Lens Basics

[James Malamatinas]

Hi,

 

I've been a camera assistant for a few years now and understand the effect of lens concepts e.g. opening up decreases DoF, however I'm not trying to understand the why, and the mechanics of a lens and how it affects the image. Can someone please help me understand a few basics, I'm going to try and understand them one at a time so my first question is below:

1. Why does changing the aperture not affect the angle of view?

I have attached a diagram from a book I'm reading showing light travelling in a lens with a wide open aperture, and also in a stopped down aperture.

 

What happens to the rays of light that I have highlighted in red, in the original diagram these lines do not exist and instead the light is shown as simply passing through the smaller aperture, however if the aperture is not affecting the actual focusing ability of the lens there must still be rays of light being refracted along these lines. What happens to these rays of light that are now hitting the closed down aperture? If these were contributing to the final image wide-open, why does the image not change when stopped down?

 

I'm sure I've made it sound more complicated than it is with that description!

 

Thanks, I'm sure they'll be plenty more to follow! Any suggestion on places to read up on the basics of lens mechanisms would be great too.

James

[Gregory Irwin]

Hi James,

I've been an First AC for a very long time and have a pretty good comprehension for optics. Let's begin by saying that the aperture has nothing to do with angle of view. When we speak of aperture, we are talking about the iris blades that control the amount of light and the shape of the path of the light passing through groups of glass elements to a measured film plane (or sensor). The construction of those element groups create the angle of view of the "taking lens", not the aperture.

 

You are correct regarding how the aperture setting affects DOF. But what it also affects is exactly how the light path is guided through the lens barrel. I'm going to generalize this for the purposes of brevity: Wide open apertures allow the light to refract and scatter more causing issues like internal flare (image looks milky; blacks are grey), chromatic and/or spherical aberrations (distortion) and possibly astigmatism. It can also adversly affect the DEPTH OF FOCUS, not field, in wider lenses. Most of this translates to images that don't look sharp and contrasty. There could be linear as well as oval distortion.

 

When the light path is guided on a more controlled path to the film plane, such as with a stopped down or closed iris, many of the above issues are eliminated. This is due to the light passing through a much tighter "funnel" and not being allowed to scatter about. Again, this is very general. Basically, lenses perform at their best when stopped down approximately 2 stops from wide open. We try and shoot most interiors around a T2.8 to T4. There are times when we can't and we must live with all of the aforementioned optical issues. These issues are not necessarily guarranteed but they do have a much greater chance of occurring depending on what generation, manufacturer of lenses, etc. are being used.

 

By the way, the red lines that you are referring to - they haven't gone away. They are there on your diagram on a different path to the film plane due to the aperture being closed down.

 

I hope I haven't completely confused you...

 

Greg

[Dom Jaeger]

1. Why does changing the aperture not affect the angle of view?

 

 

That's actually a very good question, James.

Without getting too technical, my understanding is this - the front element (or front group of elements) gathers the basic image, in terms of an angle or field of view, limited of course by the basic construction of the lens and its focal length. The rest of the glass is concerned with correcting out aberrations, and focussing the image at the focal plane. At the plane of the iris the light is travelling in such a way that rays from the entire image are passing through more or less at every point, so reducing the diameter of the opening only diminishes the amount of light passing through, rather than cutting into the image itself.

Some people believe that stopping down only uses the centre of a lens, but this isn't really the case. The front and rear elements are always passing through light all the way to their outer edges, no matter the stop. Greg's analogy of a "funnel" is a good one.

 

As mentioned, reducing the aperture also affects depth of field, depth of focus, and control of certain aberrations. It can also sometimes cause a slight focus shift (particularly in high speed lenses). Too small an aperture introduces diffraction.

[Mark Dunn]

The angle of view depends only on the relationship between the size of the sensor and the focal length of the lens. It's a matter of geometry. (Think similar triangles).

The diagram you refer to is demonstrating focus, not field of view. A ray diagram just takes a few points of light as an example; it's not a representation of the structure of the image.

Edited February 26, 2013 by Mark Dunn

[Dom Jaeger]

The angle of view depends only on the relationship between the size of the sensor and the focal length of the lens. It's a matter of geometry. (Think similar triangles).

