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Re: filmscanners: Re: Scanning problems



Hi Julie,

I certainly empathize with your concerns.  The unfortunate (and good) 
thing about the JPEG compression method is the compression setting can 
vary from making absolutely no difference in the recreated image 
(basically lossless) to one that looks like it was shot with a web-cam.

Unfortunately, I do not know of any method to determine which level of 
JPEG compression was used on any image after the fact.  A reduction of 
an image to 15% of it's original size would usually mean some loss of 
some information.  But because of the nature of the JPEG algorithm, if 
you had an image with large areas of a single color, and not very much 
detail, even the lossless compression might produce that keep of 
compression ratio.  However, if all the images are compressed to about 
that degree, you can be assured info was lost.

One way of determining this would be to look, within Photoshop, at the 
image under high zoom and see if you not a clumping or blockiness to the 
pixels or "noise" (really artifacts) around small dots with high 
contrast.  If you see this clumping, you have obviously had loss, as 
JPEGing "averages" similar colors to make less need for information from 
each pixel.

If you have some digitally stored images that were not compressed in 
JPEG (Saved as TIFFS, or Photoshop format, as examples, I suggest you 
experiment with them in Photoshop by making different JPEG compressions. 
  See what it takes to reduce a "typical image" of yours to 15% of the 
uncompressed format.  Then look at the JPEG you made and see if it has 
similar "structure" under a high zoom in Photoshop.  If so, then compare 
to the original side by side in Photoshop at 100% magnification.  That 
should give you some idea of the loss which occurred from the 
compression done by your digital lab.

Art
Cooke, Julie wrote:

> I've just had my 6x7 trannies scanned by a lab specialising in digital. I
> paid for a 50MB scan and got a 7MB jpeg back. I took the CD back to the
> manger, who told me that it was a 50MB scan, compressed to 7MB and that all
> the information would be there when I opened it up!!! I think by this he
> means that when you go to Image Size in Photoshop it does say 55MB (4800
> pixels x 3900 at 300dpi), although the file is 7MB. Is this the file size if
> saved to psd format? I've lent my Real World Photoshop book out. 
> 
> I explained that jpeg was a lossy compression, that information had been
> lost when converting to jpeg and it was no good to me. He looked at me as if
> I was mad and said that he uses jpeg for all his customers to get more
> images on the CD. I argued that jpeg does loss information and drops the
> colours that the eye cannot see. He told me that I had to specify tiff
> otherwise he saves as jpeg, then reluctantly said 'so you want me to do the
> whole job again'.
> 
> This lab specialises in digital, how can people accept jpegs? So much
> information has been lost for manipulating  (levels and curves) in
> Photoshop. I won't be going back there again...
> 
> Julie 





 




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