Pixels, and resolution, and quality, oh my!

This item probably belongs in a Photoshop forum somewhere, but I like the participants here better. Besides, this pretty much falls in the chit-chat/general conversation category. I'll find out by seeing what responses I get, if any.

I would like to be more comfortable talking about pixels, resolution, photo-quality, etc. What follows is a series of statements that describe my understanding of the subject. I'll number them to make it easier for replies to point out exactly where I've lost it completely.

1) ———————————- BASICS ———————————————
A pixel (picture element) is one tiny part of a photo that is necessarily a single color.
2) This sounds like leading the cart before the horse, but describing the characteristics of a pixel is bad enough just identifying the color, and impossible if it were multi-colored.
3) The primary colors are red, blue, and green (RBG) for electronic representation (digital camera), as opposed to red, yellow, and blue for pigment representation (paint).
4) Three 8-bit characters (bytes) are used to define a pixel's color — one byte for each of R, B, and G. Each byte can have a value of 0 to 255; thus the total number of unique colors that can be described is 256x256x256 or 16.8 million.
5) I don't know what else it takes to define a pixel, but even if the color is all that's necessary, three bytes per pixel explains why photos take up so much disk space. I suspect that they use compression techniques where umpteen pixels of the same color in a row are represented by one color triplet and a count.
6) The value of a byte can be represented by two 4-bit hexadecimal (hex) digits (0-9 and A-F), each representing the values 0-15. The minimum value of a 3-byte color in hex is 000000 (no colors, white) and the maximum value is ffffff (all colors maxed, black). FWIW, when I was first introduced to bytes and hex digits in 1957, I ascribed the term "nibl" to the hex digit, long before some young whippersnapper came up with it decades later. After all, what else would you call half of a bite but a nibble?
7) ——————— NOW FOR THE STICKIER STUFF ———————
Once you set the resolution for a photo you're going to take, you have locked in the number of pixels for that photo, regardless of how much you enlarge or reduce (resize) that photo, assuming you don't crop it.
8) As you resize the photo bigger to a ridiculous extent, you will begin to see individual pixels as single color squares, and the resulting image would be worthlessly unappealing (low resolution); on the other hand, as you resize the photo smaller, the resulting image would get sharper and sharper as the pixels got pushed closer and closer together (high resolution), although your human eye might not be able to detect the difference.
9) I just learned something, but I don't know what it is! I opened a 2"x2" head shot of me in Photoshop Elements (PSE), and saved it at the minimum file size (low quality, 225 KB) and at the maximum file size (high quality, 1.8 MB). I printed all three (original, minimum, and maximum) under Preview, scaled to fit the full letter-size page, and I couldn't detect any difference in quality. Then I opened each one in PSE and started magnifying the picture to the aforementioned ridiculous extent: The original and maximum versions showed the width of a tooth with around 50 small pixels, and the minimum version showed the same width with 6 large pixels! Did I just blow statement 7 to the winds? I'll stop now after just one more thought.

I suspect that the identical quality of the prints has something to do with dots per inch (DPI) which I think is a printer setting and an entirely different matter, but I'm not sure of anything anymore.

Comments, anyone?

One thing is for sure.... and that is this post didn't belong in the "Switcher" forum. I moved it here where it belongs. Please observe forum descriptions before posting. Thanks.

You're so right.

I looked for what I thought would be a suitable forum. I can't believe I didn't see this one.

Thanks for the help.

Print Size is Tricky

What are the actual pixel dimensions of each image? Generally speaking, the maximum enlargement that can be made from a file in inches is found by dividing each side (in pixels) by 300. This is because most printers print at 300 dots per inch (DPI).

For example, if I have a 4,800 x 6,000 pixel file I can print up to 16 x 20 inches while maintaining optimum quality (4,800 / 300 = 16)(6,000 / 300 = 20).

If you enlarge past this point your printer will begin to use multiple print dots to represent one pixel and your image will take on the look of an 80's video game. ;D

LOL yea moving it here is better. I tend to lurk in the section of the forums more then others.

I can try to help, but your question is hard to follow as a question. More like a statement really then a question.. So I will just go over the basic run down of PPI and DPI -vs- image size or resolution.

Lets get old school and say your sony digital camera only takes 800 x 600 pixel images (yea I used one like this in the Navy for work, ick..). For easy math and control purposes.
DPI refers to your Printers printing density. This started about the time of Dot Matrix Printers and carried over into the bubble jet types. Now lets say you want to print at 400 DPI (dots per inch). The maximum size you can print this image at is 2 x 1.5 inches without loosing image quality. And at 200DPI that would be 4 x 3 inches. Again without loosing image quality. You could set this to 1DPI and be able to print it large enough to fit on the size of a billboard sign, but it would look horrid up close. However billboards are meant to be read from a distance anyway and you can see when this could be an option.

Now lets get over to PPI (Pixels Per Inch), PPI normally refers to your displays pixel density. DPI refers to your Printers printing density. Normally screens have had a pixel density of 72 to 96ppi. However times are changing and retina displays have already pushed way past this.

