Image Resolution & File Size

Summary

We recommend that you use the resolution you need to get the quality you need, but not any more than that. In most cases a resolution of about 900 pixels by 600 pixels is sufficient. That may mean paying a few cents extra per slide. That is to be expected. Avoid paying dollars extra for needlessly large files (images much greater than 900x600) that don't improve the quality of your slide. For a more detailed discussion of the issues, continue reading below.

Understanding Digital Images

Digital images are images such as you might obtain from scanning a photograph or using a digital camera. JPG, TIF, GIF, and BMP are the most common formats for digital images. The default resolution of scanners and digital cameras today often exceed the requirements for most 35mm slides. Creating digital images with unnecessarily high resolution, will needlessly increase the cost of your job without providing any significant increase in quality.

Digital images consist of many dots or "pixels". The resolution of the image refers to how many pixels it contains. The resolution is best defined as how many pixels across and how many pixels down the image has. For example, an image might have a resolution of 900x600, meaning it is 900 pixels wide and 600 pixels tall. Sometimes the resolution is defined as dots per inch or dpi. To convert dpi to pixels, you must know the dimension of your image in inches and multiply dpi times inches. Because of the ambiguity of dpi and inches on a slide, we will use pixels rather than dpi to define the resolution of your image.

Each pixel is assigned a color. The color information may require one byte of memory per pixel to distinguish among 256 colors or 3 bytes per pixel to distinguish among 16 million colors. The memory required to hold a digital image depends on the number of pixels and the number of bytes per pixel. The total image size of the digital image in bytes is the total number of pixels times the number of bytes per pixel.

High resolution images cost more to process - $1/meg

The personnel time, the machine time, memory requirements, and the disk storage required to process large files containing high resolution images are significantly greater than that required to process files of a smaller size.

Because of the increased time and cost of working with large files, we add a File Size Fee of $1/meg for jobs that exceed the allowance of 1 meg for every 10 slides. That is, you are allowed 1 meg for every 10 slides, before you are charged the File Size Fee. A job is defined as all the slides that are due at the same time, and may consist of one or more files. For jobs consisting of more than one file, the job size is the sum of the sizes of the individual files. The same fee applies whether the files are uploaded over the Internet or submitted on disk or CD.

For example, if your job contains 33 slides, your allowance is 3.3 meg. The fee is $1/meg for excess beyond the allowance for that job. Therefore, if you submit a 4.5 meg job containing 33 slides, the file size fee would be $1.20 (4.5 meg - 3.3 meg).

In most cases however, if you follow the guidelines below, you can reduce or eliminate the File Size Fee without sacrificing quality.

How to reduce the File Size Fee

1. Scan at lower resolution.

The default resolution of scanners and digital cameras today often exceed the requirements for 35mm slides. Creating images with unnecessarily high resolution, will needlessly increase the cost of your job without providing any increase in quality.

If your images are too high resolution, you can reduce the resolution of your images without losing quality by resizing them in an image editing program like PhotoShop. For example, an 8x12 inch photo scanned at 300 dpi will produce a digital image with a resolution of 2400 pixels by 3600 pixels. This is much higher than necessary for a slide. Resizing in PhotoShop to 600 pixels by 900 pixels (75 dpi) will still produce an excellent quality slide, with significant savings in file size.

2. Order multiple copies of your slides.

If you think you might eventually need more copies of your slides, it is a good idea to order them all at the same time. Ordering multiple copies at the same time will increase your file size allowance without increasing the file size of your job, since we would be shooting from the same files. For example, if you send us 12 JPG files totaling 4.5 megs and you order just one copy each, your file size allowance would be 1.2 meg and the File Size Fee would be $3.30 (4.5-1.2). If instead you ordered two copies each, your file size allowance would double to 2.4 megs and your File Size Fee would drop to $2.10. Ordering 4 copies each, in this example, would increase the allowance up to 4.8 meg and reduce the File Size Fee to zero.

3. Use a more efficient image format.

Some image formats are inherently more efficient in file size than others. JPG, compressed TIF, and GIF are more efficient than uncompressed TIF, PSD, BMP, and PCX. The most efficient is JPG. The JPG compression factor should be set to "medium". This is normally the default.

4. Reduce the number of colors.

The default color resolution for your image may be set higher than necessary. If your image only needs 256 colors but is stored in a format that allows 16 million colors, try resaving your image in a format with fewer colors. Gray-scale images and computer generated color images generally don't need more that 256 colors. Black and white line art and text only need 2 colors.

Image Resolution Guidelines

There is a trade-off between slide quality and file size, up to a certain point. Beyond that point, you cannot see the difference any more even if you increase the image resolution and file size. We recommend that you use the resolution you need to get the quality you need, but that you not go beyond that. If you are ordering just one copy each of several JPG files, that may mean paying a few cents extra per slide. That is to be expected. What we want to help you avoid is paying dollars extra for needlessly large files that don't improve the quality of your slide.

We have listed specific recomendations below, which were determined by simple trial and error. These are approximations. The dimensions of your images can vary depending on the shape of your image. If you would like to test these or other resolutions for yourself, you are welcome to take advantage of our Free Test Slides offer.

1. 900x600 for digital camera files or scanned photos

Digital camera files or scanned photos need not be more than 900x600 pixels. Resolutions greater than 900x600 do not significantly improve the quality of the image on the slide. Going much beyond 900x600 will only increase our work and your expense unnecessarily. On the other hand, using resolutions significantly below 900x600 will adversely affect the quality of your slides.

2. 1500x1000 for hard edge graphics, text, and line art

Digital graphics of line art, text, charts and other objects with hard edges need higher resolution (number of pixels). Fortunately, these images usually contain fewer colors and can be stored with one byte per pixel (256 colors) instead of 3 bytes per pixel (16 million colors). A resolution of 1500x1000 should be sufficient for 256 color images of line art, text, and charts. In the case of line art, make sure the lines are thick and bold, so that the finer pixels stand out against the background.

3. 2100x1400 for cinema slides

Slides for projection in a movie theatre present a special case, since viewers in the front rows might not have the ability to see the whole screen at once. Therefore for cinema slides we recommend a much higher resolution of 2100x1400. If you want the image to match the shape of Cinemascope, use the more rectangular dimensions, 2100x930. For the US Cinema aspect ratio, use 2100x1120. (The PowerPoint dimensions in inches are 11.25 x 5.00 for Cinemascope and 11.25 x 6.00 for US Cinema.)

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