Apple Preview Image Resize

leerkeller

macrumors member
Original poster
Nov 1, 2011
97
0
Baltimore, MD
I have a bit of a concern with the way Preview handles images. I am using it to change the dpi setting in the EXIF header from 72 dpi to 300 dpi. The problem that I am having is that I believe in completing that process I am degrading the quality of the images.

When I click adjust size and leave the image dimensions and all other setting unchanged I still have the option of clicking OK. I just am not clear on what OK does, other than that it reduces file size. Clicking OK causes the file size of the image to be reduced by as much as 3/4ths despite the dimensions remaining the same. This leads me to believe that some image degradation through compression must be occurring.

Ideally what I want to be able to do is change the DPI in the EXIF tag and not change the file quality. From what I understand changing the DPI should have no effect on file size, so something else must be going on.

In the screenshot below you can see what I mean. I simply click on adjust size in the tool menu and the adjust size window pops up. The resulting file size states that the file will be reduced from 11.6 MB to 2.4 MB without changing anything. I just would like to be able to change that setting of 72 pixels/inch to 300 pixels/inch and have nothing else change. Any suggestions and can anyone tell me what is done to the file when I click OK?

 

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leerkeller

macrumors member
Original poster
Nov 1, 2011
97
0
Baltimore, MD
One more screen grab below that may make my question more comprehensible. In the adjust size window I have changed the dpi from 72 to 300 dpi, and the resulting file size calculation doesn't change, but it still has been reduced through some process unrelated to the dpi change. How can I use the adjust image feature to simply change the DPI and not reduce the file size (from 11.6MB to 2.4MB in the screengrab) and image quality that is related
to file size.
 

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swordio777

macrumors 6502
Apr 3, 2013
287
16
Scotland, UK
Hi leerkeller,

I'll cut straight to the chase, I have no idea why preview does this. But you're correct - if the file size is changing that much then the conversion is definitely compressing/throwing away data.

Personally, I definitely wouldn't recommend using preview to make this change. But I must ask why you're doing it at all? The DPI / PPI maintained in the EXIF info has absolutely nothing at all to do with anything. The fact that it's included at all only really serves to confuse.

The size of the pixels on your screen are obviously fixed, so there's really no such thing as "Pixels Per Inch" (PPI). The only way to change the image's size on screen is to reduce the number of actual pixels in the image or view it at a percentage of its actual size.

"Dots per Inch" (DPI), on the other hand, is very real, but it only affects the output device (a printer). Historically, this figure would be set by the printer operator to ensure the image was printed at the correct size, however all new software lets you simply select the output size you want and does this calculation for you (completely disregarding any PPI figure in the EXIF).

Personally, I'd recommend just leaving all your images in whatever PPI your camera shoots in - it honestly makes absolutely no difference. But if you are determined to change the PPI in the exif then I'm sure you'll be able to do it in the free image editing software "GIMP" without changing the file in any other way.

Hope that helps.
 

ijohn.8.80

macrumors 65816
Jul 7, 2012
1,246
2
Adelaide, Oztwaylya.
Go have a read of this page which explains what's going on really simply and easily.

In a nutshell, using 3 pictures at 300x400 pixels, but, at different ppi ratios, it shows:

*What happens when you print the images?

The image in 100 ppi will print 10 cm wide ([400/100] x 2,5)
The image in 300 ppi will print 3,3 cm wide ([400/300] x 2,5)
The image in 72 ppi will print 13,9 cm wide ([400/72] x 2,5)
Preview, it appears, is smart enough to take this into account!

Basically, on the web, you only need 72ppi. I'm lazy in that regard and couldn't be bothered creating another copy of my images at a lower ppi that I then need to catalogue and store as well as the originals, so my images are posted at 300ppi. :eek:
 

leerkeller

macrumors member
Original poster
Nov 1, 2011
97
0
Baltimore, MD
Hi leerkeller,



The size of the pixels on your screen are obviously fixed, so there's really no such thing as "Pixels Per Inch" (PPI). The only way to change the image's size on screen is to reduce the number of actual pixels in the image or view it at a percentage of its actual size.

"Dots per Inch" (DPI), on the other hand, is very real, but it only affects the output device (a printer). Historically, this figure would be set by the printer operator to ensure the image was printed at the correct size, however all new software lets you simply select the output size you want and does this calculation for you (completely disregarding any PPI figure in the EXIF).

Personally, I'd recommend just leaving all your images in whatever PPI your camera shoots in - it honestly makes absolutely no difference. But if you are determined to change the PPI in the exif then I'm sure you'll be able to do it in the free image editing software "GIMP" without changing the file in any other way.

