IPhoto Exporting Quality Size dpi etc. etc. Confused?

Question

Life used to be simple!
JPEG Quality:
If I wanted a photo for view on the web or my computer screen, I wanted a resolution of 72dpi (dots per inch)
If I wanted to it professionally printed, I wanted a resolution of 300dpi, and if I wanted a rough print version for
my home/business printer then I would probably be OK with 150dpi.
Now I can export from iPhoto as JPEG and I am no longer given the resolution options (72 or 150 or 300dpi etc.)
but rather the options for JPEG Quality ARE: Maximimum, High, Medium and Low [unhelpfully referred to as "Smallest size" what else would it be?]
Whatever choice I make appears to make no difference to the visual quality of the picture on screen, but makes a heck of a
difference to the file size AND gives me NO help on what the resolution of the resulting picture is.
0. This would lead me to believe that I should choose Low for ALL web or computer screen uses - as this will result in the
smallest possible file size - which in turn will reduce loading speeds for websites. Is this correct?
Unhelpfully, the iphoto Manual does not explain what these Quality Options mean and therefore I have no idea why or when I should
choose one over another.
1. Can someone out there who really knows their stuff, demystify this for me ?
What do these Quality Options mean, what dpi is assigned to each, and why and when should I coose one over another?
Maximimum
High
Medium
Low
JPEG Size:
This is confusing. If I know that for a particular use I need a particular size photograph (height vs width), I edit it in
iPhoto to the height/width that I need. For example, in the gallery I am using in Rapid Weaver, the designer suggests
a good size for  one particular design's revolving banner photos is 980 wide by 656 high - and I am assuming that these are pixel dimensions.
My photos from various sources are inevitably greater than this - and in some cases I might want to re-frame a portrait
photo into a landscape shape. To do this I use the Edit and Crop facility in iPhoto. And I make sure that the photo is edited to
the dimensions I want first.
iPhoto retains the full size photo in it's library, and I can always get it back after cropping if I wish
(by duplicating and reverting to origonal). However once cropped, it helpfully appears to EXPORT the cropped size.
The EXPORT function allows me to chose a JPEG's Size (by which I assume it means pixel width vs height)
2. Can someone tell me the exact effect/meaning of choosing any one of the following - in other words what do these mean or
put another way what pixel size am I choosing for:
Small
Medium
Large
Full Size
3. And am I choosing just the Height size or the Width size and what happens to the other dimension? And how does it
decide? And what are the Heigh or Width sizes allocated to these rather odd choices (Small, Medium, Large, Full Size.)
4. I am assuming that Small must equal a specific Width for a landscape photo, and in that case does it keep the height as it was, or does it constrain the proportions on the basis of the Width it has allocated to Small?
There is a fifth option, but again, it is unclear as to what exactly it does, and I would like help here too! (Sorry, but this is driving me demented!)
Custom
Within Custom, I have three choices:
Width
Height
Dimensions
5. What happens to the Height if I define the Width option? Or similiarly, what happens to the Width if I define the Height option?
6. What on earth does Dimensions mean when it only gives me one dimension to define?
Will it try and make a portrait picture square for example? Is it editing here?
Sorry this is so long, but I would really like to NAIL this - and I suspect I am not the only one!
All I am trying to do is to create the optimized output for the purpose I intend to use a picture for - really simple  - or is it?
Thanks to anyone who can have a go at this..... Michael

Answer 1

OK I've got it! - Thanks a thousand - and for anyone who wants a Quick Summary, here it is:
1. Cropping a photo in iPhoto sets the Aspect Ratio
Despite what it appears to do, cropping a photo in iPhoto does not affect the photos’s height or width. It does affect it’s Aspect Ratio - or shape (for example 6 x 4 ), and what is visible within that shape - but not the width or height. Note the absence of inches or cm or meters there. It also affects the content of the picture - or rather which area of the picture is visible (which is the ‘cropped’ area).
So it is OK to Crop the photo in iPhoto, but this should not be confused with the idea that you are cutting the photo into a specific dimension (width or height) - to do that you need the Export facility in iPhoto where you choose a JPEGs Size.
2. JPEG Size
Once the Aspect Ratio is determined (the shape) of the visible area of the picture - the Custom Size Option in the Export Facility can be used to select either the Width or the Height. Obviously if you chose the width, then that measurement will pre-determine the height as iPhoto will calculate the height needed to retain the Aspect Ratio you have set when you cropped it.
If you don’t crop, it will calculate based on the initial Aspect Ratio (shape) of the original photo.
Full Size is exactly that: the image at the dimensions imported from the camera. The others allow you to scale the image. Each reduces the size by about 33% or thereabouts.
Full Size = 100% of the Imported from the camera Dimensions
Large = 66% of the imported Dimensions
Medium =  44% of the imported Dimensions
Small = 29% of the imported Dimensions
As a rule of thumb a 3000 x 2000 (approx) image will reduce to 640 x 480 with the Medium setting
Thumbnails can be generated using the Export and then Web Page function, rather than the File Export function.
Otherwise, you can use the Custom setting  to specify an Exact width or an Exact height (and of course the Aspect Ratio will determine the other dimension NOT specified.)
3. JPEG Quality
JPEG Quality is setting the compression of the file which will affect the file size (not the picture Dimensions or the Aspect Ratio), but it will also affect the Quality of the visual image. So here you are seeking to minimise the file size while not losing acceptable visual quality.
Which you choose depends on what you're using the image for... Web site? Start at Low and work from there, if the quality is poor or visually unacceptable, choose a higher quality.
If you are Archiving go for max. The rest - trial and error...
4. For a deeper understanding of these issues
1. Read the thread below AND/OR
2. Take a look at this: The Myth of DPI. AND
3. This: https://discussions.apple.com/docs/DOC-4921
Clarity Rules! - And  many many thanks to Terence Devlin for the lesson...