Question about file size when saving JPEGs

I am running PS 7 on a Mac, OS 10.4. To meet the size requirement for posting an image to a certain forum, the image must be less than 100 kByte in size. So I use Save As / JPEG and as I select Quality with Preview checked it apparently shows me the file size at the bottom of the window with an estimate of the time to transfer the file. But I find quite often that if I choose Quality to get the size below 100 kB, the image is actually saved at a larger size (as confirmed by using Get Info as well as the fact that the forum site often doesn’t accept the image). So I have a couple of questions.

1. What does the size estimate based on the Quality level actually mean and why is it apparently inaccurate?

2. When I instead use Save For Web, I usually get much smaller file sizes even when I choose fairly high quality settings. What is Save For Web doing differently from Save As / JPEG?

Thanks.

While you don’t say, I do assume that you are down-sizing the pixel dimensions of your originals either at the same time or prior to saving as a Jpeg.

I am running PS 7 on a Mac, OS 10.4. To meet the size requirement for posting an image to a certain forum, the image must be less than 100 kByte in size. So I use Save As / JPEG and as I select Quality with Preview checked it apparently shows me the file size at the bottom of the window with an estimate of the time to transfer the file. But I find quite often that if I choose Quality to get the size below 100 kB, the image is actually saved at a larger size (as confirmed by using Get Info as well as the fact that the forum site often doesn’t accept the image). So I have a couple of questions.

1. What does the size estimate based on the Quality level actually mean and why is it apparently inaccurate?

2. When I instead use Save For Web, I usually get much smaller file sizes even when I choose fairly high quality settings. What is Save For Web doing differently from Save As / JPEG?

Thanks.

That’s an interesting observation.
I just created an image in PS 7 on a PC.
The image was 800 x 580 pixels, "Saved As"..at Quality 8 PS said the image was 92 KB.
I mailed it to myself via Seamonkey and it came in at 125KB. Very Perplexing.
Bob williams

1. What does the size estimate based on the Quality level actually mean and why is it apparently inaccurate?

2. When I instead use Save For Web, I usually get much smaller file sizes even when I choose fairly high quality settings. What is Save For Web doing differently from Save As / JPEG?

PS said the image was 92 KB.
I mailed it to myself via Seamonkey and it came in at 125KB.