# Technical question regarding depth of field

Started 1 week ago | Discussions thread
Re: Technical question regarding depth of field

SKersting66 wrote:

Bill Ferris wrote:

The site you linked to confirms what I wrote.

Using the default 10-foot distance to subject, select a Nikon DX camera and a 100mm, f/2 lens. Click, Calculate. The result will show a depth of field of 0.24-foot.

Next, select a Nikon full-frame camera and a 150mm, f2.8 lens. Keep the same 10-foot subject distance. Click, Calculate, and you'll get a 0.22-foot depth is field.

Divide the focal lengths used by their respective f-stops (focal ratios) and you'll see the DX camera read using a 50mm aperture while the FX camera was using a 53mm aperture. If the online depth of field calculator allowed selection of f/3, the FX lens aperture and depth of field would match that of the DX.

Try this with other combinations of camera bodies and you'll consistently get the same outcome: keeping subject distance constant and selecting focal lengths & f-stops delivering equivalent images, cameras of different format use the same lens aperture diameter to achieve the same depth of field.

It seems to me that you're convoluting multiple factors simultaneously in order to achieve "equivalence." Of the 4 factors, the only one you are keeping constant is the distance... how do you say which of the 3 changes had which affect (Ap/FL/SensorArea)?
In general, if you increase FL by 2 (2x) the DOF is reduced to 1/4. If you decrease distance by 2 (to 1/2) the DOF decreases to 1/4. If you open the aperture by 2 stops (1/2 the number) the DOF reduces to 1/2. And if you only change the image format (DX/FF) the DX sensor will have ~ 1 stop less DOF.
This holds true in any comparison as long as both are on the same side of hyper focal distance.

Keeping subject distance constant effectively removes that factor as a variable producing any change we see in depth of field when changing sensor size, focal length and aperture diameter. Comparing cameras built around different format sensors allows us to understand and discuss depth of field in a way that applies across formats.

Comparing equivalent images, we see that changing focal length does not change depth of field provided aperture diameter is constant between the two systems. This reveals that aperture diameter is the factor allowing us to maintain a desired depth of field even if we change sensor size, focal length and f-stop.

If subject distance and aperture diameter remain unchanged, depth of field remains unchanged.

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Bill Ferris Photography
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