Why Not Convert From 16 Bits to 8 Bits?

I'd appreciate help in clearing up a bit of confusion on my part.

What, in simple terms, am I doing when in Photoshop I convert a file from 16 bits to 8 bits (Image>Mode>8 Bits/Channel) so that I can gain access to Photoshop's filters gallery?

Is it accurate to say that I'm reducing the file's capacity to be manipulated without creating artifacts? If so, is that so concerning color only or concerning tonality and other changes as well? (If the latter, it seems queer that so many of Photoshop's filters, which can radically transform an image, work only on 8-bit files.)

Is there any reason to be hesitant to convert to 8 bits, especially given that I retain the 16-bit Raw file? Is this more of an issue for images I plan to print than for images I plan to display online?

I've written Photoshop actions that include such conversions, and I'm concerned that with the click of a button I'm altering imagines in a way that I might regret later.

In short, is there any good reason not to covert a 16-bit image to 8 bit?

IMO, you've got the idea.

FWIW, I process RAW to a ballpark 16 bit TIF, then drag that into PS and make any large color or luminance adjustments. Optional side trip to Neatimage for long exposure NR. Then back to PS and convert to 8 bit to apply any filters or effects needed (seldom that all much.) Resize, fine sharpen, save as, plonk on Flickr, etc.

As for why some many PS filters work only on 8 bit images?
1. Adobe are lazy bastards.
2. Hey, they've got to withhold some features! Otherwise their Dear Customers won't be so willing to pedal the never-ending 'Upgrade!' treadmill.

Tedder wrote in post #17082597
Is it accurate to say that I'm reducing the file's capacity to be manipulated without creating artifacts? If so, is that so concerning color only or concerning tonality and other changes as well?

Tones are shades of colour so yes, it impacts on tonality as well as colour. Basically you should try to do any colour/tone related adjustments before converting to 8bit.

Actually tones are grayscale ranging from black to white. With an 8-bit image there are 256 (2^8)tones (represented as 0-255); with a 16-bit image that increases to 65,536 tones (2^16).

Dan Marchant wrote in post #17082984
Tones are shades of colour so yes, it impacts on tonality as well as colour. Basically you should try to do any colour/tone related adjustments before converting to 8bit.

It does boil down, though, to doing tonal corrections in the broader range.

There is an interesting history here, though. Until a number of years ago, the "conventional wisdom" was to keep your "projects" as 16-bit files, period and only convert to 8-bits as a last, needed step.

Recently, though, there have been some major developments. For one, Raw processing has improved so that you can do more work in a Raw processor before resorting to a "pixel editor", and then if you put that together with the large resolution of modern cameras, ones that would easily produce 16-bit files of over 50 MegaBytes, well, these considerations have prompted many of us to re-think the "old school" way of approaching things!!

Geonerd wrote in post #17082628
IMO, you've got the idea.

FWIW, I process RAW to a ballpark 16 bit TIF, then drag that into PS and make any large color or luminance adjustments. Optional side trip to Neatimage for long exposure NR. Then back to PS and convert to 8 bit to apply any filters or effects needed (seldom that all much.) Resize, fine sharpen, save as, plonk on Flickr, etc.

As for why some many PS filters work only on 8 bit images?
1. Adobe are lazy bastards.
2. Hey, they've got to withhold some features! Otherwise their Dear Customers won't be so willing to pedal the never-ending 'Upgrade!' treadmill.

Thank you for describing how you work, Geonerd. I make fewer side trips but should probably save the application of filters, which I also don't use often, till late in the process when possible.

As to your two points: Whatever their motives (making money, I'd guess), it does seem to work.

Dan Marchant wrote in post #17082984
...Basically you should try to do any colour/tone related adjustments before converting to 8bit.

Thank you, Dan. That's what I halfway (or maybe three-quarters) thought.

tonylong wrote in post #17083106
It does boil down, though, to doing tonal corrections in the broader range.

There is an interesting history here, though. Until a number of years ago, the "conventional wisdom" was to keep your "projects" as 16-bit files, period and only convert to 8-bits as a last, needed step.

Recently, though, there have been some major developments. For one, Raw processing has improved so that you can do more work in a Raw processor before resorting to a "pixel editor", and then if you put that together with the large resolution of modern cameras, ones that would easily produce 16-bit files of over 50 MegaPixels, well, these considerations have prompted many of us to re-think the "old school" way of approaching things!!

Thank you, Tony, for your reply.

Somewhere along the line I must have fallen in with conventional wisdom. I had the impression that to convert to 8 bits was quite a no-no, so I've been wary of doing it at all.

Rather than being overly hesitant to convert, I should probably just be more thoughtful of where in the process I do it.