What Scan Resolution Should I Scan My Negatives At ?
Scanning resolution is probably one of the first dilemmas people are faced with when they first start out scanning negatives.
Their first reaction is to scan them at the highest values available in the scanning software thinking they are going to achieve the best scan. In my experience, this is far from reality.
Each scanner regardless of what the marketing material tells you is going to have an optimal value. The most comprehensive website I have found for testing a variety of scanners is http://www.filmscanner.info/en/FilmscannerTestberichte.html
They talk about scanning resolution, and go into a great depth about overall performance, and in my opinion, its well worth bookmarking.
Coming back to the initial question of What resolution Should I scan My Negatives At. This all depends on your final output size, what will you be doing with the final edit ?
Scanning Resolution Screen Based Output
If you are 100% sure that the final image will only ever be displayed on a screen, then you can quite easily scan at 300 pixels per inch (ppi). Why 300 you may be asking, I thought monitors were 72ppi. The reason why I suggest 300 is because we have to think about our target audience who may be using these high resolution displays and also people that like to view images on high resolution mobiles devices.
Take the Apple iPad Pro for example which a pixel count of 264, so outputting the image at 300 is enough to allow them to zoom into the image without it loosing quality.
Scanning Resolution Print Based Output
Just as screen based output, we need to workout in advance what the print size will be and adjust the scanning resolution accordingly.
What If I Don’t Know What The Final Print Size Will Be
If at this stage, you have no idea as to what the final print size will be, then scan at your scanners optimal resolution, edit the image at this resolution and save it as your master file.
When you are ready to print, open the master file, create a duplicate, close the master and then resize your duplicated file to your desired output size and then close and discard the duplicate.
What If I Know What The Final Print Size Will Be
If you know what the final output size will be, then rather than create a huge file, you can scan at the resolution for that particular size. For my printing, I use an Epson R3880 which for cut sheet papers has a maximum width of 17 inches on the shortest side and a maximum length of 22 inches on the longest side so scanning at a resolution greater than I can print is only going to produce larger than necessary files.
Caveat: Selecting a scanning resolution to match my largest print size is fine providing that I do not want to print any larger than this. Of course in a situation where you may be offering larger prints by outsourcing the printing, you may want to choose the optimal scan resolution for your scanner and then create a master file as described in the What if I don’t know what the final print size will be above.
Converting Pixels To Inches For Scanning Resolution
I cant speak for every piece of scanning software, but VueScan displays information including the pixel count in the status bar at the bottom of the program interface.
The first set of numbers represent the final pixel value of the scanned negative at whatever dpi is shown in the status bar information. The above example tells us that if we were to scan this negative at 1200dpi, we would end up with an image which has a pixel count of 4867 x 6037 and a file size of 58.8MB
To calculate the final print size in inches, we can use the formula, Inches = Pixel/Print DPI
Using the above example for an Epson printer which prints at 360ppi.
4867/360 = 14 & 6037/360 = 17 which means that we would end up with a print size of 17 x 14 inches.
Most scanning software will give you a choice for the final output resolution and the choice is left to you in what to select. Somewhere along your workflow, interpolation is going to take place unless you are printing a negative at its original size.
Whether to choose to have scanner software perform the interpolation or let some other program such as Photoshop do it, thats your choice. Through my own testing, I have not seen any visible difference between Photoshop and Vuescan. That’s not to say there is no difference, I personally just haven’t noticed any.
In Vuescan the output options are under the “Output Tab”
Shipped with Vuescan is a selection of output presets which are available from the dropdown list. Choosing a preset will force Vuescan to automatically choose an output dpi. Personally, I do not use these for scanning negatives.
Vuescan Output Scan Size
If you want the output resolution to match the input scanned resolution, then set this value to 100% magnification. For example, if you have set the scanner resolution to 2400dpi but prefer to do the final interpolation in some other software then the scanned output file will have a value of 2400dpi
Vuescan Fixed DPI
If you are ok with letting Vuescan do the interpolation for you on the final scan, then set this to the dpi value of your output device (printer). As a rule of thumb, 360 is a good value for Epson and 300 for other makes but consult your own printer user manual.
High Scanning Resolution Is Not Always The Best Choice
Just because you can set a very high scan resolution in the software doesn’t always mean you are going o achieve the best results. What it will guarantee though is a large file.
Below is a series of tests I did with the Epson V800 scanner, I scanned the same 5×4 negative in Vuescan at different scanning resolutions, 2400, 4800 and 6400
Each image represents a 100% crop at their respective scanned resolutions. I appreciate you are looking at a compressed version but as i look at them on my screen with no compression, I am unable to see any difference in image quality which tells me that 2400dpi is probably the optimal value for this scanner which was also backed up from http://www.filmscanner.info/en/FilmscannerTestberichte.html
Clicking on the image will open a larger view
The conclusion here is that choosing the correct scanning resolution is quite important and is one area which one should at least try to get a basic understanding on in order to save both time and possibly wasted hard drive space.