Effect of aperture on FOV

[AreBee]

Folks,

I'm darned if I can locate it, but I am sure that I previously have read that the FOV of the Rodenstock 23mm HR reduces (if I remember correctly) as the lens is stopped down. A few questions, if it's true:

1. Is this the case for all lenses?

2. How significant is the reduction in FOV? To take the 23mm HR as an example, if the lens was stopped down to f/11, what would be the reduction in FOV?

3. Why does FOV reduce with aperture?

Cheers,

[Tony Jay]

 Read this: http://www.rodenstock-photo.com/mediabase/original/e_Rodenstock_Digital_Lenses_3-26__8236.pdf

Tony Jay

[AreBee]

Tony,

Read this...

Thanks, but the pdf addresses none of my questions.

You have, however, succeeded in reminding me where I originally read about FOV changing with aperture. It was here (David Osborn's post).

The information in David's post is not quite correct. The Schneider Super Angulon 5.6/90 returns a FOV of 96 and 110 at f/5.6 (wide open) and f/22 respectively. In fact each lens listed on page two of the attached pdf indicates that FOV changes as a function of aperture, albeit in the reverse order to what I had thought, that is FOV reduces with an increase in aperture size.

The answer to my first question appears to be "yes" for Schneider lenses, but is it true for all lenses, not just Schneider? My 2nd and 3rd question remains unanswered.

Can anyone shed any light on this? I am interested specifically in the Rodenstock 23mm HR at f/11.

[Telecaster]

The Schneider Super Angulon 5.6/90 returns a FOV of 96 and 110 at f/5.6 (wide open) and f/22 respectively. In fact each lens listed on page two of the attached pdf indicates that FOV changes as a function of aperture, albeit in the reverse order to what I had thought, that is FOV reduces with an increase in aperture size.

My guess is this refers to maximum useful FOV taking tilt/shift/swing movements into account. When you stop down the boundary of the lens' image circle becomes more sharply defined. This has an impact on how much of the total image circle is photographically relevant.

-Dave-

[DonnaMarie]

My guess is this refers to maximum useful FOV taking tilt/shift/swing movements into account. When you stop down the boundary of the lens' image circle becomes more sharply defined. This has an impact on how much of the total image circle is photographically relevant. You may also check other websites for more information.

-Dave-

I found this to be true.  The image circle does become more defined.  Totally makes a difference.  Thanks.  

[AreBee]

Hello Dave,

My guess is this refers to maximum useful FOV taking tilt/shift/swing movements into account. When you stop down the boundary of the lens' image circle becomes more sharply defined. This has an impact on how much of the total image circle is photographically relevant.

Hmm. I appreciate your reasoning but the pdf I attached in my previous post clearly states that the image circle itself changes in diameter, so it can't simply be that optical performance within a fixed image circle improves with stopping down, though I don't doubt that that does occur - with any lens. However, you've got me thinking, and I may now have my answer:

The image circle of the Rodenstock 23mm HR may or may not increase as it is stopped down. If the 70mm diameter image circle (stated in the pdf that Tony provided) exists with the lens wide open then, when used in conjunction with a MFDB it will be conservative to assume that available FOV - i.e. from stitching - will at least equal the stated value at smaller apertures, and may be larger...except that Rodenstock apparently places a physical disc in the lens construction to visually indicate the limit of the image circle. Therefore, for the lens in question, an increase in image circle, if it occurs when the lens is stopped down, is irrelevant.

Bummer. Thanks very much for your help all the same.

EDIT: Re-reading your post I think I missed your meaning - I don't think you claimed that the image circle is fixed. Sorry!

[KevinA]

Folks,

I'm darned if I can locate it, but I am sure that I previously have read that the FOV of the Rodenstock 23mm HR reduces (if I remember correctly) as the lens is stopped down. A few questions, if it's true:

1. Is this the case for all lenses?

2. How significant is the reduction in FOV? To take the 23mm HR as an example, if the lens was stopped down to f/11, what would be the reduction in FOV?

3. Why does FOV reduce with aperture?

Cheers,

My days of LF, I stopped down to increase the useful image circle, not reduce it. What's changed?

[AreBee]

My days of LF, I stopped down to increase the useful image circle, not reduce it. What's changed?

Apparently nothing.

[ondebanks]

The Schneider Super Angulon 5.6/90 returns a FOV of 96 and 110 at f/5.6 (wide open) and f/22 respectively. In fact each lens listed on page two of the attached pdf indicates that FOV changes as a function of aperture, albeit in the reverse order to what I had thought, that is FOV reduces with an increase in aperture size.

It's just a vignetting effect. Taking the above lens as an example: at *all* f-stops, it projects an image covering a 110 degree FOV. But at the wider f-stops, there is just not enough light reaching the outer part of the image circle, to make the whole 110 degrees bright enough to be usable. So Schneider call it quits at 90 degrees. There is still a perimeter of image of a further 10 degrees all around present, but it's too vignetted to be usable. It's also possible that it is too aberrated to be useful, but vignetting is the more likely culprit: there are industry standards for image circle cutoff by intensity falloff/vignetting, but none that I know of for cutoff by general aberrations, which are much more subjective.

Think about what's happening when you stop down this (or any) lens. At f/22 you are only admitting a central pencil of light through the stop. That gives you a 110 degree image. Open it up to f/5.6. That central pencil is still coming through, isn't it? Right, so there's still a 110 degree image still coming through. But it's overwhelmed in intensity by the 15 times additional flux of the difference between the f/22 and f/5.6 beams. Most of that extra light is coming from peripheral rays near the physical edges of the lens components. So there is bound to be significant mechanical vignetting off-axis; those parts of the focal plane cannot "see" the entire circular aperture. Long story short, the centre of the FOV receives 16x the flux, the outermost degree of FOV probably receives the same flux as before or just marginally more, and points in between receive amounts of flux in between these two limits. 90 degrees out from the centre is still sufficiently bright or non-vignetted to be declared usable. I don't know what cutoff standard Schneider applies, but Japanese lens/telescope makers tend to use the diameter within which vignetting is less than 40%.

Ray

[Petrus]

To make this easier to visualize: with a large aperture the image circle edge is not sharply defined, so there is vignetting and softness at the perimeter making the usable image circle smaller (like 90 degrees or so with lens A). When the lens is stopped down to f11 or whatever, the image circle edge gets sharper and seems to expand (usable area is now 110 degrees) a bit. So the usable/constant quality area is now larger.

[AreBee]

Ray, Petrus,

Many thanks for the explanations.

Regards,