Current 40D owner contemplating upgrade

Started Nov 25, 2012 | Discussions thread
oss Regular Member • Posts: 270
Re: Current 40D owner contemplating upgrade

qianp2k wrote:

oss wrote:

Y Hafting wrote:

About EFS lenses:

When it comes to the 17-55 f2.8 you should remeber that the equivalent FF lens is 27-88 f4.5, shot at 1.6 times higher iso than you would use on a non FF. And as long as the resolution of a FF lense is not worse to an EFS counterpart than 1/1.6 (in terms of lines per picture height) tested on the same camera- the FF lense will resolve more on a FF system than the EFS lense. So even a 24-105 f4 on a FF system beats the 17-55 f2.8 hands down when it comes to resolution.


This is not correct a 17-55 f2.8 lens appears to be a 27-88 f2.8 on APS-C, the f number does not change, put an EF lens on an APS-C body and the f number remains the same, the same light per unit area falls on the sensor from the same lens, you are simply sampling a smaller area of the image, it is still the same image projected from the lens as you would get on a full frame camera.

Only thing is that changes is the apparent focal length simply because you are looking at a smaller section of the image.

edit: sorry I should have made it clear that I meant if you had a 17-55 EF lens it would appear to be a 27-88 on APS-C, such a lens does not exist in the Canon range the closest would be the 17-40 L. the principle still applies f number does not change.

I am not clear what you meant here. 17-40L will still be 17-40mm focus length on FF and Canon also has EF 16-35/2.8 lens. Still 24-105L/4.0 IS on FF is the closest to EF-S 17-55/2.8 IS on APS-C that is equivalent to 28-88mm FOV on FF but only longer and wider. Sure f/2.8 is still f/2.8 no matter on which crop format. But FF sensor is twice bigger than APS-C sensor size, collects twice more lights that effectively compensate the difference between f2.8 and f4.0 while still enjoys or suffers a bit shallower DOF.

Also don't forget EF 24-70L/2.8 mark I and II, and older EF 28-70L/2.8.

-- hide signature --

I phrased my reply badly, I used the 17-55 as the example as that had been mentioned by the previous poster, I then referred to it as if it were an EF lens, I would have been better using one of the lenses you mention as the example.

Just ignore me, it appears Y Hafting was on about apparent DOF, not aperture as such.

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