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Photoshop: resizing images questions...

Old 06-10-02, 10:28 AM
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Photoshop: resizing images questions...

I think I am going about this the wrong way.

When I scan in an image, it'll generally pop up as like 1800x1300 pixels (or some variation). So I go to IMAGE and "shrink it" down to something I think is more manageable, like 1280x960. This shrinks the image on the screen. (Does it REALLY shrink the image though??) Then when I re-save it, I am prompted to change the image quality. I always try to leave it at "high".

Am I resizing the image wrong? Is there a better way? If I change image quality to "low" versus "very high", what changes -- picture size or picture quality? Is there a way to resize the image by clicking on the edges of the picture itself and dragging the mouse?

Thanks for your photoshop help, as always.
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Old 06-10-02, 10:50 AM
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Well if you are saving as a JPEG it gives you the option for quality. Setting it to "low" does not decrease the image size...it decreases the file size. However it also decreases the quality of the image. JPEG is an image compression so quality will suffer.

I you want to change the actual image size in dimensions then you need to go to "image size" and change the actual image document size.
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Old 06-10-02, 11:59 AM
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Thanks -- I thought I was doing it a correct way, I just didn't know if it was the most efficient way.

I posed this question in another thread, but perhaps you can help (since you always do!):

If I scan at 300 dpi, and then shrink the image, is this better than scanning at 150 dpi and leaving the image alone? I get too confused when it comes to resolution (i.e. what the hell is it). Thanks!
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Old 06-10-02, 12:20 PM
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Well an easy way to think about resolution is the greater the resolution the more detail there is, which allows you more room for manipulation. The best example of this is if you wanted to scan a 35mm negative to an 8x10 scan. The negative is very small, as you know, so you would want to scan at an extremely high resolution which will produce a very detailed scan enabling you to blow it up to 8x10 without losing too much quality.

Technically speaking DPI stands for dots per inch. 300dpi means 300 dots per square inch. The dpi is proportional to the quality. Also you may see something like on a scanner that says 1200x1200 optical resolution 9600x9600 software resolution. What the software resolution does is a next pixel interpolation. It analyzes an area and looks at the white spaces between pixels and makes a determination on how to fill those white spaces best.

Now the difference between 300dpi and 150dpi depends on what you were scanning. If you are scanning a photo then you wont notice much of a difference...however you will lose detail if you blow it up. If you are scanning a picture from a magazine page then you want to scan at a high resolution becasue of the way the magazine is printed. A magazine page is printed using dots...just like a regular house-hold printer does. If you scan a magazine page at a low resolution then the quality will not be very good as you will see the actual dots on the page. The higher the resolution the less you will see the dots.

So if you are doing photos then there is no reason to scan at 300dpi if you are just going to change the dpi to 150. Also you don't need to change the dpi when you change the size of the image...there is no reason to do that. Saving it as a JPEG is going to reduce the file size for you.

Of course the files size is exponentially proportional to the resolution. My scanner does 2400x2400 optical and a 8.5x14 scan makes a file abot 450MB in size. If you don't have a lot of memory then it makes it very hard to work with.

Last edited by palebluedot; 06-10-02 at 12:23 PM.
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Old 06-10-02, 01:49 PM
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Originally posted by palebluedot
So if you are doing photos then there is no reason to scan at 300dpi if you are just going to change the dpi to 150.
I have to disagree. You will get sharper images by scanning at a higher resolution and downsampling--for example, scan at 300dpi, downsample to 150dpi.

When you scan an image at 150dpi your scanner scans 150dpi, regardless of what color or intensity is most important. It's probably right most of the time and does a pretty good job of giving you an acceptable result.

When you scan at 300dpi, your scanner scans at 300dpi, regardless of what color or intensity is most important; just like at 150. Now go into your image-editing program and downsample to 150. Downsampling is really a matter of selectively removing pixels to shrink the image.

Here's where you get a sharper image from downsampling: your image-editing program will (try) to intelligently remove the pixels that won't change how your image looks. The scan at 150 dpi saw a dot as black, the 300dpi scan saw black and white, downsampling made the better decision of keeping the white pixel.

Try it with a few very sharp images and you'll see the difference.
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Old 06-10-02, 01:50 PM
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I just want a scanned image about the size of my screen (or smaller) with good quality.

Right now when I scan, only 1/4 of the image fits on the screen...if I lower the dpi, will it the whole thing fit on there? Or is dpi -- like you said -- basically just quality?

Now, I don't "downsample" anything in Photoshop (do I?), all I do is change the image size to less pixels....is that the same as going from 300 dpi to 150 dpi? (considering I actually make the right adjustment?)

Last edited by Toad; 06-10-02 at 01:53 PM.
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Old 06-10-02, 02:09 PM
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Originally posted by Toad
I just want a scanned image about the size of my screen (or smaller) with good quality.

