Megapixels and other things

mrtoad

mrtoad

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Name
Geof
Edit My Images
Yes
my camera is 4Mp and most of the shots come out 2.5 to 1.9 Mp..sometimes larger..but not more than 3.5
the setting is highest quality so the shots are 2285 x 1552 or thereabouts
the 6 x 4 format

i have seen the pros here saying they upload their shots at 300dpi

i usually print full frame to 6 x 4

i am a bit ignorant about how these all relate...can i get some help please as to the real benefit of larger Mp/what sensor size
so that if i crop down i can still get good 6 x 4

what is a bit confusing to me is the upload criteria of 800 pixels and 20Kb

i usually work in inches..:cuckoo::cuckoo:

from a technical point of view i would like to get my head round these measurments

cheers
geof
 
Er, when you say that your shots are 2.5 to 1.9 Mp, how are you measuring that?
The size of an image on the disk, is not related to the size in MegaPixels that the image has been taken at.
The size of an image on disk is recorded in MegaBytes.
If you put your ISO up to the maximum your camera can handle, and take an image of a very colourful scene, then the size on the disk will be larger, as there will be more noise in the image, and the camera will be lessa ble to compress the image down.
The criteria for uploading to this forum, is 800px on the longest edge, and 200kb (kilo bytes) maximum,

Compression on jpegs can reduce the size of a file a lot. If a pixel were to be 3 bytes in size (for simplicity in this example, if a pixel being represented by three 1byte red green blue channels). If an image were 800px by 800px, that would be 800*800*3 bytes in size (1920000 bytes). 1 kilo byte is 1024 bytes (2 to the power 10).
That is 1.9 megabytes. A bitmap image is stored directly in this way. A JPEG image, takes the bitmap, and performs a compression on the file. Usually, compression levels in jpegs go from 1 to 10, inversely proportional to the quality of jpegs listed as 1 to 12. A level 12 jpeg is usually compressed with a lossless compression. Pretty much no compression.
If you select a quality level of 6-7, usually you cannot see any real difference at web-browser quality, but could reduce the size of the image to ~200k in this size, as it throws away some of the detail.
Hopefully this doesn't confuse things too much,
 
coldpenguin said:
Er, when you say that your shots are 2.5 to 1.9 Mp, how are you measuring that?
The size of an image on the disk, is not related to the size in MegaPixels that the image has been taken at.
The size of an image on disk is recorded in MegaBytes.
If you put your ISO up to the maximum your camera can handle, and take an image of a very colourful scene, then the size on the disk will be larger, as there will be more noise in the image, and the camera will be lessa ble to compress the image down.
The criteria for uploading to this forum, is 800px on the longest edge, and 200kb (kilo bytes) maximum,

Compression on jpegs can reduce the size of a file a lot. If a pixel were to be 3 bytes in size (for simplicity in this example, if a pixel being represented by three 1byte red green blue channels). If an image were 800px by 800px, that would be 800*800*3 bytes in size (1920000 bytes). 1 kilo byte is 1024 bytes (2 to the power 10).
That is 1.9 megabytes. A bitmap image is stored directly in this way. A JPEG image, takes the bitmap, and performs a compression on the file. Usually, compression levels in jpegs go from 1 to 10, inversely proportional to the quality of jpegs listed as 1 to 12. A level 12 jpeg is usually compressed with a lossless compression. Pretty much no compression.
If you select a quality level of 6-7, usually you cannot see any real difference at web-browser quality, but could reduce the size of the image to ~200k in this size, as it throws away some of the detail.
Hopefully this doesn't confuse things too much,
Click to expand...

that is quite a leg up on what i thought
thanks a lot!!

i have an out of camera shot of 2.45Mb and the listed details on the laptop give 2.515 Kb jpeg image...i really never noticed that change in sizing

no i do understand the selecting of differing qualities when saving to get the 200kb size etc

what i was unsure of was my method of getting 6 x 4 prints by just doing some pp on the original...not resizing it..and putting it in for printing

i do see lots of fellows here resizing to 300dpi and didnt know if that would be something i should do..dont really understand the reason

any further help would be appreciated

cheers
geof
 
The resizing is that some printers/companies work on 300dpi.
I find the easiest way to do this, is to use the crop tool in photoshop, to set the dimensions.
The use Image->Size, to resize it to the inches, and 300dpi
If I am just printing at home though, I don't bother with the second part, as I think Photoshop does the second part on the fly as it prints.
If the dpi is incorrect, and the image does not store the inches/cm dimensions, then the image will turn out of the printer badly the wrong size (doh)
 
Right, here we go.

This is all based upon a screen that measures 15" diagonal and is set to a resolution of 1024 x 768. Trouble is, nearly no-one uses that now so I'm behind the times a bt. I'll re-calculate this when I'm a bit more sober.

you say that your images are 2285 x 1552 pixels so that calculation is simple. 2285 x 1552 = 3,546,320 or 3.5mp. Now, each pixel is an item of information and a healthy human eye has an optimum resolution of 270 dots per inch - I'll not go into pica's now as thats a typographical unit and not really relevent to photography.

So, 2285 x 1552 divided by 270 D.P.I. gives a maximum print size of 8.46inches by inches by 5.7 inches.

For internet purposes and working at a maximum post size of 800 D.P.I along the longest edge, that would give an optimum image size of 2.9 inches x 1.9 inches at optimum eye resolution but as the supposed resolution doesn't resolve above 75D.P.I. it give a screen resolution of 10.6 inches x 7.1 inches.
Posting images at 300D.P.I. is great as this allow those fortunate enough to be using 24" monitors at 1440 x 900 resolution to enjoy our best efforts at optimum resolution for the big screens.
 
I've just re-read what I wrote and now my head hurts.
 
cyclone said:
A lot of it makes no sense.
Monitors ignore dpi and display images pixel by pixel.
Click to expand...

yes sheddy has another point i though of

how does ones terrific shot look on anothers screen

my laptop is 15" diagonal...IBM T60 and the screen is set at 1024 768
and some of my shots i did my PP on my old pc which had a 19" monitor
look decidedly ropey on this laptop

and i expect visa versa hola!

i am understanding the 300 part though and next time i get prints i will do some checking on what i am sending into the costco to print for me

thanks chaps:thumbs:
 
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