Vignetting Analysis - 47mm to 1200mm Lenses

[genotypewriter]

Hi everyone,

I computed the relative light gathering differences for typical LF lens focal lengths from 47mm to 1200mm, based on the cosine fourth formula (assuming non-tele and non-retro), and made the following illustrations:

http://www.flickr.com/photos/genotypewriter/5774394382

There's a bit of a write up on what the above info is and isn't in the page above which might be good to read to understand the intentions.

Would like to hear people's thoughts :)

G

[vinny]

My head hurts now. Every make of lens in the same focal length is different, as you said below your graph. I'm not sure of the practical value of this information.

[timparkin]

My head hurts now. Every make of lens in the same focal length is different, as you said below your graph. I'm not sure of the practical value of this information.

Thanks for this - it confirms a symptom that a colleague has observed through experience.

Tim

[Dan Fromm]

Example practical use of values in table: You have a 150mm lens and you're considering on getting a 65mm lens (1/2 field of view) to gain 1 stop in the amount of light needed (at same f-number) to keep the camera steady. While this benefit is valid, you will also lose slightly over 2/3 stops of overall frame brightness due to the increased vignetting in the wider lens. So the final theoretical gain would be only 1/3 stop.

WTF? 75 mm, no 65 mm, gives double the field of view of a 150 mm if both lenses cover the same format. Check your arithmetic. Proofread with care.

And, does anyone here change focal length to get more (or less) illumination? I certainly don't. The text I quoted seems very silly.

OP, are you aware that some, not all, modern wide angle lenses beat cos^4?

[GPS]

Of no practical use I can think of. If you're not happy with light fall of your lens use a centre filter.

[Leigh]

I'm inclined to use the manufacturers' data rather than yours.

Sorry.

- Leigh

[genotypewriter]

Hi folks,

Science doesn't have one magic equation that describes the whole universe yet but that hasn't stopped mankind from trying to understand it from the different things we already understand. Similarly, the purpose of this vignetting analysis is to understand the standard "cosine fourth" form of vignetting that is common to all lenses of these focal lengths, as stated, as long as the lenses are not of retrofocus or telephoto designs.

Design and build specific vignetting takes place in addition to the numbers I've given. So while the final relative illumination curve can take a different shape for a particular lens, the cosine fourth light loss constitutes to it as well.



WTF? 75 mm, no 65 mm, gives double the field of view of a 150 mm if both lenses cover the same format. Check your arithmetic. Proofread with care.Dan,
Not sure why you think arithmetic alone is sufficient. We need trigonometry to calculate the field of view:

http://en.wikipedia.org/wiki/Angle_of_view#Example

You might want to plug in the numbers.



And, does anyone here change focal length to get more (or less) illumination? I certainly don't. The text I quoted seems very silly.Sure, you and the people you know may not but the fact remains true. Under practical conditions, especially when stability is a concern, wider lenses can manage longer exposures than longer lenses of the same f number (for the same framing) giving the sense of being able to gather more light. At the same time, the cosine fourth vignetting takes its toll in the total amount of light gathered.

That is just one situation where the numbers I gave may be useful in a lens buying decision. I'm not saying that just because the table or the purpose of the table is not understood, one can't take good photos.



OP, are you aware that some, not all, modern wide angle lenses beat cos^4?These modern WA lenses that you refer to are retrofocus designs. Which is why I said in many places that these numbers can't be applied (at least directly) to retrofocus and also telephoto lens designs.

Also "beat" is a funny thing to say here because the absolute light fall off itself is not overcome. Retrofocus lenses need a longer extension from the imaging plane than an ordinary lens for the same focus distance. This makes the retrofocus WA lens have the cosine fourth vignetting of a longer ordinary lens, which would be less than that of a ordinary WA lens.

Like I said, the cosine fourth light fall off takes place after the light leaves the rear of the lens and before it hits the imaging plane. The absolute loss can't be overcome.



