Please explain me one thing rearding apperture...

Started Feb 20, 2012 | Discussions thread
Anders W
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Re: Here we go again
In reply to MadsR, Feb 25, 2012

MadsR wrote:

Louis_Dobson wrote:

Hmm?

f=FL/A Halve FL, as you do for 4/3, to keep DoF constant A must be kept constant, so f is halved as well.

So, for the same FoV and DoF as a 100mm at f4 on full frame, use a 59mm at f2 on MFT.

Why is that complicated?

Because it does not compute in the digital camera world. In the film world, this was completely true, but not any more.

You see, DoF is a perception thing, so it depends on the output media being watched by someone. Since we have different output media we need different formula to compute DoF:

For screen/web based output for PC, the output is normally full HD, that is 2MP, so to get 2MP from my 12MP 4/3 camera I need to crop or scale by a factor 6, lets compare to a FF Canon 5D2 with 21MP, that needs to be scaled 10.5 times, so you get a greater DoF difference between capture media and output media already. And for a NEX-7 you end up with 12.5 times scale. Of cause we then have to calculate the original difference and scale that, so we end up with a marginally wider DoF on the FF, but actually shallower on the NEX fow the same aperture on same FoV.

Of cause for printing, you need to calculate this difference with how many pixels are actually used and the normal viewing distance of the print. So that is why I call it "difficult" to calculate. It is a perception thing not a direct mathematical thing, like FoV or exposure. (And exposure is not even, with ISO values depending on the transformation curve and not on the raw data, but for base ISO values exposure is equal for RAW and if you take JPGs exposure is still equal.)

It's misleading to bring the sensor resolution into the calculations in the manner you do. For example, the NEX 7 (24 MP APS-C) does not have a shallower DoF than the Canon 5D2 (21 MP FF) camera (at the same aperture, FoV, and subject distance). What matters for the DoF calculations is primarily the sensor size rather than the sensor resolution. The sensor size determines the acceptable CoC (circle of confusion) of the sensor, since an image from a smaller sensor has to be enlarged more in area terms (not pixel counts) to reach the same final display size as that from a bigger sensor.

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