic range
is better, but currently the digital cameras have already
reached a DR of 6-8 stops, that is enough for the majority of
situations.
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| The
photo on the left is taken with a camera that has
low dynamic range, while the photo on the right is
taken with a camera that has wide dynamic rage,
and it shows a lot more detail both in shadows and
highlights. |
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The dynamic range of a camera
is not constant: it depends by the ISO sensitivity and
even from subjective factors. If you have a quite
noiseless image, you can brighten up considerably the shadows,
enhancing the visible dynamic range; if, instead, the photo is
noisy, you can not brighten up a lot the shadow, because the noise
becomes immediately more visible, lowering the image quality. The
amount of noise that can be considered
"acceptable" is subjective, so it can be said
that even the personal tastes of the photographer
influences the amount of dynamic range that can be
extract from a RAW file.
The bit depth is often confused with the dynamic
range. The bit depth is not the DR, it can be considered as a
container for the DR. For example, if the camera has a
DR of 3,600 levels of brightness
a bit depth of 12bit (4,096 levels) is enough to contain it; a lower
bit depth, as 8 bit (256 values), will compress it eliminating
some values; on the other hand an unnecessary high bit depth (e.g.
16 bit) will increase the file size without increasing the dynamic
range. This is one of the reasons for using the RAW format, that has
a bit depth of 12 bit, while the JPEG is just 8 bit.
The exposure has a strong impact in dynamic range. With film, the best exposure was the one that gave
an image with the right level of brightness. With digital,
instead, the best exposure is the brightest image that you can get
without overexposing the highlight, because it gives the widest
dynamic range and the lowest noise.
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| By
"overexposing", of course without
blowing the highlights, you increase the detail
in the shadows; use the "histogram" and the
"highlights" screen to check the image. It should look
quite light, but there must not be "hot"
areas (i.e. areas that blinks in the highlight screen). |
When you open the file with Photoshop, darken
the image with the Levels tool, until it has the correct
brightness. The resulting photo has the widest dynamic range
that you can get with your camera (if you need to open up the
shadows, you can extract a lot of detail). Moreover, it has
1-2 stops less noise than the noise you would usually get at a
given ISO sensitivity.
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Do you have
comments or questions?
If you have comments or questions about this
article, feel free to ask in the Juza
Nature Photography Discussion Forum!
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