Camera megapixels and print size

[David McB]

Hope this may be the right place to pose my question. Quite sure this would have come up ages ago, but I'm new to CiC and have a curiosity question. Using Canon as the example, the 5DSR has a 50 or so megapixel sensor, while the Canon 5Div is at around 30 and the 5Diii is at ~22 megapixel size. Let's say one wanted to enlarge a nice photo to about two or three feet by five or six feet for hanging on the living room wall. About where will the megapixel size take over favorably? I've always heard that, unless one wants to make a road sign, you're OK with an "x" megapixel camera. Not sure I actually heard what that "x" megapixel size would be. Any thoughts?

[Manfred M]

David - I've moved your question to a new thread. It will get more / better answers that way.

[William W]

. . . Using Canon as the example, the 5DSR has a 50 or so megapixel sensor, while the Canon 5Div is at around 30 and the 5Diii is at ~22 megapixel size. Let's say one wanted to enlarge a nice photo to about two or three feet by five or six feet for hanging on the living room wall. About where will the megapixel size take over favorably? I've always heard that, unless one wants to make a road sign, you're OK with an "x" megapixel camera. Not sure I actually heard what that "x" megapixel size would be. Any thoughts?

It is more than just a question about how many mega pixels . . . and it is "flexible":

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Viewing Distance:


The FIRST consideration is: how far away from the wall would one typically be viewing the print?

Considering 3ft x 5ft which is 36in x 60in and 3:5 Aspect Ratio and has a Diagonal = 70in. Then, applying a Rule of Thumb for the “best” Viewing Distance for Aspect Ratios around 3:4~3:5, this would be [Diagonal x2].

That’s about 140 inches (about 11’8”) for the Viewing Distance.

This “best” viewing distance is based upon the Human Eye’s capacity to take in the whole image, based on the Angle of View, such will make the typical Viewer feel comfortable when viewing the image; this is why Viewing Distance is such an important first consideration if you are going to make a large print – you need to have a large enough room in which to display it, in a manner that is artistically appealing.

Note that "in a manner that is artistically appealing" can mean that the Photographer may want the Viewer to be CLOSER than what is a comfortable viewing distance: the Photographer might want the Viewer to NOT take in all of the Photograph, in one glance.

Once you have decided upon the Viewing Distance, then obviously you want the Print to look technically good at the Viewing Distance that you’ve chosen. For this to be so, there needs to be sufficient Pixels Per Inch (PPI) to fool the eye into seeing a smooth image: that is an image which is NOT “pixelated”.

One method of determining the necessary PPI is, in part, based upon based upon Visual Acuity, or the Eye’s Clarity of Vision. Here again, we must make a Rule of Thumb by assuming that the typical Viewer will have 20/20 Vision.

Without supplying the Mathematics, the minimum PPI necessary for "OK Print" is calculated by:
3438/Viewing Distance (in inches) = Minimum PPI

Any print made at this PPI (or greater) will “look OK”, at the chosen Viewing Distance.

For your example 36 x 60 inch print at a viewing distance of 140inches: 3438/140 ≈ 25 PPI

Now 25PPI in a Print is not difficult to achieve, even from an old model Digital Camera – for example, a Canon EOS 40D has a 10.1 MP sensor and the pixels of the sensor are 3888 x 2592

The relationship of: Pixels on Sensor; PPI; and Print Size is expressed thus:

Pixels on Sensor / PPI = Dimension of Print (inches)

In your example photograph using an EOS 40D we can reckon:
3888/x = 60 (thus x ≈ 65PPI)
2592/y = 36 (thus y ≈72PPI)

So we can see that if we want to make a 36 x 60 inch print at 50PPI or greater, we can use an EOS 40D and crop either the long side or the short side and we will still be safe.

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What if the Viewer gets closer?

