# Real focal length and online focal length calculators

Started Nov 5, 2012 | Questions thread
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 Re: Real focal length and online focal length calculators In reply to Detail Man, Nov 8, 2012

Detail Man wrote:

m712 wrote:

I got an sx50 and at the longest focal length which is 215mm the field of view 14ft from the lens was 17 cm horizontal but according to online calculators only 155mm is needed to get this field of view with 1/2.3 sensor. I checked several calculator sites and checked those sites against canon's own calculator http://www.canon.com/bctv/calculator/calculator3.html

Note that the Canon "Horizontal Object Dimension Calculator" you used does not work for your 1/2.3" form-factor sensor size (6.17mm x 4.55mm, 7.66mm diagonal):

http://en.wikipedia.org/wiki/Image_sensor_format#Table_of_sensor_formats_and_sizes

It only calculates for 2/3", 1/2", and 1/3" form-factor sensor sizes - so you need to either use a different on-line calculator, or simply solve the mathematical identities for these things (below).

...

So, if you have derived your 17cm (horizontal, W) figure at 14 Feet from an actual physical measurement using the camera system, then we can derive the EFL from:

EFL = (7.66mm) x ( 4267.2mm ) / ( 1.25 x 170mm ) = 153.82mm

So, in short, what you're saying is that the OP needs to look at sites other than Canon's - as he explicitly said he's already done (and then checked them against Canon's)  - or manually solve a bunch of equations which turn out to give almost exactly the same answer for the effective focal length as he's already given. Good stuff.

The answer to the question that was actually asked is still focus breathing, though.

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Andy Westlake
dpreview.com

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