Crazy report of image size

OhLook wrote in post #18846189
My laptop shrinks images to fit the screen. I often edit a 3750 x 3000 original, but it isn't nearly that big when I work on it. In fact, it's about the size it'll be when posted at POTN.

Given that it is, whatever sharpening it gets ought to remain in a later viewing at that size, just as the white balance and other variables stay the same. I don't see why doing things in a different order changes the result. When you sharpen, you're just changing the colors of some pixels, right? Then, when you downsize, you lose pixels, but the surrounding ones are supposed to change again to preserve the detail in the larger version?

Your laptop only shrinks viewing size to fit the screen so you are able to see the image in its entirety. But, you are not seeing your image at 100%, you are seeing it at 25% (or whatever), so it has not in fact been downsized, or in any way altered! Open a full res file in Preview, look in the View menu (up top) and choose "Real Size" (or something similar - my Mac speaks Dutch). It will be too large to view it in its entirety; instead you will have sliders so you can scroll the image.

And so when you are viewing the full res image that your computer has obligingly shrunk so you can see the whole thing, and you sharpen that, it is not the same as sharpening a downsized version of the image. When you sharpen an image, you increase (edge) contrast, exactly the thing that is softened when you downsize it.

And by the way, you should ALWAYS sharpen your images whilst viewing them at 100%. It's the only way to determine the correct amount of sharpening.

Levina de Ruijter wrote in post #18846220
Open a full res file in Preview, look in the View menu (up top) and choose "Real Size" . . .

And by the way, you should ALWAYS sharpen your images whilst viewing them at 100%. It's the only way to determine the correct amount of sharpening.

I'm experimenting with applying this information, but it'd all be easier, and possibly make sense, if I understood just how a file changes. I've had no reason to think the computer would use different methods (algorithms?) for reducing an image to fit the screen and reducing it to dimensions I specify. Since it does the former without losing sharpness, it could just as well do the latter the same way.

OhLook wrote in post #18846254
I've had no reason to think the computer would use different methods (algorithms?) for reducing an image to fit the screen and reducing it to dimensions I specify. Since it does the former without losing sharpness, it could just as well do the latter the same way.

That’s just it. When an image is made to fit the screen it doesn’t get reduced. It’s just zoomed out. Whereas when you downsize the image actual pixels are cut.

Levina de Ruijter wrote in post #18846269
That’s just it. When an image is made to fit the screen it doesn’t get reduced. It’s just zoomed out. Whereas when you downsize the image actual pixels are cut.

AHA! So you're saying that the screen-fitting image is sharp because it gives the appearance of having smaller pixels?

OhLook wrote in post #18846279
AHA! So you're saying that the screen-fitting image is sharp because it gives the appearance of having smaller pixels?

Well, yes. Zoom in and pixels get bigger, zoom out they get smaller and as such look sharper. That’s why I always look at my images at 100% as it’s the only way to tell if an image is truly sharp.

There are several different algorithms for reduction of an image, Photoshop itself has something like 4 or 5 methods you can choose from when you use its resizing tool.

I ALWAYS sharpen after any resizing, for the forum or for a print. Not much but enough to redefine any edges damaged during the resize.

https://fstoppers.com …pening-your-images-228316

In general, best practice is pretty simple:

• Apply sharpening and similar adjustments at the size you are supplying as a finished product. If you are supplying various sizes that implies separate adjustments.
  
• For online images trust not the quality of auto size and change size (and check adjustments) yourself before you upload.. Some sites like this one do good work. Others do not. Supply the image at exactly the size or for sure within the size range it will be displayed.

OhLook wrote in post #18845057
I don't know what happened about those numbers. Anyway, here's a related question. I usually reduce images in landscape format to 1024 horiz. Any viewer would think I can't focus, as they always come out less sharp than before reduction. If reduced less or not at all, they take inconveniently long to upload and to copy to backups, and they use more storage space. But soupy images are disappointing. What's a good compromise, and is anything gained by reducing to a dimension greater than 1600?