Right now when I scan, only 1/4 of the image fits on the screen...if I lower the dpi, will it the whole thing fit on there? Or is dpi -- like you said -- basically just quality?

Now, I don't "downsample" anything in Photoshop (do I?), all I do is change the image size to less pixels....is that the same as going from 300 dpi to 150 dpi? (considering I actually make the right adjustment?)
"Downsampling" is the hoity-toity way of saying "resizing the image to something smaller." So yes, you're downsampling.

I don't think you're doing anything wrong. You've got the too-big image scanned. Now just use the Image Size option to resize the image to fit your screen. Save and you're done.
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Old 06-10-02, 02:30 PM
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Originally posted by danw


"Downsampling" is the hoity-toity way of saying "resizing the image to something smaller." So yes, you're downsampling.

Actually "downsampling" is not resizing the image and really isn't a term to use when describing image work. To use downsampling as an analogy for image modifications it fits the reducing of resoultion, not image size, much like in audio where you reduce or increase the sample rate...but we are pulling hairs here I guess.

Originally posted by danw
I have to disagree. You will get sharper images by scanning at a higher resolution and downsampling--for example, scan at 300dpi, downsample to 150dpi.
It depends on what you are scanning and what you are doing with it afterwards. If you are doing photos you are going to be hard-pressed to see any difference between 300dpi and 150dpi. The only time you will see a difference is if you blow that image up to a large size.

Originally posted by Toad
I just want a scanned image about the size of my screen (or smaller) with good quality.
If the images are to be used for viewing on a computer screen only (no printing) then I would scan at a decent resoultion (at least 150dpi or higher) to get a good scan, resize the image to what you want (width and height) and then change the image to 72dpi as this is the most common computer monitor resolution.

Last edited by palebluedot; 06-10-02 at 02:33 PM.
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Old 06-11-02, 08:54 AM
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Thanks guys.


I keep it at 300 resolution after re-sizing -- what does that mean I'm doing to my image? I am still confused as to whether resolution is related to image size or image quality. Sorry I'm so dense!
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Old 06-11-02, 12:48 PM
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DPI is really more important for printing than displaying on your screen. Let's say you have a 1024x768 image at 300 DPI and one at 150 DPI. You still have 786,432 pixels in both images. Displayed on your monitor, you'd have a hard time telling the difference. However, when you printed them, one would print at 300 DPI, producing a smaller image, but one that gives the appearance of being more detailed. The 150 DPI would print larger, but you'd probably see the individual pixels that make up the image.

So, if you aren't going to print, the major factor is the pixel size, not the DPI. If you plan to print, both pixel size and DPI are important. I feel like I'm confusing you, so I'm going to try one more time to explain the difference.

When you scan at a high resolution (say 1200 DPI), you are "making" 1200 pixels for every inch of the image, or 1,440,000 pixels per square inch. So, when you open the image in a computer program, it is typically displayed as actual pixels, not in DPI. So, you see a positively HUGE image. This is because a one inch scan has more pixels than a 1024x768 image (it would actually be 1200x1200). So, if you want to display it on the computer, you'll have to resize the image to something more manageable. You might resize it down to 500x500, for instance. Now you can actually see the image on the screen and it doesn't feel like you're looking at it through a magnifying glass.

So, what's the problem? Why do we have DPI? Well, most people that scan things into Photoshop are professional graphic artists, and it is assumed that at some time those images will be output to a printer. At this point, you want as much DPI as you can get and the printer can print. The greater your DPI, the better quality image you'll print -- but if you're printing a 8x10 picture, your image will have to be huge to get it at 600 DPI (decent printer quality). To print 8x10 at 600 DPI, you'll need (8 inches x 600 and 10 inches x 600) a 4800 x 6000 pixel image. If you only wanted to print it at 300 DPI, you could use a 2400 x 3000 pixel image. If you wanted 100 DPI, you only need a 800 x 1000 image. However, the 600 dpi will look very close to photo quality, the 300 a bit less so, and you will see very blocky pixels at 100 DPI.

So, to answer your question, resolution is related to both image size and image quality. However, DPI is only related to the image size and quality when printed. The less resolution you have, the smaller the image size will be, the smaller the file size will be, and the less quality you'll have.

Now, your next question is, "what resolution should I scan at?" I am of the opinion you scan at the highest available resolution, save a .tiff copy of the image, and then manipulate that image to achieve your desired result. The reason for this is that you can always size an image down and decrease the resolution, you can never size an image up and increase the resolution. So, having the original in the maximum resolution you might ever use, you give yourself a whole lot of flexibility. What if two years later you decide you want to print that image? What if you increase your screen resolution and now you want 1280x1024 images instead of 1024x768? What if you want to crop the image, and just have a 1024x768 shot of the back taillight of the car you scanned in? Having a large image will give you this flexibility. Having a smaller scan will force you to re-scan the image (provided you can find it again) to do any of these manipulations.

