Will telephoto ever be viable on NEX?

Started Aug 16, 2012 | Discussions thread
Rehabdoc Contributing Member • Posts: 919
Re: Gary: it seems premature to end the quest now

jpr2 wrote:

remember the original question? it concerned only the correct exposure
on 1.5x crop body for the 60x60mm 80/2.8 lens; I wasn't to delve
into DOF, or esp. into IQ issues at all - enough was said about it elsewhere
indeed :). As to whether image circle is irrelevant? Pls consider what would
happen if the said MF 80/2.8 lens was mounted on 1.5x such that its
image circle would cover just the 25.5x15.6mm sensor [in other words
all the four rectangle corners would lay on the circumference; this is an
approximation of course as in reality image circle needs to be somewhat
larger in order to avoid vignetting of corners, etc. - my knowledge is
lacking as to how much larger; but perhaps you do know how much].
In such situation the whole light passing through the lens would be
falling on the approximated 1.5x image circle, however... the projected
picture will be terribly OOF). So, to avoid OOF we need to preserve the
native 80/2.8 lens' flange distance -...

You're way over thinking this. When you mount a MF or FF lens on a camera with a "crop sensor" of any kind, you are cropping , not trying to place the sensor at some distance to catch close to the full image circle. That's they call APS-C a 1.5x CROP .

You preserve the original flange distance when you adapt a lens to a crop sensor because that's the point at which the lens retains all it original properties including the ability to focus at infinity.

Sure you can theoretically mount an MF lens on a 6 foot long extension tube (to put it farther away and make a microscope), or shorten the flange distance by building an improperly short adapter for NEX7 (to put the lens closer), but that is not what people mean when they talk about mounting a MF lens on a APS-C sensor . Yes it's possible to theoretically mount an FF lens to turn it into a microscope, or make it a focusing glass to burn ants, but that's not the "standard" way to mount it for use with a smaller sensor.

By convention and for practical reasons, you usually use a lens at its original register distance, so that is by convention what we mean, unless otherwise specified.

The simplest way to look at this is two words: "digital zoom"

A crop sensor is the equivalent of digital zoom. When you have a FF sensor and lens, and do a 1.5x digital zoom, the camera just displays and stores the image from an APS-C sized rectangle, while cropping the rest of your image. Wanna know what image an MFT camera would produce mounting your FF lens? Just do a 2x digital zoom. Your FF camera will just crop out an MFT sized rectangle right in the center of your sensor.

This is why the EV remains exactly the same for a lens across sensors.

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