10 Replies Latest reply on Nov 25, 2012 7:15 AM by emil emil

# Clarification of Resolution Theory

Hi,

Hope you're well.

If I scan a photo in at 600 dpi and then reduce it to 300 in Photoshop will that double it in size?

Could I go up even further? What would be the result?

Thanks!

• ###### 1. Re: Clarification of Resolution Theory

If I scan a photo in at 600 dpi and then reduce it to 300 in Photoshop will that double it in size?

Yes, if the Resample Image in the Image size is not checked. The pixel dimensions remain the same but its output size doubles.

And it will be the same if you don't do anything to the image but just scale it in the print dialog box to 200%

Could I go up even further? What would be the result?

Without resampling you are not changing the pixel dimensions and thus the detail of the image but only its output size. You can see the result in terms of size if you use Image > Image size with its Resample Image options unchecked and  try different Resolution values.

The result (appearance) on the printed output will depend on the resolution, less ppi less detail per inch and the appearance is also dependent on the viewing distance.

• ###### 2. Re: Clarification of Resolution Theory

Thanks Emil.

When you say I could also just scale it in the print dialog box to 200%, do you mean while it's still 600 dpi?

So if there is a photo you want to blow-up for print where the original is smaller than the ultimate printed size you want, if you scan it in at a resolution that is equivalent to how much larger you want to make it (ie if you want to increase a photo in size by 2x scan it in at 600 dpi, 3x at 1200 dpi, etc) will that enable you to print out a good quality image of that photo?

Thanks again!

• ###### 3. Re: Clarification of Resolution Theory

Use one of the 3 formulas below to find the unknown value, using two known values:

1) width in pixels / PPI = width in inches

2) width in inches X PPI = width in pixels

3) width in pixels / width in inches = PPI

Just replace width with height for the other half of the equation.

Or:

Image>Image size

Then play with the settings and see what happens when resample image is checked and unchecked. No need to click on OK Button when experimenting with the settings.

• ###### 4. Re: Clarification of Resolution Theory

Image size works two way.  The way Image Size works depends on the way you set the RESAMPLE check box.

If the RESAMPLE check box is NOT checked the image pixels count will not changed.  Only the print size will change.  The resize image will have the same aspect ratio and not be distorted.  When resample is NOT checked all you need do is change one of the three print setting in the middle print section of the image size dialog. Photoshop will calculate and change the other two. All that is happing is the Print DPI setting is being changed the pixel print sized is changed in size.  Dimention Pixel size divided by print DPI resolution = dimension size in measuring units. It that simple. 1,2, 3 in the above append.

When the RESAMPLE check box IS checked the other setting in the Image Size dialog bottom control section become meaningful. The image pixels will be replaced you wind up with a whole new image not a single original image pixel remains you will have more or fewer pixels then you started with.  If constrain is not checked the image can be distorted into a different aspect ratio. Checking constrain links the width and height to the current aspect ratio when checked if you change either the height or width in the Pixel Size section or the Print Size section Photoshop will calculate the other three and set them.  There is also a option check box for resizing layer styles.  Layer styles have absolute pixel size setting therefor they will look different if the images DPI resolution is changed.  If you change you images DPI resolution most likely you will want layer styles resized too.  You can change both print size and print DPI resolution using Image Size. When RESAMPLE IS checked the image pixel count will change how they are change is controlled by the interpolation method used during the resize operation. Different interpolation methods may work better the other on a particular image.  Adobe states the "Bicubic Sharper" works bests when downsizing reducing the number of pixels in an image.  It does not state it always works best.  If an image has been highly sharpened before downsizing plain "Bicubic" will work better.  Perhaps that is why the plug-in Fit Image always uses Bicubic...

• ###### 5. Re: Clarification of Resolution Theory

Try to train yourself to think foremost about how many pixels are in the image (horizontal x vertical).  Then mostly ignore ppi and physical measurements until it becomes time to commit the image to a physical medium.

