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FORUMS Post Processing, Marketing & Presenting Photos RAW, Post Processing & Printing 
Thread started 24 Dec 2017 (Sunday) 11:17
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Does reducing a jpg soften it?

 
OhLook
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Dec 24, 2017 11:17 |  #1

By the time my images land on POTN, they're much less sharp than the originals. They're also less sharp than many images by others, some in Birds being good examples. Maybe posting on POTN loses sharpness, but a lot is lost between my last look just after PPing and the next time I open the jpg file. All images are affected. What am I doing that blurs them?

I do just a little PP with the basic tools I have. The last step is to resize. Often this means reducing 3750 x 3000 to 900 by 720. (The Canon G15's sensor is 4000 x 3000.) I should think reducing would make an image crisper–with ink on paper, it does–but the opposite happens. The resizing screen has 180 pixels/inch as the default resolution, and I leave it there. Is that an appropriate fineness for Web posting? If I chose a higher number, would the result show at POTN?

Posted yesterday in the Urban Fragments thread:


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Before I saved the PPed image, the vertical grooves on the mailboxes were distinct. After saving, they were muddy. Below, at the right is a detail, SOOC with almost no reduction. At the left is the same bit from the image as posted above, enlarged to the same size. Too much is lost.

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Maybe the problem is endemic to jpgs. I'd try saving them as tiffs or something else, except that I can't find any such option in the Mac's Preview program. It doesn't have a Save As.

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Wilt
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Post edited 3 months ago by Wilt. (2 edits in all)
     
Dec 24, 2017 12:39 |  #2

You are decimating the original image resolution by about 75% and leaving only 25% of the image resolution. A lot happens on that magnitude of reduction!

I used Microsoft Paint to create a small matrix of 150 x 100 pixels, then put four lines that were 5 pixels wide and separated by 5-pixel spaces:

IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/Principles/high_zpszif5ib7c.jpg


Here is what the original stored JPG looks like, a screengrab with the original 150 x 100 pixels filling the screen, viewed in Paintshop Pro
IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/Principles/highscreen_zpsbptoqy1m.jpg

Then I resized the image to 25% of the original, ending up with about 38 x 25 pixels in the image, and here is a screengrab of that filling the screen when viewed in Paintsop Pro
IMAGE: http://i69.photobucket.com/albums/i63/wiltonw/Principles/lowscreen_zpsvcxb5fug.jpg

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Dec 24, 2017 13:23 |  #3

Wilt wrote in post #18525483 (external link)
You are decimating the original image resolution by about 75% and leaving only 25% of the image resolution. A lot happens on that magnitude of reduction!

I used Microsoft Paint to create a small matrix of 150 x 100 pixels . . .

Thank you. I want to reduce the image size but not the resolution. Here's what I don't understand: When I'm editing, the image is about the size it'll be after reduction, because the computer reduces it to fit the screen. (MacBook Pro with Retina screen.) The image is sharp then. Why doesn't resizing it produce a version with the same sharpness? A smaller version that's sharp must be possible. I know because I edited on one.

Is it better to uncheck the box labeled "Resample image"?

Presumably the bird photographers also start with images much larger than POTN allows for display, given that their cameras have bigger sensors than mine. Yet their posts preserve fine white lines in feathers. How do they do it?


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Post edited 3 months ago by Left Handed Brisket.
     
Dec 24, 2017 13:42 |  #4

In photoshop, the "resolution" of the image is the number of pixels per inch which for all intents is only used for printing. If you uncheck resample image, no pixels will be discarded or added, only the pixels per inch will change.

Overall pixel dimensions and jpg compression are the two tools you have to work with. POTN max pixel dimension is 2048x2048, I believe. If you export your jpg at that size with maximum quality (minimum compression), that will give you the best image quality.


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Dec 24, 2017 14:54 |  #5

What software are you using for resizing? I took your 100% sooc crop and reduced its size by 75%. I then re-scaled it back to the same size as before with no re sampling. There is a loss of resolution but nowhere near what you are getting. The re-scaled image is far right next to the original .