Of no practical use I can think of. If you're not happy with light fall of your lens use a centre filter.Agree that a center filter is good choice. A long lens can't replace a wide one all the time. But like I said to Dan, when the total amount of light gathered is of concern, the table can provide some insight.

Thanks everyone for the replies!

GTW

[Dan Fromm]

Not sure why you think arithmetic alone is sufficient. We need trigonometry to calculate the field of view:

Thanks for the correction. This is what I get for posting before the coffee has kicked in.


These modern WA lenses that you refer to are retrofocus designs.

Biogons retrofocus? Are you sure? With them illumination on film falls off with roughly cos^3. And with them back focus is roughly half focal length.

I think you're in 35 mm SLR territory and not thinking about the differences between lenses for them and lenses for LF cameras. There are very few retrofocus lenses for formats larger than 6x7. Name a few, please.


Under practical conditions, especially when stability is a concern, wider lenses can manage longer exposures than longer lenses of the same f number (for the same framing) giving the sense of being able to gather more light.

Explain further. I don't see how shooting at lower magnification and enlarging more in printing will reduce motion blur in the final print.

[genotypewriter]

Thanks for the correction. This is what I get for posting before the coffee has kicked in.
Always a pleasure :)


Biogons retrofocus? Are you sure? With them illumination on film falls off with roughly cos^3. And with them back focus is roughly half focal length.

I think you're in 35 mm SLR territory and not thinking about the differences between lenses for them and lenses for LF cameras. There are very few retrofocus lenses for formats larger than 6x7. Name a few, please.As I said many times, Dan, this is a simplified analysis and is not meant to be taken as the gospel of vignetting :) Yes, by optimising and increasing the complexity of the computations (e.g. incorporation BFL, etc. and other parameters) the figures can be made more accurate for some lenses, at the same time it can become less accurate for others. You might also want to read this:

http://en.wikipedia.org/wiki/No_free_lunch_in_search_and_optimization


Explain further. I don't see how shooting at lower magnification and enlarging more in printing will reduce motion blur in the final print.I don't think I ever said anything like that. Crosstalk from a different discussion perhaps? :)

GTW

[Dan Fromm]

GTW, you wrote this:


Under practical conditions, especially when stability is a concern, wider lenses can manage longer exposures than longer lenses of the same f number (for the same framing) giving the sense of being able to gather more light.

Please explain.

And while you're at it, since you're posting in an LF forum please give a list of retrofocus lenses for LF.

[genotypewriter]

GTW, you wrote this:
Please explain.
The relationship between shake and angle of view is well known. I took the liberty of making some animations for you to better illustrate this:

Constant angular movement vs. dimensional movement for a wide lens:
http://www.youtube.com/watch?v=UicWx3Y221g

Constant angular movement vs. dimensional movement for a long lens:
http://www.youtube.com/watch?v=0aq-P42QMtE


And while you're at it, since you're posting in an LF forum please give a list of retrofocus lenses for LF.Dan, you were being vague as to what "modern wide angle lenses" meant. So for the sake of the conversation, I thought you were referring to some retrofocus design. Also I find it curious as to why you put this much effort in to finding ways in which the analysis is limited when those very limitations are clearly stated to begin with.

Also, why do you refer to Biogons as "modern wide angle lenses"? They're certainly not modern.

Enjoy!

GTW

[Leigh]

Under practical conditions, especially when stability is a concern, wider lenses can manage longer exposures than longer lenses of the same f number (for the same framing) giving the sense of being able to gather more light.
That is absolute horse droppings. :eek:

Wider lenses do not "gather more light" in any sense of the word.

Camera shake is why real photographers use tripods.

- Leigh

[Dan Fromm]

Under practical conditions, especially when stability is a concern, wider lenses can manage longer exposures than longer lenses of the same f number (for the same framing) giving the sense of being able to gather more light.

At the same magnification, which is what I think your "same framing" means, angular velocity due to shake or subject motion is independent of focal length. So what do you mean when you say that focal length matters?

