View Full Version : LF Macro Lens (...and understanding bellows extension for tele lenses)

Pawlowski6132

24-Aug-2018, 06:51

Just wanting to confirm here that I understand how these lenses work. In general, will using a LF macro lens require less bellows extension than a normal lens given any magnification ratio?

thanx in advance

jrp

EdSawyer

24-Aug-2018, 06:52

bellows probably the same but the correction and performance will be better than a standard lens.

Pawlowski6132

24-Aug-2018, 06:57

Thanx. Performance issues weren't something I was trying to solve for unfortunately. The problem I was trying to solve for is that I primarily shoot portraits indoor using natural light. In order to get head and shoulders on 5x7 or 8x10 my bellows draw is so long (on a longer lens) that the exposure is also long and the subjects move resulting in blurry photos.

EdSawyer

24-Aug-2018, 07:06

What you want in those cases would be a tele lens. The Nikkor-T set would be the best choice, I'd speculate. (the 600/800/1200 set, or possibly the 360/500/720 set though I don't think that will cover 8x10 but might likely cover 5x7)

-Ed

Pawlowski6132

24-Aug-2018, 07:10

Huh. I wouldn't have thought of that. By any chance do you have experience with these? What is the principle behind needing less bellows extension than normal lens for similar magnification? I assume the trade off is IQ? Thanx Ed.

Peter De Smidt

24-Aug-2018, 08:29

Won't the bellows compensation be the same with tele and non-tele lenses? Sure, the extension will be less with the tele, but isn't it the magnification that's important for the bellows extension light loss?

Pawlowski6132

24-Aug-2018, 08:31

Won't the bellows compensation be the same with tele and non-tele lenses? Sure, the extension will be less with the tele, but isn't it the magnification that's important for the bellows extension light loss?

I don't think so. I believe that bellows extension is directly a function of the distance between the lens plane and the film plane.

Peter De Smidt

24-Aug-2018, 08:54

Hopefully someone who understands the mathematics will chime in. I'm happy to be corrected.

Dan Fromm

24-Aug-2018, 09:00

The rule is effective f/ number = f/ number set * (magnification + 1)

OP, the distance that matters is the distance between the film plane and the lens' rear node. Telephoto lenses' rear nodes are in front of the lens, that's why they require less extension than lenses of normal construction.

Emmanuel BIGLER

24-Aug-2018, 12:21

Hi!

Regarding the shorter bellows draw of a telephoto lens, the gain is interesting only for far-distant objects. When you come close to the 1:1 ratio, the additional bellows draw beyond the focal point to get a sharp image is the same, about one focal length, for all lens designs.

For example, consider a lens of focal length 360 mm.

If the lens is like an apo ronar, quasi-symmetrical, the distance between the shutter, in the middle of the lens and the focal point is about 360 mm.

In order to reach the 1:1 configuration, you have to add an other 360 mm, total bellows draw = 720 mm.

Now take a 360 Schneider Tele Arton. The flange focal distance is only 210 mm. But if you want to reach the 1:1 magnification ratio, you need to add the same additional 360 mm bellows draw, total draw = 210 + 360 = 570 mm.

Hence you have a gain of 360-210 = 150 mm which is somewhat helpful when the image is close to the focal point i.e. for far-distant subjects, but you cannot avoid the additional bellows draw required by close focusing (see below how to easily compute this).

Won't the bellows compensation be the same with tele and non-tele lenses?

In principle, bellows correction factors differ between a quasi-symmetrical lens design and a telephoto design.

It is quite simple to compute, the only additional parameter that you need to know is the pupillar magnification of your lens Mp = (diameter of exit pupil)/(diameter of entrance pupil).

Some manufacturers like Schneider Kreuznach, do provide the value of this parameter in their detailed technical data-sheet, but unfortunately, old archives on the German Schneider-Kreuznach web site are no longer directly available, however, they are stored in the Web archive "wayback machine".

The general formulae for a telephoto of pupillar magnification factor Mp, of a given (image)/(object) magnification M, is very simple.

First compute the magnification factor "M" vs. the additional bellows extension "ext"

M = ext / f

ext = additional bellows extension beyond the focal point; f = focal length.

