Dpi vs file size vs image quality?

my thoughts/experience

There is a corelation between pixels and resolution but the pixel dimension is most important.

If you have an image that is 360 pixels square and the dpi is 72, and you make a print the image would be 5 inches square.

360 divided by 72 = 5

If you used image>image size and deslected the Resample check box, you could change the dpi to 100. The file would not get any bigger so the pixels would be the same. If you printed the image it would be 3.6 inches square.

360 divided by 100 = 3.6

If you check the Resample box, Photoshop will add pixels through interpolation. You will get more pixels, and a larger image print at the same resolution, but the image will be fuzy because the program is making up the added information.

You can change the resolution to get a larger print size without changing the number of pixels in the image up to a point.

You can also resample the image to add pixels to get a bigger print size but the image will become soft.

There are two other ways to get a larger image at lower file size.

Compression
JPEG files use a losssy scheme to change the file. They average colors to lower the data. In doing this, they throw out data so image quality is degraded. How much data is tossed and how much quality is lost is determined by the amount of compression selected. Typically, the loss is less noticable on screen. Prints will look grainy if a lot of compression is used.

Fractals
There are programs that use fractals to increase image size. This is hi tech stuff like what the military uses for aerial survalence. It is available as plugin for PS but has not had wide usage.

HTH

dp

philipus wrote:

Hi everyone

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

I develop pictures using a laboratory that only gives me 50 MB web space for my albums.

I have read a lot here in the groups about dpi – pixels – ppi etc and how these can be changed in Photoshop (versio 6 in my case).
When my brother ran the marathon the participants could order and have emailed to them pictures of themselves courtesy of a professional lab. The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.
Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

All help is appreciated.

Philip

philipus wrote:

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

I develop pictures using a laboratory that only gives me 50 MB web space for my albums.

I have read a lot here in the groups about dpi – pixels – ppi etc and how these can be changed in Photoshop (versio 6 in my case).
When my brother ran the marathon the participants could order and have emailed to them pictures of themselves courtesy of a professional lab. The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.
Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

All help is appreciated.

Why do you post this question again? Don’t you read the answers people have already given you?

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

philipus wrote:

Hi everyone

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

I develop pictures using a laboratory that only gives me 50 MB web space for my albums.

I have read a lot here in the groups about dpi – pixels – ppi etc and how these can be changed in Photoshop (versio 6 in my case).

So your images measure something in the range of 2300x1770pixels. Printing them 40 cm wide prints only 57.5 pixels per cm or 146pixels per inch.

This is almost enough for the quality you get for cheap analog prints but not really good.

The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.

My Translation: The lab told you that they used higher JPG compression than usual but had enough pixels to print an image where you don’t see pixels when you don’t wear glasses.

Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

When you save an image as JPG in Photoshop you can chose the compression value on a 1-12 scale where 12 is the best quality and 1 is the smallest file.
You always trade in quality for filesize.

Maybe you should try saving one image in a series with all the compression values and compare the results.

For some images higer compression does lead to artifacts later than for others but at some level it will always be visible.

Michael

I’d say you have more than enough answers now.
Haha nice to read D. Plank gave you an almost identical anwer as I gave you. Well.. now you really should understand what we’re talking about.

Regards,
Tiemen

"Johan W. Elzenga" wrote in message

philipus wrote:

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

I develop pictures using a laboratory that only gives me 50 MB web space for my albums.

I have read a lot here in the groups about dpi – pixels – ppi etc and how these can be changed in Photoshop (versio 6 in my case).
When my brother ran the marathon the participants could order and have emailed to them pictures of themselves courtesy of a professional lab. The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.
Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

All help is appreciated.

Why do you post this question again? Don’t you read the answers people have already given you?

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

Kinda glad he did, I missed it the first time and it was good information. Thanks

"Johan W. Elzenga" wrote in message

philipus wrote:

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

I develop pictures using a laboratory that only gives me 50 MB web space for my albums.

I have read a lot here in the groups about dpi – pixels – ppi etc and how these can be changed in Photoshop (versio 6 in my case).
When my brother ran the marathon the participants could order and have emailed to them pictures of themselves courtesy of a professional lab. The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.
Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

All help is appreciated.

Why do you post this question again? Don’t you read the answers people have already given you?

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

(philipus) writes:

Hi everyone

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

Actually, it doesn’t. It takes 4MP images, and assigns them (arbitrarily) the pixel density of 72dpi. Easily changed in Photoshop without touching the actual image data. Are these JPEG images, or perhaps TIFF? 2-4MB for a 4MP JPEG sounds quite big.

I develop pictures using a laboratory that only gives me 50 MB web space for my albums.

I have read a lot here in the groups about dpi – pixels – ppi etc and how these can be changed in Photoshop (versio 6 in my case).
When my brother ran the marathon the participants could order and have emailed to them pictures of themselves courtesy of a professional lab. The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.

