Unhappy Explaining Low Res and High Res Photos and Prints

rockstarsmasher · Apr 13, 2013

I'm very confused about what the whole converting a high res image to a low res such has from 300pp to 72ppi or reversed from 240ppi to 300ppi?

I shot an image with my 5d Mark2 in RAW and opened it with Adobe Camera Raw.

Can someone explain the process of making it into a high resolution image?

Does it matter when you go to 'image size' in Photoshop and change the resolution to 300pp?

Or does only the amount of pixels (Length and Width) that the image has matter that determines if its high resolution or not?

How big can I print an image at the suggested 300 megapixels?

Can someone explain step by step?

Thanks!

Ysarex · Apr 14, 2013

A Canon 5dmkii produces an image with a resolution of 5616 x 3744 pixels.

At 300 ppi that image would print to a size of 18.72 x 12.48 inches.

300 ppi is considered photo quality.

In Photoshop the Image Size dialog has a check box labeled Resample Image. If that box is NOT checked then no matter what you set for the ppi or Document size your photo will not be physically altered. You'll find that changes to the Document size will force a ppi change and vise/versa -- the actual res of the image (5616 x 3744) is not being changed.

If the Resample Image box is checked then make absolutely sure you know what you're doing. If you are at all in doubt about what you're doing then don't check that box. Resampling your image will change those original pixel values of 5616 x 3744. You can resample down or resample up. If you resample down your photo will become a lower-res image -- Photoshop will discard that portion of your photo necessary to create the lower-res image. Note the options in the drop box below resample -- how you do it matters. When you resample up you're asking Photoshop to invent pixels to make up for the fact that you didn't have enough to begin with. Photoshop will look at what you've got and try to guess what should be there if in fact you had more. Photoshop guesses pretty good but it's still a p*ss poor substitute for not having enough pixels in the first place.

Joe

KmH · Apr 14, 2013

First a couple of distinctions.

1. For electronic display resolution is the pixel dimensions of the image, and ppi is meaningless.
2. PPI only applies to print size. PPI and the image pixel dimensions determine print size.

Your 5D MK II delivers high resolution images SOOC (straight out of the camera).

I have never understood why Canon sets the default image file ppi to 72. Doing so seems kind of stupid.

In the Photoshop Image Size dialog box, changing the ppi from 72 to a higher value has zero effect on the pixel dimensions of the image if the Resample Image box is not checked, but the stated image size in inches will change showing a smaller print size. Without resampling an image more pixel per inch (ppi) has to make an image a smaller print size.
If the Resample Image option is cheched, you malso have to chose one of the 5 or so resampling algorthms Photoshop has available.

The Image Size dialog box has some options you need to understand.
With Photoshop open click on Help > Photoshop Help, or press your keyboard F1 key. Scroll down and click on Image Size and Resolution and it will take you here - Adobe Photoshop * Image size and resolution

Ilovemycam · Apr 14, 2013

I don't understand it either. Here is what I do.

Full res, as shot in cam.
Med res is 1.5 to 2 MB
Low res which I send out is 200kb. To make a decent (just) 5 x 7 you need a min of about 800kb to 1.5 mb.

KmH · Apr 14, 2013

In the Image Size dialog, pixel dimensions only change if Resample Image is invoked.
Pixel dimensions are not the file size. File size includes the image metadata.
One way image file size is reduced for electronic display without lowering image quality is to strip all the metadata from the image file.

The Save or Save As dialog boxes have an Image Options - Quality adjustment that allows setting a smaller image file size. Image size is not affected.

Note that the Image Options - Quality adjustment (compression) is available for JPEGs but not other file types, like TIFF and PNG though TIFF has a couple of compression options choices - LZW and Zip

Compression

When discussing the quality of digital files based on file size, comparisons should only be made based on uncompressed sizes. Compression algorithms will modify each image differently depending on the subject matter of the image. Therefore it is impossible to accurately compare the file size of two digital images once they have been compressed.

There are two types of file compression, "lossy" and "lossless". Lossy compression actually changes some of the original pixels and some details are lost. The most common type of lossy compression format is JPG. While the original JPG image out of a digital camera is fine, every time the file is saved again, detail is lost. If the same file is saved as a JPG a few times, significant quality is lost and cannot be recovered. Valuable originals should always be saved in a lossless format, like TIFF or RAW. TIFF files can be edited and saved any number of times without loss of detail because the compression does not alter any pixels. The trade off is that TIFF files do not compress as well as JPG.

http://www.photography.com/articles/digital-photography/digital-compression-of-image-files/
http://en.wikipedia.org/wiki/Image_compression
http://en.wikipedia.org/wiki/LZW_compression
http://en.wikipedia.org/wiki/Zip_(file_format)
http://en.wikipedia.org/wiki/JPEG
http://en.wikipedia.org/wiki/TIFF
http://en.wikipedia.org/wiki/Portable_Network_Graphics

rockstarsmasher · Apr 14, 2013

Thanks YSarex and everyone, it's making more sense now.

Ok so say I open an image in Photoshop straight from my 5D Mark2 at 5616 x 3744 pixels.

And I want to make it a LOW RES photo for my website so that no one can steal it.

I would go to 'IMAGE SIZE' and check 'RESAMPLE IMAGE' and 'CONSTRAIN PROPORTIONS' and enter a new amount for 'PIXEL DIMENSIONS'? I would have to RESAMPLE DOWN to get rid of all those pixels to make my photo LOW RES right?

What is the usually PIXEL size that you input for a LOW RES photo? 600 Pixels by 400 Pixels?

Ysarex · Apr 15, 2013

rockstarsmasher said:
Thanks YSarex and everyone, it's making more sense now.

Ok so say I open an image in Photoshop straight from my 5D Mark2 at 5616 x 3744 pixels.

And I want to make it a LOW RES photo for my website so that no one can steal it.

I would go to 'IMAGE SIZE' and check 'RESAMPLE IMAGE' and 'CONSTRAIN PROPORTIONS' and enter a new amount for 'PIXEL DIMENSIONS'? I would have to RESAMPLE DOWN to get rid of all those pixels to make my photo LOW RES right?

What is the usually PIXEL size that you input for a LOW RES photo? 600 Pixels by 400 Pixels?

Right. You resample down to lower the resolution and resample up to have Photoshop guess at what data your next camera should provide so you don't ever try and resample up again.

Screen resolutions are considered lower res. For posting on the internet just worry about the long side maximum. 800 is a nice size so that your photo looks good but is too low res for a large print. At 1024 you can make a nice 4x6 even 5x7 print but anything larger starts to fall apart.

Joe