Now here's the "magical" formula that defines what you have do do to have a "normal" looking photo:

**viewing distance : diagonal of print = focal length : diagonal of sensor**
(Well, this is not entirely exact! But at normal magnifications this is a good approximate)

Now you can rearrange this formula to calculate e.g.:

focal length = viewing distance x diagonal of sensor : diagonal of print

Let's put some real data into this formula:

- an FF/FX sensor has a diagonal of 43mm

- a DIN A3 print (297mm × 420mm) has a diagonal of 51cm

- let's assume your viewing distance is 60cm (we'll look at this assumption later)

Then you can calculate the focal length that is needed to get a perfectly normal impression from this print as

60cm x 43mmm : 51cm = 51mm

"Bingo", I hear some of you say: "That confirms what we already knew: a 50mm lens is a normal lens for FF-format".

Yeah, but only if you print DIN A3 and view from 60cm distance

Let me give you a practical example: I'm sitting in front of a 26" monitor with a 650mm diagonal and I have a viewing distance of around 40cm from it. Now what would be the lens (on an FF/FX-body that gives me a normal viewing experience?

40cm x 43mm : 650mm =

**26mm**
Quite a difference, isn't it?!

Well, if you play a little with the formula you can clearly see that all depends on how close you look at an image. And before we dig deeper into assumptions and habits about viewing distances I encurrage you to test yourself:

How far away are you, when you want to take a detailed photo of a beautiful landscape in?

Please

**post** your distance to the image and don't forget to also mention the diagonal of that image!

_________________

Thomas (beware: Nikon-fanboy and moderator!)

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