Hello Melissa,
Actually, resampling is pretty simple yet does seem to confuse a lot of people. If you want to keep the image the same size in its linear dimensions as is indicated for a 72ppi (not dpi..a printer term) resolution, then yes, to increase the resolution to 300ppi you will need to enable resample. Your original image does not contain enough data to provide a 300ppi resolution of the same size, so you are telling Photoshop to create the data.
Let’s say your image is opened in PS and is shown to be 8×10 inches in size. If you open the Image Size dialog and see that the resolution there is shown as 72 ppi and the document size (for printing purposes mostly) is 8×10 inches, then what you’re seeing is the maximum size the available data in your image will provide without resampling. Those dimensions and resolution equate to a 576×720 pixel image, which is quite small for print, but quite suitable for a monitor display such as for a web page. For photo-quality printing, 300ppi is commonly quoted as a good image resolution to use, but if you set your image to 300ppi without resampling, then that is divided into the pixel dimensions and would provide a printed image of only 1.92×2.4 inches. To obtain an 8×10-inch print (ignoring that you really don’t have enough data there to obtain a quality image at that size), then you would enable resampling so that PS creates additional data to essentially "fill in the blanks" between your image data. That’s an exaggeration of what is done but it is easier to mentally grasp than the concept of dithering or how resampling is actually performed. With resampling enabled, you could then manually enter the desired document dimensions of 8×10 inches and a 300ppi resolution, to produce a file that is 2400×3000 pixels in size.
Hope that helps,
Daryl
Hi Daryl,
Thanks for your detailed explanation. I am more clear of the difference between resampling and not resampling.
Since unchecking resampling creates a smaller image (in inches) if the original image was 8 x 10 and I unchecked resample and the image size comes up to be 1.9 x 2.4 afer I specify 300 ppi, if that’s how small I want the image to be, will it then be okay to have resample unchecked.
The other reason why I am confused about resampling is that I have read different things about why you shouldn’t or should resample.
The following is a link that I found, that says NOT to resample in order to properly change the resolution of an image
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http://www.psprint.com/helpcenter/preparing_artwork_files/re solution.asp>
Hi Melissa,
I assume you are referring to the following paragraph from the page you referenced:
Never increase the total pixel dimension of raster image. For example, if you have an image that is 400 x 600 pixels at 100 ppi, and you increase it to 1200 x 1800 pixels by changing from 100 ppi to 300 ppi, the printed image will be fuzzy.
That, in my opinion, is poorly stated advice. While it is true a 400×600 pixel image is likely to look fuzzy or poor in quality if upsampled 300% and printed, it might look acceptable as a 4×6-inch print obtained by a 200% upsampling to 800×1200 pixels (200ppi resolution). The image resolution needed to obtain a high quality print is affected by many factors…color range, subject matter, print media, printer type, original image quality and sharpness, etc. I’m a hobbyist and pretty much always use an image resolution of 300ppi for my prints, but I’ve read others saying they have had good results with as low as 220-240ppi. Resampling a raster (pixel data rather than vector) image up in size is common practice and can provide excellent results. As one example, I have a 12×16 inch, 300ppi (3600×4800 pixel, 17.3Mpx ) image that I printed on an Epson 1270 photo printer that was upsampled from the original 6Mpx (2000×3008 pixel) photo shot in NEF (RAW) format with my Nikon D70. The original image, if set to a resolution of 300ppi without resampling, would only produce a 6.7×10-inch print, yet I obtained an excellent print with an upsampling on the order of around 180%. And that is an image represented by only about 1/3 of it being original data.
Looking at this another way, let’s say you viewed my original image at 100ppi…it would not appear photo quality except from some distance away, but it would provide a 20×30-inch image. Maintaining that size of 20×30 inches but upsampling to 300ppi, even that image might appear "fuzzy", because so much additional data had to be created through the resampling process. We now have a 6000×9000 pixel (54 Mpx) image where only 1/9 of that is original data. That is quite a difference from 1/3. But, if you view that photo from some distance away, it may still appear photo-realistic. I have heard folks speak of being able to produce a high-quality 20×30-inch image from a 6MPx original, but I have never tried. With a very high quality original, careful digital sharpening and resampling, a high quality or professional printer, etc., obtaining photo-quality results may be possible.
The bottom line though, is that if you have a particular print size in mind that you’d like to be able to produce with as much photo-realism as possible, then you need to aim at capturing the original image as near that size (in total pixels) as is practical for you. So, if a 8×10 inch photo was your goal, then a 300ppi image resolution would define an original image size of 2400×3000 pixels, which would mean a 7.2Mpx camera. In reality, you could get away with less than that, perhaps 5MPx, and with resampling still produce a very good print.
