DoF Conversion Factor – The Exercise

In my previous post I described how easy it is to calculate apertures to get equivalent depth of field on different formats. I presented there all the equations needed for DoF calculation, but left the actual “paperwork” to you.

In this post I’ll do these calculations for those of you who did not do the homework ;)

Our goal is to get the same amount of depth of field for two setups. Object distance is also the same, so we can simply work with hyperfocal distances.

H \approx \dfrac{f_F^2}{N_F c_F} \approx \dfrac {f_C^2}{N_C c_C}

Where the index F denotes full frame and the C index denotes crop sensor cameras.

We also know how the required full frame and crop factor focal lengths relate (X denotes the crop factor), so:

\dfrac{X^2 f_C^2}{N_F c_F} \approx \dfrac{f_C^2}{N_C c_C}

Now let’s see how the circle of confusion changes with the format. Having the exact same print dimensions, magnification will be higher for smaller formats.

m_C = X m_F

Viewing distance and your eye’s resolution are also the same, so:

c_C = \dfrac{\tan (\frac{\pi}{180 R_e}) D_v}{X m_F}

X c_C = \dfrac{\tan (\frac{\pi}{180 R_e}) D_v}{m_F} = c_F

To summarize what we have:

\dfrac{X^2 f_C^2}{N_F X c_C} \approx \dfrac{f_C^2}{N_C c_C}

Which after simplification leads to:

\dfrac{X}{N_F} \approx \dfrac{1}{N_C}

That is, we arrive to the result:

X N_C \approx N_F \text{.}

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