Size/resolution

[Bramela]

I wish to ask a question of rather straightforward nature :

If a photo size is reduced say from 3037 X 2033 to 800 X 536 pixels , does this permanently alter the resolution of the photo.
If the picture is taken at say 10 megapixels, does the size reduction adversely effect the resolution if the photo is enlarged in printing.
I always thought that reducing the file size does not adversely effect the printing quality as the megapixels remain the same???
Excuse my ignorance in this, I am rather naive in computer technicalities and I am having a bit of difficulty with a person I am doing a job for.

Many thanks for any help, guys!

[ozlizard]

Hi Bruce, it is my understanding that if you reduce the size of an image then blow up that same image you will lose quality. The best idea is to keep a copy of the original or shoot in RAW to retain a full size image which can then be cropped or blown up. Reducing the size of the file doesn't necessarily reduce the print quality as long as the print size is not excessive in relation to the image size. Basically the larger the file the bigger the possible print. I hope this helps and I am sure someone on here will have a more "in depth" explanation!

[rayallen]

Hi Bruce,
The only reason you would reduce a 10MP image 800 X 536 would be to post it on the web or to email it. An image of that size would not have the resolution to produce a reasonable print.

When you are printing you want to retain every pixel that you start out with.
Depending on your image editor, the image from your camera will have a resolution of 72 DPI and a size of say 53 X 36 inches. If you change the width to 12 inches, the height changes to around 8 inches and the resolution changes to 322 DPI - perfect for printing.

I use Photoshop CS3 or CS4 to do this sort of thing. What do you use?

[Wheatfield]

Originally Posted by Bramela

I wish to ask a question of rather straightforward nature :

If a photo size is reduced say from 3037 X 2033 to 800 X 536 pixels , does this permanently alter the resolution of the photo.

Yes.

If the picture is taken at say 10 megapixels, does the size reduction adversely effect the resolution if the photo is enlarged in printing.
I always thought that reducing the file size does not adversely effect the printing quality as the megapixels remain the same???
Excuse my ignorance in this, I am rather naive in computer technicalities and I am having a bit of difficulty with a person I am doing a job for.

Many thanks for any help, guys!

Again, yes. When you reduce the size as per your example you are permanently reducing the resolution of the file. The only way to get it back is to go back to the original and start again.

[Marc Sabatella]

Originally Posted by Bramela

If a photo size is reduced say from 3037 X 2033 to 800 X 536 pixels , does this permanently alter the resolution of the photo.

"Resolution" has two different meanings in with respect to digital image.

One is, total number of pixels, as in whether a camera is 6MP or 10MP or 14MP. In this case, resolution is simply width * height. 3037 * 2033 = 6,xxx,xxx = 6MP. So you are starting with a "resolution" of 6MP. You are ending with a "resolution" of 800 * 536, or about 0.5MP. Yes, you've greatly reduced the resolution - there are simply fewer pixels.

The other definition of resolution is the number of pixels per inch (ppi). This has meaning only if you have a specific print size in mind. And it literally is number of pixels divided by size in inches. The 3000x2000 image (rounding for convenience) image is 100ppi if you print at 30"x20", for example, because 3000 / 30 = 100 and 2000 / 20 = 100. Similarly, it's 200ppi if you print at 15"x10". The 800x536 image would be 100ppi at 8"x5.36", or 200ppi at 4"x2.68". So yes, by reducing the number of pixels, you've reduced the resolution in this sense too, in that for any given print size, you will have fewer ppi.

If the picture is taken at say 10 megapixels, does the size reduction adversely effect the resolution if the photo is enlarged in printing.

I always thought that reducing the file size does not adversely effect the printing quality as the megapixels remain the same???

No. You might have started with 6 million pixels, but if you reduce to 800x536, you're left with only around half a million. Thus, at any given print size, you've got *way* less resolution in ppi.

[Bramela]

Thanks all, your information has been a great help. I now have a much clearer picture () of how it works.

I appreciate all the input.

[stevebrot]

To summarize the good advice above:

• Always keep a high resolution version (whether RAW or JPEG) of your shots
• Never use this high res version for edits (write-protect the file)
• Always work from a copy when doing edits
• Resizing downward is done by selective removal of data
• Resizing upward is done by selective duplication of data
• The simple rule of resizing is that you never gain resolution/detail and may lose same

Some software makes respecting the first three points easy by supporting non-destructive edits. I am thinking Adobe Lightroom here.

Steve

[Bramela]

Steve

I appreciate that information. You certainly have spellled out in a clear manner, and I really appreciate that.