If the viewer wants to have a close squizz at your 36 x 60 inch Print (let’s say 12 inches away from it), then we have this new calculation for PPI (based upon someone with 20/20 vision) –

3438/12 ≈ 287 PPI

Hence, we can deduce for a Print 36inches x 60inches and printed at 287PPI we would require the Camera's Sensor size to be 17,220 x 10,332 pixels: (36 x 287 = 10,332 and 60 x 287 = 17,220)

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A Generally Accepted Guideline for “Gallery Quality” – how big can we print?

There is a general acceptance (another Rule of Thumb) that 300PPI is acceptable for Fine Art or Gallery Display Printing.

This Rule of Thumb is mostly independent of the consideration of the Typical Viewing Distance and relatively independent of consideration of the Final Print Size.

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The Canon EOS Cameras that you mentioned


Working with 300PPI as our “standard for High Quality Prints" let’s consider the three cameras that you cited and calculate the approximate largest size print for each at 300PPI

> EOS 5DSR Pixel Dimensions: 8688 x 5792, Largest “Gallery” Print at 300PPI = 29 x 19inches

> EOS 5DMkIV Pixel Dimensions: 6720 x 4480, Largest “Gallery” Print at 300PPI = 22 x 15 inches

> EOS 5DMkIII Pixel Dimensions: 5760 x 3840, Largest “Gallery” Print at 300PPI = 19 x 13 inches

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Summation:

If you want to make big prints, the PURPOSE of those display prints is a primary consideration for the CAMERA CHOICE.

First consider the Typical Viewing Distance and then consider the degree of interrogation that the Image might undergo.

WW

[William W]

. . . I've always heard that, unless one wants to make a road sign, you're OK with an "x" megapixel camera.

Curious, I do not understand: what does the reference to making a road sign have to do with the question?

WW

[Tronhard]

Hi Bill:

I THINK the reference is to suggest that since road signs are regarded as very large (I'm going with an advertising hoarding concept here), then I interpret it as "unless one wanted to make an extremely large picture..." e.g.

[William W]

.... I THINK the reference is to suggest that since road signs are regarded as very large

OK. Thank you. I call those billboards. Road signs are 'STOP' and '60KPM' etc. Hence my confusion.

I understand now, billboard photos typically do not require high numeral PPI due to the typical L A R G E Viewing Distance.

WW

[Manfred M]

Good explanation Bill.

My understanding is that the human eye (assuming the 20/20 vision) can resolve down to around 1 arc minute (1/60th of a degree). If I remember correctly looking at an image at a distance of 50 cm / 20 inches, that resolves to around 170 dots per inch (DPI). Drop that down to 25 cm / 10 inches, that goes down to 340 DPI; which is outside the range of both Canon and HP ink jet printers that work at 300 DPI; but still fine for Epson printers that work at a native resolution of 360 DPI. The reason I mention these two distances, they are definitely the range where people will "pixel peep" for a large print.

The other issue with making high quality prints is that the whole process depends on getting a good, sharp image to print. The high end camera also has to be used with a high quality lens that is set to shoot in its "sweet spot"; at an aperture that is small enough to reduce sharpness issues that result from wide apertures without getting into apertures that are so small that diffraction starts softening the shot. Shooting at or near the "base" ISO to get a good image with minimal noise, maximum dynamic range and colour depth are considerations as well. An appropriately high shutter speed or sturdy and likely weighted tripod are part of the equation too. There is no real use in pixel peeping if unintentional motion blur has been introduced into the image.

The of course there is always the post-processing work to prepare the image for printing. Heavy handed PP work (or not enough on the other) will also impact the final resolution of the image. Upsizing the image inherently causes some softening and not all PP software is created equal here and some algorithms do a better job at this than others, but there is a lot of debate in the photographic community regarding this subject.

Bottom line is there is no easy answer; but a good solid starting point is sensor resolution and sensor size. A poorly done image with a large, high resolution sensor may not be as good as one from a smaller, lower resolution one that is well executed.

[David McB]

WOW, you guys blew me away! THANK YOU for your sincere and instructive reply to my question. By the way, all your inferences about my comments were exactly correct (road sign = billboard, etc.). Sharpness, PPI, "sweet spot" and viewing distance all were instructive. Thank you.