Recently, I asked the question: How large a print can you make from a Leica M9 file?
For my purposes (portraiture and simple element scenes), the answer turns out to be 36 x 24 inches:
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I thought about this some more, and realized that it would be interesting to see how much of a magnification over the Leica M9 sensor those prints above represent. If you want to skip the math below, the answer turns out to be 645!
Here’s the math:
The Leica contains a so-called “full frame” sensor that is exactly 36 x 24 mm in size (that’s where the 36 x 24 comes from).
The area of this sensor
= 36 x 24 mm
= 864 mm2
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The prints I made were 36 x 24 inches.
In the equivalent (metric) mm, this translates to
= 914 x 609.6 mm
=557, 174.4 mm2
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The magnification of print-over-sensor area, therefore
= 557,174.4/864
= 645
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This is remarkable when you think about it. The digital sensor (about the size of a stamp — remember those?) produces an image which is then magnified 645 times to produce the prints you see above.
Proportionally, this is what is going on:
All of the above underscores the importance of technique, lens quality, etc., when photographing with the aim of producing large prints: any “error” will be magnified 645 times!
Of course, you always have the option of turning to the much larger and more forgiving medium format sensors, but that’s another story…
—Peter.
Prosophos 
Hi Peter.
When I first have read “645” thought “medium format camera”… 🙂
Either way, and please correct me If I’m wrong, I believe the correct line to measure a print magnification is measuring just one side, like that: 90cm / 3.6cm = 250, i.e 2500% of magnification. At least that’s the way if one should scan a 35mm film using the printing conventions…
Hello Luiz,
Yes, I thought the “645” result was an interesting number… makes one recall the Pentax 645!
My calculation involves comparing sensor *area* with print *area*, hence the result. The final print area is 645 times larger than the sensor area.
You’re absolutely right about the area comparison, however when you think on how large your print can be I would use the printing convention of magnification (just sides)… But no matters, I’m sure m9 files (specially yours) can easily achieve this huge sizes.
I think that at base ISO with a decent, sharp lens, you could go to 5’x3.3′. That is based on M9 files I’ve seen on my monitor, taking into account its DPI etc. That’s probably stretching it a little bit but it does depend on viewing distance.