Graham Harris Graham | Image Resolution

Bigger is best. Well, sometimes.

When it comes to digital sensor technology, size really does matter. Having said that, it is perfectly feasible to print huge images quite successfully using the cheapest of sensors.

Mamiya 645 AFD II The Resolution Trick

Despite massive technical improvements across all camera segments in recent years, exotic high end, medium format sensors still capture images at resolutions that budget priced sensors can only dream of. And there are other optical benefits too but consider this; a 50 inch wide, 16:9 ratio, HDTV (1080p) television set displays an image with the following resolution

             1,920 pixels / 43.58 inches = 44 dpi horizontal

             1,080 pixels / 24.51 inches = 44 dpi vertical

Ah, but that's old school, right? Television manufacturers now offer what's called 4K technology which means that the number of pixels across the width of the screen is now 3,840 pixels & 2,160 pixels vertically. Confusingly, 4K actually describes the 4,096 x 2,160 resolution first introduced in digital cinemas while UHD refers to the 3,840 x 2,160 resolution you'll find in 16:9 ratio TVs, which is what you actually take home. We call this 4K but it still results in only

3,840 pixels / 43.58 inches = 88 dpi horizontal

2,160 pixels / 24.51 inches = 88 dpi vertical

Oddly, we're supposed to marvel at this achievement even though 4K is basically a waste of money because when sitting just 7 feet from the TV, our eyes can only resolve 44 dpi, exactly the same resolution as the old school HD screen.

At this point, you might be wondering why you should bother replacing your old HD set for one of the latest 4K units. Unless you're dissatisfied with the colours or the sound quality, personally, I wouldn't bother upgrading. You won't be able to see any improvement in resolution. You might think you can but that's probably because the contrast is higher & there's nothing wrong with higher contrast so long as the shadow & highlight details are retained. But you still can't see the increase in resolution unless you like to sit just 3 feet from the TV. Apart from four year old kids, who wants to do that?

When It Matters

Nevertheless, high resolution technology is useful when it is taken advantage of. One key reason I use medium format (6 x 4.5) & large pixel count full frame (35mm) digital cameras is the capability of making huge print enlargements at resolutions that can be critiqued at near distances by the most demanding of customers. Thus I've successfully printed & framed dozens of images measuring over 4 feet wide & on occasion, up to 6 feet wide.

But I also want to demonstrate that with low resolution, low cost equipment, it is still possible to create large images & get away with it. Just like television manufacturers!

BlackrockSilver halide photographic print measuring 51" x 34" enlarged from a full frame, 35mm digital image file. BannockburnSilver halide photographic print measuring 34" x 51" enlarged from a full frame, 35mm digital image file. Here's two of my large framed prints. All the cutting is done by hand - glass, 8-ply museum grade mounting boards & mouldings. At these dimensions, it takes me about one hour to accurately cut, glue, square up & nail just the frame moulding.

Another couple of hours is needed to cut, clean, sign, mount, pin, tape up & screw (free of scratches, oil & dust of course), all the components. And they weigh about 30 lbs each!


Visual Acuity

Snellen Chart So you haven't got £20,000 to spend on an exotic bag of fancy cameras & lenses. How can one possibly create decent quality enlargements with budget level equipment?

Poor Eyesight Bonus

Well, here's some good news. Your eyes & indeed everyone else's eyes aren't actually that great at seeing detail at anything further than a few feet away. Allow me to demonstrate.

Opticians have defined someone with decent eyesight as having 20/20 vision. But what does that mean? Well, the eye chart we are asked to read during an eye test is called a Snellen chart (see image on the left).

The size of the letters on a Snellen chart are no accident. At 20 feet, your eye subtends an angle of 5 arc minutes when reading the letter E on the 8th row down. And if the room is less than 20 feet long, the optician just uses a shorter chart with smaller letters but your eye will still subtend 5 arc minutes.

One could also use a longer room but the chart & letters would have to be proportionally larger too. So 20/20 vision means that someone with this ability can see a letter of a particular size that subtends 5 arc minutes such that any other average person can also see at 20 feet. Mr. McGoo, right?

For comparison, 20/10 vision suggests above average eye sight because the person can see letters on the chart at 20 feet when the average person can only see them when standing just 10 feet from the chart. Alternatively, someone with 20/40 vision has poor eyesight because they can see a letter on the chart at 20 feet that the average person can see at 40 feet.


