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Image file size changes as a result of opening / importing into Photoshop

Forum|Forum|8 years ago
March 15, 2018
5 replies
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I am preparing to get my huge collection of 35mm slides scanned to digital and am comparing the options of TIFF vs JPEG. I am doing tests with a couple of sample slides. First I got a specialist scanning firm to do scans at different quality options. Then I've imported/opened these on my i-Mac into first Apple's i-photo (not the more recent 'Photos' app) and secondly into Photoshop Element 11 (PSE). It is in PSE that I want to do most of my editing work. Within the PSE application, I have looked at the file size (called document size in PSE - visible via a tiny tab on bottom left) in Megabites but have been totally puzzled to find this doesn't match the size of the source file. For example, one of the slides was scanned to produce a JPEG file which is 41.5 Mb in its 'source' form (prior to opening/import into PSE) but has apparently become 224 Mb having landed in PSE. Any advice welcome!

- Am I right to expect file size readings to be the same prior to opening/import and afterwards within the PSE application?

- Or does the import process bring about some kind of change to the content which change the file size?

- If so, does this have any real world implication for image quality?

- is there anything I'm doing wrong or not understanding?

Ray

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Correct answer rays33108541

Can you indicate in pixels the dimensions of a typical scanned image.

Derek, for my trial, the scanning company did scans of my sample slides at four quality levels. there are two at "HD" level which is 500 dpi and two at 400dpi. Then for each of these two file formats were used to save the image i.e. JPG and TIFF. To try and answer your questions, I went to the top menu clicked file then file info. There I was presented with a table of info and have read off under the tab 'camera data', these 'pixel dimensions'. For the two HD images (i.e. TIFF and JPG) the reading is 7229 x 10842. For the two non HD images, they are 4381 x 6571. I am outside my expertise comfort zone but with this info, I guess there can be no doubt that having HD over normal provides more richness of image data and even if one doesn't see this at normal sizes, this could be valuable for cropping, expansion of images. Does this sound right to you? My remaining uncertainty is whether working with TIFF gives a discernable benefit - again not for raw 'viewing' but for future editing. But from what I've read in this thread, it sounds like it does. By contrast having JPG means erosion of quality with editing. Again, further input very welcome.

T

Test Screen Name

Legend

It's very easy to work out the data size. If it's 8 bit RGB there are three bytes per pixel, so 7229 x 10842 you multiply 7229 x 10842 x 3 = 235,130,454 = 224 megabytes. This exactly matches the number you saw in Photoshop Elements, so it's just the number of bytes needed to store the uncompressed images. As noted you will never see an on-screen display of the size it will have in the future when compressed and saved.

Norman Sanders

Legend

File size aside, for you to be that meticulous about the quality of the scans and then to save files as jpg is counterproductive, even destructive.

D Fosse

Community Expert

This is normal.

File formats are storage formats. Jpeg is very heavily compressed, and opening a jpeg means decompressing it.

That compression is destructive, irreversible and cumulative, so jpeg should never be used as a storage/working format.

Edit - slow typing today...

T

Test Screen Name

Legend

This is entirely normal. The “document size” is probably something you can work out yourself from the width and height in pixels; the amount of menory needed to hold the pixels. When you save it is compressed. No app shows the “save size”. It isn’t known until you save. Changing the colour of just one pixel will change the file size.

To proceed in this be sure you understand your compression choices, especially that every time you edit a JPEG, quality is lost forever and that it is very bad for general scans. If you don’t want to get into this, just understand that the archiving Standard is TIFF and use it; disk space is cheap.

R

rays33108541Author

Inspiring

Yes, in preparing my collection of digital images prepared by scanning 35mm slides, I've two objectives. One is to end up with a collection to store and use (objective 1). But another is to have the option of doing edits (objective 2) on them after a specialist firm does the scanning. My understanding (reinforced by this very useful reply and others) is that JPEG is what I need for objective 1 but I should also get my scans saved to TIFF to enable my objective 2. So, presently I plan to get the scanning firm to do pass me a set saved as JPEG and a set saved in TIFF as well. Further comments/advice welcome.

Derek Cross

Community Expert

Re-reading my comment I said High Compression, of course that should be Low compression!

For most purposes the difference between JPG (without compression) and TIFF is imperceptible. And the smaller file size will make them quicker to handle and to pass onto others and to use on websites (TIFFs can't be used on websites).

If you need museum quality then choose TIFF.

Derek Cross

Community Expert

Personally I'd choose JPGs at high compression, as the files will be smaller than TIFFs (others may have a different view).

Re Elements, it's best to post in the Elements forum: Photoshop Elements

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