For the third time, please provide a list of retrofocus lenses for formats no smaller than 4" x 5".


Also I find it curious as to why you put this much effort in to finding ways in which the analysis is limited when those very limitations are clearly stated to begin with.

The person who posts as Leigh and I rarely agree about much. This time I think he's right about your analysis. I don't see the point of it and I think you are dead wrong about the effects of using a short instead of a long lens on image quality.

[genotypewriter]

That is absolute horse droppings. :eek:
Whoa, slow down there! This is the first time I'm hearing about the subject of absolute and relative facelogy ;)

However...



Wider lenses do not "gather more light" in any sense of the word.

Camera shake is why real photographers use tripods.

I see you understand the validity of my point. What is and is not photography is not a discussion we need to get in to when trying to understand physics. I think it's fair if not all of us are a bunch of bearded fellows sitting around a fire, interested only in talking about what photography is and good old times were :)




At the same magnification, which is what I think your "same framing" means, angular velocity due to shake or subject motion is independent of focal length. So what do you mean when you say that focal length matters?
I think you must pay a bit more close attention when reading, Dan. I said this same thing in several different ways and allow me to try yet another: when the angular movement of the lenses are constant, a wider lens can manage longer exposure times than a longer lens, for the same subject framing/magnification.



For the third time, please provide a list of retrofocus lenses for formats no smaller than 4" x 5".I really don't understand why you keep asking for this, Dan, when I never claimed that these numbers are applicable to retrofocus or telephoto designs.

But what's a valid curiosity is why you refer to the 1930s Biogon design as a "modern wide angle" design.




The person who posts as Leigh and I rarely agree about much. This time I think he's right about your analysis. I don't see the point of it and I think you are dead wrong about the effects of using a short instead of a long lens on image quality.As I've explained from the start, the presented analysis is not without limitations. But in my experience as scientist, I've never read or published conclusions such as "dead wrong" or "absolute horse droppings". Perhaps it's the same issues that lead one to such difficulties in articulation that cause the difficulty in understanding technical material. And by technical material, I don't mean some dusty encyclopedia of old lenses from a garage sale that one may have chosen to memorise.

Best wishes

GTW

[Dan Fromm]

Re Biogons, Biogon is really a trade name even though we use it to refer to a design type.

The Biogon you're thinking of is a Sonnar derivative, 6 elements in 4 groups. The Biogon I'm thinking of is a Russar derivative, uses the tilting exit pupil trick, has 8 elements in 5 groups. See US Patent US 2,721,499.


when the angular movement of the lenses are constant, a wider lens can manage longer exposure times than a longer lens, for the same subject framing/magnification.

Insisting won't make it so.

[Dan Fromm]

These modern WA lenses that you refer to are retrofocus designs. Which is why I said in many places that these numbers can't be applied (at least directly) to retrofocus and also telephoto lens designs.


:
Originally Posted by Dan Fromm View Post
For the third time, please provide a list of retrofocus lenses for formats no smaller than 4" x 5".
I really don't understand why you keep asking for this, Dan, when I never claimed that these numbers are applicable to retrofocus or telephoto designs.

You asserted existence. Prove it for lenses that cover at least 4x5. I clicked the links under your signature. You're a small format mainly SLR photographer, generalize wrongly from lenses made for SLRs to lenses made for cameras without mirror boxes.

[Ole Tjugen]

1: "Vignetting" is the wrong term for this: that refers to light being blocked by a physical obstruction. "Falloff" is better.

2: Most reasonably modern wide angle lenses do slightly better than cos^4, and slightly worse than cos^3. Old compact WA lenses like the Schneider Angulon are close to cos^4, but even THAT old lens does slightly better. "Modern WA" refers to what is also called "wasp waist" - i.e. oversized negative front and rear elements. The construction is old, though.

3: Camera shake in LF is usually avoided by using a tripod, not shorter exposure times or shorter lenses.