Hope that helps.
Oi Vey.....the DPI/PPI thing. There was a thread that went on forever and ever about this back in 2009. People kept coming up with metaphors of increasing complexity, finally ending with the post I am pasting below....at least that was what it ended with the last time I checked.

Nov 01, 2009; 12:09 p.m.
DPI and PPI are used in digital printing alot. DPI is a many decades old term with scanning. It is used in our 35mm slide scanner of 1989; yes eighty nine. It is used in mid 1980's DOS scan wands; it is used in patents for FAX machines and formal Government specs on scanning documents.

IF somebody comes in my shop and says they scanned there 35mm slide at home on a flatbed at 2400 ppi; I know what they mean. IF a newbie Photoshop expert walks in and says all scanners are in PPI; I am not going to argue that he is wrong. He just read a newbie Photohop books dogma and believes it. I COULD mention that all but one of the digital scanners I have owned say "DPI" over the last 30 years; but folks have rigid mindsets; and love to ignore history. One can go back to late 1970's and early 1980's books and a dot and pixel are the same. BUT a pixel can be in math a hex layout like bees honeycomb; or a quad XY layout as we know today; if it is a 1930's scoreboard sign of thousand of bulbs.

In Farming farmers before we were all born used terms like poop; s---,manure, paddies; cow-chips for that stuff on the ground around cows. In Printing one often uses "DPI" around scanners; since that is about what EVERY scanner on the planets software uses. A farmer who sells 1 ton of manure and while loading it in the customers trucks says it is 1 ton of cowpaddies probably is not confused. Nor is a printer who uses dpi and ppi all the time; and has new customers on soap boxes declaring dpi is wrong for scanners. IF a new college grad PHd farm salesman lectures an old farmer about how his terms about cow paddies are wrong; the farmer just tags the salesman as a newbie; or troublemaker; or know-it-all that has zero experience.

What really matters is communication with ones clients.

IF you tell him he is all wrong; you probably lost the sale.

If he used the terms before you were born; it looks worse too.

Pixels are like dollars; more is better.

If somebody wants to pay you 500 dollars in CASH; normal folks do NOT get bogged down in the bills used. Sane folks would be happy with five 100's; or ten 50's; or twenty five 20's. In the deep South and around some gambling places 50's are considered unlucky; thus a dealer will not give back 50's in change. In images; dpi and ppi are abit like the size of bill. IF an editor wants a 720 pixel wide image for an advert; you give him at least a 720 pixel wide image. If he wants it tagged at 72 or 300; you do it. If you pay somebody 720 dollars in one dolar bills; they might get upset. If you give an editor 100 images and each is tagged at various dpi/ppi tags to be cute; you might get labeled as a troublemaker.

****IF an image is tagged at 240 and the client wants tagged at 300; this is an easy thing to do. You just the file in photoshop etc and keep the SAME number of pixels and save it with the new tag. Pleasing a client is really nothing new; a farmer selling eggs 300 years ago might have one customer who wants his buy in dozen crates; another by crates of 36 eggs. Customer #1 might buy 1 gross of eggs and want twelve cartons of a dozen eggs; customer #2 might want 1 gross packaged as four crates of 36 eggs per crate. How "stuff" we all buy is packaged is thousands of years old. Vendor who supply in weird unwanted units often die off or go broke.

****Editors and Farmers want suppliers who are HONEST. If an image is upsized and passed off as non upsized; the editor may be concerned. If you supply diluted bags of seed or fertilizer to a farmer as full strength; he might catch on. In either case of selling images or seeds; the buyer may prefer the way the product is packaged. If you supply images tagged at 72 or 240 and they want 300; but it is a rush job; let them know. If you supply seed in 10 Lb bags instead of 25 Lb bags; tell them why. It irrates a buyer to no end when folks purposely supply what they want in the wrong units. If you do this too much they will find another who listens.

Today the world is riddled with digital experts. It really is nothing new that a fine magazine wants a 300 ppi/dpi image at the actual size of the print in the magazine. This number is driven by the line screen of the printing. Printers *think* in inches; ie the peice of paper the actual reader is going in his/her hands. This is reality; ie a physical product is going to be on a newstand. Having at least 300 pixels per inch allows a great line screen. Giving them MORE allows cropping.

A painter might want to paint a wall that has 1000 square feet; and want 10 gallons of ACME paint; in one 5 gallon pail; plus five 1 gallon cans. He purposely requests one 5 gallon pails; so Kilroy can be using a roller or sprayer; and the small gallon cans for the trim painters on ladders. One could be a expert paint supplier and "show him" and send him forty 1 quart cans; or 10 on gallon cans; by NOT listening. One could also be a total jackass and supply him with 1/4 oz Testors model airplane paint bottles too After opening 5120 bottles of paint the buyer would have enough time to create fancy nicknames for you! :) ILL inputs; MAY just make the other chaps job harder; and after awhile you get dumped as a supplier; who's brains is too full to listen.