This formula M = ext / f for the additional bellows draw "ext" beyond the focal point in order to reach a given magnification M, is universal and valid for all lenses even very asymmetric.

Bellows factor X times = (1 + M / Mp )2

The origin of this somewhat cryptic formula is explained in detail here in this article in French,

http://www.galerie-photo.com/telechargement/pupilles.pdf

The maths are here (again, in French, sorry)

http://www.galerie-photo.com/annexe-pupilles.pdf

You can just have a look at the graph attached here in pdf

This graph is a good summary of the differences between a retrofocus (Mp > 1) a quasi-symmetrical lens (Mp ~= 1, all standard LF lenses) and a telephoto (Mp < 1). For the Schneider 360 mm Tele-Arton, Mp ~= 0.57. This yields a difference of approx one f-stop at 1/1 ratio.

But in principle an assymetric lens should never be used at 1:1 ratio!!

For use at 1:1 ratio, symmetrical lens formulae are preferred, at least for view camera lenses of fixed focal length; modern macro lenses with floating elements are another issue, those lenses are a kind of a zoom lens with focal lenght and a pupillar magnification factor changing throughout the focusing range!

Hence in most usual cases of ordinary, non close-up shots like e.g. M = 1:5 = 0.2 or smaller (i.e. object is located further away than 6 times the focal length) you can safely ignore the additional correction with respect to a quasi-symmetrical lens.

For Mp ~= 0.57 like in the 360 Tele-Arton, at M = 1:5 = 0.2, the Tele-Arton would in principle require only 1/2 f-stop of additional exposure with respect to a quasi-symmetrical lens formula.

The basic formula for Mp = 1, quasi-symmetrical lens like many standarc LF lenses plus apo-repro lenses is simply :

M = ext / f

Quasi-Symmetrical Lenses Bellows factor X times = (1 + M )2

EdSawyer

24-Aug-2018, 14:04

I do have the smaller Nikkor-T set. As mentioned there's some math and theory that goes into it but the Telephoto design by nature needs less extension. The only trade off is probably smaller coverage for the Tele vs. symmetrical lens of similar focal length. (e.g. a 360 plasmat may cover larger film than a 360 tele). That, and movements like tilt or swing become a bit weird due to the location of nodal point. But really no trade-off in IQ, as far as normal shooting goes, esp. for portraits. If anything the Nikkor-T teles should have better IQ than standard plasmats of the same length, since they use more exotic glass.

Steve Goldstein

24-Aug-2018, 14:32

Bellows factor X times = (1 + M / Mp )2

Just to clarify, this is really (1 + (M/Mp))2, right?

Dan O'Farrell

24-Aug-2018, 14:37

Hi!

Regarding the shorter bellows draw of a telephoto lens, the gain is interesting only for far-distant objects. When you come close to the 1:1 ratio, the additional bellows draw beyond the focal point to get a sharp image is the same, about one focal length, for all lens designs.

For example, consider a lens of focal length 360 mm.

If the lens is like an apo ronar, quasi-symmetrical, the distance between the shutter, in the middle of the lens and the focal point is about 360 mm.

In order to reach the 1:1 configuration, you have to add an other 360 mm, total bellows draw = 720 mm.

Now take a 360 Schneider Tele Arton. The flange focal distance is only 210 mm. But if you want to reach the 1:1 magnification ratio, you need to add the same additional 360 mm bellows draw, total draw = 210 + 360 = 570 mm.

Hence you have a gain of 360-210 = 150 mm which is somewhat helpful when the image is close to the focal point i.e. for far-distant subjects, but you cannot avoid the additional bellows draw required by close focusing (see below how to easily compute this).

Won't the bellows compensation be the same with tele and non-tele lenses?

In principle, bellows correction factors differ between a quasi-symmetrical lens design and a telephoto design.

It is quite simple to compute, the only additional parameter that you need to know is the pupillar magnification of your lens Mp = (diameter of exit pupil)/(diameter of entrance pupil).

Some manufacturers like Schneider Kreuznach, do provide the value of this parameter in their detailed technical data-sheet, but unfortunately, old archives on the German Schneider-Kreuznach web site are no longer directly available, however, they are stored in the Web archive "wayback machine".

The general formulae for a telephoto of pupillar magnification factor Mp, of a given (image)/(object) magnification M, is very simple.