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Dec 24, 2017 15:47 |  #6

Left Handed Brisket wrote in post #18525529 (external link)
POTN max pixel dimension is 2048x2048, I believe.

Don't photos hosted by POTN have to fit into a box with max 1280 pixels?


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Dec 24, 2017 15:52 |  #7

Dave3222 wrote in post #18525562 (external link)
What software are you using for resizing? I took your 100% sooc crop and reduced its size by 75%. I then re-scaled it back to the same size as before with no re sampling. There is a loss of resolution but nowhere near what you are getting.

What software would you recommend? Are saying it's better to uncheck the resample box, as OneLook asked?


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Dec 24, 2017 16:16 |  #8

Left Handed Brisket wrote in post #18525529 (external link)
... POTN max pixel dimension is 2048x2048, I believe.

No.

Pippan wrote in post #18525582 (external link)
Don't photos hosted by POTN have to fit into a box with max 1280 pixels?

Yes.


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Dec 24, 2017 16:25 |  #9

Dave3222 wrote in post #18525562 (external link)
What software are you using for resizing? I took your 100% sooc crop and reduced its size by 75%. I then re-scaled it back to the same size as before with no re sampling. There is a loss of resolution but nowhere near what you are getting.

I resize in Preview. This is a fresh copy of (the upper part of) the same image SOOC with Preview's resizing screen in place, as if I were about to use it, with all defaults shown. The screenshot came out about twice the size of the actual images on the screen.


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Dec 24, 2017 16:31 |  #10

When I uncheck "Resample image," the width and height automatically change to dimensions in inches. Pixel values are then unavailable, and I need them if I'm to meet POTN's requirement.


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Dec 24, 2017 16:38 |  #11

4 inches at 300 ppi is 1200 pixels.


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Dec 24, 2017 16:42 |  #12

Left Handed Brisket wrote in post #18525609 (external link)
4 inches at 300 ppi is 1200 pixels.

Yes. How many px/in can I ask for before reaching the point of no return? That is, what resolution does the Web support?


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Dec 24, 2017 16:53 |  #13

OhLook wrote in post #18525611 (external link)
Yes. How many px/in can I ask for before reaching the point of no return? That is, what resolution does the Web support?

Doesn't matter what the resolution is, web browsers are going to ignore anything other than pixel dimensions.


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Dec 24, 2017 18:20 |  #14

As others have said the DPI/PPI resolution setting is only applicable when you are going to print the image. Whenever you are going to only view the image on screen all that matters is the pixel dimensions. One of the issues you may be having is that if you view a 900 by whatever pixel image in a photos type program using your HiDPI display it will be very small on screen. A 900 pixel image at 100% or 1:1 magnification will be 4.1" on the 5K retina display of a 27" iMac (I have a Dell monitor with the same display panel for my PC).

Once you have resized the image, if you want it to look the exact same size on the screen you will need to view it at 4.44× the magnification. So if you view at 100%/1:1 for the original, which won't quite fit your 4000 pixels across the screen, you would need to display the image at 444% or 4.44:1. I'm sure you will agree that viewing the original image at that magnification will not look that good either.

When you view the image in your web browser it will display the image at (by what I expect is the default) 200% so that the image is the correct size on screen to match the layout you would expect on a normal resolution screen. If it didn't do this all of the text on the page would be incredibly small.

One of the problems you are seeing is that monitors are fixed resolution output devices, although they vary from display model to display model once you have your monitor the output resolution is fixed. Conversely when printing you can happily vary the output resolution of a printer, well at least until you hit the maximum.

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Dec 24, 2017 19:12 |  #15

BigAl007 wrote in post #18525665 (external link)
When you view the image in your web browser it will display the image at (by what I expect is the default) 200% so that the image is the correct size on screen to match the layout you would expect on a normal resolution screen.

Before any reduction I perform, an image 4000 x 3000 px, copied from ImageBrowser to the desktop, is displayed at about 9 3/8" x 7". Vertical image, 3000 x 4000 px, about 5" x 7". In other words, the screen height limits the displayed size. I can enlarge images to 100% with a keyboard command and see only parts of them at once, but there's rarely a reason to do so.


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Does reducing a jpg soften it?
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