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Author Topic: Help needed on lens image circle expantion when focused to its min..  (Read 4418 times)

Theodoros

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Can one please provide an article on how to calculate how much image circle expands when a lens is focused to its minimum focusing distance if compared to infinity? If there is no article, but one knows the theory behind the subject, can he please comment?
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Christoph B.

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #1 on: December 26, 2015, 05:52:38 pm »

What you’re looking for is a rather basic sine function, based on a right-angled triangle.




Your initial image circle at infinity would be 2*a - so if we assume that the image circle is 90mm, a = 45mm.

b would be your focal length / bellows extension at infinity, so if you're using a 80mm lens, your (initial) bellows extension (at infinity) b = 80mm.

The angle at A (alpha) would be about 26.57 degrees and that's all we need to know to calculate the rest.

If we get closer to the subject our bellows extension grows but the angle on A stays the same (as you don't change the focal length) - if we know the bellows extension it's pretty easy to calculate the projected image circle.


I think the problem with medium and small format lenses is that through complex lens designs with internal focussing and moving lens elements the whole calculation won't be correct any more...



Edit: However I'm pretty sure you can actually calculate the theoretical bellows extension through the focal length and your magnification. But that would only work if potential moving lens elements don't interfere with the image circle! 
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Theodoros

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #2 on: December 26, 2015, 06:37:19 pm »

What you’re looking for is a rather basic sine function, based on a right-angled triangle.




Your initial image circle at infinity would be 2*a - so if we assume that the image circle is 90mm, a = 45mm.

b would be your focal length / bellows extension at infinity, so if you're using a 80mm lens, your (initial) bellows extension (at infinity) b = 80mm.

The angle at A (alpha) would be about 26.57 degrees and that's all we need to know to calculate the rest.

If we get closer to the subject our bellows extension grows but the angle on A stays the same (as you don't change the focal length) - if we know the bellows extension it's pretty easy to calculate the projected image circle.


I think the problem with medium and small format lenses is that through complex lens designs with internal focussing and moving lens elements the whole calculation won't be correct any more...



Edit: However I'm pretty sure you can actually calculate the theoretical bellows extension through the focal length and your magnification. But that would only work if potential moving lens elements don't interfere with the image circle! 

Hi, thanks for the reply but maybe I should put it practically so that people understand  better my question... What I'm looking for, is to use MF lenses on a view camera, but the lenses will be focused to their min focusing distance (instead of infinity) so that their image circle is the maximum possible, then there is provision for enough space so that the lens will be focused back to infinity (or other) by recessing the front standard to the desired focusing distance, what you describe above, does apply when one is recessing the lens... but I'm looking on how much the image circle of the lens itself will be increased (with the lens fixed at its platform's mounting distance) if it is focused to its minimum distance (using the lens focusing ring)... is there a way for one to (accurately) calculate that?
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chrismuc

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #3 on: December 27, 2015, 03:01:17 am »

No maths but a test:

FPS + IQ180 + lenses w/ 12mm shift to left side
left parts of pictures shown
upper row: Canon TSE 24f3.5 II @ f11
lower row: Contax 645 35f3.5 @ f11

findings:
- Contax has larger image circle than Canon
- enlargement of image circle towards shorter distances with Contax (longer focal length) less pronounced than with Canon (shorter focal length)
- enlargement of image circle from infinity to 1m not very pronounced, only towards 0,25 shortest distance of the Canon TSE
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Christoph B.

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #4 on: December 27, 2015, 03:17:14 am »

Hi, thanks for the reply but maybe I should put it practically so that people understand  better my question... What I'm looking for, is to use MF lenses on a view camera, but the lenses will be focused to their min focusing distance (instead of infinity) so that their image circle is the maximum possible, then there is provision for enough space so that the lens will be focused back to infinity (or other) by recessing the front standard to the desired focusing distance, what you describe above, does apply when one is recessing the lens... but I'm looking on how much the image circle of the lens itself will be increased (with the lens fixed at its platform's mounting distance) if it is focused to its minimum distance (using the lens focusing ring)... is there a way for one to (accurately) calculate that?