4: While there are no true retrofocus lenses for LF; you would be surprised at the number that are slightly retrofocus. It makes no practical difference as it is usually only by a few mm, but it does matter if you are tuning the front tilt axis location for a specific lens.

[GPS]

...
4: While there are no true retrofocus lenses for LF; you would be surprised at the number that are slightly retrofocus. It makes no practical difference as it is usually only by a few mm, but it does matter if you are tuning the front tilt axis location for a specific lens.

I think you're wrong on the 4th point, Ole. You're probably speaking about a difference between the nominal and the actual focal length of LF lenses. Nobody would make a retro-focus lens design to gain "a few mm" of the retro-focus length. Not in LF lenses.

[GPS]

Agree that a center filter is good choice. A long lens can't replace a wide one all the time. But like I said to Dan, when the total amount of light gathered is of concern, the table can provide some insight.

Thanks everyone for the replies!

GTW

When light fall off is of concern a centre filter comes to rescue. Centre filters are assigned to specific lenses. To hell with your table.

[Ole Tjugen]

I think you're wrong on the 4th point, Ole. You're probably speaking about a difference between the nominal and the actual focal length of LF lenses. Nobody would make a retro-focus lens design to gain "a few mm" of the retro-focus length. Not in LF lenses.

I agree, you misunderstand: That many lenses are slightly retrofocus is a side effect of other design criteria, not a design criterion in itself.

And no, I'm not referring to differences between stated and effective focal length, but to the lens node being behind the aperture plane. In some cases quite a long way behind, as I discovered when I was playing around with one of the few cameras to have an adjustable tilt axis position.

[GPS]

I agree, you misunderstand: That many lenses are slightly retrofocus is a side effect of other design criteria, not a design criterion in itself.

And no, I'm not referring to differences between stated and effective focal length, but to the lens node being behind the aperture plane. In some cases quite a long way behind, as I discovered when I was playing around with one of the few cameras to have an adjustable tilt axis position.


Retro-focus lenses are of a specific retro-focus design, not a side effect of other (non-retrofocus) design criteria. The difference between the nominal and the actual focal length is a result of design criteria compromises.
Now, the placement of the aperture does not make a lens retro-focus or not in its optical design. You put too many veggies into the casserole.

[Ole Tjugen]

There are other reasons that flange focal distance for making a lens with a (slightly) negative front cell and a positive rear cell - result, retrofocus. One way to discover this is to measure the positions of the principal optical points. Which essentially is what I do when I adjust the position of the tilt axis relative to the physical center of the lens.

You seen to be deliberately misunderstanding today...

[GPS]

There are other reasons that flange focal distance for making a lens with a (slightly) negative front cell and a positive rear cell - result, retrofocus. One way to discover this is to measure the positions of the principal optical points. Which essentially is what I do when I adjust the position of the tilt axis relative to the physical center of the lens.

You seen to be deliberately misunderstanding today...

Only after checking numbers on SW Nikon lenses I understood what you meant by retrofocus lenses that are not true retrofocus lenses...:) Have to say that for the same reason for which you regard them as retrofocus lenses I regard them as not retrofocus lenses...:)

[Dan Fromm]

GPS, if you look at specs for 6/4 double Gauss types, e.g., f/1.4, f/1.9, f/2.8 Boyer Saphirs, you'll see that many are a little telephoto. This is why, in the early days of 35 mm SLRs, there were so many 58/1.4 "normal" lenses for them; their designers couldn't come up with anything shorter that performed well enough and that made infinity on the cameras. In time, this changed.

The lenses that Ole mentioned are a little retrofocus. But they're not designed to clear SLRs' mirrors and none of the ones that are shorter than, say, a Graflex SLR's standard issue lens, can be used on, to keep the example consistent, a Graflex SLR. Would that that were possible.

