Aspect Ratios + Resolution = Rocket Science (not)

Have you ever noticed how many folks are confused by

The subject of Image Aspect Ratios?

Add to that the subject of

Image Resolution – & you’d think it was Rocket Science!

Ed, Don’t go there….

Too late 😉

 

.

D300_091029_093121__DSC4514 nx wbdl cepsep

The Brown House

Camp Hoover Shenandoah National Park

President Herbert Hoover’s retreat (in a simpler time)

An image I made for the National Park Service

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Image aspect ratio defined

The ratio of the width of an image to its height

Shown as two numbers separated by a colon (A:B)

Image aspect ratio explained

It has nothing to do with the image’s actual size

It is simply the relation between width & height

A D-SLR image’s aspect ratio is 3:2

“Full frame” sensors are 36 mm wide & 24 mm high

“APS-C”  sensors are 24 mm wide & 16 mm high

In spite of the different physical sizes, both full frame & APS

Produce 3:2 aspect ratio images

Would math (fractions) help?

Probably not, but

In for a penny, in for a pound

In our D-SLR example what if you thought of the ratio

Not as 3:2, but

As 3/2 (a fraction)

At this point

36/24 = 24/16 = 1.5

and equals 3/2, as well

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Aspect Ratios – A Practical Illustration

This is a common situation

Someone has an image to be matted

To make it interesting (and still common)

Assume the mat has a pre-cut window

Further, assume that window’s aspect ratio is 5:4

6-14-2013 2-53-10 PM

Along comes a photographer

She has an image with a 6:4 (3:2) aspect ratio

(She likes to shoot green abstracts with her 3:2 D-SLR))

She was told to size her image to fit a 5:4 aspect mat window

She didn’t understand what this meant

and OOPS!!

6-14-2013 2-58-47 PM

She needs to crop either the left, right, or both sides

Or – allowing the mat to cover the excess is another solution

A 3:2 ( 6:4 if you prefer) won’t fit in a 5:4 hole

Sorry, but I’ll never understand what’s so hard about this?!?!

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The parallel problem of a 5:4 image & a 3:2 hole

Is left as an exercise for the interested reader 😉

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OK, Ed – I think I understand the point

But where does image resolution fit in?

Go back and read the part that said –

….aspect ratio….

It has nothing to do with the image’s actual size

It is simply the relation between width & height

Actual Size  is where resolution comes in and

Camera sensor size is where it all starts

Assuming a 3:2 D-SLR

A 6 MP model’s sensor is 3000 pixels wide & 2000 high

(3000 x 2000 = 6,000,000)

This is the camera’s resolution

This is the resolution regardless of whether the camera is

Full-frame or APS

For purposes of resolution,

6 megapixels is 6 megapixels

(even if it’s a 4:3 point and shoot

6 megapixels is 6 megapixels)

Resolution is camera and display dependent

Depends on the physical size of a display or of a camera sensor

Typically measured in

Pixels for monitors & projectors

Inches for prints

Below is a sampling of monitor types & sizes

Shown for a given type is –

Aspect ratio (width to height relation) &

Actual size measured in pixels (width & height)

This is resolution part

There are five different 4:3 ratio monitor types shown

Note that their actual sizes (pixels) are all different

One aspect ratio, five different physical sizes (resolutions)

6-14-2013 4-05-56 PM

If the display target is a print instead of a monitor

The physical size is inches, not pixels

Swell, Ed – but the image dimensions as they come from my camera

Are in pixels, NOT inches

No problem, there’s a rough rule of thumb that says

300 pixels for every inch of print (for good quality prints)

SO – your 3000 x 2000 6 MP file will make a

10 x 6.67 inch print of high quality

I say “rough” rule because depending on lots of things

Considerably larger prints are made routinely

With less than 300 pixels per inch

This next image of mine

Was printed 3-feet high from a 6 MP file

However, rest assured that 30 pixels per inch

Won’t do the job!

6-14-2013 4-34-42 PM

The one imperative, when it come to resolution, however is

The image pixel resolution MUST MATCH

The aspect ratio of the target display

If you’re making a 6″ x 4″ print

Having more than 1800 x 1200 pixels is no problems

Having a bit less also may not be a problem

Having a lot less, say 300 x 200, will be a problem

However – getting back to the imperative 

Whatever X x Y are, the ratio of the two

Had better be 3:2

i.e., matches the target display

Don’t get confused at this level if you see DPI (dots per inch)

That’s for the printer to worry about

Just think aspect ratio, pixels (& 300 pixels per inch)

That’s enough to get you through

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To complete our photographer’s trials & tribulations –

The long side of her mat window was 15 inches

The long side of her incorrect aspect ratio file

Was just over 500 pixels

Overzealous cropping??

SO – she has not one serious error, but two

Not an uncommon situation

How do you get people to understand?

It starts with them being willing to learn….

and putting in a little effort; especially the effort….

Questions? Write to Tim Gray

He’s asked about this weekly it seems 😉

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Go in peace. Today’s Sunday sermon to the faithful is ended. 😉

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