Now in the past many people wanted images rendered at 72ppi for web work. Also many folks wanted 400dpi for printing.

However for printing we no longer have to use or worry about this nonsense anymore.

Most modern cameras shoot 18-20MP or higher and the resolution far exceeds what you can print on a 8x10 photo sheet even at 400dpi. When using Lightroom or Photoshop you can tell the software what size you want the image to print or save as and it will scale the image to that size while trying to maintain as much detail as possible.

Lightroom still has the option to allow you to choose DPI or Image Size, however you can't use both at the same time. I have a video on this BTW..

Now for website work, we still use PPI as there is no real other way to size the graphics accordingly. Using inches or centimeters for this would only end up being a rubber ruler and giving obituary results on each display you went to.

WOW! I hope I don't have to pay a tuition fee for all this education!

I think it would be smart to define where and why I'm going with this in order to keep the conversation on the straight and narrow and not waste your time giving me information that I don't need.

My grandkids grew up thinking that their mother (Holly) looked like a big round glass thing that occasionally flashed a bright light at them. At the moment, her photos occupy 138 GB of hard disk space that is beginning to get scarce. Her current camera load includes one photo taking over 9MB of disk that, according to Photoshop Elements (PSE), measures 5184x3456 pixels and can comfortably make a 72"x48" print. They're not all that bad, but most of them will make a 32" x 48" print — not exactly something you need for every shot. She says there are "a few" pics that she "may" want to blow up for a large frame wall hanging. All the rest would be fine if they just printed at 4"x6".

PSE has a feature called Process Multiple Files that will copy a folder while doing a number of things, including resizing the image, converting the type, and controlling the quality, and it's reasonably fast (over 200 photos in less than seven minutes). I ran sample 4"x6" prints on my printer at the various quality levels. The lowest level represented over 90% reduction in disk space. Holly said that she thought she could see what she calls "pixellation" at the lowest quality. I couldn't see it.

Clearly, what I'm trying to do is reduce the size of all the folders that don't contain "may blow up" photos. Apparently, Holly wants to continue shooting at these large sizes, in case she happens to come up with one of these special cases. She also seems to be afraid that I will reduce the files below the minimum acceptable file size for printing at Walgreen's.

I want to know this stuff inside and out, in order to be able to fend off all her objections. That's my story. Now to reply to your responses.

Exodist said:

I can try to help, but your question is hard to follow as a question. More like a statement really then a question..

Click to expand...

a.) I'm sorry that I wasn't clear about this, but I actually intended to present the statements as pseudo-questions, implying that each statement is to be read followed by the invisible question "Is this statement 100% true"? Would you please take another look at these statement-questions and comment on those that don't hit 100% ?

b.) DPI. I would think that a dot is a droplet of ink, and necessarily a single color. At 200 DPI, wouldn't one dot occupy the same territory as several pixels packed a lot closer than 200 per inch on my iMac, thereby losing the resolution represented by those independent pixels, conceivably each with a different color?

c.) I'm looking at a photo in PSE. At its lower-left corner it shows "21.85% 48 inches x 32 inches (72 ppi)". At the top it shows the file name followed by "@21.8% (RGB/8*)". The 48 and 32 multiplied by 72 gives the actual pixel dimensions of 3456 and 2304. I hoped that the 21.8% related the physical size of the photo on the screen (7.25" x 5") to the 48 x 32, but that doesn't compute.

d.) In my original statements 8 and 9 I said "resizing the photo bigger to a ridiculous extent". I was talking about using command-plus to blow up the image. Was I really seeing the original pixels in the resulting squares, or were they something else?

Hey Bud,
Yea if you told me what you was wanting to do I could have gave you a more clear answer. But its all good.. No worries..


Now your correct, those 20MP files 5k x 3k resolution will make huge prints. I kind of get tickled you mentioned worrying about space, I keep on average over 20k files on my system plus video and each one of my photos is about 45megabytes in size. It adds up! But I shoot RAW.

You mentioned squishing them down to 4 x 6 at 72ppi is going to look bad. I honestly recommend if you want to go this route to go 8 x 12 at 200ppi. Which would be 1600 x 2400 pixels. Still large enough for a portrait. But much smaller at only 3.8MP

Another thing I have noticed is when you take a large file and make it much smaller, the compression screws the image detail up. Actually making them hard to tell what was going on and very pixellated.

Hope this helps..

Thanks again for your help. It's great having a forum dedicated to the very area that I need help with.

Now I'm embarrassed to ask you to go one step further for me. After all, you don't owe me anything and you're doing this out of the goodness of your heart. There's no question about the value of a broadbrush answer to a problem. A good overview is essential to any analysis. Unfortunately, I'm a nitpicking detail kind of guy, and I'm not happy until I get much closer to the bottom of things.

So if you would be so kind, please give me your answers/thoughts/opinions/reactions to each of the specific items in my previous posts, numbered 7-9 and lettered a.-d.

Thank you.