Explanation of image quality and image size

tadd5181

Senior Member
I am looking for a good, short explanation to give a user of my D5100 of the difference between image quality and image size. Thanks in advance for help.
 

Moab Man

Senior Member
I'm thinking I'm understanding what your asking, so I hope this is the explanation you're looking for.

Assuming you have a good image, your print image quality will be very dependent on dots per inch (dpi). The more dots you have per inch the more fine detail can be shown. As you make the image larger you are diminishing the dots per inch which will degrade your image thus increasing the distance at which an image needs to be viewed from.
 

Eyelight

Senior Member
Hmm. What the OP may be looking for is in reference to the camera settings related to jpeg images:

Image quality would be based on the compression and defined in camera as fine, normal or basic. More compression (basic) equals smaller files and less quality.

Size is the actual pixel dimensions (width x height) of the jpeg as saved in the camera.

The camera can produce any combination of quality and size.


ETA: which all plays into what [MENTION=11881]Moab Man[/MENTION] is saying related to the final image. Did that make sense or did I just stir up the muddy water?
 
Last edited:

WayneF

Senior Member
I am looking for a good, short explanation to give a user of my D5100 of the difference between image quality and image size. Thanks in advance for help.

For JPG images, Image Size can be selected to be Large, Medium, or Small. For the D5100, these are
Large 4928x3264 pixels (16 megapixels)
Medium 3696 x 2448 pixels (9 megapixels)
Small 2464x1632 pixels (4 megapixels - but large enough to print 5x7 inches well, unless significant cropping)

Image Quality (really is JPG Quality) can be selected as Fine, Normal, or Basic.
Our RGB images are three bytes per pixel, 16 megapixels is 48 million bytes.
But these JPG settings compress the JPG file data to be much a much smaller file (JPG compression). These settings are for JPG Quality. The more compression, the smaller the file, but with less image quality. Vice versa, higher JPG quality is a larger file, and better image quality. Larger files are always better JPG quality. The maximum quality would be if no JPG compression at all.

The D5100 Ref. manual (see memory card capacity) says Large Fine file size averages around 7.1 MB, which is vastly smaller than the 48 MB (that the data actually is).
Going smaller is just asking for greater degradation.

Because, the problem is, JPG loses image quality in doing it, we don't get the same pristine image back out of the file. Normal is not "normal", it is less, not as good as Fine.
The standard advice is to NOT ruin your image with excessive JPG compression. Use Fine.
More about this at What does JPG Quality Losses Mean?


The manual (I'm looking in the downloadable Reference Manual for D5100) says Fine, Normal, Basic compression are to 1/4, 1/8, and 1/16 original size (meaning bytes, the pixels always remain the same). Confusing to me because the typical file sizes they show are more like 1/6, 1/18, 1/36 size (of the actual 48 MB pixel data size). But these have had the dickens squeezed out of them. :)

Says Large Basic is 1.8 MB files, but Small Fine is 2 MB. Both are small files, but 7 MB is already small, and of course, Fine is Fine. Generally, we like Fine. :)

If having a smaller image file is more important than maximum image quality, then maybe smaller Image Size could be considered, but do always keep Fine Image Quality. There's no substitute for quality.

Large Fine file size is not all that large.
 
Last edited:

tadd5181

Senior Member
That's a lot to digest, but is it correct that each (image quality and image size) affects the other...if so, then the sharpest possible photo is one that is both "fine" (image quality) and large (image size)?
 

WayneF

Senior Member
Yes, the image with the most detail, usable for the most critical purposes, is Large Fine. Should be the default.

Less than Large size is a smaller image, suitable for smaller purposes (video monitor screens are at most 2 megapixels, cannot use 16 megapixels). But a large print (like 16 inches) will want the large size.
Some standard print sizes (300 dpi):
4x6 inches, 1200x1800 pixels
5x7 inches, 1500x2100 pixels
8x10 inches, 2400x3000 pixels

A large video screen, 1920x1080 pixels.

Less than Fine quality is a deteriorated image, less quality, regardless of image size.
 
Last edited:
Top