First compute the magnification factor "M" vs. the additional bellows extension "ext"

M = ext / f

ext = additional bellows extension beyond the focal point; f = focal length.

This formula M = ext / f for the additional bellows draw "ext" beyond the focal point in order to reach a given magnification M, is universal and valid for all lenses even very asymmetric.

Bellows factor X times = (1 + M / Mp )2

The origin of this somewhat cryptic formula is explained in detail here in this article in French,

http://www.galerie-photo.com/telechargement/pupilles.pdf

The maths are here (again, in French, sorry)

http://www.galerie-photo.com/annexe-pupilles.pdf

You can just have a look at the graph attached here in pdf

This graph is a good summary of the differences between a retrofocus (Mp > 1) a quasi-symmetrical lens (Mp ~= 1, all standard LF lenses) and a telephoto (Mp < 1). For the Schneider 360 mm Tele-Arton, Mp ~= 0.57. This yields a difference of approx one f-stop at 1/1 ratio.

But in principle an assymetric lens should never be used at 1:1 ratio!!

For use at 1:1 ratio, symmetrical lens formulae are preferred, at least for view camera lenses of fixed focal length; modern macro lenses with floating elements are another issue, those lenses are a kind of a zoom lens with focal lenght and a pupillar magnification factor changing throughout the focusing range!

Hence in most usual cases of ordinary, non close-up shots like e.g. M = 1:5 = 0.2 or smaller (i.e. object is located further away than 6 times the focal length) you can safely ignore the additional correction with respect to a quasi-symmetrical lens.

For Mp ~= 0.57 like in the 360 Tele-Arton, at M = 1:5 = 0.2, the Tele-Arton would in principle require only 1/2 f-stop of additional exposure with respect to a quasi-symmetrical lens formula.

The basic formula for Mp = 1, quasi-symmetrical lens like many standarc LF lenses plus apo-repro lenses is simply :

M = ext / f

Quasi-Symmetrical Lenses Bellows factor X times = (1 + M )2

Wow! Interesting !

Now, let us say that we have a 360mm Tele-Xenar designed for a 4X5 frame ( at infinity ).

If we expand the draw to fill a(n) 8X10 frame, what would the additional ( or, preferrably, total) draw...lensboard to film.. be?

We're not expecting a full and finished answer to this, but how would we calculate ( or measure) it?

Dan Fromm

24-Aug-2018, 14:52

360 mm.

The diameter of the image circle at 1:1 is twice the diameter of the image circle at infinity. Extension from the infinity position to 1:1 is 1 focal length.

Pere Casals

24-Aug-2018, 14:56

Hi!

Regarding the shorter bellows draw of a telephoto lens, the gain is interesting only for far-distant objects. When you come close to the 1:1 ratio, the additional bellows draw beyond the focal point to get a sharp image is the same, about one focal length, for all lens designs.

Well, IMHO from what Dan says we can conclude: if at infinite the tele shortens X the bellows draw (compared to a regular lens), then at 1:1 the telephoto will also save the same X bellows draw compared (again) to the regular lens.

The problem I was trying to solve for is that I primarily shoot portraits indoor using natural light. In order to get head and shoulders on 5x7 or 8x10 my bellows draw is so long (on a longer lens) that the exposure is also long and the subjects move resulting in blurry photos.

Add more light or use a shorter lens. The "traditional" longer lens for head and shoulders goes out the window once you start using larger formats due to magnification. Try a normal focal length and see what you get. If you still want the additional compression of a longer lens, well, see my first point. There's no free lunch unfortunately.

Dan O'Farrell

25-Aug-2018, 07:35

360 mm.

The diameter of the image circle at 1:1 is twice the diameter of the image circle at infinity. Extension from the infinity position to 1:1 is 1 focal length.

Ah... much simpler than I had expected.

Thank you, Dan.

Emmanuel BIGLER

25-Aug-2018, 13:41

Hello all!

From Steve Goldstein

Just to clarify, this is really (1 + (M/Mp))2, right?

Yes, exactly, with the additional () inside to avoid any ambiguity.

An equivalent formula would be ((M+Mp)2) / ((Mp)2)

but definitely (1 + (M/Mp))2 is much simpler ;)

-----------------------

from Dan O'Farrell

If we expand the draw to fill a(n) 8X10 frame, what would the additional ( or, preferrably, total) draw...lensboard to film.. be?