I'm pretty sure that won't work that way for multiple reasons.

If you focus on a closer subject and then move the lens to get it back to infinity you'll face two problems:

On a lens with internal focusing you'll degrade the image quality by a huge amount and by moving closer you'll reduce the image circle again.
On a lens without internal focussing you'll end up with the same image circle as you did before.

You can't trick optics into providing a bigger image circle for free. Physics doesn't work that way.
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ErikKaffehr

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #5 on: December 27, 2015, 04:24:52 am »

Hi,

On most medium format lenses, focusing just moves the lens. So image circle at infinity will be the same, whatever the focus setting on the lens is. The only question is where you are adding extension, inside the lens or in the bellows of the camera.

Modern macro lenses are often using floating elements. Focusing them close moves a lens group in order of reducing field curvature at close distances. Not clear how that would affect image circle but focusing close would cause field curvature at longer distances.

Some older lenses, like Hasselblad FLE-s have an extra focusing ring using variable airspace between the two front groups to achieve a similar effect.

Best regards
Erik

I'm pretty sure that won't work that way for multiple reasons.

If you focus on a closer subject and then move the lens to get it back to infinity you'll face two problems:

On a lens with internal focusing you'll degrade the image quality by a huge amount and by moving closer you'll reduce the image circle again.
On a lens without internal focussing you'll end up with the same image circle as you did before.

You can't trick optics into providing a bigger image circle for free. Physics doesn't work that way.
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Christoph B.

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #6 on: December 27, 2015, 05:29:17 am »

Well not quite... sure the principle of extension stays the same but(!) as the extension happens inside the lans barrel you're not guaranteed to have a bigger image circle at a macro setting. The image circle may very well be cut off by internal elements or simply by limitations of the length of the lens barrel itself.
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ErikKaffehr

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #7 on: December 27, 2015, 05:40:56 am »

Thanks!

A good point!

Best regards
Erik

Well not quite... sure the principle of extension stays the same but(!) as the extension happens inside the lans barrel you're not guaranteed to have a bigger image circle at a macro setting. The image circle may very well be cut off by internal elements or simply by limitations of the length of the lens barrel itself.
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Theodoros

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #8 on: December 27, 2015, 09:09:04 am »

No maths but a test:

FPS + IQ180 + lenses w/ 12mm shift to left side
left parts of pictures shown
upper row: Canon TSE 24f3.5 II @ f11
lower row: Contax 645 35f3.5 @ f11

findings:
- Contax has larger image circle than Canon
- enlargement of image circle towards shorter distances with Contax (longer focal length) less pronounced than with Canon (shorter focal length)
- enlargement of image circle from infinity to 1m not very pronounced, only towards 0,25 shortest distance of the Canon TSE

Thanks for the response Chris... your contribution here is of most importance as it is Contax lenses that will be used on a (highly modified) Rollei X-Act2 camera both with 37x49mm MFDBs as well as with a mirrorless... Please follow this thread as your experience with Contax lenses will be needed (there are more detailed questions coming).

EDIT: By the way, is that tested on an ALPA FPS?  I ask this because on a view camera the image circle will expand considerably more as the lenses will be focused much nearer to the sensor... Also... I noticed that the Canon lens vignettes more to the lower corner...
« Last Edit: December 27, 2015, 11:01:52 am by Theodoros »
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Theodoros

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #9 on: December 27, 2015, 09:22:42 am »

I'm pretty sure that won't work that way for multiple reasons.

If you focus on a closer subject and then move the lens to get it back to infinity you'll face two problems:

On a lens with internal focusing you'll degrade the image quality by a huge amount and by moving closer you'll reduce the image circle again.
On a lens without internal focussing you'll end up with the same image circle as you did before.

You can't trick optics into providing a bigger image circle for free. Physics doesn't work that way.
I think you may be right to some extend with what you state for an IF lens (although Chris's example above shows some image area expansion even with the Contax 35mm f3.5 lens which is an IF lens but less so than the Canon he also posts)... But on a lens without IF, wouldn't "focus breathing" increase the image circle? (without affecting image quality)... it should, shouldn't it? ...I mean the frame on the sensor will look the same with no movements applied (as of constant focal length), but the total image circle projected should be one of wider focal length (due to focus breathing) ...shouldn't it? 

