Im confused with megapixels and jpegs etc

[adomakin]

Right. So ive got this lovely 7.2 megapixel Sony Cybershot. It takes lovely pictures etc etc but can someone please explain some of the basics for me. When I take a pic that I want to use online (sending to mates or on pelican etc) I get the image up in my photo software (adobe photodeluxe 3) and use an option that converts or 'exports' the image into a much smaller 'Jpeg' format. this seems to dramatically reduce the file size and allows me to send the image much quicker etc BUT does it make the image less detailed? If I were to have that converted image printed would it be a less detailed image than the original unmolested image

Any help would be really appreciated

Andy

[cowtown]

Yes, compression of JPEG is "lossy," and the image loses quality as it is compressed. There is a compromise involved. You should always try to print the photo using the original file from the camera. Even rotating it with your software and doing nothing else will cause compression and quality loss. Sometimes it's perceptible, and sometimes not.

There are formats that use "loseless" compression, like TIFF, but they do not allow the same compression factors.

[Saintly]

Hi,
That's the basics of it. the MP rating of the camera (7.2 for you) is basically the maximum amount of detail that your camera can record. you will be able to set the camera to record that at different levels (i.e. 3/4 max, 1/2 max etc) and doing so will affect the amount of memory each photo will require.
When you save the pics to your PC you may find that the pic is (for example) 1024x768 in size and 300dpi (dots per inch) in detail and takes 2meg of disk space.
When you convert it to a smaller size (for e-mailing or to place on a web page) the software can reduce it's disk space by either making it smaller in size (i.e. 800x600) or reducing it's dpi to something like 72dpi or both.

You can have a perfectly acceptable version of the pic that will only take 100kb or so but you would always want to make prints with the more detailed version of the pic.

btw, another way of reducing the size of the pic if you use WindowsXP is to find the files that you want in windows explorer (the file manager), select all of the files that you want to send (hold crtl and click on each file OR click the first file, hold shift and click the last required file), then right click one of the files and select "send to" and then "mail recipient". When you do that a wizard will come up and ask if you would like to reduce the pic size and also give you options for how small to make it. it works very well and is quite quick.

Hope that helps a bit

[Moneyguy1]

Try "Digital Photography or Dummies". Excellent source for understandable info on how the darn thngs work.....

[adomakin]

thanks colin and Saintly!

Saintly, If I use the XP trick for sending images does it just take a COPY of the selected images to send, leaving the original file unmolested?

Andy

[adomakin]

cheers Bob, I like a techie book now and then!

Andy

[Saintly]

Originally posted by adomakin
thanks colin and Saintly!

Saintly, If I use the XP trick for sending images does it just take a COPY of the selected images to send, leaving the original file unmolested?

Andy

Yes, thats correct and it automaticly attaches the files to a blank e-mail. when i have a lot to convert i use this method to convert them and then a save the attachments (without send ing the e-mail) to my drive (in a folder named small, under the orriginal folder) that way i have a printable version and a e-mailable version on hand when i need it.

[arcsine]

"Lossy" JPEG compression artifacts can be minimized but it is true that upon repeated saving of a JPEG file, artifacts and loss of image quality can result. Saving as a TIFF file will elimate the compression losses by using LZW compression but the file will be bigger. I always recommend to people to archive their digital images as a TIFF or PSD and never mess with them. Sure you can tweak them ans save as a JPEG/GIF/PNG etc but don't overwrite the master file.

As to your other question, yes, "exporting" does make the image less detailed and it is not recoverable. You start with an image with 7.2 million pixels (MP). That means you have 7.2 million pixels of color data in a retangular grid that is calculated by image length x image width x res2 (that is resolution squared and resolution is usually pixels per inch (ppi)). At a decent printing resolution of 150 ppi, that would be almost an 18" square image, obviously too big to email or post on the web. As computer screen resolution is roughly 72 ppi, if you take that same 7.2 MP image and make the pixels bigger thereby descreasing the resolution (pixels per inch remember) the image will be 37 inches square, way over the top too big to see on a screen. So what the export function does it start to throw a way pixels to get to a reasonable size and resolution to email/post. Once those pixels have been tossed, they cannot be recovered (Ok, you do have "undo"). Then on top of that a lossy compression can be applied in the case of a JPEG to make the file smaller which can lead to some horrific artifacts depending on how much compression you use.

[adomakin]

Thanks Saintly and co. ain't this place great!


Andy

[adomakin]

cheers Gary


Andy

[RickM]

You may want to check the size/resolution that the camera is currently taking pics at. As mentioned 7.2 MP is overkill for PC and standard prints.

Remember that reducing will distort and lose information. Enlarging will distort and show the lack of info or try to compensate and do a poor job of it.

Finding the balance for your purposes is key.

[84porsche]

What kind of software do you have for image compression. Microsoft puts out 2 really good programs. One is the extra XP Power Tools - http://www.microsoft.com/windowsxp/downloads/powertoys/xppowertoys.mspx

or if you have Microsoft Office 2003, you have the program - Office Picture Manager and this has great resizing tools.

Both of these will allow resizing for sending to friends and email.

[id10t]

What you want in a camera all depends on what you want to do with the pictures - on-screen only viewing, or printed, or both. Megapixels is simply X dots across multiplied by Y dots down, with some division (by either units of 10 or units of 2) to make it a managable number.

Printers print at anywhere from 150 to over 600 dots per inch. If your camera takes pics at 1600x1200 then you are limited to a picture anywhere from smaller than wallet (at 600dpi) or 8x10 (150dpi) using only "real" pixels. Want to double the size of the image? The software starts adding pixels in, using a guesstimation of color for each one - the one to my left is blue, the one to my right is yellow, so make me green.

Screen use on the other hand is typically at 72 dpi, so you will typically shrink down the image for web/email/whatever use.

JPEG is a format for compressing images - with loss. Some loss isn't noticable (for screen viewing), so you can do some compression with no obvious side effect. CHeck out

http://www.dsdesign.com/articles/violin.htm

for an *excellent* overview of compression for images online.

[arcsine]

"As mentioned 7.2 MP is overkill for PC and standard prints. "

True, but pixels are always good and has been mentioned making them after the fact is fraught with issues. I say shoot with all the pixels you can and tweak the image later.

"Printers print at anywhere from 150 to over 600 dots per inch."

But it is not a 1:1 correlation between an image pixel and a printer dot. Or said a different way, an image pixel does not equal a printer dot. A 150 ppi image will take full advantage of a 600 dpi printer.