Crop Factor, Low Light and Aperture with m4/3 lenses? Part 2

Started Jun 16, 2014 | Discussions thread
Eric Nepean
Eric Nepean Senior Member • Posts: 2,870
Re: Crop Factor, Low Light and Aperture with m4/3 lenses? Part 2

Just thinking out loud here.

Suppose that we have a 50mm F4 lens for a 35mm full frame, and a 25mm F2 lens for M43. These two lenses have equivalent focal length and aperture considering their designated sensors.

Now lets consider lens construction with the same front focal length and rear focal length (leave those complications for later).

The front element of the lens is the entrance pupil, it cannot be smaller than the effective maximum aperture. For the 50mm F4 lens the minimum front element diameter is 12.5mm, and it is the same for the 25mm F2 lens. (The actual element is probably slightly bigger than this)

The same consideration holds for the rear element of the lens.

This result seems to hold for my Panny 20mm F1.7 lens, for my FD50mm F1.4 lens the rear element is a little smaller - likely implying a slightly shortened rear focal length.

Now if the object plane has line pairs on it, these line pairs are shown on the M43 image plane as half the size than on the 35mm FF image plane.

Where does the image blur of the line pairs come from? The blur comes from not all the rays of a point source of the object focusing on the same point of the image on the sensor - some are focused in front of the sensor, some behind, some to the side.

Very crudely, rays which pass through an acceptable circle of confusion on the sensor are seen as sharp, those which are outside add blur.

Assuming the end images on computer screen or photographic paper are the same size, viewed from the same distance, the acceptable CoC on the M43 sensor is half the diameter of the acceptable CoC on the 35mm full frame sensor.

Now notice the the depth of focus around the M43 sensor is narrower than the depth of focus around the 35mm FF sensor, first because the CoC is half the diameter, but second because the rear focal length of the lens is half, and thus the rays from the lens come at the sensor over a cone which is twice as wide - twice the angle.

So, in order to maintain the same image sharpness, the depth of focus of the M43 lens must be nominally 4x narrower than that of the equivalent 35mm FF lens, although over 1/4 of the image area - but with the same FOV of the object plane.

I'm searching for way to compare the precsion required in the lens design and manufacture, maybe its this: - for a particular level of sharpness , the ratio of the depth of focus required to the image diagonal is a factor of two smaller for M43 than it is for 35mm FF.

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