EDIT: At least Chris's results on the Canon 24mm above, (which is not IF) show some considerable image circle expansion when the lens is focused to its minimum... Now as far as IQ is concerned, one can't be certain that it will be less... It will be altered to some degree alright, but it has as much chance to be better as much as to be worst (depending on how near to the "optimum" image circle one is)... but in any case, it can't be much different as to seriously be considered to be a problem.
« Last Edit: December 27, 2015, 11:13:27 am by Theodoros »
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Bart_van_der_Wolf

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #10 on: December 27, 2015, 11:09:14 am »

I think you may be right with what you state for an IF lens... But on a lens without IF, wouldn't "focus breathing" increase the image circle? (without affecting image quality)... it should, shouldn't it? ...I mean the frame on the sensor will look the same with no movements applied (as of constant focal length), but the total image circle projected should be one of wider focal length (due to focus breathing) ...shouldn't it?

In general, focusing closer will create a larger image circle.

The easiest way to calculate such things is with the Magnification factor, which becomes easier and more accurate to calculate with the closer we focus.

The image circle diameter at infinity focus gets magnified by a factor: 1 + MagnificationFactor / PupilFactor
If we assume symmetrical lens design (with PupilFactor=1), at infinity that would be 1+0/1 = 1, and at 1:1 magnification factor it would be 1+1/1 = 2.

The magnification factor can be calculated from the focus distance (measured from the front principle plane of the optics), or measured from a shot (# pixels x senselpitch, divided by the original subject size). If measured from focus distance it's :
MagnificationFactor = focallength / (subjectdistance - focallength).

The pupil factor for asymmetrical lens designs is the Exit / Entrance Pupil diameter ratio. Focus, and then look at the lens at arms length from the front and the rear to estimate the entrance and exit pupil diameters.

That's not the whole story if you need very high accuracy, since aperture number and barrel design can change things a bit, but the main principle is as mentioned above.

Cheers,
Bart
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Theodoros

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #11 on: December 27, 2015, 11:39:42 am »

In general, focusing closer will create a larger image circle.

The easiest way to calculate such things is with the Magnification factor, which becomes easier and more accurate to calculate with the closer we focus.

The image circle diameter at infinity focus gets magnified by a factor: 1 + MagnificationFactor / PupilFactor
If we assume symmetrical lens design (with PupilFactor=1), at infinity that would be 1+0/1 = 1, and at 1:1 magnification factor it would be 1+1/1 = 2.

The magnification factor can be calculated from the focus distance (measured from the front principle plane of the optics), or measured from a shot (# pixels x senselpitch, divided by the original subject size). If measured from focus distance it's :
MagnificationFactor = focallength / (subjectdistance - focallength).

The pupil factor for asymmetrical lens designs is the Exit / Entrance Pupil diameter ratio. Focus, and then look at the lens at arms length from the front and the rear to estimate the entrance and exit pupil diameters.

That's not the whole story if you need very high accuracy, since aperture number and barrel design can change things a bit, but the main principle is as mentioned above.

Cheers,
Bart

Great Bart, this is spot on what I wanted to be informed! It is also much in agreement with the pictures that Chris posted above...

Can you please comment on these conclusions please? Do you agree with them?

1. If one is to use MFDB with MF lenses, he better choose a view camera than a technical camera so that the image circle is first widen to the maximum by focusing the lens at its minimum possible distance and then expanded even more proportionally as the lens is focused towards infinity using the (rear) distance focusing knob of the view camera.
2. It's best for one to choose (or modify appropriately) a view camera so that there is as much space as possible between the front and the rear standard and additionally take care that the standards can be recessed as to touch one another without a bellows on...
3. For optimum image quality (depending on the movements used) one is best to frame and focus with the lens focused to its minimum focusing distance and then (after framing) re-focus the scene by readjusting the focusing ring and the camera's focusing knob as to work with an "optimum" image circle (if aiming to optimize sharpness).
 

