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View Full Version : Same camera, same focal length - same or different coverage?

Nelson Sousa
7-May-2010, 15:38
olá

another beginner's question, regarding lenses; I'm trying to understand how does it work concerning the amount of the scene it gets in the film format of choice (information in books doesn't make this clear)

As an example: comparing the Schneider Super-symmar XL Aspheric 5,6/210mm and the Schneider Apo-symmar-L 5,6/210mm.
They have the same focal length but different angles of view. The SS has a stated angle of view of 105º and the Apo-symmar 75º; they have different stated circle of image, with the Super-Symmar having the bigger.
So, does this means that, if we use both lenses for the 8x10 format, the SS will get more of the scene on the film than the Apo-symmar; or they get the same coverage of the scene and we simply have a bigger range for the movements available (for the same format of film)?

Thanks,
Nelson

Dan Fromm
7-May-2010, 16:25
Given that focal lengths and formats are the same, larger circle = larger movements possible.

drew.saunders
7-May-2010, 16:28
Lens manufacturers list angle of coverage, from which you get the image circle. If your image circle is big enough for your piece of film, then you have enough, and maybe more for movements.

Angle of view is based on the format and focal length. I made an excel spreadsheet that does the math, but a quick google search for "angle of coverage angle of view" will get you lots of examples.

From my spreadsheet, here are the horizontal angles of view for a 210mm lens and various formats:

APS 8°
35mm 10°
6x4.5 15°
6x6 15°
6x7 19°
6x9 23°
4x5 32°
5x7 44°
8x10 61°

Clearly the 210SSXL would provide a more than ample image circle for all of these formats, but would be overkill for anything smaller than 5x7.

Does that make more sense?

Nelson Sousa
7-May-2010, 16:45
So, we can assume: with the same film, the two lenses catch the same image; what differs is the circle of image available for movements.

Thanks again
Nelson

Jack Dahlgren
7-May-2010, 16:52
So, we can assume: with the same film, the two lenses catch the same image; what differs is the circle of image available for movements.

Thanks again
Nelson

Film or not, the focal length determines the size of the image projected.

The image circle determines where the edges of the projected image are - typically at infinity when the lens is closest to the image plane.

If your film fits within that circle of light, then it is all OK.

Geometry shows us that it is generally easier to make a large image circle with a longer focal length and also that a lens which marginally covers at infinity will cover better closer in.

Doremus Scudder
8-May-2010, 01:30
Nicolas,

You are correct. Two lenses with identical focal lengths will give exactly the same image with the camera in "zero" position. Same focal length = same magnification = same size rendering of a given object.

In this example, you are only using the center of the image circle of both lenses. The lens with the larger image circle simply has more "unused" image in this case.

Understanding image circle
Imagine the two lenses you mentioned outside the camera, projecting their image circles onto a flat wall. The objects in the images would be the same size, but the lens with the larger image circle would include more of the scene (it sees more at the edges). If you put 8x10 inch cards (which represent your film) in the center of both circles you will see exactly the same image projected on both cards. If you start moving the cards toward the outside of the circles (mimicking camera movements), you will find you have more "picture possibilities" with the larger image circle. The "angle of view" that is listed in lens specifications relates only to how much the lens "sees" and how large the image circle is for a given focal length (some call it "angle of coverage," which is more applicable here). Only by changing focal length can you change the size of objects in the image, however.

The advantage of a larger image circle is that one can use more shift/rise/fall (which is equivalent to moving the cards around in the example above), which use a more off-center part of the image. For example, if you needed to use a lot of front rise to keep the camera level but still get that tall building into the frame, you might run out of image circle and vignette with a lens that had a small image circle, since you would effectively be raising the bottom of the image circle above the bottom of the film. A lens with a larger image circle would still cover, allowing you the possibility of making the image.

Design, focal length and image circle size
You probably already know that lenses come in "families," grouped by design and angle of view. Most manufacturers designate these with letters, etc. to show the relative size of the angle of view. Nikkor lenses for example have designations such as SW (super wide angle of view), W (wide angle of view), M ("normal" angle of view). Schneider lenses use similar designations (Super Angulon, Symmar, Xenar) but have many more "sub-families" with differences in angles of view. There are also names for the various designs such as "Tessar," "Plasmat," etc. You will become more familiar with these as you go.

Simply put, any lens "family" will have a similar angle of view. The image circle of lenses within a family, however, will be larger for longer focal lengths; since shorter focal lengths have to be closer to the film, the image circle will be smaller even though the angle of view is the same. Two identical focal lengths from different "families" will have different image circles, since one will have a narrower angle of view than the other for the same lens-to-film distance.

