DPI & PPI calculation formular?

[Tri Tran]

Hi all,
1.I'm still confusing about the ratio of DPI & PPI.What would be the minimum resolution dpi should I scan from 4x5 or 8x10 to produce 40x50 print at 300 ppi ? Does it has a formula to calculate this or you have to test it as far as you go?
2 .For screen 1030 IA users do you use direct dpi scan or magnification scan?
3.What mode do you use for B&W negative to scan and how do you set your parameter ?

Thank you,

[Gordon Moat]

40" X 300 = 12000
50" X 300 = 15000

4x5 and 8x10 film are slightly smaller than those dimensions in inches. You would doing more than 10x enlargement of 4x5 (or >5x enlargement of 8x10) for that print size. Slightly smaller would fit a little better, unless you want to include the film edge area in the final print.

Ciao!

Gordon Moat Photography (http://www.gordonmoat.com)

[Bruce Watson]

Hi all,
1.I'm still confusing about the ratio of DPI & PPI.What would be the minimum resolution dpi should I scan from 4x5 or 8x10 to produce 40x50 print at 300 ppi ? Does it has a formula to calculate this or you have to test it as far as you go?
2 .For screen 1030 IA users do you use direct dpi scan or magnification scan?
3.What mode do you use for B&W negative to scan and how do you set your parameter ?

Thank you,

You are confused perhaps because there is no fixed relationship between the pixels per inch (PPI) resolution of an image file and the dots per inch (DPI) the printer uses to print that file.

For example, say your image file has a resolution of 300 ppi. You can print this with an inkjet printer at any number of dpi settings. Epson printers usually let you print at 360, 720, 1440, and 2880 dpi. Which you choose depends largely on the purpose of the print. Lower dpi settings typically make prints that are less sharp but which print far quicker, etc.

Since you are scanning, and drum scanning at that, I'm sure you've found that the software for the scanner typically doesn't allow the operator to set the scanner resolution directly. It wants you to tell it either 1) final print size and output resolution, or 2) level of enlargement and output resolution. These are two ways of expressing the same thing really. I find the easiest thing to do is to set my enlargement level -- I typically scan my 5x4 sheet film at 11x enlargement and 300 ppi output resolution. This is enough to let me make very large prints (100 x 125 cm) that are very sharp.

As to mode, I seem to recall that the Screen software needs an add-on module called something like "negacolor" to let it handle negatives properly. If you don't have that, it might make sense to scan your B&W film as an RGB positive and invert it in your photoeditor (and convert the RGB down to grayscale). If you have negacolor (or whatever it's name is) then you might get good results scanning as a negative with just the green channel -- this option is called "gray-green" in some software.

Sorry I can't be more specific. I've never driven a Screen scanner so all I can do is extrapolate from my other scanner experience.

[Leonard Evens]

A digital image doesn't have a resolution in pixels per inch. It just consists of pixels. Resolution only makes sense in relation to a reference length. This could be a length in the original image in the camera, or it could be the corresponding dimension in a desired print or monitor image or anything else that made sense in the specific circumstances. Photoeditors can report different figures for resolution, but they are more or less irrelevant until you get to printing. Before that, the only things that counts are the dimensions of the image in pixels. So start with whatever pixel resolution in ppi you've decided to send to your print, e.g. 300 ppi. Multiply this by the dimensions of the desired print. That will tell you the number of pixels you need in the image. If you have more than that number, just print and don't worry about it too much. If you don't, you will probably have to scale upward to the number of pixels needed. That is generally to be avoided it since it adds no more information to the image, but if you don't do it, the print may exhibit (possibly subtle) effects of pixelization. How you go about that may depend on your photoeditor. For example, you could just specify the target size in pixels or alternately you could specify the print size and the target ppi to send to the print. In the latter case, the photoeditor will calculate the number of pixels needed and do the rescaling.

In certain circumstances, you might want to rescale an image donwward. Whatever you send an inkjet RGB printer in ppi, it will modify that to what it needs and then convert the results to dots. But if the printer has an inherent ppi it likes, it might make sense to rescale to that first in the phtoeditor before printing, the idea being that it is better to do that conversion in the photoeditor than in the printer. I've seen it argued that Epson printers do better if sent a ppi which is some multiple of 360 ppi. So in that case, there would be an argument for downscaling to 360 ppi. Again, let me emphasize that in the photoeditor this only makes sense in relation to a target print size.

