ppi and output dimension questions

Since there is a finite amount of information making up any image (digital snapshots in my case), what is the trade-off between reducing output size in an effort to gain additional pixels? Does this even work without resampling and/or affecting sharpness and clarity(printed resolution)?

You don’t gain pixels. Period. Unless you resample and have Photoshop guess at what the extra pixels should be.
A photo that is 400 pixels by 600 pixels will always be 400 by 600. At 200 ppi, a 400X600 picture will print at 2 inches by 3 inches. At 100 ppi, it will be 4 inches by 6 inches.
What changes is the resolution — how dense the pixels are per inch. How many pixels you need per inch depends on the purpose and whether or not the output method can use that dense a resolution.
Resolution is not sharpness. There may be some cases where a higher resolution will actually make the photo look less sharp, if the output device can’t handle the resolution.

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Since there is a finite amount of information making up any image (digital snapshots in my case), what is the trade-off between reducing output size in an effort to gain additional pixels?

You don’t gain additional pixels. The number of pixels per inch goes up, but the total number of pixels stays the same.

i.e. take my 800 x 600 snapshot at 100 ppi, and reduce it’s output size to 400 x 300 am I (or can I) effectively double the resolution to 200 ppi?

800×600 what? 800×600 inches?

If you take a picture that is 800 pixels across and 600 pixels deep, and you make it 400 by 300 pixels, you have discarded information, and the resolution is still 100 pixels per inch.

If you take a picture that is 800 inches across and 600 inches deep, and make it 400 inches by 300 inches without changing the number of pixels, you have doubled the resolution in pixels per inch.

A pixel is like a tile in a tile mosaic. It is a square of color. The resolution in pixels per inch tells you how big each tile is. If you change resolution without changing the number of pixels, you are changing the size of each pixel.

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"LauraK" wrote in message

Since there is a finite amount of information making up any image (digital snapshots in my case), what is the trade-off between reducing output size in an effort to gain additional pixels? Does this even work without resampling and/or affecting sharpness and clarity(printed resolution)?

You don’t gain pixels. Period. Unless you resample and have Photoshop

guess at

what the extra pixels should be.
A photo that is 400 pixels by 600 pixels will always be 400 by 600. At 200 ppi, a 400X600 picture will print at 2 inches by 3 inches. At 100 ppi, it will be 4 inches by 6 inches.
What changes is the resolution — how dense the pixels are per inch. How many pixels you need per inch depends on the purpose and whether or

not the

output method can use that dense a resolution.
Resolution is not sharpness. There may be some cases where a higher

resolution

will actually make the photo look less sharp, if the output device can’t

handle

the resolution.

Very clearly explained, nice job.

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Thanks for your response Laura. I did have a few other questions though, as noted below.

(LauraK) wrote in message news:…

Since there is a finite amount of information making up any image (digital snapshots in my case), what is the trade-off between reducing output size in an effort to gain additional pixels? Does this even work without resampling and/or affecting sharpness and clarity(printed resolution)?

You don’t gain pixels. Period.

Ok, perhaps I worded that poorly, but when I turn off resampling in PS and reduce the pixel dimensions by 50%, the Resolution number (expressed as pixels per inch), doubles. This is what I am refering to with regard to the trade-off when "gaining" pixels and reducing dimensions.

Perhaps a better subject for this post would have been "Where is my data going?" or "what is happening to my data?" I guess that’s what I am really after.

Unless you resample and have Photoshop guess at what the extra pixels should be.

I understand that and that’s why I tried to leave resampling and interpolation out of the picture (excuse the pun.) I am still wondering what is happening to the data when modifying the Dimensions in the "Document Size" area of the Image Size menu option in Photoshop. To reiterate, reducing width and height by 50% doubles the resolution and conversely doubling the width and height (200%) drops the resolution by half.

What changes is the resolution — how dense the pixels are per inch.

