I've learned a few things since getting the messages from Michael Bowen and
Michael Porter about how digital cameras may save exposure information in
the JPG file.
As Michael Bowen points out, my digital camera, like most these days, does
store exposure information with the JPG image. When a camera does this, the
information is at least available in the JPG file that is stored on the
memory card that goes in the camera. This exposure information may or may
not be preserved when the file is downloaded to a computer. (The software
performing the download determines this.)
As Michael Porter points out, even if the exposure information is in the
downloaded version, it can be lost if the photo is loaded into a photo
editor and then resaved.
Here are some details of what I found. At the end I also make a few
recommendations about working with digital photos.
(1) In the Adobe line of photo-editing software I have Photoshop LE (limited
edition), I have Photoshop Elements, I have Photoshop Version 5. LE is the
older "stripped-down" version of Photoshop, and Elements is the newer
stripped-down version. I have not upgraded to Photoshop Version 7 because I
find LE and Elements both do just about everything I currently need, and I
got LE and Elements bundled with digital cameras I have purchased. In my
opinion both LE and Elements are fine products.
(2) None of these versions of Photoshop display the exposure information
even if it exists in the file. If the information exists in the original
JPG it will be lost upon resaving it if you use the older versions (LE and
PS-5). It is preserved in the file with Elements even though Elements does
not display it, i.e. does not give you access to it.
Recommendation... do not load a photo into a photo editor then resave it
using the same name. It is wise to keep the original camera version
unaltered. This is good advice even if your photo editor preserves the
exposure information.
(3) Most digital cameras come with some minimal software to transfer photos
from the camera to the computer. However, you might be able to download
photos directly from the camera using standard photo editors. For example,
all of my Adobe photo editors can download pictures directly from my Olympus
camera. However, only Photoshop Elements preserves the exposure information
(even though it won't display this information).
The downloading software that came with the camera does save the exposure
information and also allows me to open the file later and display the
exposure information as long as I have not rewritten the file using software
that does not preserve the exposure information. The camera-provided
software also stores information about multiple panoramic shots so they can
be "stitched" together. This info is also lost if the photos are downloaded
and save with older versions of Photoshop.
Recommendation.... before deleting the photo from your camera's memory card,
be sure you have downloaded the file to your computer using software that
maintains any special information your camera might have saved in the file.
(4) For the five-planet photo, the original photo from the camera was indeed
opened with Adobe Photoshop LE and I rotated the image, adjusted the overall
tonal range, adjusted contrast, applied the unsharp-mask, and did a slight
bit of cropping. Then I re-saved the file... but I re-saved it with a new
name. The edited JPG no longer contains any exposure information, but I was
able to go back to the original JPG saved from the camera, load that file
into the Olympus software, and find the following information....
ISO = 100, aperture = f/2.8, shutter = 16 s
Since the blinking airplane only shows about 12 flashes, its strobe must
have been flashing at slower than once per second.
* * * *
More recommendations....
When using a digital camera you will almost always want to adjust "levels"
and sharpen the picture. Using traditional black and white photography
terms, adjusting levels means adjusting things like latitude, contrast, and
the gamma. Several sharpening routines are available, many people,
including me, like the unsharp mask.
(a) I load an original JPG into my editing program and do the following
manipulations
rotate the image if necessary
adjust levels
apply unsharp mask
This version gets saved with the original name with a letter "a" appended.
(b) I edit-out any unwanted things and do any other corrections (such as
red-eye correction).
This version gets saved with the original name with a letter "b" appended.
(c) I apply color balancing if necessary.
This version, if it exists, gets saved with the original name with the
letter "c" appended.
(d) I perform any cropping if necessary.
This version, if it exists, gets saved with the original name with the
letter "d" appended.
There are several things at work in this sequence.
(I) I am trying to do basic things first, and more judgmental things last.
It is not uncommon for me to change my mind about cropping and color
balance. If I do those edits too early and then do level adjusting,
sharpening, and corrections, then change my mind about the cropping, then I
have to do all the other editing all over again. Hence, those things you
are least likely to change are the things you should do first.
(II) Every time a substantial amount of work is done, I save a new version.
That way I can go back to an earlier version, but don't have to repeat any
work I am already satisfied with up to that point.
(III) Some people advocate saving all the intermediate versions as TIF files
because these are not compressed and do not "degenerate" with each edited
copy. I have found that if the JPG compression is not too severe, I cannot
see much if any JPG degradation. Since TIF files are awfully large, I
typically stick with JPG unless I am working on a really special picture.
Besides, if a picture for some reason becomes really special to me, I still
have the original file from the camera.
For the planet photo, since it is somewhat scientific and documentary, I
didn't want to do much "editing." However, getting the proper orientation,
levels, sharpening, cropping are things that are pretty much essential just
like it is necessary to point the camera in the right direction, focus, and
get the correct exposure. Therefore the exposure information is missing on
the JPG that you might have observed or downloaded from the URL I gave you.
However, the "original photo" from the camera still exists, and thanks to
Michael Bowen's suggestion I was able to go back to that file and find the
exposure information.
Michael Edmiston
Professor of Physics and Chemistry
Bluffton College, Bluffton, Ohio 45817