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Camera Image Quality

Let us consider digital camera images.  A typical 7 mega pixel camera gives an image size of 3072 x 2304 pixels.  This represents the amount of detail that can be captured by the camera CCD (charge coupled device).  This detail can be stored on the camera as a raw file (e.g. TIFF).  Alternatively, to reduce the amount of memory, it can be compressed (usually in jpeg format) by the camera to give file sizes that are more easily handled by the camera memory card and computer.

The compression options on the camera are typically called, High, Fine, Normal and Basic.

In addition to the compression options, cameras often have several image output size options.  The following table describes the common sizes and corresponding resolutions.

Resolution determines the amount of detail in the image. For maximum quality printed output it is important to select the highest resolution setting possible.

Compression determines the amount of unnecessary detail that is removed.   Low amounts of compression are hardly noticeable in printed outputs.  High amounts of compression introduce more noise and artefacts into the image which become more noticeable as the size of the printed output is increased.

Actual printed image size is calculated by the image resolution in pixels divided by the resolution selected for the output.  See the table below for output resolution to be selected.

Note that digital images have an aspect ratio (height/length) of approximately 3/4 whereas traditional film camera aspect ratios are approximately 2/3.  The table below is based on digital aspect ratios.

Camera Quality Settings v Output Print Quality in dpi

dpi colour code:
### for good quality printed output
### for medium quality printed output
### for web page/email output

Saving digital camera images

If the images are important and quality is the prime objective, then the images from the camera should be saved and backed up without any further compression.  However, if the images are of a typical snapshot nature and they have perhaps been cropped or colour adjusted then resaving with a small amount of compression is acceptable. e.g. Photoshop jpeg compression setting 8 or 9 out of 10 usually maintains about the same file size.

Images that undergo complex or several manipulation stages should be frequently saved during the process using the imaging software's own format so that there is no compression and no degradation.  When manipulation is complete the final save can be in a commonly accepted format (e.g. bmp, tiff, jpeg, gif) and include compression if required to save memory space.

For general use, images with 1600 x 1200 pixel resolution, with jpeg compressed file sizes of about 400 to 600 kb are satisfactory for full frame printed picture sizes up to 6.6" x 5" .  For selective enlargements then higher resolutions should be used, requiring cameras with 3 mega pixel and above.

Resizing, re-sampling and saving camera images for web pages & email

Web pages or email do not require images with high resolution.  High resolution images with large file sizes take a long time to download with the visitor probably losing patience.  For normal viewing on a monitor an image resolution of around 72 dpi is adequate.  It is common for web pages to have quick loading thumbnail images on which the visitor can 'click' to view a larger and more detailed image if they are interested.

This is a suggested process:

For the full size image:

Resize and resample using image editing software to give an image 500 pixels high with a resolution of 72 dpi.  Save with high jpeg compression (say Photoshop level 2 or 3 out of 10) to give a file size of say 30 to 50 kb.

A 500 pixel high limit enables the image to be displayed in full on low resolution monitors allowing for space taken up by scroll bars etc.

For the thumbnail image:

Resize the above image when importing into the web page to give an image size of say 75 pixels high.  Save with high jpeg compression (say Photoshop level 2 or 3 out of 10) to give a file size of around 5 kb.  The thumbnail image has then to be hyperlinked to the larger image.