Agreed, so what is happening to these pixels when the software is changing their density? Is there any loss/gain of data in terms of quality? It would seem to me that changing the density either way would still require software intervention and thus there would be loss. Or are you speaking only relative to the final output medium and not with regard to any actual density "change" in the software? (perhaps I just answered my own question as that thought brings me back to my "finite amount of data" comment, but feel free to school me further.)

Resolution is not sharpness.

You are absolutely correct. I was speaking too casually and making the assumption (very bad thing to do) that my image resolution would never intentionally surpass the limits of my output device.

There may be some cases where a higher
resolution will actually make the photo look less sharp, if the output device can’t handle the resolution.

Interesting. I can’t say I have ever seen a case where the image looked worse – just No Better than the output’s maximum resolution. But anyway, that’s a discussion for another time.

Did I make any more sense this time around? Hope so!

Jim

On 2 Mar 2004 21:59:39 -0800, (Jim) wrote:

Thanks for your response Laura. I did have a few other questions though, as noted below.

(LauraK) wrote in message news:…

Since there is a finite amount of information making up any image (digital snapshots in my case), what is the trade-off between reducing output size in an effort to gain additional pixels? Does this even work without resampling and/or affecting sharpness and clarity(printed resolution)?

You don’t gain pixels. Period.

Ok, perhaps I worded that poorly, but when I turn off resampling in PS and reduce the pixel dimensions by 50%, the Resolution number (expressed as pixels per inch), doubles. This is what I am refering to with regard to the trade-off when "gaining" pixels and reducing dimensions.

Perhaps a better subject for this post would have been "Where is my data going?" or "what is happening to my data?" I guess that’s what I am really after.

If you change resolution without resampling the only thing that changes in the image data is – as expected – X-DPI and Y-DPI. Your pixel data is unchanged.

I am still
wondering what is happening to the data when modifying the Dimensions in the "Document Size" area of the Image Size menu option in Photoshop. To reiterate, reducing width and height by 50% doubles the resolution and conversely doubling the width and height (200%) drops the resolution by half.

What changes is the resolution — how dense the pixels are per inch.

Agreed, so what is happening to these pixels when the software is changing their density?

The "real world" representation of each pixel changes its size. A 100dpi pixel is a square of 1/100" width and length. A 200dpi pixel is a square of 1/200" width and length. A 50dpi pixel is a square of 1/50" width and lenght:

You see the pattern? Ok, then multiply the number of pixels with the size of the pixel at any given dpi value and you have your image size.

Is there any loss/gain of data in terms of
quality? It would seem to me that changing the density either way would still require software intervention and thus there would be loss.

Does a tile pattern lose quality if you use smaller tiles?

Or are you speaking only relative to the final output medium and not with regard to any actual density "change" in the software?

DPI always and only refers to real world objects on ther way into or out of the digital world. Use of dpi values within the digital world is mostly using the wrong term.

There may be some cases where a higher
resolution will actually make the photo look less sharp, if the output device can’t handle the resolution.

Interesting. I can’t say I have ever seen a case where the image looked worse – just No Better than the output’s maximum resolution.

Use an image at 167% the maximum resolution of the output device. Downsampling will introduce massive blur.

Michael

Perhaps a better subject for this post would have been "Where is my data going?" or "what is happening to my data?" I guess that’s what I am really after.

Imagine a tile mosaic. It’s up on a wall, and it is 22 feet wide. Imagine that each tile is one foot across.

Now imagine shrinking it in a magical machine that can shrink a building. Your magical machine shrinks the wall so it is 11 feet wide. Now each tile is six inches across.

That is what is happening.

If you take a 300 pixel per inch image and shrink it to half size, each pixel is half as big. Now it is 600 pixels per inch.

Think about the phrase "pixels per inch." It is a measure of *how big* a pixel is. A 300 pixel per inch image, each pixel is a little square 1/300th of an inch across. A 600 pixel per inch image, each pixel is a little square 1/600th of an inch across.

You shrink an image, each pixel gets smaller. That’s it. Conceptually, it’s a very simple idea.

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