« Last Edit: December 27, 2015, 11:50:07 am by Theodoros »
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Theodoros

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #12 on: December 27, 2015, 11:48:57 am »

No maths but a test:

FPS + IQ180 + lenses w/ 12mm shift to left side
left parts of pictures shown
upper row: Canon TSE 24f3.5 II @ f11
lower row: Contax 645 35f3.5 @ f11

findings:
- Contax has larger image circle than Canon
- enlargement of image circle towards shorter distances with Contax (longer focal length) less pronounced than with Canon (shorter focal length)
- enlargement of image circle from infinity to 1m not very pronounced, only towards 0,25 shortest distance of the Canon TSE

Chris, could you (or any other that might have checked) please comment on the image circle of the 45-90mm vario-sonnar zoom? Also... is it ALPA FPS you used for checking the image circle? (I've asked this before as it is important to know as to judge the lens behavior on a view camera where the image circle will be further increased). Any other Contax 645 lens image circle experience will be highly appreciated...
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Bart_van_der_Wolf

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #13 on: December 27, 2015, 12:05:09 pm »

Can you please comment on this conclusions please?

1. If one is to use MFDB with MF lenses, he better choose a view camera than a technical camera so that the image circle is first widen to the maximum by focusing the lens at its minimum possible distance and then expanded even more proportionally as the lens is focused towards infinity using the (rear) distance focusing knob of the view camera.

To me, a View camera and a Technical camera are the same thing. Moving the rear standard changes perspective, so I'd not consider using that for only focusing. Focusing close up will already cause the largest (bellows) extension. Subsequent infinity focus, either with the front standard or with the rear standard will reduce the bellows extension (and image circle diameter).

Quote
2. It's best for one to choose (or modify appropriately) a view camera so that there is as much space as possible between the front and the rear standard and additionally take care that the standards can be recessed as to touch one another without a bellows on...

I do not follow what that is supposed to do for the image circle. Longer focal lengths can have larger image circles, and require more distance between front and rear standards. But longer focal lengths need longer bellows, which need to be exchanged for a wide angle version if short focal lengths are used.

Quote
3. For optimum image quality (depending on the movements used) one is best to frame and focus with the lens focused to its minimum focusing distance and then (after framing) re-focus the scene by readjusting the focusing ring and the camera's focusing knob as to work with an "optimum" image circle (if aiming to optimize sharpness).

I do not follow what that's supposed to achieve. There is only one image circle dimension at optimal focus. If a lens is used with its own focus mechanism, it might be more accurate than the rail focus knobs allow, especially if the lens focus ring has a large throw (large rotation angle for small adjustment).

Cheers,
Bart
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ErikKaffehr

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #14 on: December 27, 2015, 12:12:30 pm »

Hi,

The TSE II changes image circle size with focusing and it has internal focus, which I know as I have such a lens.

The Contax has the same image circle regardless of focus. Correct?

Best regards
Erik


No maths but a test:

FPS + IQ180 + lenses w/ 12mm shift to left side
left parts of pictures shown
upper row: Canon TSE 24f3.5 II @ f11
lower row: Contax 645 35f3.5 @ f11

findings:
- Contax has larger image circle than Canon
- enlargement of image circle towards shorter distances with Contax (longer focal length) less pronounced than with Canon (shorter focal length)
- enlargement of image circle from infinity to 1m not very pronounced, only towards 0,25 shortest distance of the Canon TSE
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Theodoros

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #15 on: December 27, 2015, 01:08:41 pm »

To me, a View camera and a Technical camera are the same thing. Moving the rear standard changes perspective, so I'd not consider using that for only focusing. Focusing close up will already cause the largest (bellows) extension. Subsequent infinity focus, either with the front standard or with the rear standard will reduce the bellows extension (and image circle diameter).

I do not follow what that is supposed to do for the image circle. Longer focal lengths can have larger image circles, and require more distance between front and rear standards. But longer focal lengths need longer bellows, which need to be exchanged for a wide angle version if short focal lengths are used.

