Comparing Olympus 4/3lenses to FX "Full Frame" offerings

Started Jan 25, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 58,560
Re: Maximum Aperture

mapgraphs wrote:

Ian Stuart Forsyth wrote:

Doctor Lecter wrote:

I beg to differ. *You* can remain ignorant on the subject if you wish... DOF changes, but fstops/apertures are the same regardless of sensor size. It's simple physics — the amount of light passing through the lens.

Exactly physics, the 50mm F2.8 has a larger front element than the smaller 25mm F2.8 front element now are you going to try and convince me that the same amount of light will pass thru the smaller 25mm F2.8 as a 50mm F2.8. Has the 25mm F2.8 have some special power that allows it to suck more light thru its smaller opening.


Yes, to get a f/2.8 maximum aperture on a 50mm focal length lens, the front element is going to be larger than that of a 25mm focal length lens with a maximum aperture of f/2.8. The size of the element is about the maximum aperture per THE FOCAL LENGTH.

It's not about how many more photons go through crystal as compared to graphite or coal.

It's exactly about how many photons go through the glass and end up on the sensor. In the end, the image quality depends on how many photons get counted in the image, which depends on how many photons there are in the image projected on the sensor and how good the sensor is at counting them. So, in terms of the number of photons ending up in the projected image, there are two things that matter, how big is the aperture and what is the solid angle from which photons are collected over that aperture. The solid angle thing is why we use f-numbers to measure the 'speed' of our lenses. For instance, collecting photons over double the solid angle needs half the aperture area to collect the same number of photons. By measuring the aperture diameter relative to the FL of the lens, that correction gets made automatically.

However, if you make the solid angle bigger by making the sensor bigger, we end up collecting more photons for the same f-number. You can't compare 'speed' by f-number across formats.

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