What file format and resolution is the best choice?

The following article goes into a lot of detail about resolution and file types. If at any point you want to call and get a recommendation by phone, please feel free to use the toll free number above.

Deciding on resolution and file format depends on what the ultimate purpose will be for the digital archive, along with budget and resources available to work with the digital files.

Resolution

For our purposes resolution refers to the amount of digital information in a digital file. We are expressing this information as "points per inch" or PPI (DPI or dots per inch is usually used when referring to printer resolution). The more "points" in a single inch the smoother the image will appear ...up to a point. At some level more does not necessarily mean better. More "points" generally means bigger files that are harder to use and manipulate. More "points" can also bring increased digital noise, which can generally be removed but still requires additional work. Finally, more "points" does not necessarily make the image look better. At some point the naked eye cannot easily discern a quality difference by moving to a higher resolution.

The key is to figure out the optimum resolution needed based anticipated use of the files.

Example 1 - Printing on a home inkjet printer

To print a nice print on an inkjet printer requires approximately 240 dots per inch (DPI). For easier calculations we will increase that to 300 DPI. (Side note: You should experiment on your own printer with the paper you use. Smaller resolution will use less ink. Go to the minimum you feel yields an image that you like on the media you use.)

If you want to print an 8 x 10 image you would need 2400 dots (8 x 300 DPI) of information by 3000 dots (10 x 300 DPI) of information.

If your starting image is a traditional 4 x 6 photo you would need to scan it at 600 PPI on your scanner.

2400 dots (needed to print) divided by 4 = 600 PPI

3000 dots (needed to print) divided by 6 = 500 PPI

Choose the higher resolution needed to be safe.

Example 2 - converting a slide for the same inkjet print

We know that we still need approximately 2400 x 3000 dots to make an 8x10 print on an inkjet printer. So, to convert a 35 mm slide to a digital file that will be reprinted as an 8x10 print we calculate the needed resolution this way:

A slide measures approximately: 1 inch x 1 inch

2400 / 1 = 2400 PPI

3000 / 1 = 3000 PPI

In this example we would want to scan a slide at 3000 PPI to be able to print a quality 8x10 print.

Example 3 - large format printing

At some point you may want to print a very large print. In this case you would want to choose a high resolution on conversion and then would have to rely on photo editing software to resample the image for large prints. Different programs resample images in different ways. However, the goal is the same in each instance. The software looks at the pixels and extrapolates the information to add more resolution. Thus the goal is for the software to create a smooth looking image at enlarged sizes.


Example 4 - for viewing on TV or Computer

In this case you do not need as much resolution compared to printing. Even the resolution on HDTV is only 1280 x 720.

A 1 x 1 35mm slide could be converted at 1500 PPI and still have plenty of resolution to display beautifully on an HDTV. A standard television is about half of that resolution. The question comes about the future. How much resolution will TVs display in the future. As mentioned above at some point the naked eye will not see a noticeable difference so it is hard to tell how much more resolution will be needed.

Example 5 - cropping and zooming

One thing to keep in mind is that after converting an image to a digital format you may want to crop and then zoom in on a part of that image. This would have the effect of having scanned in a smaller photo which would require greater scan resolution to get to the same size as the original. If you think you will be cropping and zooming your images, add to the initial scan resolution to account for this.

File Formats

The most popular choices for file format are:

JPG (also written as JPEG): This is a popular format because the file sizes are smaller (which makes them easier to use and manipulate). It is also very compatible and able to be opened by all kinds of software and operating systems. This is the most popular choice. The downside is that to get these benefits, the file data is compressed and some data is taken out. Some color information will be lost and the ability to do post capture editing will be more limited. Of course you can still do some editing on the image. However, the amount and level of editing will be less compared to some of the other format choices. For most uses this format is ideal and is the most popular choice. Save the Image will provide 100% quality / minimum compression files to give the highest level JPG possible.

TIFF: This is an uncompressed file format that retains full image information. It is an excellent format for color reproduction and for reprinting images. The downside is that the files tend to be quite a bit larger and require more processing power and space to manipulate, store and edit.

DNG (Digital Negative): This is a relatively new format developed by Adobe. It is intended to be a universal format that has the advantages of a RAW file (a digitally captured image that has minimal processing done to it and allows for a wide range of editing and manipulation). This format tends to take up more space and can be more difficult to manipulate (usually only higher end photo editing software can handle these files).

So why bother with a DNG file? It is the most image information available and will allow for the greatest level of post capture processing and editing. Versions of the data can be saved in various formats and with various adjustments while preserving all of the original data captured at the time of conversion.

The best of both worlds...

One recommendation is that you can get both DNG + JPG at the same time. This way you have the full information files but also a set of workable, smaller sized files to use immediately.

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