[Ron Marshall]

The scanning resolution depends on the viewing distance; for close viewing 300 ppi would be obtimal. At greater distances 240 or 200 would suffice.

Assuming you want 300 ppi on a 40x50 print (10x linear enlargement) from a 4x5: 300 dpi x 10 =3000 ppi scan resolution. This will give you a scan that has the required number of pixels in each dimension, as specified in Gordons post above: 4 inch (film) x 3000 ppi = 12,000.

For a 40x50 (5x enlargement) from 8x10, you need only a 1500 ppi scan.

The actual film dimensions are slightly less than the nominal 4x5, and you may need some room for cropping, so better to scan at a slightly higher resolution such as 3200.

Hope this helps!

[Ted Harris]

Tri,

As I recall you are working with a Screen 1030 Scanner. I am assuming that it uses a version of Color Genius software. That being the case you CAN set your spi directly (scanners scan in samples per inch, monitors display in pixels per inch and printers print in dots per inch). I find the simplest way is to scan at 100% image size and the maximum resolution possible for a manageable file size. For 4x5 fim scanned at 2400 spi, for example, you will get a file of ~ 650 mb depending on your crop. This then becomes your master scan that you can use for prints of varying sizes. I then do all my photoshop work (working on a separate layer for each manipulation of the image). When I have done everything except sized the file and done final touchup sharpening I save it as my master file. Then, working on a renamed copy of the file I size it for print as follows: 1) uncheck the resample box and set the resolution to either 300/600 for HP and Canon printers or 360/720 for Epson. 2) recheck the resample box and set the final print dimensions ... note that you should only use 600 or 720 in step 1 if it will not result in uprezing in step 2. 3) do any sharpening and print.

[Ken Lee]

"scanners scan in samples per inch, monitors display in pixels per inch and printers print in dots per inch"

As long as the inch remains a constant... we're safe !

[Ted Harris]

or we could change it all to meters and stop being the only holdouts left in the world :)

[Tri Tran]

Thank you all and appreciated your time to answer my questions. I’m still in the learning curve with this Screen scanner and printing stage. Perhaps someone here in Orange County , CA can give me a training session how to use this thing as a drum scanner operator? Thanks so much.

[Maretzo]

"scanners scan in samples per inch, monitors display in pixels per inch and printers print in dots per inch"

As long as the inch remains a constant... we're safe !

Resolution of lenses is expressed in lines/mm and an inch is about 25 mm. Since such lens resolution rarely exceeds 100 lines/mm, does it make a difference to scan with a resolution higher than 2500 dpi? (Which is equivalent to about 100 d/mm.):confused:

[Tri Tran]

Resolution of lenses is expressed in lines/mm and an inch is about 25 mm. Since such lens resolution rarely exceeds 100 lines/mm, does it make a difference to scan with a resolution higher than 2500 dpi? (Which is equivalent to about 100 d/mm.):confused:

You guys are too technical.These thing drive me crazy but I 'm willing to learn.Cheers.

[Joanna Carter]

You guys are too technical.These thing drive me crazy but I 'm willing to learn.Cheers.
Try this for a simplified description of determining resolution for scanning:

Most flatbed scanners, despite the manufacturers claims, rarely improve the resolution acquired from film above 2200dpi.

Decide on the maximum size of print that you are ever likely to make from a given image. (for example, 20x25 from a 4x5)

For most purposes 240ppi is a perfectly adequate resolution for producing prints on an inkjet printer.

Take the ratio of the desired print size over the original film size (20:4 = 5:1)

Multiply the print resolution by this ratio (240 x 5 = 1200)

Therefore, 1200dpi is the correct resolution to scan a 4x5 film to produce a 20x25 print.

After scanning, set the image resolution in Photoshop to 240dpi (disable the "resample image" checkbox) and you wil end up with an image that should measure 20x25 at 240dpi.

When printing the image, set your printer resolution to "photographic" or "best photo", depending on your preferences, the actual printing resolution number (1440/2880/etc) has nothing to do with the resolution of the image you are editing, only the quality that the printer works at. Do not attempt to alter the image resolution to match the numbers for the printer.

If you are sending files to a lab for lambda printing or similar, then they usually require 300dpi files; simply multiply the print size/film size ratio by 300 instead of 240 to get the correct scanning resolution.