FZ35/38 JPEG Compression

 


Variation in JPG Compression at Different Settings on the FZ35/38


In a private communication, Dr J. C. Brown told me that he had observed differences in size when comparing JPG image files taken using the same exposure settings and the same test chart.  He found that the file sizes were somewhat smaller when using the “Auto Bracket” setting than when taking single shots, implying that a different amount of compression was being applied.


To see whether this also applied to my FZ38, I carried out some tests with a tripod at a fixed distance from a target on a wall under constant lighting indoors, using the same exposure and zoom settings throughout.  I examined the file sizes produced when using single shot mode, “Auto Bracket” mode and “Burst” mode, and I also compared the file sizes for shots taken in “Fine” and “Standard” quality modes.  I carried out replicate shots to determine how consistent the differences were.  I used “EXIF Viewer” to determine the image compression parameters for each shot, as shown below.


Results (file sizes are shown in MB)


1. Fine compression setting, tripod, 2 sec timer, A mode, f/5.0, 100 ISO:


Single shot  (compression all shown as 4.3bpp, 6x, compression mode 4)

6.4,

6.5,

6.4,

6.5


Burst Mode (compression all shown as 3.8bpp, 6x, compression mode 4)

5.8, 5.8, 5.8;

5.7, 5.7, 5.7;

5.7, 5.7, 5.7


Auto bracket (compression all shown as 3.5bpp*, 7x, compression mode 4)

5.3, 5.2, 5.3;

5.2, 5.1*, 5.2;

5.2, 5.1*, 5.2;

5.3, 5.1*, 5.3


*shown as 3.4bpp


2. Standard compression setting, tripod, 2 sec timer, A mode, f/5.0, 100 ISO:


Single shot  (compression all shown as 1.9bpp, 12x, compression mode 2)

2.9,

2.9,

2.9


Burst Mode

2.8,  (compression shown as 1.9bpp, 13x, compression mode 2)

2.8,  (compression shown as 1.8bpp, 13x, compression mode 2)

2.5;  (compression shown as 1.7bpp, 14x, compression mode 2)


2.8,  (compression shown as 1.9bpp, 13x, compression mode 2)

2.8,  (compression shown as 1.8bpp, 13x, compression mode 2)

2.5   (compression shown as 1.7bpp, 14x, compression mode 2)


Auto bracket

2.7, (compression shown as 1.8bpp, 13x, compression mode 2)

2.6, (compression shown as 1.7bpp, 14x, compression mode 2)

2.4; (compression shown as 1.6bpp, 15x, compression mode 2)


2.7, (compression shown as 1.8bpp, 14x, compression mode 2)

2.6, (compression shown as 1.7bpp, 14x, compression mode 2)

2.4  (compression shown as 1.6bpp, 15x, compression mode 2)



Conclusions


Although there were no obvious differences in visible IQ, it is clear that slightly different amounts of compression are applied to JPGs in single shot, Auto Bracket and Burst modes.


At the Standard quality setting, the bpp (bits per pixel) is about half of that at the Fine setting.


At the Fine setting, the single shot mode gives the largest files size (lowest compression, highest bbp) with 4.3bbp, in Burst mode, the compression is about 3.8 bbp; and in Auto Bracket the file size is smallest (highest compression) at about 3.4-3.5bbp.


At the Standard setting, the single shot mode gives the largest files size (lowest compression) with 1.9bbp, in Burst mode, the compression ranges from 1.9-1.7bbp; and in Auto Bracket the file size is smallest (highest compression) at about 1.8-1.6bbp.


It can be observed that at the Standard quality setting for both Burst and Auto Bracket, there is a consistent trend with the compression increasing slightly with successive shots.  In other words, for the three shots, the first shot has a slightly lower compression than the last shot.


However, to get these figures into perspective it is necessary to consider the effects of these different levels of compression on IQ.  The highest compression of all of the shots was 15x, using Standard quality; in other words, a compression of 15:1.


From the Wikipedia article on JPEG compression (http://en.wikipedia.org/wiki/JPEG) it is noted that

10:1 compression “usually results in an image that cannot be distinguished by eye from the original”; and that “Average” quality (15:1) shows "Initial signs of subimage artifacts".


Therefore, while my tests showed there are some differences in compression using the different modes, the greatest compression in Standard mode was found to be no more than 15:1, which means even that image would be expected to show only the very first signs of artifacts appearing.  At Fine quality, with compression between 6:1 and 7:1, there would be no discernible loss of quality.


SD cards now have very high speeds and capacities, and their cost has become very low, and that applies similarly for computer hard disks.  Therefore there seems no reason not to always use “Fine” quality for JPG images when using the FZ35/38.




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