Does using a FF lens with adapter lose light?

Started Feb 14, 2013 | Discussions thread
viking79
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Re: Does using a FF lens with adapter lose light?
In reply to dpyy, Feb 14, 2013

dpyy wrote:

It has been mentioned a few times here on the forum that if you use a FF A lens with one of the EA adapters you are basically going to crop the image. As a result, you really have to move your location to frame the subject the same way, effectively "changing" the focal length. I think I understood that correctly.

However, what I'm not sure is, wouldn't this mean you are losing light through this cropping step? If so, would it total up to something significant? Say one stop of light? If my guess is right, does that mean if you use a 80mm/2.8 with EA2 you are really "effectively" using a 50mm/4.0 at the widest aperture?



Light shining through diffusor on front of lens mounted at flange distance from paper sensor models.



Even smaller image circle (not quite centered), Sony has been tending to run the image circle on the smaller side (this is from a Samsung lens)

The inner rectangle is approximately APS-C sized, the outer rectangle approximately Full Frame size. This lens happens to be an APS-C lens so it doesn't quite cover full frame.

The point being is that if you shoot full frame, the total light hitting the sensor is greater than the APS-C sensor. The intensity is the same at the same aperture.

This is why the APS-C is considered a crop sensor, it is exactly the same as cropping away part of the full frame image.

To correct for this, you adjust by the crop factor, 1.52 or so for Sony APS-C, and so a 35mm lens will be fairly close to a 50mm lens on full frame for field of view. However, a 50mm has shallower depth of field at the same aperture as a 35mm, so to get the same composition depth of field you also need to adjust the aperture by the crop factor, so if you were at f/1.8 you would take that * 1.52 which is about f/2.8. However, if you do this, the light intensity on the full frame camera is now 1 and 1/3 or so stops darker than the APS-C. To compensate for this you also need to adjust the ISO by the same amount, so if you were at 100 on the APS-C you would adjust to 250 on the full frame. Since you don't have to enlarge the full frame image as much to reach some screen or print size, the noise is the same at the image level between the two (given similar technology sensors, e.g. Sony 16 MP 4/3" sensor, Sony 16/24 MP APS-C sensor, or Sony 24/36 MP full frame sensor).

Eric

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