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

Started 9 months ago | Discussions thread
Great Bustard
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Re: Cha-ching!
In reply to Ontario Gone, 9 months ago

Ontario Gone wrote:

I have a very technical question for you. I know i could easily just go by the MFT is -2 stops to FF, but i prefer to understand the logistics of things. I will try to keep this as clear as possible

If i take a FF lens and use it on a D800, then the same lens on a GH3, speaking only about light gathering and SS/ISO (DOF isn't part of this question), plz explain what is going on here. It was my impression that the image circle coming through the lens is suited to just fit a FF sensor, 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?

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. 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?

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?

I read your earlier post and im not sure about the terms. Does the entrance pupil equate to overall light gathered, or is that what the F stop signifies? I know in telescopes, most people don't mention F stop, it's the objective lens that determines the amount of light gathered. Why do lenses not work that way? Or do they?

Correct this thought if it's wrong: F2 on a 10mm lens will yield the same SS as F2 on a 300mm lens. Since that is so, can we assume the total amount of light gathered is also exactly the same? If so, then aperture isn't as important, since the F2 on 300mm is 150mm, vs 5mm on the other. When i picture a 50mm F1 FF lens, and a 50mm F1 MFT lens, the FF lens would be physically bigger. The elements would be bigger, and the aperture would be bigger no? This makes the F stop equation misleading, because according to the math, they are both 50mm in aperture. So what part of the lens does that 50mm refer to?

I believe most all of your questions are answered here:

http://www.josephjamesphotography.com/equivalence/index.htm#exposure

Please give that a read first, and then let me know if you still have questions on the matter.

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