Photoshop file size how to determine

In article , Nathan Gutman
wrote:

[…]
How can I determine what the jpg file size will be when using different image quality options when saving?

Use Save for Web to make it easy. The file size will be shown before saving.

"Nathan Gutman" wrote in message

Document size is displayed in the lower left corner. Not really sure what that means.
For example:
Document size shows 5.49M/5.49M. File size (jpg) on computer shows only 346K. Why, what does that mean?

It means there are 1.83 Mpixels in the image (5.49 / 3 bytes per pixel for 8-bit color depth), which is stored as a 346K file when compressed.

How can I determine what the jpg file size will be when using different image quality options when saving?

When you click "Save as" you will get a pop-up screen that allows you to set the file name and type (JPEG, TIFF, GIF, etc.). Assuming you select JPEG and click "Save" you will get a second screen that allows you to set the amount of compression. At the bottom of that screen (you may have to wait a few seconds) it will tell you the file size to expect, which will change as you change the amount of compression.

What really is 2.5 compatibility. User guide says "file to be used by other programs". What other programs?
I understand that unchecking that option will result in a smaller file.

Beats me – I’ve not seen that in my version 7.0.1 of PS.

Don

Nathan Gutman wrote:

Document size is displayed in the lower left corner. Not really sure what that means.
For example:
Document size shows 5.49M/5.49M. File size (jpg) on computer shows only 346K. Why, what does that mean?

The file size Photoshop is talking about is the UNCOMPRESSED size. If you would save the file as uncompressed TIFF or PSD, that would be the the file size on disk. JPEG is a compressed size, and that is why JPEG is much smaller.

How can I determine what the jpg file size will be when using different image quality options when saving?

Both "Save for the web" as well as "Save as…. <JPEG>" will show you the file size at different quality settings.

What really is 2.5 compatibility. User guide says "file to be used by other programs". What other programs?
I understand that unchecking that option will result in a smaller file.

AFAIK "2.5 compatibility" means that the PSD file can be opened in Photoshop 2.5, which didn’t know layers yet. It is a way of making sure that other programs (who support Photoshop files) can open the image.

—
Johan W. Elzenga johan<<at>>johanfoto.nl Editor / Photographer http://www.johanfoto.nl/

For example:
Document size shows 5.49M/5.49M. File size (jpg) on computer shows only 346K. Why, what does that mean?

JPEG files are compressed. The compression makes the file smaller on disk. It does this by degrading the quality of the image. JPEG is only useful for situations where file size is important and quality is not important. Saving as JPEG always degrades the image; never save as JPEG unless you have to.

If you save in a format like TIFF, the file size on disk will be the same as the file size when opened.

How can I determine what the jpg file size will be when using different image quality options when saving?

The file size is displayed in the JPEG Options dialog when you do a Save As.

What really is 2.5 compatibility.

It means that when you save a .PSD, the image is saved into the file TWICE–once in layers, and then once again flattened. This makes the file considerably bigger.

Some programs can only read a .PSD file if it is flattened; that’s why this option is used.

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Nanohazard T-shirts now available! http://www.villaintees.com Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

In article , says…

"Nathan Gutman" wrote in message

Document size is displayed in the lower left corner. Not really sure what that means.
For example:
Document size shows 5.49M/5.49M. File size (jpg) on computer shows only 346K. Why, what does that mean?

It means there are 1.83 Mpixels in the image (5.49 / 3 bytes per pixel for 8-bit color depth), which is stored as a 346K file when compressed.

How can I determine what the jpg file size will be when using different image quality options when saving?

When you click "Save as" you will get a pop-up screen that allows you to set the file name and type (JPEG, TIFF, GIF, etc.). Assuming you select JPEG and click "Save" you will get a second screen that allows you to set the amount of compression. At the bottom of that screen (you may have to wait a few seconds) it will tell you the file size to expect, which will change as you change the amount of compression.

What really is 2.5 compatibility. User guide says "file to be used by other programs". What other programs?
I understand that unchecking that option will result in a smaller file.

Beats me – I’ve not seen that in my version 7.0.1 of PS.
Don

This refers to the backward capability of PS 2.5 to read the file. I haven’t seen if since about v5 (?). In CS it has been replaced with a pop-up stating that Save(ing) with backward compatibility increases the file size, which it does, but allows those with older versions of PS to still access the file.

Hunt

In article <1gek8gc.vdsk7e2nlev4N%>,
says…

Nathan Gutman wrote:

Document size is displayed in the lower left corner. Not really sure what that means.
For example:
Document size shows 5.49M/5.49M. File size (jpg) on computer shows only 346K. Why, what does that mean?

The file size Photoshop is talking about is the UNCOMPRESSED size. If you would save the file as uncompressed TIFF or PSD, that would be the the file size on disk. JPEG is a compressed size, and that is why JPEG is much smaller.

How can I determine what the jpg file size will be when using different image quality options when saving?

