The quality setting for JPEG in Photo Mechanic does not correspond to Photoshop's quality setting, so there is no direct comparison.
Photo Mechanic will not go as low quality as Photoshop, meaning the lowest quality setting in Photo Mechanic is higher quality (less compression) than the lowest quality setting in Photoshop.
Furthermore, a setting of 100 in Photo Mechanic is essentially "lossless" JPEG (assuming you also turn off chroma subsampling). It isn't truly lossless because we still go through the YCC & DCT math that JPEG uses, and therefore there will be some occasional rounding errors. But we are talking about a very small number of pixels having one of their RGB values changed by no more than one level (of 256 levels). You will not be able to detect any compression artifacts and if you subtract a quality 100 (no chroma subsample) JPEG saved with PM with a TIFF file you will see they are nearly identical. However, these JPEG files will be quite large (for a JPEG), especially if you start with an image that is noisy or already heavily compressed, because JPEG will reproduce every bit of that noise and original artifacting. And if you start with a heavily compressed (e.g. mosquito noise) JPEG and apply a crop whose top-left origin does not align on an 8x8 boundary (especially if the offsets are odd), then this exacerbates the problem of exactly encoding those artifacts at quality 100.
The quality setting controls the quantization of the 12-bit intermediate YCC values. This is essentially the divisor Q, and this number must be a whole number (i.e. 1, 2, 3, 4 etc not something like 1.2). The lower the quality the larger the divisor, therefore the smaller the number that is recorded in Huffman encoding, therefore the fewer number of bits required to encode these values, therefore the smaller the file size.
PM uses the following quantization table based on quality setting:
200, 175, 155, 140, 132, 123, 114, 105, 98, 93, // 0-9
86, 78, 70, 63, 56, 49, 43, 39, 35, 32, // 10-19
30, 29, 28, 27, 26, 25, 24, 23, 22, 21, // 20-29
20, 19, 19, 18, 18, 17, 17, 16, 16, 15, // 30-39
15, 14, 14, 13, 13, 12, 12, 11, 11, 10, // 40-49
10, 9, 9, 9, 8, 8, 8, 7, 7, 7, // 50-59
6, 6, 6, 6, 6, 5, 5, 5, 5, 5, // 60-69
4, 4, 4, 4, 4, 4, 4, 4, 4, 4, // 70-79
3, 3, 3, 3, 3, 3, 3, 3, 3, 3, // 80-89
2, 2, 2, 2, 2, 2, 2, 2, 2, 2, // 90-99
As you can see, quality settings 90-99 are all the same exact quality, and same with 80-89 and 70-79. The quality setting of 100 is a divisor of 1 or no quantization (meaning we encode all 12 bit intermediate values without loss - the only "loss" being rounding errors).
Again, please don't make comparisons of JPEG quality between Photo Mechanic and Photoshop - they are different beasts.
HTH.
--dennis