Regards,
Daryl
Hi Daryl,
I was refering to the link under the referenced website where it states "How do I safely change my resolution?" (it’s at the bottom of the website / link) and then click on adobe photoshop link. It basically says that in order to "safely" change resolution of an image, uncheck resample.
Melissa
Aha!
OK, I see what you’re referring to. So, that to that referenced bit, I’ll add this:
"Safely" changing the resolution by ensuring resample is not checked, simply ensures you are not modifying any data. Instead, you are simply tagging the image header with data defining the resolution that image file should be handled at. This does not mean it is unsafe to resample however.
The comment that states "pay careful attention NOT TO SCALE A RASTER OBJECT so that the print output is less than 300 ppi" is a good example of where someone is using PPI terminology poorly. The image file resolution is in ppi, but the printer output will be in dpi…often used interchangeably to mean the same thing, but actually they are different. But, it appears that what this print shop is saying really, is that they want the files they are printing to be provided as 300ppi image files at the print dimensions desired.
So, if you want them to print a 4×6-inch photo for you, they want an image file that is 1200×1800 pixels with a 300ppi resolution. If all you had was a 1000×1500 pixel file, printing it at 4×6 inches would be a 250ppi resolution which is lower than they are requesting be provided, so you would have to enable resample, set the resolution at 300ppi while keeping the document size at 4×6 inches, and apply that Image Size change. A small resolution change like that is likely to be of little consequence, whereas if you’d had to double or triple your image resolution to get to the desired print size, then you will probably see some loss of quality in the image.
I hope this isn’t just confusing you…is there a question I’ve not answered?
Daryl
No,
you are not confusing me, I appreciate your replies in effort to answer my question.
Sorry if I seem a little ignorant with this whole resampling concept!
Thanks
Melissa, Daryl’s info is spot on, but I know how difficult the concept can be, so let me try telling you much the same thing from a different angle.
First, here is a basic image formula: Linear Pixels = Linear Size (inches) x PPI. (You could also write that as "PPI = Pixels / Size" or as "Size = Pixels / PPI".) If you change any one term, a second term must change in order to keep the equation balanced. The good part is that you get to decide which of the three terms you want to remain constant.
A very important characteristic of an image is its actual number of pixels. Let’s suppose you have a 1-megapixel camera that delivers images of 1000 x 1000 pixels. Now let’s see what happens when you change the image size without, and then with, resampling. Some of my following comments oversimplify the way printers work, but I am concerned here only with the basic concept.
With resampling turned off, a change in print size merely alters a "size tag" that is stored in the image file. Printing the image sends that tag to the printer, telling it how far apart to space those 1000 (horizontal or vertical) pixels. If the size is to be 4", the printer will place 250 image pixels per linear inch. If you size it to 8", the printer must cover the increased width by spreading the 1000 pixels to only 125 pixels per inch. Note that, at either print size, the physical number of pixels in the image file remains at 1000 x 1000. The potential quality of the image does not change because the total pixel count does not change.
Now let’s turn resampling on and use the same starting point: 1000 pixels wide, a 4" print at 250ppi. From there, if you resize to 8" while maintaining 250ppi, Photoshop must somehow increase 1000 pixels to 2000 pixels (2000 pixels = 8" x 250ppi). To create them, it resamples the existing pixels and makes its best guess at inventing new pixels of the optimum color and luminosity to plug in amongst the existing ones.
Your need is to increase the PPI by a factor of about 4 while keeping the Size constant. Crank that through the formula and you will see that the linear pixel count must also increase by a factor of about 4. Of course, images have 2 dimensions, so your total pixel count will increase by a factor of more than 16, and the file size will grow by about the same amount!
A good general rule is to avoid resampling whenever possible because it forces Photoshop to invent new pixels when upsampling, and to throw away perfectly good pixels when downsampling. The result is always some degree of degradation. But, yes, you must resample in order to accomplish your stated goal. Just be sure to save the resampled image with a different file name so as not to overwrite your original.
As bad a choice as resampling is, it is still generally better to let Photoshop resample and invent new pixels than to ask your printer to produce a decent image with an insufficient pixel count. You pays your money, and you takes your choice…
The same basic mathematical principles that are behind resolution and its relationship to pixels and linear dimension is also behind velocity and its relationship to distance and time traveled, yet most people will grasp the latter more intuitively than the former.
More miles traveled in less time = higher velocity :: More pixels in less linear measure = higher resolution.
Velocity=miles/hour :: Resolution=pixels/inch
Which makes resampling = shifting gears? 😉
Hmmmm….although I guess it’s more of an inverse relationship however, as a higher resolution would yield a shorter distance traveled whereas a higher gear would yiled a longer distance.
Good analogy John.
Higher resolution = shorter distance just like higher speed = shorter time for the trip