Hi OhLook, one way to reduce file size and therefore speed upload and download times is to run your 1600 px (or any other size) image through a reducing program like JPEGmini. I always use it (and wish others would as I have a slow internet connection) and can see no degradation of image quality whatsoever, while reducing the file size by between half and three-quarters.

Levina de Ruijter wrote in post #18846220
And by the way, you should ALWAYS sharpen your images whilst viewing them at 100%. It's the only way to determine the correct amount of sharpening.

AZGeorge wrote in post #18852491
Apply sharpening and similar adjustments at the size you are supplying as a finished product.

Thank you, Levina and George, for enlightening me. I've learned the principle now–but there's a glitch, discovered when I posted a moth image recently, POST 18850582 for anyone who's curious. Although I sharpened it while viewing at 100% ("Actual Size," or Command-zero), it seemed less sharp when posted. Indeed. On my Desktop at 100%, the wingtips are 8.5 cm apart. On POTN, they're 10 cm apart.

How to get actual actual size?

OhLook wrote in post #18852521
Thank you, Levina and George, for enlightening me. I've learned the principle now–but there's a glitch, discovered when I posted a moth image recently, POST 18850582 for anyone who's curious. Although I sharpened it while viewing at 100% ("Actual Size," or Command-zero), it seemed less sharp when posted. Indeed. On my Desktop at 100%, the wingtips are 8.5 cm apart. On POTN, they're 10 cm apart.

How to get actual actual size?

That's a vertical photo and I think there is an issue with them on POTN, as a few others have pointed out. We are asked to size photos 1600px on the long side and when they are horizontal (1600px wide) they are good. But when it's a vertical photo it's 1600px high, so might be only 1200px or even 1000px wide, but it is presented at the same page width on screen as a horizontal photo, so it is enlarged and usually looks worse. Maybe if all photos were 1600px wide this wouldn't happen but then vertical photos would be a much larger file size, take longer to upload and download, and take more storage space on POTN's servers.

Pippan wrote in post #18852525
That's a vertical photo and I think there is an issue with them on POTN, as a few others have pointed out. We are asked to size photos 1600px on the long side and when they are horizontal (1600px wide) they are good. But when it's a vertical photo it's 1600px high, so might be only 1200px or even 1000px wide, but it is presented at the same page width on screen as a horizontal photo, so it is enlarged and usually looks worse.

The image is only 700 x 847. It wasn't enlarged to anywhere near the full column width. If it was enlarged at all, I don't know why.

I keep heights to not much over 800 so they'll fit screens about the size of mine.

OhLook wrote in post #18852521
Thank you, Levina and George, for enlightening me. I've learned the principle now–but there's a glitch, discovered when I posted a moth image recently, POST 18850582 for anyone who's curious. Although I sharpened it while viewing at 100% ("Actual Size," or Command-zero), it seemed less sharp when posted. Indeed. On my Desktop at 100%, the wingtips are 8.5 cm apart. On POTN, they're 10 cm apart.

How to get actual actual size?

When viewed on my MacBook Pro at 100% the wingtips are 7.5cm apart. (That is viewing at 100% on POTN)

john crossley wrote in post #18852562
When viewed on my MacBook Pro at 100% the wingtips are 7.5cm apart. (That is viewing at 100% on POTN)

So the size may vary depending on model? This is mine:

HOSTED PHOTO
please log in to view hosted photos in full size.

OhLook wrote in post #18852720
So the size may vary depending on model? This is mine:

Hosted photo: posted by OhLook in
./showthread.php?p=188​52720&i=i182044803
forum: Forum Talk

Yes, especially monitor size and resolution.

Details in your POST 18850582 image hold up very well on my monitor even though it puts the wingtips about 14 cm apart. Asking Chrome to zoom way in putting them about 40 cm apart still shows a nice level of detail.

AZGeorge wrote in post #18852797
Yes, especially monitor size and resolution.

Details in your POST 18850582 image hold up very well on my monitor even though it puts the wingtips about 14 mm apart. Asking Chrome to zoom way in putting them about 40 mm apart still shows a nice level of detail.

Blimey you must have bloody good eyesight. 14mm is just over ½ inch in real money.