It is my understanding that a well exposed photo will show up as an even-ish
bell curve roughly in the middle of a histogram display...I am finding that
some of my shots are showing the nice bell-shaped curve, but oftentimes i
get a very sharp , narrow spike on the extreme right of the histogram.
what does this mean, and what settings should I alter in order to get the
smooth bell curve to indicate a decent exposure?
"the_niner_nation" <the_niner_nation@sf49ers.com> wrote in message
news:464891dc$0$16393$88260bb3@free.teranews.com.. .
> Hi, I have a histogram enquirey...
>
> It is my understanding that a well exposed photo will show up as an
> even-ish bell curve roughly in the middle of a histogram display...I am
> finding that some of my shots are showing the nice bell-shaped curve, but
> oftentimes i get a very sharp , narrow spike on the extreme right of the
> histogram.
>
> what does this mean, and what settings should I alter in order to get the
> smooth bell curve to indicate a decent exposure?
>
> thanks in advance!
"the_niner_nation" <the_niner_nation@sf49ers.com> writes:
>Hi, I have a histogram enquirey...
>It is my understanding that a well exposed photo will show up as an even-ish
>bell curve roughly in the middle of a histogram display...I am finding that
>some of my shots are showing the nice bell-shaped curve, but oftentimes i
>get a very sharp , narrow spike on the extreme right of the histogram.
>what does this mean, and what settings should I alter in order to get the
>smooth bell curve to indicate a decent exposure?
The sharp spike is due to all of the areas of the image that are too
bright to represent correctly being recorded as a value of 255, which
artificially inflates the height of that one histogram entry.
As for what to do, it depends on whether those areas being overexposed
is a problem or not. It's very common to get this if you're shooting
outdoors and your subject is in shade but some portion of the frame is
in full sunlight, or your image includes clouds in the sky. If you
don't care about the overexposed area, and the subject you do care about
is well exposed, don't change anything.
On the other hand, if you want all areas of the image to be captured in
the available brightness range, you'll need to do one or more of:
- reduce exposure (faster shutter or smaller aperture in manual, minus
exposure compensation in auto
- reduce contrast (via a menu on some cameras)
- capture in RAW, which usually preserves more highlight range
- capture multiple images at different exposures and combine them via
HDR techniques.
the_niner_nation wrote:
> Hi, I have a histogram enquirey...
>
> It is my understanding that a well exposed photo will show up as an even-ish
> bell curve roughly in the middle of a histogram display...I am finding that
> some of my shots are showing the nice bell-shaped curve, but oftentimes i
> get a very sharp , narrow spike on the extreme right of the histogram.
>
> what does this mean, and what settings should I alter in order to get the
> smooth bell curve to indicate a decent exposure?
>
> thanks in advance!
What the curve "should" look like depends entirely on what you're
shooting. If it's the photo in the "My image is way too dark" thread
above, for example, where most of the image is SUPPOSED to be dark with
a couple of very bright spots, then the histogram "should" be a lot
heavier to the left, with a narrow spike toward (but not all the way at)
the right. If you're shooting a snowboarder on a sunny day, you WANT
the histogram to be massed towared the right, lest the snow be underexposed.
the_niner_nation wrote:
> Hi, I have a histogram enquirey...
>
> It is my understanding that a well exposed photo will show up as an even-ish
> bell curve roughly in the middle of a histogram display...I am finding that
> some of my shots are showing the nice bell-shaped curve, but oftentimes i
> get a very sharp , narrow spike on the extreme right of the histogram.
>
> what does this mean, and what settings should I alter in order to get the
> smooth bell curve to indicate a decent exposure?
There's no actual "right" or "wrong", since you may want to render a
scene all sorts of different ways for artistic purposes.
However, a big spike at one end means lots of clipping at that end. You
may or may not wish to clip; there's nearly always *some* clipping that
you want (or at least can't avoid). With clipping on the bright end
(usually the right is the bright), you're losing highlight detail in the
brightest areas. If those are specular reflections, that's generally
okay, otherwise it's at best a compromise you may be forced to make.
If you don't want to clip the highlights, decrease exposure -- shorter
exposure time, smaller aperture (larger f number), or lower ISO speed.
In most real-world scenes, you have to clip either the highlights or the
shadows *some*, because the brightness range simply exceeds what your
camera can record.
On Wed, 16 May 2007 14:08:10 -0500, David Dyer-Bennet <dd-b@dd-b.net>
wrote:
>the_niner_nation wrote:
>> Hi, I have a histogram enquirey...
>>
>> It is my understanding that a well exposed photo will show up as an even-ish
>> bell curve roughly in the middle of a histogram display...I am finding that
>> some of my shots are showing the nice bell-shaped curve, but oftentimes i
>> get a very sharp , narrow spike on the extreme right of the histogram.
>>
>> what does this mean,
>
>.............. a big spike at one end means lots of clipping at that end.
A narrow spike at the extreme right of the histogram is probably
propagation light waves reflected from glass or metal in the scene.
There is usually not much of this specular reflection in an image
which is why it is narrow and it is blinding white which is why it is
a spike. Don't worry about it.
the_niner_nation wrote:
> Hi, I have a histogram enquirey...
>
> It is my understanding that a well exposed photo will show up as an even-ish
> bell curve roughly in the middle of a histogram display...I am finding that
> some of my shots are showing the nice bell-shaped curve, but oftentimes i
> get a very sharp , narrow spike on the extreme right of the histogram.
>
> what does this mean, and what settings should I alter in order to get the
> smooth bell curve to indicate a decent exposure?
>
> thanks in advance!
>
Your idea of a bell curve is not correct with regard to histograms.
There are more exhaustive explanations on the net, but briefly, a
histogram is a particular kind of graph which shows a range of values on
the x-axis, and the relative quantity of the data relating to a
particular x-axis value plotted on the y-axis. For example, a group of
people are measured for height, and a histogram is plotted for the
number of people of a given height against a range of heights along the
x-axis. If the x-axis is calibrated from 2 feet high at the left, to 7
feet high on the right, then it's fairly obvious that there will be few
people at the left or the right end, i.e. 2 feet high, or 7 feet high.
But through the range of common heights, the vertical axis will show the
number of people whose height falls into each height 'slot'. In this
case, one would expect a bell curve, of course.
In the case of a digital histogram, the height of the graph at any point
on the x-axis is indicative of the number of pixels in the image that
correspond to the brightness at that point. Since a scene can have
almost any distribution of tones, the curve can be almost any shape. A
large, dark foreground with a bright sky background will produce a
histogram with both ends high but a low middle range, directly
reflecting the tone distribution of the original scene. There is no
correlation at all with bell curves in this instance.
Histogram readings.. 
Linear Mode