[GPS]

GPS, if you look at specs for 6/4 double Gauss types, e.g., f/1.4, f/1.9, f/2.8 Boyer Saphirs, you'll see that many are a little telephoto. This is why, in the early days of 35 mm SLRs, there were so many 58/1.4 "normal" lenses for them; their designers couldn't come up with anything shorter that performed well enough and that made infinity on the cameras. In time, this changed.

The lenses that Ole mentioned are a little retrofocus. But they're not designed to clear SLRs' mirrors and none of the ones that are shorter than, say, a Graflex SLR's standard issue lens, can be used on, to keep the example consistent, a Graflex SLR. Would that that were possible.

Dan,
in photographic optics, a retrofocus design (Ole would say - true retrofocus - design, but that expression, unlike its telephoto equivalent, is not normally used speaking about retrofocus lenses) is meant as having the back focal length about at least twice the lens focal length. A "little" retrofocus has different reasons to be (Ole knows it, it seems) - hence my misunderstanding when reading his retrofocus remarks. I was too used to the scholar retrofocus definition.
But I wouldn't like to honour this thread with this discussion - as I consider it simply too a stupid thread... :)
BTW I agree with you that the OP misunderstands the LF photography world and forces on it his SLR photography knowledge.

[Dan Fromm]

GPS, I'm glad that you checked your assumptions.

I just checked mine with respect to the idea that at the same magnification using a shorter lens will reduce blur caused by camera motion (rotation about some point near it, probably) or by subject motion (translation in the subject plane, the two are roughly equivalent). Made a simple geometric model, found that when a point in the subject plane moves the distance the corresponding point in the image plane moves is independent of the lens' focal length.

Would that we had blur-reducing magic!

[GPS]

Hat down for your effort - you should be paid...:)

[Ole Tjugen]

One of my many "oddities" is a Schneider Xenar Typ D. I was surprised to find that first of all it is NOT a triplet as all literature says, but a "reverse Tessar". The second surprise was that the front cell has a small but definite negative power, not enough to make it a "mirror clearing retrofocus lens", but enough that it is right at the limit of the axis shift on my Carbon Infinity. Not that I'd like to use front tilt with that lens anyway, it barely covers 4x5" straight on..

[GPS]

One of my many "oddities" is a Schneider Xenar Typ D. ...

Yup. Typ D - one of those lenses Typ Dan with blur-reducing magic. Keep it - as Kerry Thalmann would say - a future classic...:)

[rdenney]

Back to the confusion about "modern" wides.

Modern in the large-format world is roughly post-war. WWII, that is. In some cases, the Great War.

In wide angle lenses, it's really shift from double anastigmats to biogon-derived designs like the Super Angulon. If you look through a Super Angulon from a shallow angle, you'll see that the aperture remains quite round, rather than becoming oval as with other designs. This is only apparent when stopped down far enough to avoid vignetting by the barrel. This fall-off thus is only caused by the shallow angle, not also by the ovalization of the aperture when viewed obliquely.

Rick "noting that the Super Angulon defines 'modern' wide-field design, despite being 60 years old" Denney

[GPS]

Back to the confusion about "modern" wides.

Modern in the large-format world is roughly post-war. WWII, that is. In some cases, the Great War.

...

In some cases also the Great Patriotic War...;)

[Dan Fromm]

Rick "noting that the Super Angulon defines 'modern' wide-field design, despite being 60 years old" Denney

Why not the f/4.5 Biogon or its precessor from Wild Heerbrugg the Aviogon?

[genotypewriter]

You asserted existence. Prove it for lenses that cover at least 4x5. I clicked the links under your signature. You're a small format mainly SLR photographer, generalize wrongly from lenses made for SLRs to lenses made for cameras without mirror boxes.Actually I didn't assert, Dan... seem like this thread has brought up quite a few things you need to look up. Also it has no relevance to the correctness of this analysis as it states its limitations.

