"Understanding Resolution" Thu 08-Nov-12 07:27 PM by Robes
I'm having a difficult time understanding the resolution situation. I understand how in Photoshop(PS) you can take an iPhone5 72ppi (3264x2448) image and it does not need to be upsampled to print at 306ppi (2448x3264pix) giving a 8x10 print using all the pixels and if printing at a wanted ~300ppi. If I wanted to print this image at 24x32 it would print at 102ppi indicating I may wish to upsample, add more pixels.
If I save the iPhone pic at 306ppi & 8x10" file size (no upsampling) it uses all the pixels supplied. If I open this pic to view it, it looks identical to the original, being fuzzy with loss of detail.
If I take a pic from a real camera the info from PS gives:
300ppi 13.3x10"(W/Ht) 4000x3000pix 2.6MB
A photo from iPhone5 info from PS gives:
72ppi 45.3x34'(W/Ht) 3264x2448pix 3.49MB
** I can take this iPhone pic & upsample in PS to give the following info:
So at this point,except for file size (2.6MB vs 4.45MB) both pics (one from a home camera & one from iPhone5) are sized the same. If I open each on my iMac the home camera pic is far superior in detail, sharp & crisp to the iPhone pic, even though the iPhone pic has almost double the total MP. The iPhone pic always looks the same on the monitor compared to its untouched original. How can the iPhone image have so much more total MB comparable number of pixels& size yet look so much inferior? What am I missing here? Why does the home camera pics look so much better even when I supposedly make both (home camera & iPhone) pics of equal quality?
The home pocket camera takes pics at 300ppi resolution. The iPhone takes at 72ppi resolution. Import both pics to my computer (27" iMac 2.8 GHz Quad Core i7; 8 GB DDR3 SDRAM 2x4; 2 TB Serial ATA Drive; ATI Radeon HD 4850 Graphics with 512 MB) and they are not the same quality when viewed on monitor. My monitor is set to 2560x1440 resolution and the monitor views both pics at 108.79ppi. When the iPhone pic is processed to match the camera pic via PS (changed from 72 to 300ppi & equal size) it still looks significantly of lower quality. I don't understand how this can be when the iPhone pic has the pixels to work with. How can the two similar resolution/sized pics look so much different in quality?
The iPhone5 display is 1136x640 with 325.97ppi. So, the 3264x2448 72ppi pic it takes looks excellent on it's display. I cannot make this pic look good once downloaded to my iMac but why not? Why does the 300ppi camera pic look so good on my iMac display when compared to iPhone pic yet both are displayed at my monitors 108.79ppi?
I've read that the ppi of the camera makes no difference on a computer screen, if so, what makes one image so much better looking then the other?
That discussion seems to be on camera-vs-camera more than software-vs-screen, but maybe it will serve as a starting point. If not helpful, maybe you can search for a better discussion using www.nikoscope.com.
#2. "RE: Understanding Resolution" In response to Reply # 0 Thu 08-Nov-12 10:10 PM by barrywesthead
The ppi information in an image file is of no significance to the quality of the image and no effect on how well it displays on any specific display or print whether it be cell phone or computer. The ppi number just a number that is used to tell a print service how big you want a print to be made.
The only pixel specification that affects image quality or how well it displays is the total width and height of the image in pixels.
If you display two images 1000 and 10,000 pixels wide on a screen that is 500 pixels wide they will both be displayed exactly 500 pixels wide.
The ppi number is just that – a number in the header of the image file that tells someone how many inches wide and tall you want your image printed. All that matters is the number of pixels in your image (i.e. width and height in pixels). The optional ppi number in an image file (if it exists in the image header) is completely ignored by any software that displays the image on a screen and used only by third party print services, who often ignore it because so few understand the proper meaning and use of the ppi specification.
So, simply put you can totally ignore the ppi number unless you are wanting to specify to a print service exactly what size you would like them to print your image.
I believe I’ve said the same thing in three different ways above – hope it answers your question
As to why images of similar resolutions look different on an iPhone -- perhaps someone else on the forum can help with that. You can rest assured it has nothing to do with the ppi numbner.
KenLPhotos Stewartstown, US Nikonian since 26th Jul 2009
Fri 09-Nov-12 02:03 AM
#3. "RE: Understanding Resolution" In response to Reply # 2 Fri 09-Nov-12 02:58 AM by KenLPhotos
>As to why images of similar resolutions look different on an >iPhone -- perhaps someone else on the forum can help with >that. You can rest assured it has nothing to do with the ppi >numbner. > I would say it has a lot to do with lens quality, sensor quality and maybe a few other parts of the systems.
#4. "RE: Understanding Resolution" In response to Reply # 3
>>As to why images of similar resolutions look different on >an >>iPhone -- perhaps someone else on the forum can help with >>that. You can rest assured it has nothing to do with the >ppi >>numbner. >> >I would say it has a lot to do with lens quality, sensor >quality and maybe a few more parts of the systems.
