MFT Users: Do you miss the shallower depth-of-field of bigger sensor cameras?

Started 5 months ago | Discussions thread
Heyseuss Hoolio
Regular MemberPosts: 336
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Re: Light gathering, aperture, and DOF all go hand-in-hand.
In reply to Great Bustard, 5 months ago

Great Bustard wrote:

MrScorpio wrote:

Sometimes yes, but sometimes it is good to have the good light gathering with a large Aperture without having to have the very shallow DoF.

This is a profound misunderstanding that many have, which causes a great deal of confusion. First of all, we need to distinguish between the relative aperture (f-ratio) and the virtual aperture (entrance pupil), where the relative aperture is the quotient of the focal length and the diameter of the virtual aperture. For example, a 25mm lens with an 18mm aperture diameter will have a relative aperture of 25mm / 18mm = 1.4. Likewise, a 50mm lens with an 18mm aperture diameter will have a relative aperture of 50mm / 18mm = 2.8. Thus, 25mm f/1.4 and 50mm f/2.8 both have the same aperture diameter.

As it turns out, for a given perspective, framing, and display size of the photo, the same aperture diameter results in the same DOF. If we also include the same scene luminance and shutter speed, it also results in the same total amount of light falling on the sensor, which, in turn, will result in the same noise for equally efficient sensors.

In other words, your statement that "sometimes it is good to have the good light gathering with a large Aperture without having to have the very shallow DoF" is a physical contradiction. Specifically, 25mm f/1.4 does not have a "larger aperture" than 50mm f/2.8 -- in fact, they are the same. Of course, f/1.4 is a lower f-ratio than f/2.8, but that is neither here nor there in terms of cross-format comparisons.

Yes what you're saying is partly true and the concept/idea is being looked at the wrong way. Aperture diameter is different than f/stop. I can see you know that. And that the light hitting the sensor formats as a whole is the same. Although, that same concept is what changes the intensity of light that hits the sensor surface itself.

The f/stop is the amount of concentrated(focused) light hitting the sensor (m43 or FF). If you crop your FF 50mm f2.8 in the center with a m43 sensor crop, it's still f2.8 on the m43 sensor crop. That's why if you adapt a FF lens to a m43 body the f/stop is still the same, unless you have a speed booster which takes the uncropped light and concentrates it.

Unlike uncropping, you wouldn't want to add a m43 lens to a FF body because all that concentrated light at f1.8 that is being focused onto a m43 sensor will need to expand onto a FF sensor size. For a far fetched example, you add an adapter to the m43 lens to project on a larger area, if you can get the light circle to cover a FF sensor, though that same f1.8 light will be spread out larger reducing it's concentrated intensity, less light on the surface, darker f/stop.

"So, if we took a photo of a scene at 25mm f/1.4 1/100 on mFT and 50mm f/2.8 1/100 on FF from the same position, and displayed the photos at the same size, they would have the same DOF and the same amount of light would fall on the mFT and FF sensors, resulting in the same noise if the sensors were equally efficient."

No, not necessarily, I think I see what you're saying but it can't be presented in that manner, it's not as simple as that and there's factors that are at play; there's crop factor, there's distance to focal point, there's ISO(if you wanna go there, but we're just talking about light). Yes they have the same "aperture diameter" but the intensity of light is different across the sensor format as a whole. Put a squarish piece of tape on the wall, take a flashlight and hold it over the tape to cover the corners with the light, it has concentrated brightness, now keep stepping backwards while pointing at the same area. The projection of light gets bigger but that center you were shining on before is getting dimmer, that's f/stop (luminosity) on that square, not solely aperture diameter decisive.

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