Image pixels and sizes...

[Jamesc359]

I'm not a mathematician, but for me this works.

WIDTH / (WIDTH - HEIGHT)
HEIGHT / (WIDTH - HEIGHT)

In English, lets say I want to find out the ratio of a picture that is 640x480. I start by subtracting the short side from the long side.

640 - 480 = 160

Then I divide the long side by the result, and then the short side by the result.

640 / 160 = 4
480 / 160 = 3

Our result is a 4:3

Another example would be 5x7

7 - 5 = 2
7 / 2 = 3.5
5 / 2 = 2.5

Now 3.5:2.5 isn't quite right. So we find the lowest common denominator (I believe... My math is so rusty it stinks!) for 3.5 and 2.5. Which is 7:5.

[Nicole]

Originally Posted by Arvid

Nicole, sorry but I am still confused. Again, mathematically, how do I get from a ratio of 3:4 to a fraction of .75. Or, vice versa, how do we get from .75 to a ratio of 3:4? I can see what is happening but, I cannot see the numbers required to make the conversion. There must be some formula this.

Dividing the smaller number by the larger number usually gets you the decimal.

And don't worry about hijacking the thread. If you have the question then odds are there's someone else out there with the same question

Originally Posted by Jamesc359

I'm not a mathematician, but for me this works.

That works pretty well as far as I can see

[porterd2nz]

Originally Posted by Arvid

Nicole, sorry but I am still confused. Again, mathematically, how do I get from a ratio of 3:4 to a fraction of .75. Or, vice versa, how do we get from .75 to a ratio of 3:4? I can see what is happening but, I cannot see the numbers required to make the conversion. There must be some formula this.

Ratios are generally written with the largest number first: i.e. 4:3 or 16:9 being two common ones.

To explain 4:3 = .75...

Simply, 3 divided by 4 equals .75. The larger number is the width if the image is in landscape mode (the natural mode for most computer displays and the default orientation for most digital cameras). This essentially means that the height is .75 of the width.

Example: if the width is 800, the the height of an image to fit into a 4:3 ratio image has to be 800 x .75 = 600. Or, showing up the numbers in the ratio; (800 [divided by] 4) x 3 = 600.

to go the other way is a little more difficult:

Here's how to convert .75 to a fraction...

There is not much that can be done to figure out how to write .75 as a fraction, except to literally use what the decimal portion of your number, the .75, means.

a) Since there are 2 digits in 75, the very last digit is the "100th" decimal place. So we can just say that .75 is the same as 75/100.

b) The fraction is not reduced to lowest terms. We can reduce this fraction to lowest terms by dividing both the numerator and denominator by 25. Why divide by 25? 25 is the Greatest Common Divisor (GCD) or Greatest Common Factor (GCF) of the numbers 75 and 100. (I wont show the method of calculating the GCD/GCF here, as the quickest way using prime number decomposition will probably create more questions than answers.)

c) So, this fraction reduced to lowest terms is 3/4. So your final answer is: .75 can be written as the fraction 3/4, which as a ratio is written as 4:3.


If you can follow that, good. If not try Working with Ratios or this one Ratios.

[Arvid]

A big thank you to all that have responded to my dilemma. I think it is slowly getting into this old head of mine. I have copied everything and will go off in a corner, with pencil and paper in hand, and work out different combinations until I totally understand the whole process.

Again, thank you all,
Arvid

[Arvid]

I now have a red forehead It all fits together so nicely when one knows what is going on. I now know that:
My sensor size, 22.2x14.8mm=3888x2592 pixels=1.5=.6666=3:2=6x4
And, I can easily convert from one measurement to another.
I also see why most people say “6x4” rather than 4x6 as I usually do.

Again, thank you!
Arvid

[Nicole]

Glad you got it all figured out Arvid!

[DennisL]

Thanks for the info! It helps clear things up some!
I sent a few photos to the printer yesterday. Most came out how I thought they would, but a couple didn't, so I'm a work in progress! LOL
BTW, thanks for moving it to the proper forum... I realized after the fact that I was in the wrong place.
*Embarassed*

[cjbnc]

A while back I scanned some old, square shots. Naturally, when they came back from the photo printer, they were badly cropped.

To fix:
- open square image in photoshop
- make sure background color is white (press D)
- Image -> Canvas Size..., set width to "150" and units of width to "percent", click OK
- should now have the square picture in the middle of two white bars, scaled perfectly for a 4x6 print.
- send it off for prints. When the prints come back, trim the white areas off yourself

The crop tool in photoshop also lets you select the dimensions before you select the area to crop. That way you can just plug in your print size and let PS worry about keeping the ratios right.