Angle of view and Focal Length

[chema_perez]

Hi. I post this in here because I supose the LF shooters have better knowlege

of these matters.

 

My dout is

 

Could two different design lenses have exact equal focal length and different

angle of view?

 

Not angle of coverge, this is a different matter. And comparing the two of them

focused at infinity as they should.

 

I ask because often we can find lenses specs with same focal and different

angles. I know that they rarely give the real focal, but if we calculate it,

would we find the mathematical corelation?

 

I've read this would be the formula:

 

angle of view = 2 * tan^-1 [(1/2 * diagnoal of format)/focal length]

 

The question is : Is this true at 100%? or is just something to give an

approximate response.

 

Thanks.

[ronald_moravec1]

Could two different design lenses have exact equal focal length and different angle of view?

 

Not if used on the same format film assuming sufficient coverage.

 

But if you were to use a 110XL on 8x10, it almost covers. But a 120 Angulon will not.

[bob_gentile]

It's physics. All lenses of the same focal length have the same angle of view for the same format. You're right to make the distinction between angle of view and coverage, because not all lenses of the same focal length will cover the format.

[michael_briggs2]

The equation that you give is correct, for the angle of view across the format diagonal, when focused on infinity. The equation is derived assuming that a lens renders perspective in the normal manner, not using any other properties of the optical design; it even applies to a pinhole. The only property that appears in the equation is the focal length f, so all lenses of the same focal length have the same angle of view for a particular format. It's just geometry.

 

If you find specs that give a different angle of view, it might be from rounding, or from diagonal versus long side, or a mistake. There is one current Schneider document in which the summary table of lens properties mistakenly labels the "coverage" column as "view".

[ilkka]

It can be a bit confusing. Why not think of it in this way: The focal length of the lens decides the image size. The size that an actual subject in front of the lens is reproduced on film. When two lenses with same focal length are focused on same distance (eg infinity), the image sizes will be identical. One of the lenses can cover wider angle, ie. have larger coverage, a larger image circle. So it shows more area around the main subject. This lens could be used on 8x10 camera while the other lens with narrower angle of view would only cover 4x5.

 

The actual field of view depends not only on the lens but also on the film size. A bit like in digital, there is a crop involved if you use a 150mm lens made for 8x10 and use a 4x5 film back. This has been very simple and well known for a century already. Strange how now with digital, people are making a big fuss about it and make it seem more complex than it actually is. You can buy a 150 mm normal lens or a 150 mm wide angle. On 4x5 camera, both produce identical images. On 8x10 camera, the subject will be identical height with both lenses, but only the wide angle lens will cover the whole 8x10 area. The normal 150 lens will produce circular image or at least darkened corners.

[chema_perez]

Well. Thanks for the answers, in fact this is to confirm what I already know.

 

The thing is that in another forum, someone who actualy is a professional fotographer of arquitecture and uses LF, is claiming that not only the angle of coverage can be different depending the lens design, but also the angle of view in one same format can change!!

 

It's clear now. Thanks!

[ilkka]

Professional photographers don't necessarily know what they are talking about. There are plenty of books and magazine articles written by professional photographers that contain snippets of information that are misleading or just plain wrong.

[mike_collins5]

Hello,

 

I'm decided to make my own lens shades for my 8x10 lenses. If I understand the discussion above correctly I would

need to know the angle view in order to keep my lens shade from vignetting the image. Is the worst case being the

lens stopped down to maximum and focused at infinity? BTW my lenses are: 300 dagor (Berlin), 480 red dot altar

and 210mm Wollensak Pro Raptar. Since my HP-16C batterys died 20 years ago, can anyone give me the angle of view

for these 3 lenses?

[michael_briggs2]

You could use the angle of view to design a lens shade to cutoff light outside the view of the lens when the

camera movements were zeroed, but then if you used movements such as rise or shift, the shade would impinge into

the view. You can use google as a scientific calculator.

[mike_collins5]

Thanks for your response Micheal.

But now I'm confused. If the lens shade is attached to the lens how could camera movements change the relationship between the shade and the lens? If you move the lens your moving the image circle over the film plane. If you move the camera back your moving the film plane through the image circle. The angle of view remains constant, being inherent in the design of lens.

If this is not true then I don't understand what "angle of view" means. I mean it to mean what the lens sees, (a diagonal moving outwards from the front edge of lens barrel.)

 

Thanks,

 

Mike