Hi,
I need to scan a colour picture-for an output device requiring 75lpi. The original image size is 5.94in x8.867in. As my image is larger than what's needed for print, I need to scale this image 25%. At what dpi do I scan the picture considering the image will be reduced? I will complete the downsizing process in photoshop.
#1
DPI scan resolution = (scale%) x (2 x LPI)
.... [ if scale% is unknown, use (final size)/(source size)x100 ]
So yours is ...
DPI scan resolution = 25% x 2 x 75 = 37.5dpi
Your picture will be ( 5.94 in x8.867in ) x 25% = ( 1.485 in x
2.21675 in ) @ 150dpi = 223 x 332 pixels.
Oh God, this is math, isn't it, I hope I did it right.
#2
I need to scale this image 25%.
You did mean "scale TO 25%" right?
Or did you mean "scale DOWN BY 25%"?
In the latter case, your scale% is 75, not 25.
#3
To get 75 lpi at 100%, you would scan at 150 ppi. Since you want to have a smaller image (I am assuming 25% means 50% of width and 50% of height, you could scan as low as 75 ppi. But I would scan bigger, like 300 ppi, since you lose very little data downsampling. Besides, 75 lpi is pretty cruddy printing quality anyway.
#4
Besides, if you scan bigger, you already have the larger scan should you need it at a later date
#5
Thanks for your reply.
I have one other question. If I scan the image at 100% at 150ppi, and I scale it 25% in photoshop (unconstrained), the resolution jumps to 600ppi. I know the ratio of lpi to dpi is 1:2. So would that not make the finale lpi 300?
Thanks
Sue Townshend
#6
Sue,
I know the ratio of lpi to dpi is 1:2<<
The relationship isn't a direct one. The lpi is chosen when making the film. The same file can be imaged at any lpi the imagesetter supports, regardless of the dpi on the image.
In fact having a file rez at twice the lpi is overkill - 1.4x is quite sufficient over-sampling for the vast majority of images. So you could successfully print a 300 ppi image at any lpi up to 214 lpi with no problems as far as image rez is concerned.
#7