I agree with Dan's answer. However the rule of thumb regarding doubling the image circle at 1:1 ratio is just an estimate, not something very precise.

But even for a telephoto design, this should also work, but this property is not easy to explain without doing some ray tracing.

-----------------------

From Pere Casals

if at infinite the tele shortens X the bellows draw (compared to a regular lens), then at 1:1 the telephoto will also save the same X bellows draw compared (again) to the regular lens.

Yes, exactly, so you cannot avoid the additional extension of exactly one focal length beyond the focal point in order to reach the 1:1 ratio with all lenses.

With a telephoto, you keep the gain in distance between the lens and a sharp image throughout the whole focusing range.

For example with the 360 tele Arton, the gain in flange focal distance with respect to a quasi symetrical 360 lens (like most standard lenses, plus all apo-repro lenses) is 150 mm at any magnification ratio.

In fact the only possibility to have less distance between the lens and the image at 1;1 ratio is ... to use a telephoto with a bigger asymmetry, or a lens of shorter focal lenght!

Actually if you have an unknown unmarked positive lens, i.e. allowing you to get a sharp image on a ground glass for a distant object, by measuring the additional extension beyond focal point needed to reach the 1:1 ratio, you get a good estimate of the focal length, even if the lens is a telephoto or whichever thick and complex compound lens design.

In other words, with this old method due to MM. Davanne & Martin at the end of the XIXst century, you can measure the focal length without any knowledge of where the image principal plane H' is located, no need to guess where an hypothetical "optical center" would be located ;)

Pere Casals

25-Aug-2018, 16:31

In other words, with this old method due to MM. Davanne & Martin at the end of the XIXst century, you can measure the focal length without any knowledge of where the image principal plane H' is located, no need to guess where an hypothetical "optical center" would be located ;)

hmmm, this is interesting...

Emmanuel BIGLER

26-Aug-2018, 10:24

from Pere Casals

hmmm, this is interesting...

I realize that Davanne & Martin's method to measure the focal length of a photographic lens has been explained here by Leigh

http://www.largeformatphotography.info/forum/showthread.php?114173-How-to-determine-focal-length-of-unmarked-lens&p=1144455&viewfull=1#post1144455

The attached diagram below explains the principles. I have chosen an hypothetical lens with principal planes H and H' "crossed" and located far from each other.

The method appears in « Traité encyclopédique de photographie, » by Charles Fabre, Paris, Gauthier-Villars et fils, 1889-1906, 1st supplement A, page 19

http://cnum.cnam.fr/CGI/fpage.cgi?8KE304.5/20/100/402/11/381

(thanks to Dan Fromm for finding this golden treasury of XIXst century photographic literature)

---------

Regarding what happens in real life with a real asymmetric tele lens, the other diagram (as an external link next line) shows the 360 Schneider Tele-Arton (effective focal length = 353 mm) with its principal planes H & H' and its pupils, entrance and exit.

http://bigler.blog.free.fr/public/docs-en-pdf/2018-08-26-tirage-tele-arton-360-EN.pdf

The pupils are not located at the principal planes, H and H' are not crossed but H & H' are floating in air in front of the first lens vertex.

Regarding the increase in image circle at magnification 1:1 or so, looking for the distance requested to fill the 8x10" format, after thinking about it twice, I think that we have to count distances from the exit pupil of the lens and not the principal plane H'. It is when the distance between the exit pupil and the image point is doubled, that the illuminated image circle is doubled.

If we plot extreme rays crossing the exit pupil and corresponding to the absolute limit of the illuminated circle, we define a cone which is fixed with respect to the lens. Hence the maximum illuminated circle for any image position is defined by the intersection of the image plane and the extreme cone of rays propagating from the center of the exit pupil and blocked by some parts of the lens mount.

Again in all questions related to image illumination, or limits of illuminated image field, principal planes play no role, only the position of the main iris (stop) and other obstructing diaphragms, and the position of their images in image space actually count.

However F & F' the focal points; H and H' the principal (or nodal points N & N', they are identical to F & F') of course, tell us where to look for a sharp image for a given object distance, and what the magnification will be, regardless of pupils.

