Potential dead horse: how bad is FF's deep DoF disadvantage?

Started Mar 5, 2014 | Discussions thread
Erik Magnuson Forum Pro • Posts: 12,247
Re: Cha-ching!

Ontario Gone wrote:

It was my impression that the image circle coming through the lens is suited to just fit a FF sensor,

It's usually larger then "just fits" to avoid vignetting in the corners.

so when the MFT sensor is back there, a good chunk of that light hits outside the MFT sensor, i assume bouncing around inside the lens?

No, all of that light still exits the lens and goes into the adapter and sensor chamber.  Hopefully such areas are designed to not bounce light around.

Assuming this is correct, MFT camera makers set the gain for each ISO stop two stops higher than FF, that way at a certain ISO/aperture you get close to the same SS on both systems.

No, the light per unit area is the same - just different areas.

Here is what i don't get. If that lens is on the D800, and is a 50mm F1, then the aperture is 50mm (when i say aperture, i mean wide open for sake of simplicity). When i put that lens on the GH3, what is the effective aperture? Since some of that light is landing outside of the sensor, it can't possibly still be 50mm no?

The lens is still 50mm but the angle-of-view intercepted by the smaller sensor is similar to the angle-of-view of a 100mm lens on the D800.  The aperture is still f/1.  But to shoot the same scene with the same framing, you have to back up quite a bit with the GH3 which changes the camera-to-subject distance which changes the DOF.

Now take a 50mm MFT lens, also F1. If we do the math, both the FF lens on a D800 and the MFT lens on a GH3 come out to a 50mm aperture (50/1=50). I know your earlier post said if DOF, subject distance, and FOV are even, then aperture is equal too correct?


But what if FOV isn't the same, since a 50mm FL on a MFT is actually 100mm. In other words, is the F stop equation accurate between systems for finding the actual total light gathered by the sensor?

If FOV is not the same, it's a different image.  To make it the same image, you could crop out a 17.5x13mm area from the D800 sensor.  Obviously by cropping, you are losing some of the "total light" gathered by the sensor.

Correct this thought if it's wrong: F2 on a 10mm lens will yield the same SS as F2 on a 300mm lens.

The issue is over what area of coverage.

Since that is so, can we assume the total amount of light gathered is also exactly the same?

Not unless they have exactly the same image circle or you are sampling exactly the same area.  If you are not capturing it on a sensor, then the "light gathered" is lost.

When i picture a 50mm F1 FF lens, and a 50mm F1 MFT lens, the FF lens would be physically bigger.

Not a lot of easy direct comparisons but, no. Not unless they require very different lens designs.

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