More on Pixel Resolution

In my previous post on pixel resolution  I mentioned that it had some serious ramifications for print.

The major one is PHYSICAL or LINEAR image dimension.

In that previous post I said:

• Pixel dimension divided by pixel resolution = linear dimension

Now, as we saw in the previous post, linear dimension has zero effect on ‘digital display’ image size – here’s those two snake jpegs again:

Digital display size is driven by pixel dimension – NOT linear dimension or pixel resolution.

Print on the other hand is directly driven by image linear dimension – the physical length and width of our image in inches, centimeters or millimeters.

Now I teach this ‘stuff’ all the time at my Calumet workshops and I know it’s hard for some folk to get their heads around print size and printer output, but it really is simple and straightforward if you just think about it logically for minute.

Let’s get away from snakes and consider this image of a cute Red Squirrel:

Yeah yeah – he’s a bit big in the frame for my taste but it’s a seller so boo-hoo – what do I know ! !

Shot on a Nikon D4 – the relevance of which is this:

• The D4 has a sensor with a linear dimension of 36 x 24 millimeters, but more importantly a photosite dimension of 4928 x 3280. (this is the effective imaging area – total photosite area is 4992 x 3292 according to DXO Labs).

Importing this image into Lightroom, ACR, Bridge, CapOne Pro etc will take that photosite dimension as a pixel dimension.

They also attach the default standard pixel resolution of 300 PPI to the image.

So now the image has a set of physical or linear dimensions:

• 4928/300  x  3280/300 inches  or  16.43″ x 10.93″

or

• 417.24 x 277.71 mm for those of you with a metric inclination!

So how big CAN we print this image?

 

Pixel Resolution & Image Physical Dimension

Let’s get back to that sensor for a moment and ask ourselves a question:

• “Does a sensor contain pixels, and can it have a PPI resolution attached to it?

• Well, the strict answer would be No and No not really.

But because the photosite dimensions end up being ‘converted’ to pixel dimensions then let’s just for a moment pretend that it can.

The ‘effective’ PPI value for the D4 sensor could be easily derived from its long edge ‘pixel’ count of the FX frame divided by the linear length which is just shy of 36mm or 1.4″ – 3520 PPI or thereabouts.

So, if we take this all literally our camera captures and stores a file that has linear dimensions of  1.4″ x 0.9″, pixel dimensions of  4928 x 3280 and a pixel resolution of 3520 PPI.

Import this file into Lightroom for instance, and that pixel resolution is reduced to 300 PPI.  It’s this very act that renders the image on our monitor at a size we can work with.  Otherwise we’d be working on postage stamps!

And what has that pixel resolution done to the linear image dimensions?  Well it’s basically ‘magnified’ the image – but by how much?

 

Magnification & Image Size

Magnification factors are an important part of digital imaging and image reproduction, so you need to understand something – magnification factors are always calculated on the diagonal.

So we need to identify the diagonals of both our sensor, and our 300 PPI image before we can go any further.

Here is a table of typical sensor diagonals:

And here is a table of metric print media sizes:

To get back to our 300 PPI image derived from our D4 sensor,  Pythagoras tells us that our 16.43″ x 10.93″ image has a diagonal of 19.73″ – or 501.14mm

So with a sensor diagonal of 43.2mm we arrive at a magnification factor of around 11.6x for our 300 PPI native image as displayed on our monitor.

This means that EVERYTHING on the sensor – photosites/pixels, dust bunnies, logs, lumps of coal, circles of confusion, Airy Discs – the lot – are magnified by that factor.

Just to add variety, a D800/800E produces native 300 PPI images at 24.53″ x 16.37″ – a magnification factor of 17.3x over the sensor size.

So you can now begin to see why pixel resolution is so important when we print.

 

How To Blow Up A Squirrel !

Let’s get back to ‘his cuteness’ and open him up in Photoshop:

See how I keep you on your toes – I’ve switched to millimeters now!

The image is 417 x 277 mm – in other words it’s basically A3.

What happens if we hit print using A3 paper?

Whoops – that’s not good at all because there is no margin.  We need workable margins for print handling and for mounting in cut mattes for framing.

Do not print borderless – it’s tacky, messy and it screws your printer up!

What happens if we move up a full A size and print A2:

Now that’s just over kill.

But let’s open him back up in Photoshop and take a look at that image size dialogue again:

If we remove the check mark from the resample section of the image size dialogue box (circled red) and make one simple change:

All we need to do is to change the pixel resolution figure from 300 PPI to 240 PPI and click OK.

We make NO apparent change to the image on the monitor display because we haven’t changed any physical dimension and we haven’t resampled the image.

All we have done is tell the print pipeline that every 240 pixels of this image must occupy 1 liner inch of paper – instead of 300 pixels per linear inch of paper.

Let’s have a look at the final outcome:

Perfick… as Pop Larkin would say!

Now we have workable margins to the print for both handling and mounting purposes.

But here’s the big thing – printed at 2880+ DPI printer output resolution you would see no difference in visual print quality.  Indeed, 240 PPI was the Adobe Lightroom, ACR default pixel resolution until fairly recently.

So there we go, how big can you print?? – Bigger than you might think!

And it’s all down to pixel resolution – learn to understand it and you’ll find a lot of  the “murky stuff” in photography suddenly becomes very simple!

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