I do not follow what that's supposed to achieve. There is only one image circle dimension at optimal focus. If a lens is used with its own focus mechanism, it might be more accurate than the rail focus knobs allow, especially if the lens focus ring has a large throw (large rotation angle for small adjustment).

Cheers,
Bart
Sorry... my bad! I forgot to explain that I modify a Rollei X-Act2 as to accept both MFDBs and Sony mirrorless on it... (starting with an α7ΙΙ). The requirement of the design (and hence the modification on the rear standard) is for one to be able to use the mirrorless instead of the MFDB without any further modification on the rear standard other than just unplug the MFDB and plug the mirrorless instead (on the same MFDB hinges via an appropriate plate)... So the requirements in my case change as one has to count for the extra space needed as to compensate for the 18mm sensor depth and the extra recessing of the mirrorless sensor because of the grip size...
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Christoph B.

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #16 on: December 27, 2015, 05:36:11 pm »

I think you may be right to some extend with what you state for an IF lens (although Chris's example above shows some image area expansion even with the Contax 35mm f3.5 lens which is an IF lens but less so than the Canon he also posts)... But on a lens without IF, wouldn't "focus breathing" increase the image circle? (without affecting image quality)... it should, shouldn't it? ...I mean the frame on the sensor will look the same with no movements applied (as of constant focal length), but the total image circle projected should be one of wider focal length (due to focus breathing) ...shouldn't it? 

EDIT: At least Chris's results on the Canon 24mm above, (which is not IF) show some considerable image circle expansion when the lens is focused to its minimum... Now as far as IQ is concerned, one can't be certain that it will be less... It will be altered to some degree alright, but it has as much chance to be better as much as to be worst (depending on how near to the "optimum" image circle one is)... but in any case, it can't be much different as to seriously be considered to be a problem.

On a lens without IF there is 100% no focus breathing, if you just move the lens back and forth you don't change the angle of the lens.
If you have a lens that's affected by focus breathing, the image circle may also change - for better or worse. Without testing the lens one can't tell you that for sure, it depends on which element moves where.

The image quality will definitely be worse if you use an IF lens at close focus and then back-focus to infinity with the camera. The internal focusing is (or rather should) be set to achieve the best performance at the adjusted focus point, at close focus position certain elements will most likely be moved back and/or forth to improve the quality at a larger magnification - whether that affects field curvature, central or corner sharpness or any other aspect of the image.

As I already mentioned there's another possible problem with the technique you want to use: depending on the lens design, internal elements or the length of the lens barrel could possibly reduce the image circle more than expected. Normally that wouldn't be an issue as they're not made to be used in tech cameras with movements - but in your case you could get a smaller image circle than you expect.
Not trying to get you down, I just want you to be prepared for that.

Now after reading your last comment I think I understand what you're trying to do but either I'm completely wrong or you've mixed up some things:

If you set the lens to its closest possible focus setting, the point where infinity focus lies is CLOSER to the lens. Not further back.
If you put the camera on an adapter (which increases the distance) and your cameras sensor is another 18mm further back - then it's like introducing a bunch of extension rings. You won't get closer to infinity, you'll do the opposite.
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Theodoros

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #17 on: December 27, 2015, 06:20:52 pm »

On a lens without IF there is 100% no focus breathing, if you just move the lens back and forth you don't change the angle of the lens.
If you have a lens that's affected by focus breathing, the image circle may also change - for better or worse. Without testing the lens one can't tell you that for sure, it depends on which element moves where.

The image quality will definitely be worse if you use an IF lens at close focus and then back-focus to infinity with the camera. The internal focusing is (or rather should) be set to achieve the best performance at the adjusted focus point, at close focus position certain elements will most likely be moved back and/or forth to improve the quality at a larger magnification - whether that affects field curvature, central or corner sharpness or any other aspect of the image.

