Is my thinking about equivalence right?

Started 3 months ago | Questions thread
bobn2 Forum Pro • Posts: 69,811
Re: Is my thinking about equivalence right?

rogerstpierre wrote:

bobn2 wrote:

Olympus has to produce double the resolution of the Sony just to match it in a final image, due to being magnified twice as much, then there's not much of a comparison. You have to compare the Olympus lines with the lined for half the lp/mm on the Sony. On FF 10 lp/mm corresponds to 215 lp/ph, whilst on FF that final resolution is given by 20 lp/mm, so if we compare the two at the same final resolution, we find that the Sony gives an MTF of 0.93 in the centre and 0.85 at the edge, whilst the Olympus gives 0.77 in the centre and 0.69 at the edge.

You can't talk "magnification" with a digital sensor like you do with a film substrate. On a digital sensor the number of pixels, or data points you may want to call it, defines the size of the image, not the size of the sensor itself. Hence a sensor 1/2 the size with twice the number of pixels will produce an image twice larger when viewed at the same "magnification". This is where I don't understand when magnification is used as a variable to compare DoF between different sensor size. It has no relevance in a digital world, rather the total number of pixels that makes up an image does.

I'm talking about the dimensional magnification from the size of the camera's image frame to the viewing size. Call it 'enlargement', or what you want. Whether the mechanism is chemical or electronic, the image that the lens projects has to be enlarged to the size that you want to view it. If your image frame is 17.3x13mm it needs to be enlarged twice as much to get any given size output as it does if it is 36x24mm. Your point has no impact on what I was saying, that the image the lens projects must be enlarged twice as much on mFT as on FF. It also has no impact on DoF, where the important factor is the saze of the blur in the viewed image. To take into account the doubled enlargement DoF calculations for mFT use a CoC that is half the size as they do for FF, resulting in the same size CoC in the viewed image.

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