Author Topic: Scan at 600ppi, convert to 300ppi, how much quality loss?  (Read 5948 times)

Offline Charlie Bucket

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Scan at 600ppi, convert to 300ppi, how much quality loss?
« on: Saturday 24 November 12 00:27 GMT (UK) »
Hello

Often when scanning a photo, I am not sure what printed size I will eventually want the photo to be. So I will scan at 600ppi just in case I want to double the printed size at some stage. But if I convert it to 300ppi in Photoshop, how much quality is lost? A lot, a little, not noticeable?

To be more specific: I am writing a family history book. The printing firm wants 300ppi illustrations. So whatever physical size it is on the book page, it needs to be 300ppi. However I am not always sure on this physical size until I experiment with it.

At home, once I know the size I can rescan appropriately. But if I am away from home I can't so may have to "downsample" to 300ppi hence my question on quality loss if I do.
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Offline jim1

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Re: Scan at 600ppi, convert to 300ppi, how much quality loss?
« Reply #1 on: Saturday 24 November 12 10:26 GMT (UK) »
When you save the image in Photoshop you will see the dialogue box "jpeg options".Set to 12 (max.).

jim
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Offline Ray T

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Re: Scan at 600ppi, convert to 300ppi, how much quality loss?
« Reply #2 on: Friday 28 December 12 19:43 GMT (UK) »
The file format (e.g. JPEG) has no impact on the image resolution. It's simply an image compression algorithm set up by the Joint Photographic Experts Group. Whilst good at reducing the size of files, JPEG compression throws away information - the amount of which varies with the degree of compression.

You will be able to see artefacts of the compression process in an over-compressed image and one thing you definitely shouldn't do is save a JPEG file as a JPEG as this progressively decreases the quality.

To get back to the original question, you should see no discernable difference reducing a 600dpi image to 300dpi image (assuming the latter is printed to only half the size of the former)

Offline Charlie Bucket

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Re: Scan at 600ppi, convert to 300ppi, how much quality loss?
« Reply #3 on: Sunday 30 December 12 02:10 GMT (UK) »
Thank you both for your replies.

Charlie
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Offline Treetotal

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Re: Scan at 600ppi, convert to 300ppi, how much quality loss?
« Reply #4 on: Monday 31 December 12 10:56 GMT (UK) »
Hi Charlie...why not save the original scan of 600dpi and reduce it in photoshop to 300dpi and and save to highest quality...rename the file adding "Reduced" that way you have one copy as per the book requirements and you still have your original scan for future printing.
Carol
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Offline Ray T

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Re: Scan at 600ppi, convert to 300ppi, how much quality loss?
« Reply #5 on: Tuesday 01 January 13 11:26 GMT (UK) »
The standard way would be to open the scan in Photoshop - which creates a .PSD file. You can do what you wish with the .PSD file and save it as many times as you want. You only need to convert this to a JPEG when you have to (e.g. if this is what the publisher wants or if you want to email a copy to somebody) The only drawback is that .PSD files take up significantly more space than JPEGs but, unless you have thousands of them and a small hard drive, this shouldn't be much of a problem.

This way you keep your original file at its best and you can tailor as many different versions as you want to the use they're going to be put (i.e. your publisher wants them the right size at 300dpi so they can be printed or your grandson wants a photo of auntie Nellie on his new iPhone 5)

Offline Rachels

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Re: Scan at 600ppi, convert to 300ppi, how much quality loss?
« Reply #6 on: Sunday 06 January 13 23:09 GMT (UK) »
Scan at 300 and increase the scale for the best results



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Offline Charlie Bucket

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Re: Scan at 600ppi, convert to 300ppi, how much quality loss?
« Reply #7 on: Monday 07 January 13 05:00 GMT (UK) »
Thanks, everyone. I think what I'll do is scan each image twice, at 300dpi and 600dpi, and keep them as .psd files in photoshop.

This will give me plenty of options in the future.

Ray, you say that there should be no discernible difference going from 600 to 300 "provided the latter is printed to only half the size of the former". What if it is printed to the same size?

Charlie




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Offline Ray T

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Re: Scan at 600ppi, convert to 300ppi, how much quality loss?
« Reply #8 on: Monday 07 January 13 10:34 GMT (UK) »
Thanks, everyone. I think what I'll do is scan each image twice, at 300dpi and 600dpi, and keep them as .psd files in photoshop.

This will give me plenty of options in the future.

Ray, you say that there should be no discernible difference going from 600 to 300 "provided the latter is printed to only half the size of the former". What if it is printed to the same size?

Charlie

Hi Charlie,
Image resolution is one of those things which everyone finds difficulty in getting their head round at some stage - I certainly did when I converted from darkroom to digital photography over 12 years ago.

As I've said, the "magic" figure to work at is 300dpi. In reality, there is nothing magical about this figure, it's simply that there is general agreement that printing an image which has a higher resolution than this will not give you a better print. As you increase image resolution, say from 150dpi towards 250dpi, your prints will appear progressively clearer. Print ones between 250dpi and 300dpi and you'll be hard pushed to tell the difference. Go higher than 300dpi and I'd defy anyone to tell the difference.

Whilst you're thinking about resolution you also need to think about image size. Double the size of a 300dpi image and, in theory, you'll end up with an image at 150dpi. I say "in theory" because you'll find that, in Photoshop, you will be able to keep the image resolution at 300dpi whilst doubling the size. In reality, Photoshop is moving all your pixels apart and cleverly making up new ones to fit in between. In 99% of cases this will simply result in a less clear image but you can get software which does this quire successfully - e.g. Genuine Fractals or whatever its called nowadays - but I digress.

What you need to aim for is your final image to be 300dpi at the dimensions you want to print it. There's no harm in having it bigger than this - you can easily reduce the size - but try not to store it as a jpeg until you have to - i.e. until you're sure that nothing else needs to be done to it. As I've said jpeg is known as a "lossy" file format and, in compressing your image to a smaller file size, the process throws away a fair bit of information.

To get back to your original question (!), let me answer it like this; if you have a 600dpi image the size of an A4 sheet, it will print happily at A3. If you were to reduce the image to 300dpi, it would print happily at A4 but would look poor printed at A3 as, in effect, it would then be a 150dpi image.

Essay over!
Ray