As I already mentioned there's another possible problem with the technique you want to use: depending on the lens design, internal elements or the length of the lens barrel could possibly reduce the image circle more than expected. Normally that wouldn't be an issue as they're not made to be used in tech cameras with movements - but in your case you could get a smaller image circle than you expect.
Not trying to get you down, I just want you to be prepared for that.

Now after reading your last comment I think I understand what you're trying to do but either I'm completely wrong or you've mixed up some things:

If you set the lens to its closest possible focus setting, the point where infinity focus lies is CLOSER to the lens. Not further back.
If you put the camera on an adapter (which increases the distance) and your cameras sensor is another 18mm further back - then it's like introducing a bunch of extension rings. You won't get closer to infinity, you'll do the opposite.

Christoph, I don't question your knowledge on the subject here... to the contradict, I find your posts very helpful indeed...., but out of my testing (and as Chris posted with pictures and Bart explained) all lenses expand their image circle if focused to their minimum focusing distance, additionally, the phenomenon is more pronounced (I find) with non IF lenses rather than with the IF ones.... Now, about my camera project... by converting the rear standard of the X-act2 as to be able to accept both an MFDB & a mirrorless, it doesn't mean that the two image areas will be positioned the same... There is 3.2cm of difference between the two (up to now) and I'm working further as if I 'll be able to redesign the rear standard as to avoid the mirrorless (only) being recessed by the grip...

My problem with the later, (allowing for the mirrorless grip) is that (after having the first conversion made) the mirrorless leaves only 6mm of space for the front standard (lens set to infinity) which is too close to the rear standard to allow serious tilts... I believe I can gain another 2cm (!!!) if I redesign the rear standard and overcome the recessing because of the grip (with the mirrorless only)... Actually I already thought how this can be done (it requires a totally different approach though) and I'll start working on it by tomorrow... Still the MFDB will be at the same distance as before (which is plenty with the lens set to infinity) and it will have a focal plane shutter included on the (MFDB) adapter too...

Please, keep an eye on this thread as your comments can be very helpful to me...

EDIT: 6mm of space (mentioned above) is with a Contax lens on, it means that a Mamiya 645 lens would be (about) 5mm & a Hassy-H only 3mm... Of course a 35mm lens won't be usable at all... If I "gain" 2cm with the mirrorless, 35mm lenses will be usable...
LOL  ;D I think my view camera will end up with an L-bracket rear standard and the adapters slotted on the L-bracket from its "free" side..., as not having rotate-able by 90 degs image sensors on it (either MFDB or mirrorless) is out of the question...  ;)
« Last Edit: December 27, 2015, 06:41:20 pm by Theodoros »
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Christoph B.

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #18 on: December 28, 2015, 07:43:05 am »

Yes, the image circle expands with close focus - but if you move the sensor closer to the lens you re-focus again. You lose the image circle and get back to where you started from.

As I understand it with your setup you're trying to get more distance between the two standards in order to have a little bit of movement. But when you're using the min focus setting for your lens, you're actually losing distance. Your method for the mirrorless is like adding extension rings, meaning your lens will move forward as will the focus point to infinity.

In short - not only do you not gain anything, you actually have to put the camera even closer to the lens to get to infinity  than you would otherwise.

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Petrus

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Re: Help needed on lens image circle expantion when focused to its min..
« Reply #19 on: December 28, 2015, 09:30:46 am »

On a lens without IF there is 100% no focus breathing, if you just move the lens back and forth you don't change the angle of the lens.

I thought what you are describing is called focus breathing: changing the angle (framing) when focusing a lens without IF, as the lens moves closer or further away from the sensor. For that reason cine lenses are designed so that they actually slightly change the focal length during focusing to keep the angle/framing constant while being focused.

Lenses with internal focus systems can behave whichever way they choose. They often also actually focus by changing the focal length instead of moving the lens away or towards the sensor, but they do not do it as precisely and controlled way as a $$$$$ cine lens does.

The idea OP suggests to squeeze more image circle out of a lens is a brain dead one anyway. Take a flashlight with a tight beam, move it twice as far to make the beam twice as large, then move back to half distance to get the same brightness again? What was gained?
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