Discovered Something New

Started Mar 18, 2014 | Discussions thread
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Eric Nepean
Eric Nepean Veteran Member • Posts: 3,217
Discovered Something New

Being stuck at home with a nasty cold I'm reduced to a low level of activity - I was looking at all the various lens I have and wondering what exactly is the depth of field in various configurations - and I discovered something which is new to me.

Consider that you have a sensor, such as M43, and a bagful of lenses of various focal lengths (what?? who? me )

Then pick some horizontal distance you want to cover with your sensor, say 1m or 3ft 3in for those in the imperial system. It is well understood that the camera to object distance for each lens depends on the lens focal length, and there are formulas and online calculators that can help you determine this, for example

What surprised me though, is that if you put all of these lenses with different focal lengths to the same aperture, for example f2.8, and then focus them each for the object distance required for the same horizontal coverage, then the depth of field distance is the about the same for all focal lengths.

Example: Assume the width of the sensor is 17.3mm (M4/3), lets say with each lens you want to cover a 104cm horizontal distance - about 41" - so portrait size.

This is the table of lens to object distance for some of my lenses calculated using the tool mentioned above. I calculated the depth of field for each using

Foc len _ Obj Dist _ f2.8 DOF _ f1.8 DOF

20mm __ 120cm __ 30.5cm __ 19.4cm

35mm __ 215cm __ 31.7cm __ 19.9cm

50mm __ 305cm __ 31.1cm __ 19.6cm (Note: reference calculation - 50mm lens @10ft)

75mm __ 460cm __ 31.4cm __ 19.8cm

100mm _ 615cm __ 31.6cm __ 19.9cm

135mm _ 825cm __ 31.2cm __ 19.6cm

It doesn't look like an exact relationship, and it probably only holds for narrow depth of field - I see it breaking down as the DOF becomes greater than about 20% of the object distance.

I had no idea this was so.

Practical Real World Implication: If I take a portrait of someone of a certain size, the depth of field distance is determined mostly by the image size and the aperture f-ratio. I can choose any focal length -  for example to set background perspective, or to manage the camera to subject distance.

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