[Leica] Digital cameras with large aperture lenses

[s.jessurun95 at chello.nl (animal)]

----- Original Message ----- 
From: "Henning Wulff" <henningw@archiphoto.com>
To: "Leica Users Group" <lug@leica-users.org>
Sent: Sunday, March 20, 2005 2:07 AM
Subject: Re: [Leica] Digital cameras with large aperture lenses


> At 12:21 PM -0800 3/19/05, Adam Bridge wrote:
>>Okay - I STILL don't understand what you say is happening.
>>
>>I can understand that less light may be captured by the microlenses
>>over the sensor elements located at the edges of the sensors because
>>the lenses, I suppose, are designed for light arriving perpendicular
>>to the plane of the sensor when, in fact, they are not.
>>
>>BUT  how does this affect what's going on in the middle of the sensor?
>>It doesn't make any sense to me at all. If you open from f2 to f1.4
>>you should receive twice the number of photons arriving at the
>>micro-lens (since the lens works identically as that for film up to
>>this point.)
>
> First, think of the image sensors being at the bottom of a small well, so 
> that it has trouble seeing anything except what is in a narrow angle 
> directly above them. The microlenses try to alleviate this by providing a 
> wider angle of view, but their efficiency falls off the greater the angle 
> of the incident light off the perpendicular.
>
> Now think of a lens with a large aperture, that has its rear nodal point 
> close to the censor. For the sake of this argument, consider the rear 
> nodal point as the center of the rear element, and the rear nodal plane as 
> being the cross section of the rear lens parallel to the sensor. The 
> diameter of the rear element is the same as the diagonal of the sensor in 
> this lens. Now: a ray that hits the very corner of the sensor, coming from 
> the center of the rear nodal point, arrives at an angle say, 45 degrees 
> from the perpendicular. As it has to make a 45 degree turn at the entrance 
> of the sensor 'well' to make it down through the filters and into the 
> sensor, a lot of light is lost and vignetting results, right?
>
> Now, imagine that you are using the same lens wide open to shoot 
> something, and therefore some of the rays go through the middle of the 
> lens and hit the central sensor well (middle pixel of the image) head on. 
> Optimal usage of the light. However, with a very fast lens wide open, over 
> _half_ of the light comes from the outer 1/3 of the lens's diameter (you 
> can check the geometry). This means that a lot of the light is expected to 
> go through the outer zone of the lens, make a 40 to 45 degree turn through 
> refraction of the lens, and then enter that central sensor well site by 
> making another 40 to 45 degree turn, which it finds just as difficult to 
> do as the previous ray trying to get to the corners. The sensor can't make 
> very efficient use of this light, and instead of getting one full 
> additional stop's worth of light, it only gets 50% or even less of that.
>
> I hope I'm making this clearer, and not muddier. :-)
>
> Anyway, it's mostly just geometry, and if you draw it out on paper, it 
> might become clearer. Remember, it's the rays that have to make a sharp 
> turn that have trouble, and in the end don't contribute much to the light 
> gathering. Rays come from the full rear nodal plane in a real lens, and 
> the closer the rear nodal plane gets to the sensor, and the larger the 
> rear nodal plane is, the more rays have to make sharp turns to get into 
> the sensor wells.
>
> In so-called 'telecentric' lenses, or specifically the ones that are 
> closer to the telecentric ideal in image space, the light rays are all 
> more parallel, and as far as the sensor is concerned, come from a greater 
> distance (a rear nodal point and plane far from the sensor) so that sharp 
> angles are no longer involved at the sensor plane. They rays all head 
> 'straight in', and are no longer required to make a sharp turn.
>
>>So you are saying that something non-linear is happening at this
>>point. What would this be? Is the reflection at the air/microlens
>>interface involved? The microlens/sensor interface? It's about
>>photons, right? I can't believe we've dropped into the quantum realm
>>where QED explainations are required in order to understand the
>>interactions. But I sure could be wrong.
>>
>>anyway - I'd like to understand the physics and so far it's all
>>hand-waving and assertions.
>>
>>There must be a way to do some science here - but I think the camera
>>software, even in RAW mode, tends to cover up any effects that might
>>be happening.
>>
>>I'm really curious and dumbfounded - I'm not doing this for the sake
>>of argument.
>>
>>Adam
>>_______________________________________________
>>Leica Users Group.
>>See http://leica-users.org/mailman/listinfo/lug for more information
>
>
> -- 
>    *            Henning J. Wulff
>   /|\      Wulff Photography & Design
>  /###\   mailto:henningw@archiphoto.com
>  |[ ]|     http://www.archiphoto.com
> _______________________________________________
>
Hi Henning i can visualise the sharp turn of the rays and the bouncing 
around in the little well .just forgot where the energy goes.
if the angle of attack between the ray and the surface it hit,s first is the 
same for the center as the edge of the array lets presume the angle of 
attack is ninety degrees for simplicity then all photons will arrive at the 
bottom of the well or not?
i understand they will arrive slightly later.

Still as i replied to Don,s post my reason for starting this thread was 
merely to point out that large aperture optical systems are built and that 
they use digital sensors.
and i would be very disappointed if my noctilux changes into a F4 with the 
digital M
thanks for your reply
simon