Sensor size affect on effective aperture

Started Sep 7, 2011 | Discussions thread
Barrie Davis
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Re: Sensor size affect on effective aperture
In reply to Everlast66, Sep 8, 2011

Everlast66 wrote:

It is often commented that a cropped sensors result in a narrower viewing angle and respectively capture less from whats in front of the camera, i.e. equivalent to a higher focal length than using the same lens and a larger sensor.

That's a rather odd way of saying that smaller sensors need smaller f-lengths in order to get the same Field of View, but yes, that is correct.

Does sensor size also affect the effective aperture (and F-number) of a lens? (I assume yes, but this is never commented, people only mention equivalent focal lengths.

In what regard do you mean "affect?"

It doesn't make any difference to exposure lightness/darkness...or, put another way, all f/5.6 apertures admit the same brightness of light, and therefore require the same shutter speed for correct exposure.. (at any one ISO and subject light level).

For example if I get a 50mm f1.4 full frame lens and use on a APS-C size sensor, I would be getting the equivalent view of a 75mm lens. Would I also be getting an equivalent aperture of, say, f2, as far as the amount of light and the DOF are concerned?

As already stated, you get the full f/1.4's worth of light admitted.

However, DoF will be similar to the Full Frame's DoF at 75mm, not 50mm. This is because the smaller format will need enlarging more..... (all enlarging reduces apparent sharpness, and DoF is only ever apparant sharpness).

Does this mean that if I get the above lens to use on a APS-C size sensor I would only be getting an F2 (or F2.8 or similar) lens?

Absolutely not.

Then what about a F2 lens designed for a cropped sensor, would we be getting the same result as of F1.4 designed for full frame?

No. In exposure terms, f/2 is f/2... (is f/2).
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Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

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