JPEG file size much smaller when converting from TIFF

[Miles Astray]

Hi there community,

I have recently started using Nikon Capture NX-D to "pre-convert" the RAW files of my Nikon D750. The Nikon software just does a great job with the colors and camera settings, etc. In a second step, however, I import the images into Lightroom, which is obviously the richer tool for fine tuning. 

I have tried two paths to do this: 

1. Export the RAW file as a JPEG from NX-D

2. Export the RAW file as a 16 bit TIFF from NX-D

When I import the JPEG and TIFF into Lightroom and then export both as JPEGs, I notice that the JPED-based JPEG file is significantly larger (from 1/3 larger up to twice the size) than the TIFF-based JPEG file. However, the quality seems to be almost identical. At 100 % the difference is imperceptable. At 200, 300, 400, 500 % some difference becomes visible but is basically negligable (and at that level of maginification the slightly less detailed TIFF-based JPEG doesn't necessarily look worse to me). 

Does anybody have an explanation for such a big difference in file size with hardly any difference in image quality? Am I missing something? If anything, I would have expected the difference in file size to be the other way around, with the lossless TIFF file as a base producing the larger JPEG file with more detail.

If there is truly no (noticable) difference in quality, I would of course prefer the smaller files to save disk space. Even for large scale printing I guess the difference would be imperceptible. 

I am attching two sample images (TIFF-based JPEG and JPEG-based JPEG) and screenshots of their 100 % and 500 % magnification.

Thanks a lot for your help!

[Pigprof]

I regularly post images on the 'Dartefacts' website and in the normal way, I shoot in RAW and then export 16 x 9 images at 1188 x 688 pixels (to give the right display size on the website) with a maximum file size set at 200 kb. No problem and very acceptable image quality. Today, I inadvertently shot in jpeg rather than RAW (I know, big mistake!) but I thought one image was still good enough to post. So, I tried to export the jpeg using the same settings I use for a raw image, only to find that LrC reports it cannot produce an image of that size and I need to increase the file size. What? You can squeeze a 19.2 MB RAW file down to 200kb with very little loss of quality (for a website that is) but you cannot squeeze a 9.34 MB jpeg, with poorer initial quality, down to that file size?

I can only assume that the algorithm that strips out data from a RAW file to produce a jpeg in the camera is much less efficient than the one in Lightroom Classic and that when LrC tries to strip out (more) data and compress it can not do it. Incidentally, I tried exporting the jpeg with the quality down at 20% and it still would not get down the image size to 200 kb. I really cannot see why this should be the case.

[Jim Wilde]

I just exported from LrC a 19.73MB Jpeg (from a 45MP sensor) with a long edge of 1188 px and a size limit of 200KB, and successfully produced a file of 189KB. Unless your image is so incredibly detailed that it cannot be compressed that small, it would seem that it should be possible to do what you want. Have you double/triple checked the export settings?

[Pigprof]

Having read your post (and some others) I think it must be due to the
detail in the image. It was a woodland scene with a LOT of twig details. I
have managed to export and compress some other images from the same shoot
that had less detail. Thanks for that, but I still don't understand why a
RAW file compresses better. Thanks for your help.

[Pigprof]

I have revisited this image (See below).  When exporting, if I pull the 'Image quality' slider down to 0% it exports at a file size of 208.3 kb; which explains why it will not export if I set the max size to 200 kb. 

I also tried exporting using the 'JPEG optimizer' plugin.  With recompression, the file size goes up to 563 kb!  Without recompression, if I remove just the EXIF thumbnail the file size comes down to 194.7 kb.  Removing ALL the metadata brings it down to 188.9 kb.  An interesting learning exercise. 

[Jao vdL]

JPEG compression works by analying the image for repetitive elements using Fourier transform math. It doesn't matter what the source of your image is, what matters is the complexity of the image. An image of a wall with no detail and just solid color will be very compressable. An image like what you show here with basically a lot of chaos and very little repetition will be very hard to compress down.