Fewer Pixels or More Compression?
posted April 16, 2006
At the last HAL-PC Clear Lake Digital Photography SIG (Saturday 4/15/06) the question came up whether increasing JPEG compression or reducing the number of pixels recorded will result in the better image.
Sometime you may find yourself in the situation where you don’t have enough memory/storage space to record all the pictures you want to take — for example, you’re on a weekend trip and only have a memory card that can record 100 pictures at the best quality. Depending on your camera you may be able to increase this to 400-800 pictures by sacrificing a little image quality that you may never miss.
There are two way to increase the number of pictures you can record in the same space. One way is to decrease the number of megapixels you record for each picture. The other is to increase the JPEG compression used to store each picture. Either method sacrifices some picture quality. This raises the question of which method will result in the better image — which prompted this article.
NOTE: In this article the terms megapixels, MP, number of pixels,
and resolution all mean the same thing and are used interchangeably
JPEG is a “lossy” file format. The JPEG method of image storage can achieve a remarkable amount of compression (reduction in file size). It can reduce the file size by a factor of 20 and still have a reasonably good image. But, there’s no free lunch. To achieve its remarkable compression it throws away small amounts of the original image, and the greater the compression the more of the image gets thrown away. On the other hand, using a lower number of pixels to record the image results in an innately lower fidelity image. So, which method is best?
To create this demonstration I took two shots. They were taken with my 8 MP (megapixel), Olympus 8080 camera. The high resolution shot was taken at 8 MP (3254 x 2448), and the lower resolution shot at 2.8 MP (2048 x 1336). 2.8 MP was selected for the lower resolution because it produces roughly the same file size as the higher resolution when using increased JPEG compression. The target was a piece of striped cloth. The two shots were taken one after the other with the camera on a tripod.
The image on the left in both the upper and lower pair is the same crop from the shot taken at higher resolution and lower quality (higher JEPG compression). The image on the right in both pairs is same crop from the shot taken at lower resolution and high quality (low compression).
For the images to be comparable they must be the same size (same resolution/PPI). To do this, the low resolution image must be up-sampled (made larger) or the high resolution must be down sampled (made smaller). Anticipating an issue over which of these methods might yield the best results I have done it both ways. The pair on the top were done by up-sampling the low resolution image, the bottom pair were done by down-sampling the high resolution image (and magnified to make them large enough for comparison). Other than this, the upper and lower pair are the same — that is, the same crops from the same two images.
File Size 1503Kb
File Size 1605Kb
|The low resolution shot above has been up-sampled to make the images the same resolution/PPI|
|The high resolution shot has been sampled down to make the images the same resolution/PPI. The down-sampled pair has been magnified so the images are large enough to compare conveniently
To my eye the higher resolution/higher compression
images on the left, are clearly (pun intended) sharper
So, if I’m right and you need space for more pictures, first increase the JPEG compression. That is, choose a lower “quality” setting. If that still doesn’t give you enough pictures, start lowering the number of pixels (resolution). Step down through each lower resolution until you reach the number of pictures you need. I probably wouldn’t go below 1600×1200 (2 megapixels) if you intend to print your pictures. If you only intend to display them on a computer or TV you can go down to 1200×900 (1 megapixels). It’s important to remember, reducing the number of pixels you record limits the amount of cropping you can do and still have enough pixels for a good print.
I recommend you experiment with this by shooting and printing at the higher quantity settings before using them on something you can’t do over if the results aren’t good enough for you. Following this procedure on my camera with a 512 MB memory card, you would see the following increases in space available for pictures.
|Resolution/Compression Setting||Megapixels||photos||Shots Times X|
|3264×2448 / low compression||8 MP||130|
|3264×2448 / high compression||8 MP||260||2|
|2592×1944 / high compression||5 MP||410||3|
|2288×1712 / high compression||4 MP||515||4|
|2048×1536 / high compression||2.8 MP||638||5|
|1600×1200 / high compression||2 MP||1000||8|
|1280×960 / high compression||1.2 MP||1600||12|
One thousand shots is the equivalent of about 30, 36 exposure rolls of 35mm film; or 50, 20 exposure rolls..