Now, let's talk about .jpg. You'll notice I mentioned saving the image as .tiff. I recommend this format because there is no compression (or using the Photoshop tiff compressions, no lossy compression), and your image will forever be as pristine as the second you scanned it in. JPG uses a compression algorithm that is very similar to the one used for mp3s. There is a problem with both these compressions, however. They rely on the fact that the human eye and human ear are not perfect. They trade off some image quality, in a way that human eyes and ears are not likely to detect, to get rid of a lot of file size. However, to do this, some of the image information is lost -- forever. A first generation jpg or mp3 doesn't lose enough detail to generally be an issue -- provided it was saved at a low enough compression. This is what the slider bar is for on photoshop. You can pick how much compression you're comfortable with. At the high end (10, high quality), there's almost no compression, and you might be better off saving the image in a non-compression format like .bmp or .tiff. At the low end (1, poor quality), you'll see a huge decrease in quality. You'll see jpg "artifacts", and there should be heavy pixilization and loss of color. It's the difference between 256kbps mp3s and 56kpbs mp3s. The former is difficult to distinguish from a CD, the latter sounds worse than an AM broadcast.

This problem is compounded when you start manipulating and re-saving jpg images. Consider a photocopier. I can make a copy of an article, and it will look very close to the original. Now, if I manipulate that copy and then make a copy of that copy, some of the artifacts of the photocopy process are magnified. If I do this 10 or 12 times, I'll end up with a big, muddy mess that's barely legible. In essence, this is what happens when you save as jpg, manipulate, and then resave as jpg again. You should only save as .jpg when you are done with your manipulation, and need to use the image for use across a wide variety of computer applications.

The thing you should really do is play with photoshop. Try printing a 300 dpi image. Set it to 150 and reprint. Size the image up and down and see what that does. Try saving a jpg at quality level one, and then open it again and see how it looks. Try saving a jpg over and over again as jpg. Use the filters (which are amazingly fun, if somewhat overdone). Play. The more you play, the more you'll understand.
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Old 06-11-02, 09:37 PM
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Wow! Thanks a lot for that post!! I appreciate people like you putting your time into a reply like that!

I'm still trying to absorb it, but I will take your advice and play play play!!
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Old 06-12-02, 10:04 AM
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When I resize the images in "IMAGE SIZE", the scanned images' print sizes drop dramatically from 6x4" to like 3.3x2.2" when I scale back the pixels. Now, when I scale back digital pictures, the print size stays the same. What's up with that?

Also, regarding loss of data in JPEG...For whatever reason, either my scanner or Photoshop won't let me scan as a TIFF, so I scan as BMP, and then re-save as JPEG when I'm done doing stuff.

Here's my question: If I open up a JPEG, alter its size, shape, etc., and then save it as a different name, does the original JPEG still lose some quality? (like the photocopies mentioned above)
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Old 06-12-02, 12:07 PM
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Originally posted by Toad
When I resize the images in "IMAGE SIZE", the scanned images' print sizes drop dramatically from 6x4" to like 3.3x2.2" when I scale back the pixels. Now, when I scale back digital pictures, the print size stays the same. What's up with that?


Check the DPI. The scanner scans things in as a certain DPI. When you resize, it keeps that DPI, but since the pixels are less, the printed image size becomes smaller. Your digital pictures are probably brought in with a "print size" as the primary attribute rather than a DPI. When you cut down the pixel size of these images, the print size stays the same, but the DPI changes.

Also, regarding loss of data in JPEG...For whatever reason, either my scanner or Photoshop won't let me scan as a TIFF, so I scan as BMP, and then re-save as JPEG when I'm done doing stuff.

Here's my question: If I open up a JPEG, alter its size, shape, etc., and then save it as a different name, does the original JPEG still lose some quality? (like the photocopies mentioned above)
Photoshop always lets me scan directly into Photoshop, then choose the format I'd like to save in. I prefer TIFF just because it's the standard commercial format, and the available compression isn't bad and non-lossy. But any format that doesn't use lossy compression is fine, even .bmp.

However, I do have the problem you describe when I bring in digital camera photos. They are stored as .jpg on the memory card on the camera, so the only way to bring them across is in jpg format. Usually, I move them directly to the hard drive and flag them as read only, then manipulate from there. I typically don't make a tiff image of these files, because it doesn't really gain me anything, and takes up a ton of hard drive space. However, if I manipulate the jpg, and expect to do more manipulation later, I will save that image as a tiff or psd (if I have layers I'm playing with). The image won't get saved as .jpg until I'm done with it.

