Help with equivalence calculations please

Started 1 month ago | Discussions thread
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kohinoor Senior Member • Posts: 1,406
Help with equivalence calculations please

I am currently shooting a 500mm f4.5 Lens on a 2X crop body of 3888x5184px resolution (3/4 ratio). When shooting base ISO I can crop my best pics down to screen size (1200px high) and still be satisfied with IQ.

Now I want to compare this to a 600mm f6.3 lens on a1.5X crop body of 4000x6000px resolution (2/3ratio).

My question is how big ore small (expressed in equFL-figures) will the same subject be in comparison when I crop the 2/3 sensor to the same 1200px high. I'm emphasising 1200px height here because the subject won't show up any bigger on my screen when cropping the horizontal dimension as I'm exclusively displaying in landscape orientation on a wide screen.
And I'd also like to know the equivalent apertures for the two crops.

Let's assume we are comparing at infinity focus and the FL-figures from the manufacturers are correctly stated so they are as stated above.

Please correct me where I'm wrong:

For the 500mm lens:

500mm f4.5 X 2crop = 1000mm f9 equFF

3888px high sensor divided by 1200px high final pic = 3.24 crop factor

gives 3240mm f29.16 equFF

For the 600mm lens:

600mm f6.3 X 1.5crop = 900mm f9.45 equFF

And now comes where I'm uncertain. -Do I have to convert the 3/2 format to 4/3 for comparison before applying the crop to 1200px high?
I did so by calculating a 4/3 sensor of same diagonal (FL). That sensor would be 4286px high. (If I've correctly done the two triangles of same base length!)

that would give

4286px high sensor divided by 1200px high final pic = 3.57 crop factor

gives 3213mm f33.74 equFF

After correction/approval of above please also tell me if the difference in pixel density of the two sensors (89600px/mm2 for the 4/3 sensor versus 65500 for the 3/2 sensor) can be expressed in ISO performance difference.


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