LX7 - does its IQ compete with e.g. Sony RX100?

Started Apr 6, 2013 | Discussions thread
kkardster Veteran Member • Posts: 9,132
Re: LX7 - does its IQ compete with e.g. Sony RX100?

tedandtricia wrote:

kkardster wrote:

ultimitsu wrote:

kkardster wrote:

The sensor specs favor the RX100 but the brighter and sharper LX7 lens often makes up for its lesser sensor.

This is a common misconception.

LX7 does not have a faster lens.

LX7's lens is 4.7mm F1.4, aperture is 3.6mm in diameter, actual opening is 10.2mm2

RX100's lens is 10mm F1.8, aperture is 5.6mm in diameter, actual opening is 24.6mm2

RX100's lens lets in 150% more light. it is much much brighter.

It appears to me that you have the misconception here.  Basic photography has taught us that f/1.4 is faster than f/1.8, period - it's a basic photographic quality.

I don't have the time nor specs to challenge your calculations, so I'll take them at face value.  However, I find that your equations are incomplete, leaving out the fact that the larger RX100 opening needs to feed its additional light across a larger sensor area.

I can't challenge or confirm ultimisu's concept directly but it seems like the reason f stop is sufficient to describe "brightness" may be due to optic physics and something called the inverse square law. I tried googling for a good tutorial on how focal length, aperture diameter and sensor size all interact, together with the inverse square law, but came up with nothing really good.

This article mentions the inverse square law but not in a way that I found satisfactory.

I guess my problem is with ultimisu's assertion that a bigger opening is brighter.  Take a 32-80mm zoom lens, for example.  The lens diameter is obviously fixed, and for this exercise lets fix the f-number as well to f/8.0.  As move from 32mm to 80mm, the size of the lens opening increases from a diameter of 4mm to 10mm - yet the lens "brightness" remains a constant f/8.0 even though the opening area changes from 12.6 sq/mm to 78.5 sq/mm!

The fact that the minimum f-number was used in the original calculations to determine opening diameters makes the final "actual opening" size irrelevant as the LX7 opening calculation pertains to f/1.4 while the RX100 opening calculation pertains to f/1.8 - somewhat of an apples-to-oranges comparison.

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