Bipod

Exposure. Off-topic, but relevant to gray cards.

One thing you can't change in post-processing is exposure.
You can lighten and darken the image you've got, but lost
detail in blown highlights or burned out shadows cannot
be magically restored. What the sensor failed to capture
is gone forever.

So exposure needs to be accurate. If you always use flash,
then the shutter is not involved in exposure. In all other
cases, it is.

All digital cameras (except the very cheapest ones) have
electro-mechanical shutters. Like everything mechanical,
they are not perfect, even when brand new. Tolerances
are as good as they can make them, where they make them--
which is more likely to be in China that in Switzerland. :-)

If its an absolutely top quality camera -- the factory may test every
unit, and includ a shutter calibration card in the box. When's
the last time you saw one of those? :-)

Over time shutters wear, lubricants dry out, and so speeds change
(often very greatly!). It's more common for shutters to get slower
than faster, so old cameras tend to underexpose.

May other things can make exposure inaccurate: variations between
lenses (apertures, contrast lost to stray light).

In film cameras it was possible to measure shutter speed directly
with a very simple set up (desk lamp, photocell & cable, PC
and free audio software -- yes, I mean audio). On a digital
camera, it's not possible.

So on a digital camera, you have to calibrate exposure the
old fashioned way: using a grey card and an extremely
elaborat and time-consuming procedure:

* constant, uniform light of known EV
* large black backdrop to prevent stray light from
entering the lens.
* keeping all other variables: lens, aperture, etc constant
* shooting multiple shots in raw mode at every combination of aperture
and shutter speed
* examining the raw mode image files with software (or on film camera,
the developed negative is measured with a densiometer).

Sound like fun?

But if you don't do it, you'll never know why you blew so many shots,
never learn that on your camera, shutter setting 100 is actually
1/80th of a second, not 1/100th, while all other speeds are fine.
Life really sucks, sometimes.

My impression is that most people adopt the scattergun approach:
"shoot lots and cull". The slightly more evolved bracket exposure.
Ansel Adams is rolling in his grave. :-)

There are shooting situtations when anyone would bracket. But not
calibrating your camera -- white balance and exposure -- is like not
tuning up your car. It may have electronic ignition and a computer,
but it still has timing.

rcarol

"Over time shutters wear, lubricants dry out, and so speeds change
(often very greatly!). It's more common for shutters to get slower
than faster, so old cameras tend to underexpose."

It seems to me if shutters get slower, then they would over expose rather than under expose.

burkphoto Loc: High Point, NC

Ah, the daily dose of proofreading failure on UHH... Yes, slower shutters overexpose.

PeterBergh

Sort of. Calibrating a monitor involves setting its color temperature, which for most purposes is close to white balance.

burkphoto Loc: High Point, NC

That is only a very small part of the process...

PeterBergh

I didn't intend to imply that it is not a small part..

burkphoto Loc: High Point, NC

I didn't take it that way. I sort of meant I would reply [to the group] later:

Calibration is something a lot of newbies here don't get. In a nutshell:

Calibration makes sure that the three color channels of a monitor (RGB) are producing perfectly neutral gray from black to white, at all levels. In other words, the output response is linear. It also conforms to target aims for black brightness level, white brightness level, color temperature, gamma...

ICC PROFILING makes sure that the color you see on your monitor is realistic, according to internationally recognized standards. When that is the case, images (potentially, at least) can be printed as they appear on the monitor. They will also look their best on the vast majority of other peoples' monitors, whether or not they're in perfect calibration and have freshly made custom ICC profiles.

Many factors influence color. It is darn near impossible to get truly PERFECT color. But you can make a reasonable facsimile of it. White balance, camera profiles, the "matching or working color space" used by the operating system, monitor profiles, and output profiles (sRGB, Adobe RGB, or printer/ink/paper profiles) are the key touch points. For WYSIWYG prints, all of these must be in line with each other.

Bipod

It is impossible to stitch together "sound bites" into a symphony,
or statements of opinion into a conclusion.

All day birds twitter in the trees--they make a bucolic sound,
but they never get any wiser.

steve03 Loc: long Lsland

I shoot in Raw and my WB on auto and make adjustments in LR. I very rarely have to make adjustments. I do have a gray card and color checker but I never have to use them any more.

Pablo8 Loc: Nottingham UK.

Over time shutters wear, lubricants dry out, and so speeds change
(often very greatly!). It's more common for shutters to get slower
than faster, so old cameras tend to underexpose.
***********************************************
Sorry, Bipod...………. If the shutter gets slower..... The resultant image will be OVER exposed. The shutter is open LONGER...Lets more light in!!

Bipod

Of course. Good catch.