Converting a 16 bit image

[Roy C]

If you convert a 16 bit image to a 8 bit and then back again to a 16 bit do you lose anything in the way of quality?

[Don Hoey]

Roy,

I just realized other than taking a 16 bit TIFF and converting it to JPEG which is 8 bit, I don't know how to do the conversion and retain it as a TIFF to give it a try.

I will watch this thread with interest.

Don

[yelvertoft]

Originally Posted by Roy C

If you convert a 16 bit image to a 8 bit and then back again to a 16 bit do you lose anything in the way of quality?

Once you've converted to 8 bit, you can't get the "true" 16 bits back again. If you convert back, the 16 bit file will be an interpolation of the 8 bit data, not the original 16 bit data.

Whether you'd ever be able to tell the difference is another matter completely, I suspect not unless you are making large prints of images with exceptionally subtle colour variations.

You could convert to JPEG2000, lossless 16 bit format.

Duncan

[wolfie]

I doubt that you will see a difference a quick test shows that after conversion from a 16.5mb Tiff to 1.4 mb Jpg, then back to Tiff, the file size is again 16.5mb.

Even if you save the 16.5mb as Jpg with 50% compression (263Kb), the conversion back to 16 bit is again 16.5mb.

Given all the above I feel that something as been lost, so all my files are saved as Tiffs.

Harry

[robski]

Yes coverting to 8 bit and back to 16 bits does affect the image. It adds a very small amount of digital noise. In 99.99999% of cases I expect you would not notice it as you don't notice the digital noise when listening to music on a CD compared to the analogue version.

I made a simple image using the gradient tool in PS. On the original version you could see very light bands as the level changed. After the conversion to 8bit and back again the bands disappeared and a random speckling of noise was seen. I then amplified the effect by increasing the contrast on both versions to show the difference in the levels histogram. The upper window is the original and the lower is the converted. In the converted you can see a number of levels are missing. This misplacement of data produces the noise effect.

Ducan is on the right track with his reply.

Sorry for the techno babble but that is the nature of the question.

[sassan]

I fully agree with Ducan. Seems like said it all.
Please pay attention to the 2nd paragraph. There is a reason manufacturers have adopted one formate more universally than others for general use.
Sassan.