 

It's also dependent on what format the lens was designed for. You can't get a wider angle of view by putting a lens on a larger format camera if the image circle won't cover it. Since lenses are designed and optimised for a particular format, you could make an argument that in terms of mechanical design the angle of view is built in, even if someone chooses to crop the image circle (and thus reduce the angle of view) by using the lens on a camera with a smaller sensor/film gauge than it was designed for.

[Gregory Irwin]

 

. Since lenses are designed and optimised for a particular format, you could make an argument that in terms of mechanical design the angle of view is built in, even if someone chooses to crop the image circle (and thus reduce the angle of view) by using the lens on a camera with a smaller sensor/film gauge than it was designed for.

Very true. You mean like, using 35mm lenses on the 16mm format, correct? That effectively reduces the particular lens' angle of view but it's more of a product of the format - not the lens.

 

G

[Gregory Irwin]

In other words, in terms of 35mm lenses, a 25mm lens on the 16mm format would have the equivalent angle of view of a 50mm lens. Great discussion!

 

G

[Tom Jensen]

Yes, but keep in might that all a gate does is crop the image of the lens.

[James Malamatinas]

Thanks for all of your input and sorry for not following up sooner. I realise now that I may not have been too clear on what I meant but Dom seemed to hit the nail on the head and understood me correctly.

 

I understand the image is demonstrating focus but it was the best image I could find to try and illustrate the actual question I was asking, and Mark I think you made an important point to in that it illustrates a few points of light and not really the structure of the image.

The way I was interpreting the image was that the top rays (as illustrated in the diagram) represented the top part of the image - imagine an image of a person with the top rays representing their head. As per Mark's explanation however this would be incorrect and instead the entire image is passed along the lens at the point of the iris and thus only the amount of light is reduced and not the angle of view.


Does anyone have any links where I can read more about this, or about the actual structure of the image as it hits and passes through the lens?

Thanks and sorry for the confusion!

James

[Dom Jaeger]

Can't say I've found much on the net, lots of misinformation though!

 

I enjoy Roger Cicala's blog at http://www.lensrentals.com/blog, some interesting articles in there, but not much on cine lenses (or this particular question). There's a great French site on older lenses at http://dioptrique.info/sommaire/sommaire.HTM which is worth using an online translator to explore.

 

But the best info I find is in good old fashioned books - and often the old ones are the goodies. Kingslake's "A History of the Photographic Lens" or Cox's "Photographic Optics" are two of my favourites, both written by lens designers.

 

The problem you'll find is that most ray diagrams simplify the path of light going through a lens, just showing rays travelling parallel to the lens axis or passing through a focal or nodal point, and only depicted as a 2 dimensional cross section. In reality, every point from a focussed object within the field of view of a lens is sending a cone-shaped multitude of rays through the glass elements, each one being bent and spread into separate wavelengths only to be re-focussed again out the back of the lens to a point (or close to a point) on the image plane. With a wide open aperture, all these rays need to be corrected, leading to combinations of aberrations (3rd order, 5th order, 7th order etc), but as the aperture closes the cone of rays from each point becomes narrower, reducing the complexity of correction required, and increasing the clarity of other points near the focus plane. So you get more depth of field, and a sharper image. But the entire image is still visible because rays from each point are still passing through the lens, just through the inner zone of the aperture.

 

 

 

[James Malamatinas]

Cheers Dom, I'll check both of those sites out and try and get the books. I've been recommended a few other books too which I'm trying to get hold of; Applied Photographic Optics: Lenses and Optical Systems for Photography by Sidney Ray, and Hands-on Manual for Cinematographers for David Samuelson. These, the first especially, seem to be fairly advanced and physics led and I was looking to develop more of a foundation before getting stuck in.

 

I was definitely misinterpreting the theory, mainly because I was stuck thinking in 2D. Your explanation does a great job of clearing that up and explaining the actual reality. Hopefully when I go on to look at lens aberrations, depth-of-field, the effect with tilt and shift’setc it will make a lot more sense. I’m definitely going to look for more in-depth information on this though so thanks for starting points.

It would be nice if there were a 3D simulation of light passing through a lens so you could interact, change aperture, focal length etc to see the effect! The closest I’ve found is this:

http://nagykrisztian.com/synthrays/synthrays.html