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[quote name='Yakim' timestamp='1331675905' post='16681']

I remember a test I did about a year ago. I had a 5D with 100/2.8 and 7D with 60/2.8. I set both cameras to the same ISO and used Av at f/2.8. I pointed both to an evenly lit wall and shot with both from the same spot. Result, SS was the same. Apparently I was obscured. <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='' />



But I do not wish to quarrel with you. I know I can never win no matter what I say.

[/quote]

No, you just simply are not getting what we are talking about.



I have explained quite clearly that ISO in its own is quite meaningless. It just equalizes differences in sensor sensitivity, to make it easier for the user of the camera.



Of course, when you make cameras have different amplifications for a certain ISO setting so that they will "need" a similar exposure time, of course all lenses with a similar t-value will give you similar exposure times.



But what does that in any way have to do with equivalence? Nothing.



Now lets examine your two combinations.

The 60mm was on a camera with a 1.6x crop factor, when we take the 135 format as benchmark.

That is equivalent to 96mm FF, it will give the same field of view as 96mm focal length on FF.

I guess we can say 96mm is close enough to 100mm to assume equivalence here.



So, that covers field of view.



Now the other defining element for the image: DOF.

DOF is mostly determined by aperture size.

What is the aperture you used on the APS-C + 60mm combination?

60 / 2.8 = 21.4mm

What f-value will be equivalent on the FF combination?

100 / 21.4 = f4.67

(you can also just multiply the f-value by 1.6... faster way. by the way, note that the f-value is a bit higher than if the focal lengths were totally equivalent... so we will just take f4.5 as equivalent).



That covers similar field of view AND similar DOF. We will get the same photo with both cameras now (more or less).

Now if we want to also get the same exposure time, we also have to set equivalent ISO settings.

So you should set the FF camera's ISO 1 1/3rd stop up compared to the APS-C camera. SO something like ISO 100 on the APS-C and ISO 250 on the FF.



Now the field of view is similar, the DOF is similar, the exposure time will be the same.



When we are talking about equivalent lenses... and you start to mention f2.8 for both.... you simply do not get the concept. If you were right and I was wrong, of course you could win. Everyone can learn, including me. But you are right in that with this, you can not win. Because I am not wrong here. It is simply very simple laws of physics and optics.