50mm and Q2

[M11 for me]

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You are completely right that a lens keeps its focal length and aperture when changing from one camera to another with different sensor size. This was never questioned by anyone and by me not either. Its the result that differs though. And if one asks by how much it differs then equivalence might be very helpul in the understanding the effects. And in order to help us as consumers most manufacturers (incl. Leica) indicate in the technical specs of their lenses  the "ff equivalent". 

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[jaapv]

A conversion for convenience with no technical consequences. On my MFT cameras it would be a "70 mm" lens, on a medium format (not really possible) a "15 to 10"mm one. Both are meaningless. The angle of view is what counts. Nor does  the aperture change. Leica has stopped indicating the equivalent focal length on their lenses. With the Digilux2 they still used equivalency.

Changes in grain/noise, resolution and apparent DOF can only be defined through the changing angle of view, if it is compensated by corresponding magnification.

The point is that the whole light-gathering stuff makes a complete mess out of a simple phenomenon.

[danielmfrank]

On 12/30/2019 at 12:29 AM, jaapv said:

Please, not that "light gathering" stuff again. A 1.7 lens remains a 1.7 lens, whatever crop you apply. The light per square mm will remain the same, and the exposure will remain the same. The sensor noise is the only real difference, and can be better explained by the difference in pixel count/relative pixel size. The difference in DOF is created by the difference in magnification through the system. If you print the full frame image large and crop with a pair of scissors you will get an identical result to a print to the smaller size that has been cropped on the sensor or in the computer.

I stand corrected on that point. Thanks for clarifying, Jaap. This stuff can be a little confusing.

[jay968]

Hate to revive an older thread like this but Jaapv is soooo right. I really get so annoyed that so many people have fallen for stuff written by bloggers who really have no idea what they are talking about. This entire "equivalence" discussion has done so much harm to proper understand of how sensor size and aperture relate. A 28mm f1.7 is a 28mm f1.7 lens no matter what the sensor size. Changing that sensor does no in any way shape or form change the resulting exposure nor the focal length of the lens. It just CROPS the image giving the IMPRESSION that a different focal length lens was used.

I wrote to a very well known blogger about his misconception. He always claims that since a larger sensor has more area, the result is more light having been gathered, so a smaller aperture must be used to get equivalent exposure. What he doesn't realize is that when talking about light gathering, it's the light per sq mm that's important, not the amount of light collected outside the image area. This light per sq mm cannot possibly change when changing the size of a sensor.

Again, sorry to rehash this but I was looking for some info and stumbled upon this thread. I couldn't resist.

Edited May 3, 2020 by jay968

[RustyBug]

Light per sq. mm ... that's the physics involved. 

Aperture and f-stop are a "photographer's" lingo to simplify the terminology of physics of how much light is being projected by the lens projection ,,, OVER A GIVEN AREA (i.e. the image circle coverage of the format).  The projection remains the same (for a given lens) whether it is mounted on FF or APS-C.  Here, one is simply making a crop of the projection.

OTOH ... a TL designed lens is designed to cover a smaller sensor area, thus requiring a smaller image circle projection.  Hang on, cuz here's where it gets tricky for some folks.  Mind you that when the sensor area is reduced, a lens DESIGNED for the smaller area (APS-C), projects a smaller image circle, vs mounting a lens designed for a larger (FF).  Now, for a given physical / optical aperture SIZE (i.e. mm), the ratio of that aperture size (which is an area ... diameter * pi) to the image circle / sensor size (which is an area) is what yields the f/stop number (ratio) that we refer to as the aperture.  It is a ratio of the area from which the light is passing through the lens, to the area upon which the light is being distributed.  This is the ratio of "light gathering", "light passing", "light distribution".  Taking a central crop, doesn't change the density of light (amount / area) to somehow change the f/stop ratio.

 

Because the f/stop number already is a RATIO, representing the density of light being projected onto the film plane, that density (vignetting notwithstanding, as another aspect of optical projection ISL) does not change from the central portion of a cropped area.  For a given designed projection area (MF, FF, APS-C) the math has already be assigned to indicate the ratio of light for the given distance (ISL influence on distribution) and area of coverage for that format (which is also influenced by the distance from the exit to the film plane, per ISL vs. lens format size, i.e. larger physical, still = same f/stop ratio).

