DPI question

[rowghani]

So I got some imacon scans done at 5000 dpi and going for 16x20 prints and the print shop told me to change the Image size to 16x20 in Photoshop but to choose 300 dpi for printing. I'm confused. Why scan so high then print so much lower? Thoughts?

 

[Doyle Thomas]

because you wont see any difference in the print. the reason the print shop wants 300dpi is because that is a multiple of the native resolution of their printer and no interpalation is needed. the main reason for higher scan resolution is larger prints. maybe some time in the future print resolution will improve, the major problem at this time is what is called "dot gain", when the ink hits the paper it spreads out due to capillary action.

 

[rowghani]

Im still not getting it. Is scanning dpi different from the dpi setting in the Image Size panel of photoshop? so I scan at a high dpi to get the most detail but then resize the file to 300 dpi ready for printing?

 

[Doyle Thomas]

no, u scan at a high dpi to make a larger print or crop. resolution is resolution don't matter scan or PS. zoom deep into the 5k (pixel peep) file then resize you will see the loss of detail so save the 300 with a different file name to keep options open with the 5k. I always scan at the max optical resolution of my scanner (4800dpi) and print at 360dpi (the native resolution of my printer) what is not used gets tossed.

I do use ON ONE resize which converts the file to a vector file then resizes and rastorizes. its manly for making the file larger and does a great job when adding and does seem to do slightly better when subtracting.

 

[StoneNYC]

Im still not getting it. Is scanning dpi different from the dpi setting in the Image Size panel of photoshop? so I scan at a high dpi to get the most detail but then resize the file to 300 dpi ready for printing?

Yes scan DPI is different than printing DPI.

The printer they use is 300dpi, if THEY downrez it on their end, all sorts of issues can arise like subtracting the wrong color pixels which can change the color tones of the image, where if you do it on your end you ensure the image comes out properly before sending it to them.

Just do as they say. Lol.

 

[L Gebhardt]

Why scan so high then print so much lower?

Because the scan resolution determines how many total pixels you pull from the film. Print resolution determines how many pixels you send to the printer.

Assuming you are scanning 35mm film you have an film image that's about 1 x1.5 inches. If you scan at 5000dpi you will have a digital image with 5000 x 7500 pixels. Note there is no set size associated with this. It's the pixel count that's important. The image file has a setting for dpi, which is used to calculate an image size by dividing the pixel dimensions by the dpi. As long as you don't resample the file it will always contain 5000x7500 pixels.

When you print this file at 300dpi it will be 16.67" x 25" (5000/300 x 7500x300). This will send every pixel in the file to the printer, no more and no less. The difference with printing is the printer needs to know how big you want it on paper. With scanning the film has a fixed size that you can't change (without using a different format).

With printing the printer will take the pixels you give it and either up or down resample the image so it has 300dpi (or some other native value depending on the printer). If you give it 300 dpi it doesn't do any resampling.