Both "Save for the web" as well as "Save as…. <JPEG>" will show you the file size at different quality settings.

What really is 2.5 compatibility. User guide says "file to be used by other programs". What other programs?
I understand that unchecking that option will result in a smaller file.

AFAIK "2.5 compatibility" means that the PSD file can be opened in Photoshop 2.5, which didn’t know layers yet. It is a way of making sure that other programs (who support Photoshop files) can open the image.

Oops, sorry! I see you have already answered that.
Hunt

When you click "Save as" you will get a pop-up screen that allows you to set the file name and type (JPEG, TIFF, GIF, etc.). Assuming you select JPEG and click "Save" you will get a second screen that allows you to set the amount of compression. At the bottom of that screen (you may have to wait a few seconds) it will tell you the file size to expect, which will change as you change the amount of compression.

I’m using Photoshop CS for Mac.

The actual saved file size is approximately 2 times the
file size indicated in the photoshop dialog (in this
case, I’m saving as a JPEG file). E.g. It predicts
81.1k, and the file size is 170k after saving.

Any ideas on what could be causing this discrepancy
between the predicted file size and the actual file size?

Thanks.
DW

The actual saved file size is approximately 2 times the
file size indicated in the photoshop dialog (in this
case, I’m saving as a JPEG file). E.g. It predicts
81.1k, and the file size is 170k after saving.

Yep–but not really.

This is one of the things that’s really tricky to understand, because it involves understanding how a computer allocates space on a hard drive.

When you save a 1-byte file on a hard drive, it does not take up 1 byte of space on the drive. A computer has to allocate space on a hard drive in "chunks," called "allocation blocks." The minimum size of an allocation bloack depends on many things–the total size of the hard drive, the operating system of the computer, and the way the hard drive is formatted.

Let’s say that your computer’s minimum allocation size is 64K. A one-byte file will show as 64K on disk. A 100-byte file will show as 64K on disk, as will a 2K file, and a 63K file. All space on the disk must be allocated in 64K chunks. the one-byte file occupies only one byte of that 64K chunk; the rest is wasted. if you upload it to a Web server, only 1 byte will get transferred–but the file still takes up 64K on your hard disk.

Now, a 66K file is going to occupy 128K of space on the disk–because it’s too big to fit into one chunk, so it takes up two.

If you click on a file on your Mac and go to File->get info, you will see the file’s size listed twice. The Get Info window will show the amount of space taken up on disk, and also the actual "true" size of the file.

So in our hypothetical example, let’s say your allocation size is 64K. When you do a Get Info, you will see something like this:

Size on disk: 64K (19,230 bytes).

This tells you that the file is approximately 19K in size–iuf you upload it to the Web, 19,230 bytes will be sent. however, it takes up 64K of space on the hard disk, because that’s the minimum allocation size.

Another factor which might be important as well is the fact that you might be instructing Photoshop to save an icon, a thumbnail, or an ICC color profile. The file size in the Save As window is only the pure JPEG information. Adding a thumbnail, a preview, a color profile, or other metadata will increase this size.

—
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

Tacit wrote:

The actual saved file size is approximately 2 times the
file size indicated in the photoshop dialog (in this
case, I’m saving as a JPEG file). E.g. It predicts
81.1k, and the file size is 170k after saving.

Yep–but not really.

This is one of the things that’s really tricky to understand, because it involves understanding how a computer allocates space on a hard drive.
When you save a 1-byte file on a hard drive, it does not take up 1 byte of space on the drive. A computer has to allocate space on a hard drive in "chunks," called "allocation blocks." The minimum size of an allocation bloack depends on many things–the total size of the hard drive, the operating system of the computer, and the way the hard drive is formatted.
Let’s say that your computer’s minimum allocation size is 64K. A one-byte file will show as 64K on disk. A 100-byte file will show as 64K on disk, as will a 2K file, and a 63K file. All space on the disk must be allocated in 64K chunks. the one-byte file occupies only one byte of that 64K chunk; the rest is wasted. if you upload it to a Web server, only 1 byte will get transferred–but the file still takes up 64K on your hard disk.

Now, a 66K file is going to occupy 128K of space on the disk–because it’s too big to fit into one chunk, so it takes up two.

If you click on a file on your Mac and go to File->get info, you will see the file’s size listed twice. The Get Info window will show the amount of space taken up on disk, and also the actual "true" size of the file.
So in our hypothetical example, let’s say your allocation size is 64K. When you do a Get Info, you will see something like this:

Size on disk: 64K (19,230 bytes).

This tells you that the file is approximately 19K in size–iuf you upload it to the Web, 19,230 bytes will be sent. however, it takes up 64K of space on the hard disk, because that’s the minimum allocation size.

Another factor which might be important as well is the fact that you might be instructing Photoshop to save an icon, a thumbnail, or an ICC color profile. The file size in the Save As window is only the pure JPEG information. Adding a thumbnail, a preview, a color profile, or other metadata will increase this size.