Hmm. That might be optimistic, but I have provided 6MP images to a national magazine for its cover. As I recall, three would fit on a 1.44MB floppy.

Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

Forget dpi. What you want to do is play with JPEG quality settings. You probably want to save a series of JPEGs of the same image with quality 10, 9, 8, 7, 6 and have them printed. Decide which of them has adequate quality, and use that setting. NB: You probably want to reload the original image before each save, as Photoshop will otherwise show the results of the last JPEG compression. You want clean compression from the original.

—
-Stephen H. Westin
Any information or opinions in this message are mine: they do not represent the position of Cornell University or any of its sponsors.

Hmmm here we are again at the sensor-resolution-render conundrum

A sensor samples data – nothing more, nothing less

That data is converted into digital data using something called Analogue to Digital Converter (ADC)

the data are processed using digital signal processing techniques (DSP)

the data are processed again to match the input-output needs of display devices

A display device may be the camera lcd, a computer monitor or a printer

These output devices have their own way of doing things

Don’t be anxious or worried about it. It doesn’t happen all in one stage – there are many stages involved. Understand this and you will grasp the elegance and sophistication behind digital imagery

Artie

"Stephen H. Westin" <westin*> wrote in message

(philipus) writes:

Hi everyone

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

Actually, it doesn’t. It takes 4MP images, and assigns them (arbitrarily) the pixel density of 72dpi. Easily changed in Photoshop without touching the actual image data. Are these JPEG images, or perhaps TIFF? 2-4MB for a 4MP JPEG sounds quite big.

I develop pictures using a laboratory that only gives me 50 MB web space for my albums.

I have read a lot here in the groups about dpi – pixels – ppi etc and how these can be changed in Photoshop (versio 6 in my case).
When my brother ran the marathon the participants could order and have emailed to them pictures of themselves courtesy of a professional lab. The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.

Hmm. That might be optimistic, but I have provided 6MP images to a national magazine for its cover. As I recall, three would fit on a 1.44MB floppy.

Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

Forget dpi. What you want to do is play with JPEG quality settings. You probably want to save a series of JPEGs of the same image with quality 10, 9, 8, 7, 6 and have them printed. Decide which of them has adequate quality, and use that setting. NB: You probably want to reload the original image before each save, as Photoshop will otherwise show the results of the last JPEG compression. You want clean compression from the original.

—
-Stephen H. Westin
Any information or opinions in this message are mine: they do not represent the position of Cornell University or any of its sponsors.

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

Actually, your 4 megapixel digital camera takes pictures with no "DPI" setting at all. When the picture is taken, it’s just pixels, no "DPI"–the resolution is set by the program you first open the image in.

The file size is 2-4 MB *compressed.* Your camera takes pictures, then compresses the files to make them smaller. The compression it uses is "lossy." In English, that means that image detail is thrown away, degrading the quality of the image in order to make it smaller. This image degredation is permanent and irrevokable. The quality can never be restored.

The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.

There are two possible reasons why the images were smaller:

1. The number of pixels was smaller.

2. More compression was used, which degrades the image quality more, but makes a smaller file.

A 30×40 cm print can be made from any image. You can make a 30×40 cm print from a 2K file, if you want–but it will look like crap.

Whether or not the 1MB file was good enough to give a *high quality* print is a different story. Many people, especially people who know little or nothing about photography or image quality, will look at a picture made from a low-resolution file and say "Oh, yeah, sure, that looks fine." Those same people who look at the same picture made from a high-resolution file, side-by-side with the picture from the low resolution file, will say "Oh, yeah, that one is better," although they probably can’t explain WHY it’s better.

A picture that looks "just fine" to one person, however, may look like garbage to a more skilled or more critical viewer. In general, if you care about the picture enough to want to make a print from it, you should care about the picture enough to want to get the highest quality print you can. Can you make a smaller file? Yes, you can. Will it make the print worse? Yes, it will. Can people tell that the print is worse? If they see it next to the high-quality print, yes. If they are critical observers, yes.

Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

Can you shrink the file? Yes. Will it degrade quality? Yes. Can you shrink the file without degrading quality? No.

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

"Tacit" wrote in message

[…]
There are two possible reasons why the images were smaller:
[… good stuff…]

Actually there are three reasons: number three being he stands farther away. I strongly suggest the technique to digital photography mavens – stand as far from your pictures as you can. Leave the country if you have too.

Hi Tacit – after recent discussions on rec.photo.digital at the word "pixel" my brain sorta hurts

Yup – with you on the JPEG lossy compression and output specs

I think the trend to have "all inhouse" with digital cameras does give an impression that the sensor does all the work. Sensel = ouch?

Artie (really just the same ol’ bagal – change in nom de NG s’all)

"Tacit" wrote in message

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

Actually, your 4 megapixel digital camera takes pictures with no "DPI"

setting

at all. When the picture is taken, it’s just pixels, no "DPI"–the

resolution

is set by the program you first open the image in.

