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<blockquote data-quote="WayneF" data-source="post: 317140" data-attributes="member: 12496">No.&nbsp;&nbsp; fstop = focal length / effective aperture diameter.&nbsp;&nbsp; A 200 mm f/4 lens has a 4x larger aperture diameter than does a 50 mm f/4 lens, but (unadjusted) the focal length is a factor of exposure too (it is a principle of magnification).&nbsp;&nbsp; So, we use the fstop numbering system instead of the diameter.&nbsp;&nbsp; It is designed like this specifically so that a f/1.8 lens compares to another f/1.8 lens.&nbsp; All lenses of same fstop should see the same exposure.&nbsp; It is the single purpose of fstop.The DSLR (with larger sensor) will have much less depth of field at f/1.8 than would a tiny video camera with a tiny sensor (like compact cameras).&nbsp;&nbsp; You might not like depth of field in videos shot at f/1.8 with the larger sensor.&nbsp; But.... the larger sensor can use much higher ISO (less noise) than the tiny sensor, so f/5 at higher ISO might do as well.</blockquote>

[QUOTE="WayneF, post: 317140, member: 12496"] No. fstop = focal length / effective aperture diameter. A 200 mm f/4 lens has a 4x larger aperture diameter than does a 50 mm f/4 lens, but (unadjusted) the focal length is a factor of exposure too (it is a principle of magnification). So, we use the fstop numbering system instead of the diameter. It is designed like this specifically so that a f/1.8 lens compares to another f/1.8 lens. All lenses of same fstop should see the same exposure. It is the single purpose of fstop. The DSLR (with larger sensor) will have much less depth of field at f/1.8 than would a tiny video camera with a tiny sensor (like compact cameras). You might not like depth of field in videos shot at f/1.8 with the larger sensor. But.... the larger sensor can use much higher ISO (less noise) than the tiny sensor, so f/5 at higher ISO might do as well. [/QUOTE]

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