A couple of examples will illustrate: Compare image circles for a Schneider Apo-Symmar 150mm and 360mm. Both these "wide" designed lenses have about a 70° angle of view, but the image circles are 220mm and 491mm respectively. The 150mm would not cover 8x10 whereas the 360 has room to spare. In order to get a 150mm lens that covered 8x10, you would need a "very wide" design like the much larger Super Symmar XL with a 105° angle of view and a 386mm image circle.

Conversely, lenses of the same focal length from different "familes" will have different image circles. An example in the Nikkor lens selection will illustrate. A "normal" Nikkor M f/9 300mm lens has a 57° angle of coverage (or view) and a 320mm image circle. It is very small and lightweight (also due to its smaller maximum aperture). It will just cover 8x10 but allows for little or no movements. A "wide" (or plasmat) design lens like the Nikkor W in the 300mm focal length has a 70° angle of view and a 420mm image circle, which allows for moderate movements on 8x10. The trade-off is the size and weight. The latter lens is almost four times as heavy and would be overkill for a 4x5 camera.

How image circle affects lens choice
Most of us weigh the advantages and disadvantages of lens image circle when choosing a lens. Lenses with larger image circles are usually more complex in design, more expensive, and larger than lenses of the same focal length with smaller circles.

Determining how much image circle your camera can use and how much you really need will help in lens choice. If you will be doing a lot of close architectural work with short focal lengths, you will need a camera capable of a lot of movements and big lenses with large image circles. If you will be backpacking with a wooden field camera with limited movements, you probably would not be able to use all the image circle some lenses, plus you would likely want to have smaller, lighter lenses to carry.

I remember how confused I was with LF lenses when I first encountered them. I hope this (rather simplified) explanation helps.

Best,

Doremus Scudder

Nelson Sousa
8-May-2010, 03:58
simplified is good, very good! :)

now I can definitely work out the choice of my first lens (never thought it would be so complex, after fifteen years being serious about photography) - just have to decide wich focal will be more usefull to start with

a very photogenic summer for you all!

nelson

ic-racer
8-May-2010, 07:13
"Angle of View" is just not the best way to compare lens image circles because it can mean different things in different context as you have discovered. Best to convert "Angle of View" to "Lens Image Circle." Not sure why some manufacturers do it that way and not just give the image circle.

Jack Dahlgren
8-May-2010, 07:38
"Angle of View" is just not the best way to compare lens image circles because it can mean different things in different context as you have discovered. Best to convert "Angle of View" to "Lens Image Circle." Not sure why some manufacturers do it that way and not just give the image circle.

Image circle is based on more than angle of view. It is dependent on distance to the lens and the angle of the focal plane. I think I've seen manufacturers give both numbers, but angle of view would seem like it is a more primary measure from which you can derive the other values.

rdenney
8-May-2010, 18:21
The first and most confusing part of large-format photography is understanding the difference between coverage and angle of view.

As long as the coverage is enough to illuminate the frame, all lenses of the same focal length present the same field of view. The difference between a 200mm lens for a 35mm camera and a 210mm view-camera lens is that the former will only make a circular image maybe a couple of inches in diameter when focused onto 4x5 film. Within that circle, though, both will produce the same image. We buy coverage with view camera lenses so that we can use larger formats, more movements, or both.

Rick "who first learned this trying to enlarge a 6x6 negative from a Yashica-mat using a 50mm enlarger lens intended for small format" Denney

Rui Morais de Sousa
9-Jun-2010, 16:45
Olá Nelson,
Já respondi noutro post, aqui deixo mais uma palavrinha simples a ver se me "encontras"... Realmente, neste nosso pequeno país, somos poucos a practicar grande-formato... Fico contente de te ver por aqui. É bom saber que não sou o único "maluco"... Não leves a mal, estou óbviamente a brincar!

Mr. Doremus Scudder,
I very much enjoyed your explanation, which shows that you really like to help (and know how to...).
I didn't know you or your work, and I must say that I was very pleased to look at your site. Congratulations! Your work and personality are very interesting.
Very rewarding site to look at and read.
Cheers,
Rui

Bruce Watson
10-Jun-2010, 09:27
As an example: comparing the Schneider Super-symmar XL Aspheric 5,6/210mm and the Schneider Apo-symmar-L 5,6/210mm. They have the same focal length but different angles of view.

Your use of the jargon is wrong. All lenses of a given focal length have the same angle-of-view (http://en.wikipedia.org/wiki/Angle_of_view) for a given format. They may have different angles-of-coverage however, which varies with the design of the lens in question.

Dan Fromm
11-Jun-2010, 17:22
So, Bruce, I'd get as good results on 2x3 with my 105/2.8 MicroNikkor AIS as with my 101/4.5 Ektar? Are you sure? Did I misunderstand what you wrote?

Cheers,

Dan