How many pixels does man need, three sides of a coin...

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Erik Kaffehr
Erik Kaffehr Senior Member • Posts: 3,306
How many pixels does man need, three sides of a coin...
5

Hi,

Here is a simple formula for calculating Megapixels needed from 24x36 mm aspect ratio.

(PPI * PH )^2 * 1.5 / 1e6

My Epson printer has two native resolutions 720 PPI and 360 PPI and I normally print at 8"x23". The formula above gives:

720 PPI -> 199 MP

360 PPI -> 50 MP

So if I print from my 42 MP Sony Alpha 7rII, some upscaling would be needed. That is one side of the coin.

On the other hand, there is a lot of conventional wisdom, stating that about 180 PPI is needed for an excellent print.

At 180 PPI, 12.4 MP would be needed. That is another side of that coin.

But, unlike men, all pixels are not created equal.

Lets regard the 180 PPI criteria. For my 16"x23" print. 180 * 16 -> 2880 lines. That is 1440 line pairs. Let's now look at an MTF plot of sharpened output from an A7rIV (62 MP) and an (A7III) (24 MP).

If we look at 1440 cy/PH (which is similar to 1440 lp/PH) , the A7III would transmit around 80 contrast, while A7rIV would transfer 1015 contrast. My understanding is that about 35% NTF is needed for a subject to be seen as sharp. So the print from the A7III will be plenty sharp, but the A7rIV would be significantly sharper. Would you make a 26"x40" print from the A7rIV, it would be about as sharp as the 16"x23" print from the A7III.

The amount of sharpening applied in the figure above is quite a bit, BTW.

The edge of the coin is aliasing. If the lens delivers detail that the sensor cannot resolve that detail will be converted into coarser fake detail. In worst of cases, the fake detail will arise at different locations for red, green and blue channels, know as color moiré.

The Sigma sd Quattro H has a Foevon sensor that samples all three color channels simultaneously. So, it doesn't show color moiré. But it has a lot of fake detail, look at the pattern on the frame on the stand in the lower left part. The Fuji GFX 50 has a lot of color moiré, while the Fujifilm GFX 100 has very little. The Sony A7rIV shown here combines 16 exposures with the sensor shifted between the exposures to produce a very clean image. 24x36 mm cameras at 24 MP typically would use an antialiasing filter to reduce color moiré.

It may make sense to decide how big you will print or view you images and choose megapixels according. Done that, it may be feasible to choose a camera that delivers that resolution at excellent quality.
That camera may be 4/3, APS-C, 24x36 mm, 33x44 mm, 54x41 mm or even a scanning back.

Best regards
Erik

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Erik Kaffehr
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Sigma sd Quattro H
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