The lossy compression always affects the end result--but whether or not you
can see it depends on how you display or print the image, and on subject,
and on the particular algorithm.
You may think you are controlling what PhotoShop does to your image file,
But digital filters peform very complex numerical transformations--which
may or may not lose information.
And you are viewing the results "though a glass, darkly" -- a very imperfect
monitor. And what-you-see is never what-you-get when making a print.
All often, probelms aren't spotted until the print is drying.
When you look at a final image (e.g, a print), you only care about subjective
impression. But when processing an intermediate image (a negative or an
image file), you need objective information.--an accurate assessment of image
qualtiies: tonal values, acutance, resolution, gradation, global contrast, etc.
It's impossible to do that using only the naked eye and an image displayed on
a montor.
We didn't hold our negatives up and peer at them. "I like that one--it's
pretty!" We put them on a light table and examined them with a high
quality loupe and a densiometer. (I also use a binocular microscope
with a reticle)
Can you tell black from gray by looking? No, you can't. What looks pure black
to the eye may contain three or four tones. If you scan the negative or lighten
the image file in processing, you can print those tones. This is as true of
image files as it was of negatives.
There are other problems with "seeing is believing":
* The eye is easily fooled into thinking tones are different that are in fact the same:
http://www.illusions.org/* It is easy to miss changes that are gradual. If you apply five different filters and each
degrades your image a little bit, it may not be obvious. But if you compared the
original to the last revision, you might be shocked.
* Viewing size matters. Anything that increases acutance (at the price of gradation and
resolution) will look good (sharper) on a small display. But when you go to print, the loss
of gradation and detail will be glaringly obvious.
* Background lighting matters. Process a phtoto in a dimly lit room and it will look
very different than if it is processed in a brightly lit room.
* A monitor is brighter than any paper. What looks luminous (and is luminous!)
on a monitor may look murky on paper.
* Maximum black on paper is much blacker than black on any montior (except a CRT).
though OLEDs come pretty close. But you should be aware that there can be
much more contrast in your image file than you can see on a particular montior
(e.g., an LCD/LED screen).
* Or you may be assuming there is more conrtrast in the image file, and there isn't! You
can't tell by looking at an LCD/LED screen. But global contrast is important in deciding
how best to print the image.
* Any digital filter such as "sharpen" that increases acuance (at the cost of gradation and resolution)
will a small image look better. But when you print it, the loss of gradation and detail will be glaringly
obvious.
Some "sharpen" filters are better than others. When you shop for processing software, how do you
compare the filters between one package and another? There are hundreds.
Only the final image matters. Your subjective eye can judge the final image--but not a
negative and not an image file.
An image file is not a photograph--it's a table of numbers an encoding of an image.
Digital filtes operated upon the number--not upon an optical image. So just about
any sort of transformation is possible.
It might, for example, increase the redness of every prime-numbered pixel by 1.
How you gonna spot that?
A playful programmer my chose to encode his initials -- or face! -- into your image,
so it could only be seen with a colored filter. Wouldn't that be a laugh!
There are "concealment ciphers" the encrypt a text message into an image file
(not the header or EXIF -- the image itself) in such a way that it can be extracted
with the right software. (This is a big problem for the NSA.)
A digital filter algortihm may perform a dozen steps. You don't know what's done
to the numbers, and you can't control it.
You wouldn't develop film by dipping your finger into the developer to judge the temperature,
then counting "one-Mississipi, two-Mississipi...". You'd use a thermometer and a timer!
Photographer used to be scientific instruments. Everyone knew that a desniometer saw things
the eye cannot see.
And because dodging and burning were optical, you knew exactly was and wasn't being
affected. In any optical processing, we know what kinds of degradation are possible, so
we know what to look for.
But a digital filters is a secret algorhitm -- it can do
anything. Nobody is going to pan all
over their image on the montior, magnifying every part and looking for every possible kind
of degreation. (And some -- like loss of contrast -- may be invisible on a paricular monitor.)
Your photographic results are now limited by extremely complex algorithms that you didn't write,
haven't read, and don't understand. And you have substituted your eye for a densiometer, making
the whole process erratic.
If that isn't a loss of control, I don't know what is.
The lossy compression always affects the end resul... (