Optimal Resolution for Derivatives?

[danaltick]

I currently use 300dpi as my resolution setting in ACR, but am beginning to think that 240 is sufficient for creating the derivatives.  It seems like I remember reading somewhere that the human eye cannot really detect resolutions any greater than that; and I don't want to make my derivative files any larger than they need to be.  Any advice on this.  Thanks.

Dan

[Rick McCleary]

In ACR, the resolution setting only adjusts the ppi of the final output, not the pixel dimensions of the file.

The actual pixel dimensions of the file are controlled in the "size" pop-up.  The native resolution of your camera will be listed in the middle of the list.  To interpolate to a larger file size (pixel dimension), you would select one of the "+" sizes; to interpolate to a smaller file size (pixel dimension), select one of the "-" sizes.  

The native resolution of a Canon 20D is 3504 pixels x 2336 pixels.
 At 240ppi, that creates a 14.6" x 9.733" print.  At 300ppi, it creates a 11.68" x 7.787" print.
In either case, the file size is the same; it contains the exact same number of pixels -- 3504 X 2336 = 8,185344, or 8.2MP.

So, the setting you select under "resolution" will have no effect on the final file size.  

As a rule of thumb, 240ppi is fine for all inkjet ouput and most offset output.  Old school myth clings to using 300ppi for offset, however, I've been using 240ppi for my work (high-end annual reports) for the past two years.  There are some cases where 300ppi will make a difference, but it's rare.

The only output that requires 300ppi (or even 400ppi in some cases) is Lambda/Lighjet printing.

Hope that clears it up.

[danaltick]

Thanks Rick.  Sounds like 240 will work fine then for my deliverables, keeping them smaller.  I like to go ahead and start with that in ACR though, even though it doesn't affect the size of the image until I actually resize/resample in PS near the end of my workflow, but before final sharpening.

Best Regards,
Dan