Image Size

[JEB]

Sorry to raise this old issue again but can somebody help me put this old image size/resolution nutmeg to rest?

I have read most of what has been said on the forum in recent years on this subject, which has helped me enormously – I think! But I still get confused from time to time!

Perhaps if I say what I believe to be the case in my own words somebody will confirm or otherwise that I am correct.

My camera provides an image size of 3504 x 2336 pixels.

If I resize in PS using the Image/Image Size Dialog box using the conventional 1024x768 pixel dimensions clearly I lose a portion of my original image because of the different proportions.

My understanding is that PS effectively disposes of unwanted pixels in this case with minimal deterioration of quality - correct?

Resolution is therefore not an issue at this point – correct?

If I set the Crop Tool to 1024x768 and crop from top to bottom over the original image I get the same result as with the Image Size Dialog method – correct?

Roughly speaking 1024x768 pixels is about one third of 3504x2336 pixels. Therefore, when using the Crop Tool to resize a portion of the original image, care has to be taken not to crop too tightly.

Can I assume that were I to crop with the 1024x768 setting an area less than one third (or whatever the correct fraction is) then PS will add pixels - correct?

I have run some tests to try to demonstrate this to myself and find that I have to go substantially below the 33% before serious deterioration is detectable. Perhaps a different image may show this up more dramatically or perhaps my eyes are not discerning enough!

I have also played about with the resolution setting when using the crop tool and this seems to have no effect on the quality of the image. This leads me to believe that resolution is irrelevant – correct?

Thanks in anticipation.

John

PS I will not be able to respond to any reply for a few days.

[Lin Evans]

Sorry to raise this old issue again but can somebody help me put this old image size/resolution nutmeg to rest?

I have read most of what has been said on the forum in recent years on this subject, which has helped me enormously – I think! But I still get confused from time to time!

Perhaps if I say what I believe to be the case in my own words somebody will confirm or otherwise that I am correct.

My camera provides an image size of 3504 x 2336 pixels.

If I resize in PS using the Image/Image Size Dialog box using the conventional 1024x768 pixel dimensions clearly I lose a portion of my original image because of the different proportions.

Yes, the camera produces an aspect ration designed to print a 4x6 or 8x12 inch image. When you crop to 1024x768 in addition to loosing considerable resolution you toss a couple inches of horizontal image out.

My understanding is that PS effectively disposes of unwanted pixels in this case with minimal deterioration of quality - correct?

Only in the sense that at a screen resolution of 1024 x 768 you would see little difference. It's another issue, however, if you choose to zoom in tight on the image. Whenever you loose pixels you loose detail, that's unavoidable. But because a 1024x768 resolution screen can only display 1024x768 pixels, the appearance of the full sized image whether being downsampled by PTE or Photoshop or whatever software will not reveal any substantial difference. On the other hand, if you zoom in, and your "actual" resolution is 1024x768 then you will not have the same quality or detail as if the program were using the full resolution file. When you zoom in on a high resolution file, to the extent that you eventually zoom to 100% size for that portion of the full image displayed on your 1024x768 image screen the full sized image will be displaying the "native" capture pixels complete with all details captured. The 1024x768 resolution image will be simply interpolating (adding pixels) to fill the screen and considerable display sharpness and detail will be lost.

Resolution is therefore not an issue at this point – correct?'

No, it is an issue if you zoom in. Only if you display the full image is there no issue

If I set the Crop Tool to 1024x768 and crop from top to bottom over the original image I get the same result as with the Image Size Dialog method – correct?

No. When you "crop" with the crop tool, you are saying to the program "please limit the height and width of this selection to 1024x768. What the crop tool then will do is simply go horizontal for 1024 pixels and vertical for 768 pixels and perform a 100% crop meaning it will select the original pixels but limit which portion of the screen according to how you draw the rectangle. If you draw the rectangle on fewer than the original pixels (very tight crop) then the program will "interpolate" or add pixels to maintain your desired 1024x768 pixel dimensions. If you an area larger than that portion of the original image containing 1024x768 pixels the program will discard the unused pixels.

In the Image Size Dialog method, you are telling the program to "resample" the original image and throw away all but 1024x768 pixels but also to toss the excess beyond 1024 pixels out from the horizontal width of the image.

Roughly speaking 1024x768 pixels is about one third of 3504x2336 pixels. Therefore, when using the Crop Tool to resize a portion of the original image, care has to be taken not to crop too tightly.

Yes. A simple way to tell is to look at the image at full resolution, use the rectangular selection (not crop tool) tool and turn on the XY coordinates. See when you have reached 1024 pixels on width and 768 on height. Then you can move that rectangle about on the screen to see the relationships of the images in terms of the overall width and height. If you crop tighter then you stand to loose some detail via interpolation.

Can I assume that were I to crop with the 1024x768 setting an area less than one third (or whatever the correct fraction is) then PS will add pixels - correct?

Yes.

I have run some tests to try to demonstrate this to myself and find that I have to go substantially below the 33% before serious deterioration is detectable. Perhaps a different image may show this up more dramatically or perhaps my eyes are not discerning enough!

That's because you can often crop tighter without seeing any significant difference, depending on how well the original was focused and which type technology (bayer or Foveon X3) you used for the original capture. It's like performing a zoom beyond 100% in photoshop. On some images you can't see a great deal of difference.

I have also played about with the resolution setting when using the crop tool and this seems to have no effect on the quality of the image. This leads me to believe that resolution is irrelevant – correct?

Don't confuse the EXIF tag in "DPI" for resolution. They have no relationship. The "dpi" tag simply tells the printer how "dense" you want the pixels to be placed on the print. It has absolutely nothing to do with screen display, only print quality.

For example. If you have a 72 dpi tag set your print pixels will be spaced (assuming the print driver doesn't automatically over-rule your decision - it usually does) at 72 pixels per inch on the print. So if you print the entire image and the image contains 1600 pixels in the horizontal direction and 1200 pixes in the vertical direction, the print size will be 22.22 inches by 16.66 inches (1600 pixels wide/72 pixels per inch by 1200 pixels wide/72 pixels per inch). On the other hand if you elected to have 300 dpi (or more correctly 300ppi), then your print would be 5.33 inches by 4 inches. In general, most inkjet printers when "told" to print at best print quality will automatically interpolate (add pixels) to get to the optimum print density. For Hewlett-Packard and Canon inkjets this is 600 pixels per inch and for Epson this is 720 pixels per inch. So regardless of what you "feed" the printer in terms of dpi tags, if you select best print quality they will automatically print at either 600 pixels per inch density or 720 pixels per inch print density.

To further complicate the issue, the printer itself may use up to six or more "dots" of ink to make a single pixel so when the printer manufacturer tells you that the printer has 5600 dpi resolution, etc., what they are saying is that the droplets of ink are very tiny and they overlay a number of them to create a single pixel of print. You print itself is not 5600 dpi but rather 720 dpi in terms of individual printed pixels. Some or all of these "pixels" may be composed of as many as six individual drops of ink depending on the technology used.

I hope I haven't confused this further for you.

Best regards,

Lin

John

I will not be able to respond to any reply for a few days.