Jumping ships - DoF

Started Sep 29, 2012 | Discussions thread
Macx
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Re: Values are wrong
In reply to pannumon, Sep 30, 2012

pannumon wrote:

The values in the table are wrong. DOF is only dependent on the lens, not the sensor. Therefore, 19mm f2.8 will give same DOF on MFT and NEX and FF. It's a different case if you are talking about equivalent focal length (same FOV).

Not true. Or at least not true if we talk about lenses in terms of focal length and focal ratio (or f-stop). An image from a smaller sensor is magnified more to reach the final output size than the image from a larger sensor, and because of this the depth of field is smaller on a micro-four thirds than it is on a larger sensor, all else being equal.

Depth of Field is not something inherent to a lens. A bit like the old chestnut about the tree falling in the woods, DoF doesn't exist without an output image and a viewer. If you decided to do away with those and imagine an image where the sharpness was unconstrained by the output and the eyesight of the viewer, all lenses would give you a infinitesimal DoF.

In practice this then translates into the fact, that all other thins being equal, that 50 mm/4 will produce the same amount of depth of field on a M43, as a 50 mm/2 would on a 135 full frame. And that if we keep the sensor size constant, that the same 50 mm/4 will produce the same amount of depth of field as 25 mm/1.

However, as several has pointed out, the most meaningful comparisons are between lenses that give you the same angle of view, and if you instead define lenses by their angle of view and their aperture by their actual physical size, then you can indeed claim that you get the same DoF from lenses with these qualities, no matter the sensor. But we would, of course, have to change the focal length and thereby the focal ratio to keep the angle of view constant when we change the sensor.

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