 

Alternatively, as the f/stop is a ratio, it is derived from a measurement of the aperture diameter (optical).  That distance, relative to the focal length, relative to subject distance (magnification) now becomes an indicator of how the DOF will correlate.  All that to say that when you REALLY get down to the making the physics involved actually equivalent ... ummm, then things are equivalent.  Much of the internet discussion about equivalency are attempts to take photographic terminology and apply it without truly understanding the physics from which the photographic terminology is derived.

Then what happens is you get a smattering of folks who have never understood all the pieces of the math, trying to sway others to the differences through half-equivalent efforts to describe what is / isn't happening.  For those who don't understand the physics, they bite.  For those who do understand the physics, they contest, the fallacies and omissions involved.

Long story short ... mount it, shoot it.  Like it, keep it.  Don't like it, don't keep it. 😉

I can tell you more about the physics of why I shoot MF on FF or when I choose to shoot m43 or APS-C ... but, you'd much rather enjoy shooting than listening to more of me.  

 

Oh, and as to the Q2 vs. CL ... they should make a great pair.  That's what I'm saving some pennies for.

 

Edited March 28, 2021 by RustyBug

[Coppereye1960]

On 3/28/2021 at 7:14 PM, RustyBug said:

Light per sq. mm ... that's the physics involved. 

Aperture and f-stop are a "photographer's" lingo to simplify the terminology of physics of how much light is being projected by the lens projection ,,, OVER A GIVEN AREA (i.e. the image circle coverage of the format).  The projection remains the same (for a given lens) whether it is mounted on FF or APS-C.  Here, one is simply making a crop of the projection.

OTOH ... a TL designed lens is designed to cover a smaller sensor area, thus requiring a smaller image circle projection.  Hang on, cuz here's where it gets tricky for some folks.  Mind you that when the sensor area is reduced, a lens DESIGNED for the smaller area (APS-C), projects a smaller image circle, vs mounting a lens designed for a larger (FF).  Now, for a given physical / optical aperture SIZE (i.e. mm), the ratio of that aperture size (which is an area ... diameter * pi) to the image circle / sensor size (which is an area) is what yields the f/stop number (ratio) that we refer to as the aperture.  It is a ratio of the area from which the light is passing through the lens, to the area upon which the light is being distributed.  This is the ratio of "light gathering", "light passing", "light distribution".  Taking a central crop, doesn't change the density of light (amount / area) to somehow change the f/stop ratio.

 

Because the f/stop number already is a RATIO, representing the density of light being projected onto the film plane, that density (vignetting notwithstanding, as another aspect of optical projection ISL) does not change from the central portion of a cropped area.  For a given designed projection area (MF, FF, APS-C) the math has already be assigned to indicate the ratio of light for the given distance (ISL influence on distribution) and area of coverage for that format (which is also influenced by the distance from the exit to the film plane, per ISL vs. lens format size, i.e. larger physical, still = same f/stop ratio).

 

Alternatively, as the f/stop is a ratio, it is derived from a measurement of the aperture diameter (optical).  That distance, relative to the focal length, relative to subject distance (magnification) now becomes an indicator of how the DOF will correlate.  All that to say that when you REALLY get down to the making the physics involved actually equivalent ... ummm, then things are equivalent.  Much of the internet discussion about equivalency are attempts to take photographic terminology and apply it without truly understanding the physics from which the photographic terminology is derived.

Then what happens is you get a smattering of folks who have never understood all the pieces of the math, trying to sway others to the differences through half-equivalent efforts to describe what is / isn't happening.  For those who don't understand the physics, they bite.  For those who do understand the physics, they contest, the fallacies and omissions involved.

Long story short ... mount it, shoot it.  Like it, keep it.  Don't like it, don't keep it. 😉

I can tell you more about the physics of why I shoot MF on FF or when I choose to shoot m43 or APS-C ... but, you'd much rather enjoy shooting than listening to more of me.  

 

Oh, and as to the Q2 vs. CL ... they should make a great pair.  That's what I'm saving some pennies for.

 

Thank you for pointing out the pomposity over this issue.  At the end of the day Leica cameras and lenses are made to make pictures.  We choose them because we like the Image Quality / attention to detail you need.  
 

as my old boss ( Yorkshireman ) would say the “rest is just propaganda”

[M11 for me]

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So the equation would be propaganda = maths? Something went wrong here. I think that maths brings the answer to the confusion. For the ones who do not think that its usefull to understand just leave it appart. Nothing wrong and it does not affect the quality of your images.