Dan Fromm

26-Aug-2018, 10:57

For those of you who'd like to peruse Fabre's monumental Traité encyclopédique de photographie, there are links to it in the list's Ancient lenses section. Thanks, Emmanuel, for mentioning it. My piece on Berthiot's anastigmats, there's a link to it in the list, has a brief biographical sketch of Fabre. There were giants in those days.

Peter De Smidt

26-Aug-2018, 12:45

I've read through the thread, but I'm a bit slow. At 1:1, is there any exposure advantage for a telephoto versus a regular lens? How about at 1:2, or whatever head-shot magnification is with an 8x10?

Dan Fromm

26-Aug-2018, 13:14

Peter, depth of field is controlled by magnification and aperture. Focal length and lens construction have nothing to do with it.

Exposure (aperture and shutter speed to use) given magnification, which you asked about, is controlled by film speed and illumination. Focal length and lens construction have nothing to do with it. If you want to shoot at a smaller aperture or with a higher shutter speed, use faster film or turn the lights up.

Teles need less extension than lenses of normal construction to get the magnification desired. That's the tele advantage. Teles have less coverage and, usually, worse distortion than lenses of normal construction. That's the tele disadvantage.

Peter De Smidt

26-Aug-2018, 13:28

This was the original problem: " The problem I was trying to solve for is that I primarily shoot portraits indoor using natural light. In order to get head and shoulders on 5x7 or 8x10 my bellows draw is so long (on a longer lens) that the exposure is also long and the subjects move resulting in blurry photos." -Pawlowski6132 Someone suggested getting a telephoto lens to help with this. My view was that a telephoto lens wouldn't allow a shorter exposure given a fixed scene/film magnification size and depth of field. I take Dan's helpful summary to confirm this. Is that correct?

Dan Fromm

26-Aug-2018, 14:09

That's right. Can you use faster film?

Emmanuel BIGLER

26-Aug-2018, 14:58

From Peter De Smidt

At 1:1, is there any exposure advantage for a telephoto versus a regular lens?

Hi, Peter, unfortunately, on the contrary, there is a loss ;)

For a 360 mm Tele Arton with pupillar magnification = 0.57, the loss at 1:1 is about one additional f-stop with respect to a quasi-symmetrical lens formula.

How about at 1:2, or whatever head-shot magnification is with an 8x10?

Well at 1:2 i.e. M = 0.5, the additional loss is about 2/3 of a f-stop for a pupillar magnification = 0.57 (tele arton 360), the total correction factor with respect to infinity-focus is about 1+2/3 f-stop.

Bob Salomon

26-Aug-2018, 15:09

That's right. Can you use faster film?

Or learn how to use flash to simulate natural light.

Pere Casals

26-Aug-2018, 15:19

Here there is an interesting reading about effective aperture depending on magnification, explaining what Emmanuel points:

http://www.coinimaging.com/effap_pupil.html

https://web.archive.org/web/20180101064344/http://www.coinimaging.com/effap_pupil.html

I guess that a quasi symetric glass holds its speed for macro, while a tele not.

Pere Casals

26-Aug-2018, 15:24

Or learn how to use flash to simulate natural light.

good point...

Emmanuel BIGLER

27-Aug-2018, 10:25

From Pere Casals

http://www.coinimaging.com/effap_pupil.html

https://web.archive.org/web/20180101064344/http://www.coinimaging.com/effap_pupil.html

Many thanks, Pere, for this very clear explanation. the only comment I would have is that the f-number for an assymetric lens is defined with respect to the relative size of the entrance pupil.

The diagrams shown here http://www.coinimaging.com/effap_pupil.html

explain very clearly how the angle under which the exit pupil is seen from the image plane varies for an asymmetric lens when increasing the image magnification.

Hence it is a good formulation to summarize: in macro work, you have a gain in effective speed for a retrofocus and a loss for a telephoto.

I keep this brief sentence for further use on my favourite photo forums ;)

Pere Casals

27-Aug-2018, 10:29

Hence it is a good formulation to summarize: in macro work, you have a gain in effective speed for a retrofocus and a loss for a telephoto.

I keep this brief sentence for further use on my favourite photo forums ;)

I was suspecting that... but I was not sure, thanks for making it clear.

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