Editors tire with the constant cost of dealing with digital inputs.The math is really not all that hard at all.
The number of pixels per inch required in printing depends on the application. National Geographic is higher than newspaper; which is higher than hockey dasher boards; which is higher than billboards. One has the physical size plus viewing distance too. Folks who print this stuff have a feel for what is required.

Whether selling eggs; seeds; manure; paint or images the buyer has reasons why they want the product packaged. The farmer grows tired constantly explaining what a dozen is; or what pounds or acres are. The painter will grow tired too if one has to be told what a gallon is; or a brush; ore a roller.

The editor grows tired too of explaining what pixels are; or dpi or ppi; or why they want a 300 ppi/dpi image. Real editors known hat they want; so do Farmers; Painters; and Carpenters too. A real carpenter in the USA probably knows what a 2x4 is in actual size; or what a worm drive saw looks like. Those who think printers and editors are confused about dpi and ppi and images probably also believe a farmer doesnt know what an acre or tractor is; or a carpenter a 2x4; or a painter what a gallon or brush is.

Part of being a successfull vendor is learning what client wants and understanding why; understanding the terms/words used to communicate. Folks new to any industry tend to coin new terms for the same old stuff and often embark on a crusade to educate the older established group on why they are all wrong. Many folks have jobs to do and do not have time for this petty monkey business.
IF one supplies up-sized images to a client; or diluted paint to a client; let the client know.
 

swordio777

macrumors 6502
Apr 3, 2013
287
16
Scotland, UK
Hi Leerkeller,

Apologies if you didn't find my previous post useful. I was trying to keep things as simple as possible in case you were new to this, but it looks like you've done quite a bit of investigation into this in the past.

Your post above is basically saying much the same as I did before: essentially, the PPI/DPI tag does not matter.

I'll try to avoid all the metaphors. The key paragraph from that whole post is this:

"It really is nothing new that a fine magazine wants a 300 ppi/dpi image at the actual size of the print in the magazine. This number is driven by the line screen of the printing. Printers *think* in inches; ie the peice of paper the actual reader is going in his/her hands. This is reality; ie a physical product is going to be on a newstand. Having at least 300 pixels per inch allows a great line screen. Giving them MORE allows cropping."

What this paragraph is saying is not that the image's exif tag needs to say 300dpi, but that your digital photograph must have 300 pixels of width / height for each inch you want to print. For example, a 6 megapixel image is 2000 pixels tall x 3000 pixels wide (rounding down from 3008 for ease of calculation).

If you want to print this image 10 inches wide at magazine quality then you can do so, because there are enough pixels in the image to allow for 300 pixels per printed inch (10" x 300 DPI = 3000 pixels). This is true even if the photograph's tag says that the DPI is only 72.

However if you wanted to print that same image at 15" wide then your print shop may tell you it can't be done. This is because there are not enough pixels in the image to allow 300 pixels for every printed inch. Regardless of whether the EXIF tag says the image is 1 DPI, 72 DPI, 300 DPI, or 5000 DPI, it simply doesn't change the fact that there are not enough pixels in the image to print at that large size. The image would need to be 4500 pixels wide to print it 15" wide at 300DPI (15 x 300 = 4500).

When a print shop asks for an image that's 300 DPI, this is what they're asking for. They're not asking for an image that has an exif tag which states the image is 300 DPI.

If you want to make prints that are 6" x 4" then your image only needs to be 1800 pixels wide (6 x 300) and 1200 pixels tall (4 x 300). That's a 2.2 megapixel image. The larger you want to print, the more pixels you need if you want to retain your 300DPI print quality.

From the image in your original post I can see that the photograph in question is 4000 x 3000 pixels. That means you'll be able to print this image at any size up to 13" x 10" while still retaining full magazine print quality of 300 DPI.

If you want to print larger than this, but there are not enough pixels in your image to print at the size you want, then you must reduce the print quality to compensate.

For example, you cannot print a 6 megapixel image (3000 x 2000 pixels) at 15 inches wide and still retain 300DPI print quality. However you can print it that wide if you reduce the print quality to 200DPI, assuming your print shop can do this (15" x 200 DPI = 3000 pixels). I hope that clarifies what I was saying previously.

As I mentioned before, if you really do need to change the exif tag to please a client then you'll be able to do this in most photo manipulation software (GIMP, Photoshop, Lightroom, Aperture, Elements, etc). If you choose to do this, make sure you DO NOT "resample" the image. You only want to change the tag, not the actual image dimensions.

Best regards.
 
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