Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

le boecere

This is becoming an interesting conversation, whitewolf. Can you tell us why (s)he is "Wrong again,...." (about the forensics statement)? I have a close friend who owns and operates a forensics lab. I'd like to take your answer to him.

Thank you.

CatMarley Loc: North Carolina

Don't gang up on the poor guy. It's not a fair fight!

burkphoto Loc: High Point, NC

You have to RE-SAVE a JPEG to RE-COMPRESS it.

If you just copy the file, using the operating system, you get an exact digital copy.

If you open and print the file without saving it, nothing changes.

If you open and view the file and then close it without saving it, nothing changes.

But if you open a file and then SAVE it or EXPORT it as a JPEG, you lose data.

That said, the operant question is always, "Does it matter?" It can matter a lot, or a negligibly little amount, depending on the source of the original file (adjusted in a raw converter vs. straight-out-of-the-camera), the scene itself, and how good the JPEG was to start with. Did you save the highest quality, largest file size possible? Were your raw adjustments made perfectly, in reference to a calibrated and ICC profiled monitor? Did you set the camera menus (ALL of them!) perfectly for the scene?

The irony of JPEG photography is that the closer you are to perfect exposure and white balance at the camera, the more latitude you have to adjust a straight-out-of-the-camera JPEG later.

The largest data loss is on conversion from raw data, regardless of where that happens. So if you have accurate exposure and white balance, and adjust the menu settings correctly for the scene, little visible data will be lost... IF the scene brightness range can be captured using the menu settings and exposure settings available.

That said, there is one helluva lot more data in a raw file. It may be impossible to save it all at the camera, due to the limits of the camera's JPEG processing menus. But manipulating the raw image in Lightroom or a similar software may give you a much better JPEG image, because of the range of control and the wider range of available tones.

Architect1776 Loc: In my mind

Here is a simple answer to various ways of saving and not of JPEGs.
https://www.thoughtco.com/jpeg-myths-and-facts-1701548

burkphoto Loc: High Point, NC

The Finder. The Macintosh Finder is what you see when you click on a drive or folder on the Desktop.

I always loved how Microsoft called their original web browser Internet Explorer, and their file browser, Windows Explorer. It was so unintuitive, it was baffling, and led to plenty of new user confusion (I know, I'm a trainer). Fortunately, they changed that mess. Apple calls its browser Safari. The Finder has always been the Finder, even when MS Explorer was the popular browser on the Mac.

CatMarley Loc: North Carolina

And of course then, the obverse is also true: Lightroom may NOT give you a much better JPEG image with that particular photo, because all the settings were appropriate for the scene. Where Lightroom and RAW pay off is when there are difficult conditions that the usual camera settings cannot adequately compensate and convert to JPEG efficiently. The human operator can then employ the software to tease out of the raw data, the details HE wants to preserve at the expense of some others he values less.

burkphoto Loc: High Point, NC

True! Fortunately, you can preserve the camera's look by changing the defaults LR uses when it opens a raw image... At least I can with my GH4. I don't know how well it works with other brands.

jenny Loc: in hiding:)

You can make any # of copies of your original JPEG without harm to the original, work on those copies, and never
need to degrade your original.

wesm Loc: Los Altos CA

File COPY only moves the bits in the file to another location, with no modification of those bits.

If you open a jpg file and then save it in a PP program, the compression algorithm may run again, which would mean the saved copy will be different from the original. An easy way to test this is to look at the file sizes in bytes for both the original and the saved copy. If different, you know the file was modified. The converse is NOT true.

dsmeltz Loc: Philadelphia

The word "little" is a comparative. If you think that the amount you can do with a JPEG is equivalent to how much you can do with a RAW file, then you do not understand basic comparative statements.

burkphoto Loc: High Point, NC

Or use a program like Lightroom, which never touches the original pixels of any image. You see your changes applied only to proxy images. They are "baked into" only exported or printed images.