The file size is 2-4 MB *compressed.* Your camera takes pictures, then compresses the files to make them smaller. The compression it uses is

"lossy."

In English, that means that image detail is thrown away, degrading the

quality

of the image in order to make it smaller. This image degredation is

permanent

and irrevokable. The quality can never be restored.

The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.

There are two possible reasons why the images were smaller:
1. The number of pixels was smaller.

2. More compression was used, which degrades the image quality more, but

makes

a smaller file.

A 30×40 cm print can be made from any image. You can make a 30×40 cm print

from

a 2K file, if you want–but it will look like crap.

Whether or not the 1MB file was good enough to give a *high quality* print

is a

different story. Many people, especially people who know little or nothing about photography or image quality, will look at a picture made from a low-resolution file and say "Oh, yeah, sure, that looks fine." Those same people who look at the same picture made from a high-resolution file, side-by-side with the picture from the low resolution file, will say "Oh,

yeah,

that one is better," although they probably can’t explain WHY it’s better.
A picture that looks "just fine" to one person, however, may look like

garbage

to a more skilled or more critical viewer. In general, if you care about

the

picture enough to want to make a print from it, you should care about the picture enough to want to get the highest quality print you can. Can you

make a

smaller file? Yes, you can. Will it make the print worse? Yes, it will.

Can

people tell that the print is worse? If they see it next to the

high-quality

print, yes. If they are critical observers, yes.

Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

Can you shrink the file? Yes. Will it degrade quality? Yes. Can you shrink

the

file without degrading quality? No.

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

heh heh heh

FWIW my 35mm kit has been gathering dust over the years

Bought a digital camera about 6 weeks ago

I have now filled best part of 3 large photo albums and 3 small ones 🙂

But there again, I suppose I am easily pleased?

Artie

"jjs" wrote in message

"Tacit" wrote in message

[…]
There are two possible reasons why the images were smaller:
[… good stuff…]

Actually there are three reasons: number three being he stands farther

away.

I strongly suggest the technique to digital photography mavens – stand as far from your pictures as you can. Leave the country if you have too.

Thank you very much, I really appreciate this. I knew of the compression aspect of jpegs contra file size but thought there was some other correlation between the various settings in PS and file size. Basically I was looking for a smoking gun I guess.

As for the double-posting, I waited for more than a day before posting again because I could not find that my original post had been published, which I, admittedly, thought was strange since Google claims it will take between 3 and, is it, 6 or 9 (?) hours before a post appears. I really didn’t mean to offend but seeing how the two people who commented on the double-posting have Dutch email addresses, and therefore, I admit, prejudicially assuming that you are Dutch nationals, and how I, during my three years here in Holland have regularly experienced this rather rude directness (or is it just that the lack of sufficent language skills prevents accurate delivery?), I do not take offence but thank you humbly for so kindly pointing out to me that I obviously failed to meet the posting standards.

Really, there are more important things to get upset about

Again, thank you
Philip

(Tacit) wrote in message news:…

My 4 megapixel digital camera takes pictures with 72 dpi and each file is typically 2-4 MB.

Actually, your 4 megapixel digital camera takes pictures with no "DPI" setting at all. When the picture is taken, it’s just pixels, no "DPI"–the resolution is set by the program you first open the image in.

The file size is 2-4 MB *compressed.* Your camera takes pictures, then compresses the files to make them smaller. The compression it uses is "lossy." In English, that means that image detail is thrown away, degrading the quality of the image in order to make it smaller. This image degredation is permanent and irrevokable. The quality can never be restored.

The lab told me that each image file was less than 1 MB but was still of such quality that a print 30×40 cms could be printed from it.

There are two possible reasons why the images were smaller:
1. The number of pixels was smaller.

2. More compression was used, which degrades the image quality more, but makes a smaller file.

A 30×40 cm print can be made from any image. You can make a 30×40 cm print from a 2K file, if you want–but it will look like crap.

Whether or not the 1MB file was good enough to give a *high quality* print is a different story. Many people, especially people who know little or nothing about photography or image quality, will look at a picture made from a low-resolution file and say "Oh, yeah, sure, that looks fine." Those same people who look at the same picture made from a high-resolution file, side-by-side with the picture from the low resolution file, will say "Oh, yeah, that one is better," although they probably can’t explain WHY it’s better.
A picture that looks "just fine" to one person, however, may look like garbage to a more skilled or more critical viewer. In general, if you care about the picture enough to want to make a print from it, you should care about the picture enough to want to get the highest quality print you can. Can you make a smaller file? Yes, you can. Will it make the print worse? Yes, it will. Can people tell that the print is worse? If they see it next to the high-quality print, yes. If they are critical observers, yes.

Since space is an issue at my lab, I would like to know if I can "transform" my images somehow so that they shrink in size while preserving image quality.

Can you shrink the file? Yes. Will it degrade quality? Yes. Can you shrink the file without degrading quality? No.