Opening a jpg, manipulating, and then saving under a different filename (or different location) does not hurt the original image. However, if you open it, and then re-save it to the same spot(even without manipulation), the image is degraded. You can copy the files themselves from one area to another without loss, but if you open them, and then resave them as jpg, there will be a degredation.

JPG is actually a really good format, but it's really poor if you're going to keep manipulating the image or doing multiple saves. It really should only be used as a "final draft".
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Old 06-12-02, 02:00 PM
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OK, so it is essentially okay for me to open up a JPG file, alter it in any way I see fit, then save it under a different name (but still a JPG) without losing any data on the original JPG....GOOD!! I don't really do a whole lot of altering, so generally the first JPG I get is going to be the last -- except when I scan something in and need to shrink it.

Okay einTier for some reason I just double-checked, and actually the print size for digital images DOES change...it's just that at first, they scan in at like 17x14"!! (which is pretty big). I actually don't touch the dpi at all.

The scanned images I resize, which drops the print size down to like 3x2" actually scan at much bigger than 3x2"...why is that? Shouldn't the print size in Photoshop be accurate? If I change the print size back to like 6x4", the pixels jump up and I get a HUGE picture on my screen!! Doesn't make sense to me...

Thanks -- and I apologize -- just trying to find out new things about this!!

Last edited by Toad; 06-12-02 at 02:03 PM.
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Old 06-12-02, 06:31 PM
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Originally posted by Toad

Okay einTier for some reason I just double-checked, and actually the print size for digital images DOES change...it's just that at first, they scan in at like 17x14"!! (which is pretty big). I actually don't touch the dpi at all.


In that case, they are acting just like the scanned photos. I was just trying to come up with a reason why you might be experiencing what you described. You aren't experiencing it, so forget my description.

The image goes down in print size because you have decreased the pixel size (like we talked about earlier) but left the DPI the same. So, before you had say, 9600 x 12000 pixel image at 1200 DPI, which would print at 8"x10". Let's say you decreased that to 4800 x 6000 pixels, but still left it at 1200 DPI. 4800 pixels divided by 1200 pixels per inch leaves you with 4 inches, or a 4x8 inch printed image. As we used to do it in chemistry:


4800 pixels | 1 inch
------------------------------------ = 4 inches
..................| 1200 pixels

Remember, DPI means Dots Per Inch, or in other words, pixels per inch or in a fraction 1inch/xxxx pixels (dots)

9600 pixels | 1 inch
------------------------------------ = 8 inches
...................| 1200 pixels

Once you downsize the image, you simply have less dots (pixels) to spread across those inches, so your printed size goes down, unless you increase the DPI. Keep in mind, if you decrease the DPI, your printed image will get progressively more pixeled (blocky).


The scanned images I resize, which drops the print size down to like 3x2" actually scan at much bigger than 3x2"...why is that? Shouldn't the print size in Photoshop be accurate? If I change the print size back to like 6x4", the pixels jump up and I get a HUGE picture on my screen!! Doesn't make sense to me...

Thanks -- and I apologize -- just trying to find out new things about this!!
Your initial statement doesn't make sense to me. I hope I'm addressing it correctly. When you initially scan the image, it's at some ungodly pixel size, like the 9600x12000 described above, and it says it will print at 8x10. When you decrease the pixel size to something more manageable for the computer, your print size goes down to something like 4x5. You don't understand why this is so. I think I explained it above.

If you want to print the images, I recommend having the largest pixel-size image you can find. This might mean that you have more than one version of the exact same image. I typically have at least three versions of the initial scanned image. I have the original, at it's highest resolution, strictly for editing and printing purposes. Then, I have a smaller image for use on the computer, and another, even smaller image for thumbnail purposes. If I edit, I'll typically make three more copies, exactly as described above.

The reason you're having problems is you're addressing the wrong thing. If you decrease the pixels and want to increase the print size, you need to decrease the DPI, not try to reset the print size.

Let me see if I can explain this to you. I've explained why the image goes down in print size as you decrease the pixels. Now, you're trying to reset the print size. However, when you do this with Photoshop, it can do it one of two ways. One is to decrease the DPI. This is what you want. The other is to increase the pixels so that it matches the DPI. This is not preferable to you, because you're right back where you started from -- but with a blockier image. You'll need to tell it to reset the DPI, not the pixels. I don't do a lot of this kind of thing, so I can't tell you where the setting is. Better yet, keep a big image for printing and a small one for computer work.

You should really try printing some of these, and see what an effect DPI has on your printing. Then, see what an effect pixel size has. Then, try to figure out how they relate (if it's still not clear to you). It took me a long time to understand that even though a 1024x768 72 DPI image looked great on my computer, it was a big blocky, chunky mess when I tried to print it to fill a standard sheet of paper.

Last edited by einTier; 06-12-02 at 06:42 PM.
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