So, what exactly does shooting at f/2 bring you?

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bobn2
OP bobn2 Forum Pro • Posts: 65,596
Re: So, what exactly does shooting at f/2 bring you?
1

OM-G wrote:

bobn2 wrote:

OM-G wrote:

Low F-number is always good for AF.

There is no automatic advantage to low light AF for larger sensor.
AF has more to do with light intensity per area, and in that regard F2=F2.

I don't think you're right, for any kind of AF. Please explain why you believe this to be the case.

Example: A 1/2.3" sensor will focus fine at F5.6, but you will have to look hard to find a FF camera that will focus past F8, and then just in the centre. The equivalent aperture on FF to the mentioned F5.6 is F32. You wont find any camera that will focus at F32. On a 0.5x medium format the equivalent aperture would be F64 and the sensor would be totally in the dark.

You're confusing different things here. You're essentially comparing DSLR focus modules with CDAF. CDAF doesn't have any f-number restriction, DSLR focus modules do, because they are designed to be bale to make a precise measure of subject distance, which means they take their light from a very small patch of the exit pupil. It is that design for discrimination which leads to the f-number limit, not an amount of light.

A mFT camera with a F2 lens will have around a 2 stop advantage in low light AF compared to an equivalent FF F4 lens.

Example:

Focus sensitivity for the Sony A7S II is EV-4 (ISO 100 equivalent with F2.0 lens)
Focus sensitivity for the Olympus EM1 III is EV-4.5 (ISO 100 equivalent with F2.0 lens)

You can't tell much from claimed figures. It's marketing spin.

Where do you get this from? Sony has a EV-3 for metering on the mentioned camera in the specs, and EV-3 AF for the A7RIV. Do you not think they measure this? Why not put EV-6 on all cameras if its just marketing spin? You usually have to dig in to specs to find these numbers.

By and large,  manufacturers marketing claims cannot be directly compared, you don't know that they are using the same methods to determine. You can't really base any rational argument on what is essentially advertising material.

Same thing applies to exposure metering.

I don't believe that is the case, either. please explain your reasoning.

Se above, or Sony specs.

That's not reasoning, it's a single, very poor example.

The point is that you cant just take a FF system with a 2-stop slower lens and think its equivalent in focus and metering. An eqivalent FF lens in this regard would have to be F2 and way bigger.

Since you're just presenting you're beliefs as fact, there's not much I can do except say 'I think you're wrong'. If you provided some theory, reasoning and evidence to back up your statement, perhaps we could discuss it.

I think if you see the specs and compare over more than two stops (mFT/FF), you might see that light intensity plays an important part, nom the size of the sensor.

I prefer to work on knowledge rather than hunches based on selection of examples that fits the hunch.

Here is an old blogpost from lensrentals

https://www.lensrentals.com/blog/2010/07/how-autofocus-often-works/

Look at the part on aperture on phase detection sensors

"Effect of Lens Aperture

No matter what the sensor type, however, it will usually be more accurate with a wider aperture lens."

It's wrong, so far as DSLR AF sensors are concerned. Look up Marianne Oelund's excellent series on these forums explaining just how DSLR AF works. As above, it's highly selectve, so a smaller f-number than it was designed to work with does not give an advantage, it just can't 'see' the extra light.

For on-sensor PDAF and CDAF, that doesn't apply, they aren't selective in the same way. For CDAF the more 'total light' the better - not the f-number. For on-sensor PDAF, the alerger th aperture (as opposed to the lower the f-number) the better, since that gives it a wider baseline.

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