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jay_9030
Participant
September 14, 2024
質問

Increasing photo size with minimal loss of detail?

  • September 14, 2024
  • 返信数 3.
  • 820 ビュー

I have a landscape photo that is originally 5893x7071 pixels, 300 dpi.  A chart on my preferred photo developer's website says that the optimal size is 16x20 inches, but I would like to develop it much larger to place above my desk or reading chair.  The same developer's website says that I could go as large as 30x45, but it would not look so good. 

 

Questions:

-- How are large can I really go before I lose detail?

-- Can I enlarge it without a significant loss of detail?

 

Here is what I have already done: I cropped it to 5071x7071 for a 5x7 (20x28 inches) photo.  Then I went to Image > Image Size and set the paremeters at 20x28 inches, 300 ppi, Resample: Preserve Details (enlargement). 

-- Will that do the trick?  And if that's the right technique, how large can I reasonably go with this image?

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返信数 3

Conrad_C
Community Expert
Conrad_CCommunity Expert
Community Expert
September 14, 2024

If the image is now 5071 x 7071 px, and you want to print it 20 x 28 inches, then we can easily work out both the ppi at the print, and also roughly at some typical viewing distances.

 

For the resolution on the print surface, 7071 pixels divided by 28 inches equals a little more than 252 ppi. In theory, it falls short of 300 ppi. But let’s take a closer look at that in practice.

 

In practice, it might actually be difficult to tell the difference between 252 ppi and 300 ppi. If you’re going to see the difference, it will have to be in details that are fine enough that the difference is easily visible. Those details will probably need to be very sharp: Accurately focused, low noise, zero motion blur. Remember, if there is even one pixel of motion blur or focus blur in a 300 ppi image, then the effective resolution of that image is actually 150 ppi or less because details have been blurred across two or more pixels. So there are a lot of handheld images where 252 ppi in the file is more resolution than there actually is in the image content itself.

 

300 ppi is just one easy number that’s safe for most things, so everyone asks for it. But it might not always be necessary. 

 

For one thing, it can depend on the printing method and paper type. Some less expensive papers might not actually hold 300 ppi of detail. The chance that it will show 300 ppi is higher if it’s a higher grade of paper, and if it’s glossy. (And if the image content actually has 300 ppi details.)

 

The other factor is viewing distance. Requiring 300 ppi on the print surface assumes you’re viewing it up close. Step twice as far away and the print only needs 150 ppi at the print to appear 300 ppi at the given distance, and the resolution resolvable by the eye drops another 1/2 for every 2x that the viewing distance increases. (At least, I hope I got that math right…does eye resolution follow the inverse square law?) Regardless of the exact math, the truth here is that the farther away the print will be viewed, the less it actually needs to be 300 ppi.

 

If you are going to place the print above your desk or reading chair, that sounds like nobody is going to be able to be able to see the print up close unless they step on or lean over the furniture that’s in the way. So maybe a minimum of 3 or 4 feet typical viewing distance. If you hang it above eye level, then it could be more like a typical viewing distance of 5 feet or more depending on how far up it is on the wall. At those distances, a print of 150 to 200 ppi could appear to provide as much detail as a 300 ppi print that you hold six inches from your face.

 

That's a long way of saying that if the pixels per inch at 20 x 28 inches works out to 252 ppi at the print surface, and it’s going to be viewed from 4 or 5 feet away, there’s plenty of resolution in that image for that size and viewing distance.

    J
    jay_9030作成者
    Participant
    September 14, 2024

    Thanks for the information.

     

    I'd like to run something else by you:

    Right now, if I recropped the original image to 5657x7071 px and printed it at 24x30 inches, it would be 236 ppi, which may be fine for looking at it from 36 inches away (the depth of the reading chair). But I can upscale using Image > Image Size and take the image to 7200x9000, at which point it would ostensibly 300 ppi.

     

    Do you think that upscaling is worth it, or does it do more harm than good to a JPG file?  (Unfortunately, it was not sent to me as a TIFF.)

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    September 14, 2024

    236 ppi is more than enough, more than a pair of very good eyes can actually resolve at the intended viewing distance. You will not be able to see individual pixels! I can guarantee you that using the image as-is will look better than an upsampled version. The only valid reason for upsampling is if you can see pixels.

     

    Again, it is much more important the image is optimally prepared and sharpened.

     

    Just a little background on where the 300 ppi number comes from. It's almost universally misunderstood.

     

    It's based on standard offset priinting processes for books and magazines to be viewed at normal reading distance. The standard process uses a halftone screen frequency of 150 lines per inch (lpi). If you feed a pixel image into this, 150 ppi is exactly one pixel per line. In other words, you can see the pixels. If you double that number, individual pixels can no longer be distinguished. Hence 300.

     

    So it's important to realize two things here:

    • it's not a sharpness parameter, it's a smoothness parameter. The sharpness is determined by the 150 lpi screen frequency.
    • it's a purely theoretical upper limit, beyond which no further improvement can possibly happen.

     

    This is not the same as optical resolution in the eye. That depends on viewing distance. A billboard can be 10 ppi and look razor sharp.

     

     

     

     

    D Fosse
    Community Expert
    D FosseCommunity Expert
    Community Expert
    September 14, 2024

    5893 x 7071 is big enough for anything - if the image is of excellent technical quality! If it isn't, upsampling will just make it worse and there's certainly no point. Upsampling never improves anything, it's just a matter of minimizing the damage. But why inflict any damage at all when you don't need to?

     

    If it's a jpeg, don't even think about it. Jpeg compression artifacts become really ugly really fast.

     

    It's much more important to prepare the file well, with optimal sharpening.

     

    Here's how ppi works (and why you don't need 300 ppi):

     

    I assume you have a retina screen, which is about 220 ppi. Take a close look and see if you can see any pixels, or if it isn't sharp enough.

    J
    jay_9030作成者
    Participant
    September 14, 2024

    Thanks for the reply.

     

    Some more detail: Someone else took the photo and sent it to me. It came to me in JPG format. I cropped it from 5:6 (20x24), 5893x7071 px, to 5:7 (20x28), 5071x7071 px. (I cut some of the sky.) 

     

    Having done that one crop, is it ready to go without any upsamling or further editing?

     

    Thanks.

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