Ok. That makes sense.

However, when I look at the file info, I get the following for a file that was estimated to be 54.08KB in PS:

Size on Disk: 128kB (126,681 bytes).

When I email the image to someone, the size of the
attachment is about this size.

So something still doesn’t make sense. You would think that this file would be saved as 64k under your example.

DW

DW wrote in
news:3yEKc.12485$:

Tacit wrote:

The actual saved file size is approximately 2 times the
file size indicated in the photoshop dialog (in this
case, I’m saving as a JPEG file). E.g. It predicts
81.1k, and the file size is 170k after saving.

Yep–but not really.

This is one of the things that’s really tricky to understand, because it involves understanding how a computer allocates space on a hard drive.

When you save a 1-byte file on a hard drive, it does not take up 1 byte of space on the drive. A computer has to allocate space on a hard drive in "chunks," called "allocation blocks." The minimum size of an allocation bloack depends on many things–the total size of the hard drive, the operating system of the computer, and the way the hard drive is formatted.

Let’s say that your computer’s minimum allocation size is 64K. A one-byte file will show as 64K on disk. A 100-byte file will show as 64K on disk, as will a 2K file, and a 63K file. All space on the disk must be allocated in 64K chunks. the one-byte file occupies only one byte of that 64K chunk; the rest is wasted. if you upload it to a Web server, only 1 byte will get transferred–but the file still takes up 64K on your hard disk.

Now, a 66K file is going to occupy 128K of space on the disk–because it’s too big to fit into one chunk, so it takes up two.

If you click on a file on your Mac and go to File->get info, you will see the file’s size listed twice. The Get Info window will show the amount of space taken up on disk, and also the actual "true" size of the file.

So in our hypothetical example, let’s say your allocation size is 64K. When you do a Get Info, you will see something like this:
Size on disk: 64K (19,230 bytes).

This tells you that the file is approximately 19K in size–iuf you upload it to the Web, 19,230 bytes will be sent. however, it takes up 64K of space on the hard disk, because that’s the minimum allocation size.

Another factor which might be important as well is the fact that you might be instructing Photoshop to save an icon, a thumbnail, or an ICC color profile. The file size in the Save As window is only the pure JPEG information. Adding a thumbnail, a preview, a color profile, or other metadata will increase this size.

Ok. That makes sense.

However, when I look at the file info, I get the following for a file that was estimated to be 54.08KB in PS:

Size on Disk: 128kB (126,681 bytes).

Look again. What does Size: report?

When I email the image to someone, the size of the
attachment is about this size.

How are you determining the email size?

Tacit wrote:

The actual saved file size is approximately 2 times the
file size indicated in the photoshop dialog (in this
case, I’m saving as a JPEG file). E.g. It predicts
81.1k, and the file size is 170k after saving.

Yep–but not really.

This is one of the things that’s really tricky to understand, because it involves understanding how a computer allocates space on a hard drive.

When you save a 1-byte file on a hard drive, it does not take up 1 byte of space on the drive. A computer has to allocate space on a hard drive in "chunks," called "allocation blocks." The minimum size of an allocation bloack depends on many things–the total size of the hard drive, the operating system of the computer, and the way the hard drive is formatted.

Let’s say that your computer’s minimum allocation size is 64K. A one-byte file will show as 64K on disk. A 100-byte file will show as 64K on disk, as will a 2K file, and a 63K file. All space on the disk must be allocated in 64K chunks. the one-byte file occupies only one byte of that 64K chunk; the rest is wasted. if you upload it to a Web server, only 1 byte will get transferred–but the file still takes up 64K on your hard disk.

Is this concept of 64K chunks still valid with OS X? I thought there was the UNIX standard of 2KB chunks used now.

Even if this is the case, the given file of 81.1 KB should result in 128kb, not 170.

You final assumption about additional information saved in the file (Thumbnail, color profile, etc.) seems to be much more probable.

Michael

Is this concept of 64K chunks still valid with OS X? I thought there was the UNIX standard of 2KB chunks used now.

OS X is normally installed on an HFS+ formatted disk, not a UFS/EXT2/EXT3-formatted disk. With HFS+, the allocation block size can vary, anywhere between 4K and 32K depending on the parameters with which it was formatted. (Alsoft makes a neat little program called PlusOptimizer which can reduce the allocation block size on an HFS+-formatted volume to 4K without reformatting.)

—
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html

However, when I look at the file info, I get the following for a file that was estimated to be 54.08KB in PS:

Size on Disk: 128kB (126,681 bytes).

If the file is that large, it’s quite likely you’re saving an icon, a preview, and possibly a color profile in the image. You can turn these off in Edit->preferences; does that help?

—
Art, literature, shareware, polyamory, kink, and more:
http://www.xeromag.com/franklin.html