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Lens construction
cab.in.boston
Registered Users Posts: 634 Major grins
I don't know where to put this, so please move the thread if another forum is more appropriate.
This has probably been discussed before, but I'm still new to this stuff, so please forgive me if it's a dead horse. I'm very curious about the engineering and optics behind photography. I'm an EE and I understand the basics of how the body works and records images, etc, but the lenses... not so much. Does anyone have a good tutorial about how a modern lens is physically built?
I understand the basic concepts of focal lengths, apertures, etc, but am just curious how a few things really work. Like, where is the aperture, physically? I don't understand how a highly focused beam of light (say, f/22) and a much more spread out beam (f/1.8 or larger) could cover the same area of the sensor. It seems like the f/22 light would only illuminate a bright spot in the center of the sensor, since the physical distance between aperture blades and sensor remains fixed.
Regarding variable vs. constant apertures... say you have (for ease of numbers) a 25-50mm f/2 zoom. At each FL, you have a max aperture of FL/2. Doesn't that mean that the aperture is physically capable of being 25mm in diameter? And if so, why would it not be advantageous to use the same larger aperture at 25mm, resulting in a 25-50 f/1-2? You wouldn't lose anything at the tele end, but gain two stops at the wide, so why not do this? Or in a constant aperture zoom, is the physical size of the aperture the same throughout the range, and only the "equivalent" FL changes? For variable aperture zooms, does the physical size of the opening actually change, or do they design it such that the same size aperture is f/3.5 at the wide end and f/5.6 at the tele?
On focal lengths... my 55-200 (cheap-o) lens changes physical length when zoomed, but it doesn't actually change from 55mm to 200mm, so there must be some way of using internal groups/elements to create an equivalent FL. I always see lens "recipes" described in terms of groups and elements. I understand what this means, but not really how it works. The spacing between groups will change when zoomed, do the elements within a group move with respect to each other as well?
I know a lot of this stuff doesn't really matter to many (most?) photographers, who just want to understand the controls to get the shot they want and maybe don't care about the technical details, but there must be other science geeks out there who are curious about this, or have already researched it and have a perfect understanding of how everything actually works.
If anyone has any links to sites that describe this type of stuff, or simply has the answers, I'd love to hear them!
This has probably been discussed before, but I'm still new to this stuff, so please forgive me if it's a dead horse. I'm very curious about the engineering and optics behind photography. I'm an EE and I understand the basics of how the body works and records images, etc, but the lenses... not so much. Does anyone have a good tutorial about how a modern lens is physically built?
I understand the basic concepts of focal lengths, apertures, etc, but am just curious how a few things really work. Like, where is the aperture, physically? I don't understand how a highly focused beam of light (say, f/22) and a much more spread out beam (f/1.8 or larger) could cover the same area of the sensor. It seems like the f/22 light would only illuminate a bright spot in the center of the sensor, since the physical distance between aperture blades and sensor remains fixed.
Regarding variable vs. constant apertures... say you have (for ease of numbers) a 25-50mm f/2 zoom. At each FL, you have a max aperture of FL/2. Doesn't that mean that the aperture is physically capable of being 25mm in diameter? And if so, why would it not be advantageous to use the same larger aperture at 25mm, resulting in a 25-50 f/1-2? You wouldn't lose anything at the tele end, but gain two stops at the wide, so why not do this? Or in a constant aperture zoom, is the physical size of the aperture the same throughout the range, and only the "equivalent" FL changes? For variable aperture zooms, does the physical size of the opening actually change, or do they design it such that the same size aperture is f/3.5 at the wide end and f/5.6 at the tele?
On focal lengths... my 55-200 (cheap-o) lens changes physical length when zoomed, but it doesn't actually change from 55mm to 200mm, so there must be some way of using internal groups/elements to create an equivalent FL. I always see lens "recipes" described in terms of groups and elements. I understand what this means, but not really how it works. The spacing between groups will change when zoomed, do the elements within a group move with respect to each other as well?
I know a lot of this stuff doesn't really matter to many (most?) photographers, who just want to understand the controls to get the shot they want and maybe don't care about the technical details, but there must be other science geeks out there who are curious about this, or have already researched it and have a perfect understanding of how everything actually works.
If anyone has any links to sites that describe this type of stuff, or simply has the answers, I'd love to hear them!
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I don't know much more about lenses than you do, but here are my impressions (I won't claim they're more than that) about the answers to some of your questions.
Regarding f/22 not making a tiny dot in the middle of the sensor: The way lenses behave when stopped down seems to imply that the aperture iris has to be positioned at just the right place so that the light from any point in the scene is evenly distributed around the aperture opening. The light from different points would then be distinguishable only by the angle at which it crosses the aperture. The elements in front of the iris would have to be responsible for arranging the light this way, and the elements behind the iris would have to rearrange it into a spatial image that comes into focus at the sensor. Does that make sense?
I think that elements in a group are permanently attached to each other and therefore only move with respect to other groups.
