When I save an image to jpeg the save dialogue box has a drop down box called scans..it is normally set at 3 but can be riased or reduced.can some one tell me what Scans means and what effect it has on the saved file.
I already asked this in the Photoshop forum but no one could be bothered to reply so as I know a lot of people here use Photoshop I thought I would give it a try.
Thanks

It appears to me to be meant for progressive opening of the image, like web shots that open from top to bottom, etc., instead of all at once. When all was dialup I belive I saw a lot more of progressive saves than now with highspeed more available.  I don't use progressive on my saves and really can't tell how or why number of scans is elective.    Thanks, I really had not noticed that option.

Hi,
Thanks for the info
Eddie

I might try it and see how it does...If I knew more about it I would be on the Photoshop forum. : )

if I knew more about it I wouldn't ask :-)

The effect it has (apart from the physical effect of its loading explained above), is for those who have a slow connection or that the page is taking a long time to load for some reason, the person who is viewing the page will get the "full" image a few seconds earlier (albeit "pixellated"), and know that it is loading, rather than a partly blank image holder (white space).  It is basically a trick to catch your attention on the image so you wait for it to fully resolve rather than navigate away from the page.  3 scans is generally a good idea. Also it seems that using Progressive (multiple scans) over Linear (default) reduces the filesize and for that reason i use it all the time.

ETA - two exerts from wikipedia on progressive jpg's:

"It is also possible to transform between baseline and progressive formats without any loss of quality, since the only difference is the order in which the coefficients are placed in the file."

"There is also an interlaced "Progressive JPEG" format, in which data is compressed in multiple passes of progressively higher detail. This is ideal for large images that will be displayed while downloading over a slow connection, allowing a reasonable preview after receiving only a portion of the data. However, progressive JPEGs are not as widely supported."

Hi Rich,
Thanks for a very detailed explanation...very interesting.I am sure anyone reading this will be much the wiser.Thank you very much.
Eddie

Rich, thanks for an answer with added depth.  I tried an image to compare file sizes and this is what I got:

Least Compression                                  Compression at "5"
Baseline "standard"     4174kb                           207kb
Optomized                  3778kb                           162kb
Progressive(3scan)       3671kb                           166kb   

There is some savings on file size.

Interesting.          Tom.                 

Another question then.
Am I correct in assuming that for say printing purposes it is better to save the file as Baseline "Standard"
Thanks Tom for taking the time to look into this further...I never thought of it :-(

I'd say it was a safer bet to use it - as well as not using anything but 12 quality and the save as function (colour space is stripped in the optimise save function i seem to recall).  But i cant think that progressive scans wont print the same as baseline.

Thanks Rich

For print you can better use either psd or tif.
Jpg is lossy, meaning information is lost as pixels are averaged. If you shoot jpg, your camera already does this calculation a first time, so the file size is smaller. When you then open in PS or whatever, you get a full-size file becasue all pixels are shown, When you then save again as jpg, even when you set to maximum quality!!!,  the compression works again and your already averaged pixels are further averaged. For the web, seen the fact that you usually reduce the size, this is not a real problem. But for print you inevitably  move towards visual artefacts.

Thanks for the reply on printing.I only print stuff for myself and my labs always want jpeg.but if I ever get beyond that then this info will be of great help.