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Converting 16bit to 8bit

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  #1  
Old 09-28-2010, 11:49 AM
SymbolPhoto SymbolPhoto is offline
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Converting 16bit to 8bit

Hello All,

We just recently started to shoot with a Hasselblad and the files are converted to 16bit Tiffs. We put them through photoshop doing our usual thing and then convert to JPG which means we have to go from 16 to 8bits.

Is there a preferred method of doing this? Should i not do this as the last step before saving?

I haven't seen any loss of depth in gradients etc, but i figured i'd ask. Just to ensure i wasn't doing anything incorrectly.

thx!
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Old 09-28-2010, 01:04 PM
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mistermonday mistermonday is offline
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Re: Converting 16bit to 8bit

JPG is a very lossy compression format. Each time you open a JPG and save it again as a JPG you continue to degrade the image and by the time you see it, you have done too much damage. If you have to export or archive in JPG format do a File>Save As and select the highest possible setting (12) which creates the largest size file but applies the least amount of compression.
Regards, Murray
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Old 09-28-2010, 01:43 PM
SymbolPhoto SymbolPhoto is offline
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Re: Converting 16bit to 8bit

I have no choice, i'm mostly displaying on the web. So nothing on the converting itself? I don't reopen and save again. I just save once.
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Old 09-28-2010, 01:50 PM
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Re: Converting 16bit to 8bit

OK, your original post did not mention that the output was for web. In that case there is a lot less to worry about. For web output I suggest you use Photoshop's File>Save for Web & Devices.
Regards, Murray
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Old 09-28-2010, 03:19 PM
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Re: Converting 16bit to 8bit

In CS5, a new feature is that if you ask to save as a JPEG from a high bit file, it will be converted to 8-bit for you. One less step.
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Old 09-28-2010, 03:35 PM
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Aladdin Aladdin is offline
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Re: Converting 16bit to 8bit

opening a jpg file in PS million times and closing/saving it will not degrade the file if you don't touch the pixels! also, if some edit take place, only the "region" where the edit took place is affected (degraded).
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Old 09-29-2010, 03:10 AM
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Mike Needham Mike Needham is offline
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Re: Converting 16bit to 8bit

Perhaps this page will shed some light on the subject JPEG Myths & Facts
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Old 09-29-2010, 04:33 AM
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Chain Chain is offline
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Re: Converting 16bit to 8bit

Quote:
Originally Posted by Aladdin View Post
opening a jpg file in PS million times and closing/saving it will not degrade the file if you don't touch the pixels! also, if some edit take place, only the "region" where the edit took place is affected (degraded).
This is wrong.

Opening a JPEG in Photoshop, and saving it back down (even if you haven't changed a single pixel value) will recompress the entire file – degrading it further. This is soften called "generation loss".

Closing without saving will obviously not change it. Also keeping the document open and clicking save 100 times will not do more damage than saving once; and i will look uncompressed in Photoshop (until you close it and open the compressed version)... Maybe it is the latter that confused you?

Last edited by Chain; 09-29-2010 at 04:46 AM.
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Old 09-29-2010, 04:41 AM
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Post Re: Converting 16bit to 8bit

Quote:
Originally Posted by andrewrodney View Post
In CS5, a new feature is that if you ask to save as a JPEG from a high bit file, it will be converted to 8-bit for you. One less step.
That's right, but when the output is for web he should be using Save-for-web, never Save As...

SymbolPhoto:
You can let Save-for-web handle the conversion automatically (it will do it just fine) but if you want to do it manually you can go to Image > Mode > 8 bit. It will give the same result.

When going from 16 bit to 8 bit there will be some dithering taking place (could be a filesize issue issue if it was an illustration to be saved as png) but this should not be noticeable to the human eye and you have no option to turn it off.

To manually convert the profile to sRGB (as it should be on web) you can go to Edit > Convert to Profile...
Ps: The manual conversion allows you to turn off dithering when converting the color space if you want (but for a photo you shouldn't see any difference).

Last edited by Chain; 09-29-2010 at 06:36 AM.
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