EF-S Aperture vs EF Aperture

[Sponge1971]

Hi all,

I was comparing an EF vs EF-S lens (I've got the EF-S and love it...my buddy is still deciding between...) (We've both got APS sensors.)

After a revelation that I have to do the 1.6 multiplication on both EF and EF-S lenses (right?  ) I got to thinking:

Since the EF-S lens design is made for APS sensors and EF lens design is for FF sensors...the end result is the same-ish.  A properly sized cone of light falling on the appropriately sized sensor.  But what about the physical aperture size?

My assumptions:

The EF-S lens has an f2.8 hole diameter suited to an APS sensor, to get the 2.8 ratio.
-and-
The EF lens has an f2.8 hole diameter suited to a FF sensor, to get the 2.8 ratio.
-so-
Since the FF sensor is bigger than the APS sensor...the Ef lens has a physicaly bigger diameter hole, than the EF-S (even though they both say f2.8 )
-so-
The EF-S lens has a physically smaller hole, than the EF lens.
-and-
I'm pretty sure the front glass diameters are sized accordingly too.


Now, let's pretend that we 'can' put an EF-S lens on a FF body:
I say, the EF-S lens would not be f2.8 on the FF sensor, it's too small a hole.  It would be more like f4, let's say.

However, we can (and do) put EF lenses on APS bodies.

Does this mean that an EF f2.8 lens is actualy faster, say f2 on an APS body? (I'm assuming the lens just get's told to go wide open, by the camera body)  the EF f2.8 hole is bigger than the EF-S 2.8 hole, so there should be more brightness? available...even if the APS sensor only sees the center of the light cone.
-or-
Does the APS body automaticaly stop the aperture down to the APS-sized f2.8 diameter...wasting the extra available opening)?
-or-
Am I way off base and forgetting something about/not fully understanding the "APS only sees the central part of the lens cone" truth?


I'm sure the actual math would show this isn't a huge deal, but I'm just curious.  If it was a big enough difference, it would sway my buddy towards the EF lenses.  He shoots in 'dark-ish' hockey arenas and needs every micron of aperture diameter he can get.

Anyway, hope I made sense here.  Thanks for the input!

Eric

p.s.
as a side note, depending on the reality/answer.
When I look at the EXIF specs on a photo (EF lens on an APS body), it says: 70mm @ f2.8....not that it matters....but is that true?  or is it really 112mm @ F2.0?  I'm pretty sure the EXIF is just recording what the lens says it's set to.  Another one to ponder...

[Bob Atkins]

The physical aperture is constant.  An f2 lens is an f2 lens is an f2 lens. Doesn't matter what you mount it on, it's always f2

The best way to think about it is this. The lens projects an image of a certain brightness onto the focal plane. That brightness is determined by the physical size of the aperture and the diatance from that plane (the f-stop). Obviously for a given lens, that's a constant.

It doesn't matter what size film or digital sensor you put in that plane to record the image. You can put an APS-C sized sensor or you can put a sheet of 8x10 film. They both see an image of the same brightness (i.e same lens speed). Of course the difference is with the 8x10 film you get a small image in the middle surrounded by lots of black whereas the image fills the APS-C frame.

I think where you have gone wrong is assumiing that he physical size of the aperture changes between an EF-S lens and an EF lens. it doesn't. A 100mm f2 lens designed for APS-C has to have an aperture of 50mm. A 100mm f2 lens designed for full frame has to have an aperture of 50mm. Exectly the same. The speed of the lens is the focal length divided by the aperture. Format size doesn't come into it.

So why can EF-S lenses be smaller? Because a 100mm lens on full frame covers the same angular view as a 62.5mm lens on APS-C (the 1.6x multiplier comes in here). On the full frame camera an f2 lens still needs the 50mm apertrure, while on the APS-C camera the f2 lens now needs an aperture of 32.25mm. - But the focal lengths are different! The field of view is the same because of the differnt format size, but the focal lengths are different, so the physical aperture is different. That's why you can get away with a physically smaller lens. Not because the "virtual multipler" affects aperture, but because it affects focal length.

EXIF records true focal length. If you put a 100mm lens on a full frame camera it recoreds 100mm. If you put the same lens on an APS-C camera, it records 100mm, becaise the lens still has a focal length of 100mm. If EXIF recored the angular field of view, then that would be different on full frame and APS-C bodies.

You might want to read http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html which goes into a more detailed explanation of crop sensors and lenses

[Sponge1971]

Phew...that was a brainfull...thanks Bob!

I should have looked more into the definition of 'aperture' before making that assumption...

Shucks...and here I thought I'd made some great discovery, that every expert had overlooked <sigh>...foiled again. 

Sorry for your having to write so much, to make your point...you shoulda just smacked me upside the head and directed me to a post where you've probably already explained this...for the third time.  Thanks though!  I'll read that link tomorrow!



psssst....between you'n me......do you think there's a hope in hell that Canon is going to make a big-brother to the EF-S 17-55....or should I just give in and scoop up an EF 70-200 IS f2.8, as an early xmas present?
I'm so disapointed, I've been waiting since spring...hoping Photokina would unveil some super duper EF-S 55-250 IS f2.8....but no....instead they went for the super zoom. (not that I mind too much...I kinda want it for my wife's XSI anyway...but the Tamrom 18-270 is looking pretty darn cool too)....

Thanks again,
Eric