1. ## Lens viewing angles

Hi all,

I'm new to LF and doing some sums ...

I take the data available for the 80mm Super Symmar and find this ...
It has an image circle (@ f22) of 212mm at which point the lens will be 80mm from the film. The lens has a stated field of view of 105 degrees. Simple geometry indicates the useable data on the 'input' side of the lens (the side where the subject is !) is in the same 'angle of view' as the 'output' side of the lens. Both being at 105 degrees.

Is it possible for a lens to have a 105 degree 'input' side and say an 80 degree 'output' side ?

This would mean a 150mm lens might have a image circle of 212mm and a field of view of 105 degrees yet also have a focus distance of say 120mm ... (YES! - I can fit the thing on a camera !) [I also get a little worried by the light striking my film at such an oblique angle ...]

Does anyone know of such technology ? I have put numerous lens data through my spreadsheet and they're all looking the same in that the angle of view on one side of the lens is the same angle of view on the other side. The final element on my 35mm 17mm lens is no closer to the film plane than the final element of my 200mm lens !

Cheers,
Steve

3. ## Re: Lens viewing angles

Originally Posted by Steve McLevie
Hi all,
Is it possible for a lens to have a 105 degree 'input' side and say an 80 degree 'output' side ?

This would mean a 150mm lens might have a image circle of 212mm and a field of view of 105 degrees yet also have a focus distance of say 120mm ... (YES! - I can fit the thing on a camera !)
No, at infinity I do not believe it is possible for a lens to have an 'input' of 105 degrees and an 'output' of 80 degrees. If the lens has an output (angle of coverage) of 80 degrees it is not a super wide angle design and must therefore be used on a smaller format (when compared to a ultra wide design). On the smaller format it would be closer to a semi-wide at best.

If you focus close enough, any lens will eventually have enough coverage. You do see very short focal lengths for ultra macro lenses. However, the extra extension will alter the effective focal length, and the drastic reduction in depth of field at such close focus really makes the angle of view ('input' side) irrelevant.

4. ## Re: Lens viewing angles

Thanks guys,

I appreciate your comments. In a discussion about it here at my workplace I likened it to 35mm photography. In 35mm, many of the lenses have their last element at a similar distance from the film plane. So a 35mm lens and a 70mm lens and a 135mm lens all have the same lens-film distance (pretty much). Not so the case in LF.

If lens manufacturers for LF were to make all their lenses have say a 100mm distance from film to last lens element (for infinity focus) then all LF cameras (for a given size like 4x5) would only need bellows that extended from say 100mm to 200mm. Easy !

I reckon its got something to do with optics or shifts. It may be that to allow a lens to shift it needs to be of a certain design and the one I'm thinking would be nice wouldn't allow shifts because it would need to be central to the film area.

Any other thoughts welcome !

5. ## Re: Lens viewing angles

does your spreadsheet take into consideration the design of each lens you are doing the calculation for, or are you just assuming that your formulas will work for all lens designs just because you have used the generic formulas printed in some publication?

If it's the latter, which I suspect, then you are wasting your time except for learning that since you don't have the design parameters for each individual lens design, you can't do what you are trying to do with any accuracy.

6. ## Re: Lens viewing angles

Originally Posted by Steve McLevie
Hi all,

I'm new to LF and doing some sums ...

I take the data available for the 80mm Super Symmar and find this ...
It has an image circle (@ f22) of 212mm at which point the lens will be 80mm from the film. The lens has a stated field of view of 105 degrees. Simple geometry indicates the useable data on the 'input' side of the lens (the side where the subject is !) is in the same 'angle of view' as the 'output' side of the lens. Both being at 105 degrees.
The angle of view is the same on both sides of the lens. But you don't want to confuse the angle of coverage with the angle of view. The latter is determined by the format size and the distance of the frame from the lens. The former is determined by the design of the lens and relates to light fall off and loss of resolution as you move from the center to the peripherry. For a 4 x 5 frame, which has a diagonal of about 153 mm, the angle of view of an 80 mm lens when focused at infinity is about 87 degrees. As you extend the bellows, the angle of view decreases, although until you are in the close-up range, the change is negligible. The figures you give for the angle of coverage---105 degrees---and the diameter of the circle of coverage---212 mm---are consistent with one another. As you extend the bellows, the angle of coverage stays roughly the same, and the circle of coverage increases. Again, this increase is negligible until you are in the close-up range.

Is it possible for a lens to have a 105 degree 'input' side and say an 80 degree 'output' side ?
No.

This would mean a 150mm lens might have a image circle of 212mm and a field of view of 105 degrees yet also have a focus distance of say 120mm ... (YES! - I can fit the thing on a camera !)
It is not clear what you mean by that. The angle of view of a 150 mm lens at infinity is always about 52 degrees, but its angle of coverage and its image circle at infinity would depend on the design of the lens. The distance of the lens to the film must always be not less than the focal length. But that distance has to be measured from what is called the rear principal plane. The position of that plane for most lenses is pretty close to the front of the lensboard, but for some lenses, e.g., those of telephoto design, it could be elsewhere, including some considerable distance in front of the front element of the lens. The actual distance of the lensboard from the film when you are focused at infinity is called the rear flange focal length and is usually specified in the lens specifications. If you know that and the focal length, you can calculate the position of the principal plane. None of this affects the angle of view at infinity, which is determined by the focal length and the frame size.

Does anyone know of such technology ? I have put numerous lens data through my spreadsheet and they're all looking the same in that the angle of view on one side of the lens is the same angle of view on the other side. The final element on my 35mm 17mm lens is no closer to the film plane than the final element of my 200mm lens !
But you can be sure the rear principlal plane for your 17 mm lens is 17 mm from the film when focused at infinity and that for your 200 mm lens it is 200 mm from the film when focused at infinity. Your 200 mm lens is almost certainly a telephoto lens and its rear principal plane is considerably in front of the rear element of the lens and may in fact even be in front of the lens entirely. Your 17 mm lens is probably of inverted telephoto design and its rear principal plane is probably in back of the inside lens element. Its rear flange focal length would be greater than its focal length. That is typically the case for wide angle lense for 35 mm SLRs because room has to be left for the mirror.

7. ## Re: Lens viewing angles

A "normal" lens has the same angle input and output. But there are telephoto lenses and inverted telephoto or retrofocus lenses, which have positive and negative sections with a large spacing. These are common on 35, the retrofocus for wide angle lenses where room behind the lens for the mirror is required, and telephoto to keep the lens short. These have different input and output angles. Think of the telephoto as a normal lens with a built in telextender, and the retrofocus with the reverse. Telephotos are available for large format, the retrofocus as far as I know not. Most LF lenses are "normal".

8. ## Re: Lens viewing angles

Originally Posted by James E Galvin
...These have different input and output angles...
How are you defining 'angle' in those cases?

Thanks,
Helen

9. ## Re: Lens viewing angles

I am using the terms as Steve originally used them. The angle of coverage is the input angle, the output angle is the angle between the lens and film. These are the same for most lenses, but different for telephoto and retrofocus lenses. Consider the wide angle and telephoto converter lenses that screw onto the front of lenses for 35. They change the input angle (the angle of coverage), but not the output angle. It would be nice to have retrofocus lenses for LF, no bag bellows or recessed lensboards, no need for center filter! But looking at the size of very wide lenses for 35, our lenses would be huge.

10. ## Re: Lens viewing angles

Sorry, I was wondering which part of the lens you were measuring the 'output angle' from. You do not appear to be measuring from the rear node to the film plane.

Best,
Helen

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