Of course. I had misread the OP thinking that the iPhone image was the superior one.
#5. "RE: Understanding Resolution" In response to Reply # 0 Tue 13-Nov-12 06:18 PM by elec164
>So at this point,except for file size (2.6MB vs 4.45MB) both >pics (one from a home camera & one from iPhone5) are sized >the same. If I open each on my iMac the home camera pic is far >superior in detail, sharp & crisp to the iPhone pic, even >though the iPhone pic has almost double the total MP.
I think it pertinent to point out that it’s important to keep your metrics straight lest you get confused!
In your example, the real camera (aren’t all cameras real ) actually has about 50% more MP (12) than the iPhone (8). But the camera file has about 60% less MB than the iPhone’s. But now you’re probably asking, “but Pete that makes it even worse, how can an image file have more MP yet less MB”? Hold onto that question, I’ll get to it later.
As for image quality, I believe you have stumbled upon what is commonly referred to as the ‘Megapixel Myth’; or “the more pixels the better”. That’s true when you have good quality pixel information, but not all pixel information is created equal.
What I believe you are facing here is the law of physics, and numerous factors can affect image quality.
For example, pixel pitch plays a very important role in quality. On Apples site they list limited information as to the camera phones specs, but an internet search seems to suggest that the iPhone 5 uses a Sony 1/3.2 inch sensor. So the iPhone camera has a pixel pitch of 1.3 microns. I don’t know what camera you are comparing with, but a D7000 has a pixel pitch of 4.7 and a D90 about 5.5 microns. What that means is the iPhone is collecting significantly less signal than a DSLR which means there will be more apparent noise. In comparison it would be like shooting a DSLR at say ISO6400. And please don’t take that ISO comparison as gospel; there is a way to calculate it, but I don’t know how but it will be in the ball park. It also probably means that the iPhone will have a lower DNR and lower tonal accuracy.
The next quality limiting factor is diffraction. A 1/3.2 inch sensor has a crop factor of about 7. When testing with my D7000 I estimate that the focal length of my iPhone 4s to be about 2mm is 4.28mm. So even at f/1 with an f-stop of f/2.4, given the amount of enlargement needed to create an 8x10 print which is usually the standard used for DOF and diffraction affects, the iPhone appears to be a diffraction limited device. So while the MP statistic looks good at 8MP, I would surmise that the actual spatial resolution achieved is only about 4MP. It’s important to note that there is a difference between spatial and pixel resolution. You need a specific pixel resolution to achieve a given spatial resolution. But you can have a lower spatial resolution than you have pixel resolution. The former will provide finer detail; the later will aid you in avoiding artifacts such as pixilation and stair step affect.
Now back to file size. It may at first blush seem that things are awry with the iPhone having a larger file size. JPEG’s are compressed and all compression schemes are not equal. But that is most likely only part of the reason. Earlier I mentioned that the iPhone was equivalent to shooting a DSLR at about ISO 6400 with the apparent noise that goes with it. JPEG compression works best when there are greater areas of similar tonality. So a scene that contains a greater portion of sky, or say a blank wall of solid color will be more compressible than an image with significant fine detail of varying tonality. Unfortunately the compression scheme cannot differentiate between what’s fine detail or noise, so the iPhone file would be less compressible and will be larger file size.
At least that is my present understanding.
Edited to make corrections in post. Little did I know that there was EXIF information and using Photome I was able to determine the actual focal length and f/number.
Clint S Chula Vista, US Nikonian since 02nd Jan 2011
Sun 25-Nov-12 10:32 AM
#6. "RE: Understanding Resolution" In response to Reply # 0
The short the answer, GIGO. Or garbage in = garbage out. The longer answer - In addition to what Barry said that ppi has no significance and KenL about equipment, there is also the compression in every jpg file.
So let's forget about the ppi.
Your iPhone lacks the capabilities to take images of the same quality as your camera. The camera probably has a larger sensor with more photo sites, better processing software, and better optics. So it produces comparatively better images than your iPhone.
Now for the files sizes. You cannot tell the quality of an image by the Megabyte size of a file, partially because of compression that is used to make the files digitally smaller. And the files, at best, can only contain the image quality of what was put into the file.
Because your camera image quality is so much better than your iPhone, it can easily handle more compression (smaller file size) and still have a relatively better image than your iPhone. So the camera digital file size was smaller when originally created, but the quality was not in your iPhone image to begin with, and it was apparently not saved with a like amount of compression making a larger digital file. Probably as a means to try and keep image quality.
When you took your iPhone image into Photoshop and resampled it to a larger size, the resampling process has to guess as to what to fill the new pixels with. Programs do this by sampling the pixels around where new pixels are being added and is scientifically guessing at what needs to be filled in, so the image does not improve, it just gets larger.
And then when you saved the resampled image, you used a compression that allowed the file size to be more than what the camera image was. But since you started with an imaged that was not as good as the comparison, you could not end up with a better picture just by resampling. You just ended up with a bigger file. In some case this might referred to as GIGO.