A reminder for some memebers as to what the F stop is all about.

Started Jan 26, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,009
Re: You think so?

Sergey_Green wrote:

Great Bustard wrote:

Of course, 4/3 lenses are sharper than FF lenses, which offsets that advantage for FF somewhat. That is, if a 4/3 lens is twice as sharp as a FF lens, it will resolve as well as a FF lens for sensors with the same pixel count and AA filter.

I don't believe this is necessarily true.

Not necessarily true, and it does depend on the particular lenses being compared as well as where in the frame we are looking.

They are told to be sharper because they resolve more lines per mm, but that is only because those lenses are tested with the sensors that have the highest pixel density. Third party lenses that are ported and adapted to different formats show that twice as sharp Olympus lenses are nothing but the myth.

For a given lens sharpness, the difference in resolution (lw/ph) between two systems will be between the ratio of the sensor heights (enlargement ratio) and the ratio of picture height in pixels (linear pixel density). The sharper the lens, the closer the balance will be to the ratio of linear pixel density, the less sharp the lens, the closer the balance will be to the enlargement ratio.

For example, if a lens were very sharp relative to the pixel size of either system, the resolution advantage would go to the sensor with more pixels.  If the lens were not so sharp relative to both systems, the resolution advantage would go to the larger sensor.

Take Sigmas for example, many on this very same forum will agree that Sigma 17-50/2.8 will indeed deliver more detail than the celebrated Zuiko 14-54 (Bootstrap made several comments on it, even Ray agreed and echoed it), whereas the same Sigma is not necessarily the best or better lens than many Nikon equivalents when used on APS-C cameras. Or Sigma 150 macro, whenever there is a mention of it on this very same forum you will hear nothing but a praise. Yet again, Nikon 105 and Nikon 200 are at least as good if not better alternatives. I tried Tamron sp90 in the past, it was very much the same story - excellent lens on FT, good but not the best on APS-C.

And of course, how many times do we hear how good 50-200 is, some will not even change to the system that does not have such a lens, yet it is not really that great when you spend time and really compare it.

Again, it will depend on the particular pair of lenses you compare.  Also, lenses for 4/3 have to be twice as sharp as lenses for FF to resolve as well for sensors with the same pixel count and AA filter, but FF sensors tend to have more pixels, so they have to be more than twice as sharp to resolve as well.  So, let's say the 4/3 lens was 1.5x as sharp.  That would help narrow the gap, but the gap would be there, nonetheless.

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