As I see it, you're just trying to be plain dismissive but it can only be tasteful and impressive on your part if you could come up with a limitation that the analysis did not point out about itself, right? Otherwise it's just plagiarism, at best. Getting feedback on what I may have missed was the reason why I posted this here. And right now, I just thought of one possible problem with it and no one even questioned about, pointing it out aside... I'll double check the numbers and re-upload if necessary. Will let you in on the surprise later... ;)



1: "Vignetting" is the wrong term for this: that refers to light being blocked by a physical obstruction. "Falloff" is better.

2: Most reasonably modern wide angle lenses do slightly better than cos^4, and slightly worse than cos^3. Old compact WA lenses like the Schneider Angulon are close to cos^4, but even THAT old lens does slightly better. "Modern WA" refers to what is also called "wasp waist" - i.e. oversized negative front and rear elements. The construction is old, though.

3: Camera shake in LF is usually avoided by using a tripod, not shorter exposure times or shorter lenses.

4: While there are no true retrofocus lenses for LF; you would be surprised at the number that are slightly retrofocus. It makes no practical difference as it is usually only by a few mm, but it does matter if you are tuning the front tilt axis location for a specific lens.Interesting stuff, Ole... thanks for sharing. Re. 3, I do agree that most people use tripods but:

1. There are limitations to what tripods can do. E.g. Say, if you're on a bridge.

2. I solely think that "large format" is about the size of the film and an optic that covers it. So the use of tripods or any other support/stabilisation methods depend on the weight of the camera and the subject matter that's being shot.



When light fall off is of concern a centre filter comes to rescue. Centre filters are assigned to specific lenses. To hell with your table.I'm sure you know, a center filter doesn't increase the amount of light a lens already gathers, which is what this table is about. A center filter worsens the overall light fall off. Like I've said earlier, not understanding the purpose of the table doesn't mean you can't take good pictures... but understanding it might be of use to some.


GTW

[genotypewriter]

GPS, I'm glad that you checked your assumptions.

I just checked mine with respect to the idea that at the same magnification using a shorter lens will reduce blur caused by camera motion (rotation about some point near it, probably) or by subject motion (translation in the subject plane, the two are roughly equivalent). Made a simple geometric model, found that when a point in the subject plane moves the distance the corresponding point in the image plane moves is independent of the lens' focal length.

Would that we had blur-reducing magic!The problem with your model, Dan, is you put in subject motion... please point to where I said subject motion induced blur reduced with wider lenses? Otherwise I hope you understand the problem in your understanding.

Also who needs a geometric model to understand that subject motion is the same regardless of the angle of view if the framing/magnification is the same? That's like proving an apple is edible. Might want to patent that.

GTW

[genotypewriter]

I was surprised to find that first of all it is NOT a triplet as all literature says, but a "reverse Tessar".Ole, I have some lenses I want to accurately count the number of elements on. And some of these elements maybe cemented (would shining a light and counting be accurate enough?).

Any suggestions as to how I could count them accurately without doing serious dismantling would be great. The front and back lens cells can be taken apart.

Thanks,
GTW

[Ole Tjugen]

Counting reflections is usually enough, in the difficult cases I use a binocular loupe to focus on successive surfaces. Not even the best cemented surfaces are completely perfect, at 40x magnification it is always possible to see some small impurities. Some very odd lenses have one or more nearly flat internal surfaces which can be extremely difficult to see with reflections, and then the very strong loupe is the only way. Like the 10-in-3 Schneider Göttingen Aerotar (a 4+3+3 "ÜberHeliar").

[genotypewriter]

Counting reflections is usually enough, in the difficult cases I use a binocular loupe to focus on successive surfaces. Not even the best cemented surfaces are completely perfect, at 40x magnification it is always possible to see some small impurities. Some very odd lenses have one or more nearly flat internal surfaces which can be extremely difficult to see with reflections, and then the very strong loupe is the only way. Like the 10-in-3 Schneider Göttingen Aerotar (a 4+3+3 "ÜberHeliar").Thanks for the tip on focusing on surfaces... might also work if I use a SLR with a macro lens and a focusing rail. Will try that...

G