-Noel

• ###### 6. Re: Clarification of Resolution Theory

Noel,

Those are good tips, and should also be posted to a FAQ in the Premiere (Video-editing program) Forum. There, DPI and PPI have no meaning (for Video, and not printing, or display, per-se, on a computer screen), and we have the hardest time getting folks' minds around "pixel x pixel," as they always think of "resolution" as being Inches x Inches x either PPI, or DPI. In Video, it is only pixel x pixel that will count.

Well-stated,

Hunt

• ###### 7. Re: Clarification of Resolution Theory

I wish people would stop saying dpi. That is nonly meaningful as a printer output these days and doesn't have a resolution

connotation even there.

• ###### 8. Re: Clarification of Resolution Theory

Resolution in very meaningful without its use programs like Photoshop would not be able to do what they do.   Without referencing resolution how would a program like Photoshop be able to add 1" of canvas.  It would only be able to add an absolute number of pixels to a canvas.  Pixels have no physical size without resolution. It this relative term names resolution that defines pixel size  relative values normally stated in terms of DPI or PPI.  It does not matter if you label it DPI, PPI, XYZ or any other.

Without resolution Photoshop could not work in a relative way.  In fact some thing in Photoshop do not support resolution like Layer Style settings are Absolute pixel setting.  They are created for a particular resolution. If you change and images resolution Layer styles will not look the same.   To resolve this problem Photoshop had to add a menu item in the Layer menu.  You can target a layer with a layer style the use menu Layer>Layer Style>Scale Effects... Without Scale Effects if you change a layered document that has layer styles added resolution you would need to edit each all layer style effect in all layers with layer style by hand.  Resolution is very important I wish Photoshop supported it in all its features.

• ###### 9. Re: Clarification of Resolution Theory

The point is that Photoshop doesn't add 1" of canvas.  Photoshop only adds a certain number of pixels, derived by math.

It allows YOU to specify 1" given the pixels per inch value you've set, on the principle that if you set things up right it can help you work in units that relate to your work products to help make your job easier.   But that doesn't always make sense (imagining, for example, a project with a number of different work products).

As with anything, to use something most effecitvely the user has to come to grips with how it actually works.

• A person with no knowledge of how a car works can certainly drive to the store.

• But you'd better believe a race car driver, working to extract the maximum possible performance and reliability, knows every detail and nuance of how his car is put together and functions.

Photoshop first and foremost manipulates pixels, not inches or canvases.  Think about pixels first and envision how all the abstractions relate to those pixels, and you won't go far wrong.

Beyond that, there is the "PPI vs. DPI" controversy over terminology...  Certainly pixels aren't dots.  Use of proper terminology is important, especially when trying to work out technical things that are not fully understood.  Perhaps it is less important to correct its misuse everywhere it's seen however.

Literally, Photoshop dialogs are labeled as "pixels / inch" or "pixels / cm" (depending on unit preferences).

-Noel

• ###### 10. Re: Clarification of Resolution Theory

When you say I could also just scale it in the print dialog box to 200%, do you mean while it's still 600 dpi?

Yes

So if there is a photo you want to blow-up for print where the original is smaller than the ultimate printed size you want, if you scan it in at a resolution that is equivalent to how much larger you want to make it (ie if you want to increase a photo in size by 2x scan it in at 600 dpi, 3x at 1200 dpi, etc) will that enable you to print out a good quality image of that photo?

when scanning images that will be enlarged, scan them with the maximum resolution (samples) your scanner can use. But have in mind that unlike taking a photo of real object with higher resolution, sampling a 2d image with a scanner or camera will not add more details than what's already in the photo. So the quality of the enlarged image, no matter what you do cannot be better than the original and scaling it up will be interpolation of the existing detail. So, the question is how worse in comparison to the original it can get. This depends on the physical size, the viewing distance, and also on what's in the image.  An mage of clouds which are fuzzy anyway will not suffer significant difference when enlarged but if the image for example has sharp fine text then the blurring will be obvious when viewed from the same distance. The good news is larger the image, larger the viewing distance needed to see the whole image comfortably. So, Depending on these factors, and with some image adjustments like sharpening, doubling the size may give you decent result.