With regard to maximum apertures in zoom lenses, this is probably a complex subject. It may be that in a constant-aperture zoom, the aperture does not move with respect to something else (the sensor, the rear nodal point, or whatever). It also could be that manufacturers limit some zoom lenses to constant apertures because if they allow the lens to open up more at the wide end, they would have to make sure it performed acceptably at those settings. I'm also not sure that "effective aperture" and "actual physical aperture" have a simple relationship, considering how many different ways there are to design lenses.
Got bored with digital and went back to film.
I am not sure if that will help, but my $0.02!
WildViper
From Nikon D70s > Nikon D300s & D700
Nikon 50/1.8, Tamron 28-75/2.8 1st gen, Nikkor 12-24/4, Nikkor 70-200/2.8 ED VR, SB600, SB900, SB-26 and Gitzo 2 Series Carbon Fiber with Kirk Ballhead
http://www.canon-europe.com/Support/Documents/digital_slr_educational_tools/en/ef_lens_work_iii_en.asp
and
http://www.canon.com/camera-museum/tech/index.html
(look at the virtual lens plant...pretty detailed)
There is no singular lens formula and zoom lenses complicate things dramatically. As an example, replaceable lens camera designs all have a fixed flange to image plane distance, yet some lenses are capable of a focal length of less distance than this "flange-focal" distance. This implies that those lenses have some sort of optic relay added to the basic lens design, allowing for the greater distances involved. If you take 2 lenses, one reversed and mounted in front of the camera-mounted lens, you create an example of this sort of image relay system and the combination of a wide-angle lens mounted in front of a "normal" lens or telephoto can yield very short effective focal lengths (with very high magnifications).
There is a lot of mis-information on the Internet about how "constant aperture" zooms work, but the most lucid explanation comes from Bob Shell:
The lens diaphragm is generally located near the "nodal point" of the lens (although generally ahead of the nodal point). This is the point where the rays of light cross inside of the lens and can be considered the optic center of the lens, not to be confused with the physical center of the lens.
The following digram is somewhat representative:
http://www.photoreview.com.au/guides/digitalslr/Diaphragm-1350.jpg
You might consider the following as a simple primer for lens design:
http://en.wikipedia.org/wiki/Photographic_lens_design
Moderator of the Cameras and Accessories forums
So ziggy, is it perhaps fair to say that in a constant aperture zoom, the physical maximum of the diaphragm used may be the same (or not, depending on design) for every FL to which the lens is set, but it's the positioning of the elements that determine the apparent aperture size? And then in my imaginary 25-50 f/2, the aperture may not actually be 25mm in diameter for 50mm, but may in fact be the same physical dimension at both 25 and 50 mm, and the groups are manipulated such that it appears to be f/2 for all lengths.
Neat stuff. I love the results a good photographer can get with an image, but the technology behind it is fantastic.
My site 365 Project
The "f" value of an aperture in a simple lens is based on the physical size of the aperture as it relates to the opening of the lens (basically the diameter of the front element that is visible).
In a constant aperture zoom, the size of the front element remains as a constant but the "apparent" size of the aperture changes with focal length, and it changes at a rate that keeps the effective "f" value constant. The change is due to the variable internal "diopter" elements of the zoom design. The position of all of the groups generally changes to accommodate the design features of the zoom, and the most complicated designs are those that are both constant aperture "and" internal focus (IF), implying that overall length does not change.
These new optical designs are thanks to a number of technological advances in lens design, lens element manufacture, lens element composition, lens group design and composition (including mounts and glues), barrel design (including complicated cams and multiple cams to move elements and groups) and motor design (to adjust focus.)
Moderator of the Cameras and Accessories forums
But just as in modern lens designs the physical length is not the same as the focal length, I guess the physical diameter of the aperture is not necessarily what it would be if one simply divided the FL by the f-number. So my 50mm f/2 lens wouldn't necessarily be physically 50mm long, and the aperture wouldn't necessarily have a 25mm diameter, but it would be equivalent to a simple lens having those measurements, and then at 25mm f/2, it would be equivalent to a simple lens 25mm long with a 12.5mm diameter aperture. However, in practice, the physical size of the diaphragm may actually be the same in both cases.
Good links, I'm enjoying the research!
My site 365 Project
That's a great book, I just got a hard copy with my CPS gold subscription.
EFS 17-55 f/2.8 & 10-22 // Sigma 30mm f/1.4 & 50mm f/1.4
Sigma Bigma OS // Canon 70-200 IS f/2.8
This is a thread I wrote a few years ago about aperture and f numbers in three different lenses
I only mention it, because as I point out, lenses may be sold as the same focal lengths or apertures, but may not measure exactly the same.
Moderator of the Technique Forum and Finishing School on Dgrin
http://www.kenrockwell.com/nikon/d3/images/D3R_4636-cutaway-950.jpg
The "maximum" "f" value is indeed a relationship between lens opening and the focal length. At reduced "f" settings the diaphragm is engaged and in a constant aperture lens ("aperture" this time refers to the "f" setting) the diopter elements vary the effective diaphragm size to keep the resulting light transmission a constant "f" value.
Note that I stayed away from the term "aperture" as much as possible because it can have different meanings depending on context, which may not always be clear.
Moderator of the Cameras and Accessories forums
Ah, so that's how it works. Thanks for the explanation.
Got bored with digital and went back to film.