Easy Trigonometry Lesson

An arc minute is just a subdivision of 1 arc degree. There are 360 arc degrees in a complete circle & there are 60 arc minutes in each arc degree. And as you can see from the diagram below, we can also subtend 1 arc minute because someone with 20/20 vision can see that the letter E on line 8 on the Snellen chart, is actually created using five equally spaced, horizontal lines; three black & two white (clear).


20/20 VisionAt 20 ft one can only resolve 1 arcminute

With a simple equation we can calculate the dimension, (P) in millimeters that we can observe at a known distance, (d) also measured in millimeters. Then we can convert that dimension in a dots per inch (dpi) number. Thus,

P = 2 x d x tan (α / 2)   where

P = pixel width (mm), d = distance (mm), α = angle (degrees), 1 inch = 25.4 mm, 1 arc minute = 1/60th degree


Now, the average adult can comfortably focus no closer than about 4 inches (about 100 mm). So with 20/20 vision, one can resolve a dot with a diameter in mm ...

P = 2 x 100 x tan ((1/60)/2)

= 0.0290 mm

This can be converted to a dpi resolution by dividing the number of millimeters in one inch by the diameter of the dot in millimeters, thus ...

=> 25.4 mm / 0.0290 mm

= 876 dpi

So at 4 inches, we can resolve 876 dpi. But I know of no one who reads anything from 4 inches away. In reality we read magazines at a distance of about 12 inches (305 mm), A4 wall art from about 1.5 x the diagonal which is 21.4 inches (546 mm), laptop screens from about 30 inches (762 mm) & large television sets from about 96 inches (2,438 mm).


And why does any of this matter & what does it have to do with digital sensors & print enlargements? Well, we might be able to resolve almost 900 dpi at 4 inches but as we step back from whatever subject we are viewing, our ability to resolve detail quickly diminishes as the table below demonstrates. Visual Resolution is the dpi our eye can resolve at a distance from a subject as indicated in the 1st/2nd column.

Glasgow University




(metres) (feet) (dpi)
1 3.3 87.4
2 6.6 43.7
3 9.8 29.1
4 13.1 21.8
5 16.4 17.5
6 19.7 14.6
7 23.0 12.5
8 26.2 10.9
9 29.5 9.7
10 32.8 8.7


It's obvious from the table immediately above that when standing just three feet away from an object, we can't even resolve 90 dpi. When we read a magazine from about 12 inches (305 mm), we can still only resolve 300 dpi.

That's why magazines are typically printed at 300 dpi. It's good enough for most people with 20/20 vision. And that's why an HD 50 inch television set displaying a lousy 44 dpi looks sharp from just 7 feet away. Our eyes are brilliant at many things but are simply unable to resolve high levels of detail at any distance longer than our arm.

Distance Matters

Consequently, we don't always need large & expensive digital sensors to print large images because it ultimately depends on the Canon Powershot viewing distance. After all, a 4k movie being projected inside a cinema can only display somewhere between 5 dpi & 7 dpi depending on where the viewer is seated & the movie still appears to be acceptably sharp.


Budget Solution

Let's assume for a moment that our budget has limited us to having a camera with a 16 megapixel sensor, a size which is common in many low cost cameras produced by Canon & Nikon. e.g. CANON PowerShot SX530 HS Bridge Camera. As of December 2016, it costs about £160 & records images on a 6.17 x 4.55 mm CMOS sensor with 4,608 pixels on the longest side & 3,456 pixels on the shortest side.


Using this budget friendly sensor, we could print an image at 100 dpi, measuring 3 feet wide & well over 2 feet tall. The recommended viewing distance is 1.5 x the diagonal length of the print & a bit of trigonometry arrives at a distance of ... 

1.5 x √{(36.48)2 x (27.36)2} = 68.4 inches

And we can calculate our visual acuity using the equation; P = 2 x d x tan (α / 2) to arrive at a number which describes our ability to resolve at 68.4 inches. Thus,

P = 2 x (68.4 inches x 25.4 mm) x tan ((1/60)/2)

= 0.5054 mm

=> 25.4 mm / 0.5054 mm

= 50.3 dpi

Who Needs a Hasselblad?

This is great news. With a 16 megapixel camera that cost only £160, we can print a 3ft x 2ft image at a resolution of 100 dpi. By standing at the recommended viewing distance which here would be 68.4 inches, our eyes can only resolve 50.3 dpi, half the resolution of the 100 dpi print. The print then will appear perfectly sharp as if it had been created using a camera costing ten times as much.

Still have some questions? You can always contact me at