Focal length of macro lenses at 1:1

[Lenshood]

Hi everyone,

I have a bit of a tech question.
- What happens with the focal length of modern macro lenses (e.g. my Canon 100 mm f2.8) at 1:1 magnification?
(- and how does the lens do it?)

I read https://www.cambridgeincolour.com/tu...cro-lenses.htm about the increase in effective f-stop and understand that.

However I am confused about what happens to the real focal lentgh. I am juggling with four options, that contradict each other:
- the link mentions that at 1:1 the effective focal length moves out to twice the focal lenggh of the camera (f = 200mm?).
- when I look through my 100 mm macro lens and decrease the focus from infinity to very close I see the FOV narrows, implying a longer focal length. Or is this effective focal length?
- however my canon 100 mm has subject distance of 300mm at 1:1. This gives a focal length of 75 mm.
- most Internet foca discussions state that focal distances of macro lens decrease at 1:1. Expensive 180mm macro lenses may even reduce to 120-130mm and 100mm lenses usually have 70mm at 1:1

So what to make of this? Anyone who knows how this works? Or that can give more background info?

Regards

[tclune]

Well, I learned photography from my father on his old Crown Graphic. That analog camera showed everything that it was doing, which was great for learning. If you wanted to shoot 1:1 macro, you would double-extend the bellows. So in my experience, you double the focal length to do macro work with a normal lens. FWIW.

[xpatUSA]

Hi everyone,

I have a bit of a tech question.
- What happens with the focal length of modern macro lenses (e.g. my Canon 100 mm f2.8) at 1:1 magnification?
(- and how does the lens do it?)

So what to make of this? Anyone who knows how this works? Or that can give more background info?

Regards

My two pence:

None of my three older Sigma macro lenses conform exactly to the thin lens formula, the one that gives 2X the focal length at 1:1:

105mm:- 313mm
70mm:- 257mm
50mm:- 189mm

All three extend when adjusted if that means anything.

Which brings us to entrance and exit pupils and their effect on focal length. I'm not knowledgeable on that, so here's some links that may - or may not - help:

http://photo.net/nikon-camera-forum/008dgO

https://en.wikipedia.org/wiki/Talk:C...t_%28optics%29

http://www.panoguide.com/forums/qna/3312/

http://photo.net/large-format-photography-forum/004ybL

[edit] also found this in Wikipedia for entrance pupil:

The geometric location of the entrance pupil is the vertex of the camera's angle of view and consequently its center of perspective, perspective point, view point, projection centre or no-parallax point.

. . . .

Increasing the focal length of a lens (i.e., zooming in) will usually cause the f-number to increase, and the entrance pupil location to move further back along the optical axis.

Looks relevant, especially if we know which way is "further back"

good luck . . .

[xpatUSA]

Found more in good ole' Wiki-land:

The effective focal length is nearly equal to the stated focal length of the lens (F), except in macro photography where the lens-to-object distance is comparable to the focal length. In this case, the magnification factor (m) must be taken into account:


[this is the simple formula that doubles the focal length at m = 1]

(In photography is usually defined to be positive, despite the inverted image.) For example, with a magnification ratio of 1:2, we find and thus the angle of view is reduced by 33% compared to focusing on a distant object with the same lens.
A second effect which comes into play in macro photography is lens asymmetry (an asymmetric lens is a lens where the aperture appears to have different dimensions when viewed from the front and from the back). The lens asymmetry causes an offset between the nodal plane and pupil positions. The effect can be quantified using the ratio (P) between apparent exit pupil diameter and entrance pupil diameter. The full formula for angle of view now becomes:^[5]


[this might be why the effective focal length at m=1 is not 2X]

N.B. Click on the formulae above to see them a bit mo' better.

[george013]

Hi everyone,

I have a bit of a tech question.
- What happens with the focal length of modern macro lenses (e.g. my Canon 100 mm f2.8) at 1:1 magnification?
(- and how does the lens do it?)

I read https://www.cambridgeincolour.com/tu...cro-lenses.htm about the increase in effective f-stop and understand that.

However I am confused about what happens to the real focal lentgh. I am juggling with four options, that contradict each other:
- the link mentions that at 1:1 the effective focal length moves out to twice the focal lenggh of the camera (f = 200mm?).
- when I look through my 100 mm macro lens and decrease the focus from infinity to very close I see the FOV narrows, implying a longer focal length. Or is this effective focal length?
- however my canon 100 mm has subject distance of 300mm at 1:1. This gives a focal length of 75 mm.
- most Internet foca discussions state that focal distances of macro lens decrease at 1:1. Expensive 180mm macro lenses may even reduce to 120-130mm and 100mm lenses usually have 70mm at 1:1

So what to make of this? Anyone who knows how this works? Or that can give more background info?

Regards

I haven't a straight answer for you. But what I've learned is that the focal length decreases with macro. The magic is floating elements. This link might give you some direction to look at. http://www.outdoorphotographer.com/g...ens-guide.html
What's called here as "effective focal length" is the image distance.
How do you measure the figures you give?

George

[xpatUSA]

I haven't a straight answer for you. But what I've learned is that the focal length decreases with macro. The magic is floating elements. This link might give you some direction to look at. http://www.outdoorphotographer.com/g...ens-guide.html

George, could you explain what is meant by "the focal length decreases with macro" a little more explicity?

Some people might find it confusing and I'm wondering if it should say "the focusing distance is smaller for macro lens" - or words to that effect? Like it says in your link:

There are a number of things that make macro lenses different from non-macro lenses, but the biggest is that macro lenses can focus much closer than non-macro lenses.

I'm asking because your statement appears to be opposite to what is said in my Wiki quote in the preceding post.

[george013]

[QUOTE=xpatUSA;460744]George, could you explain what is meant by "the focal length decreases with macro" a little more explicity?

Some people might find it confusing and I'm wondering if it should say "the focusing distance is smaller for macro lens" - or words to that effect? Like it says in your link:

I'm asking because your statement appears to be opposite to what is said in my Wiki quote in the preceding post.

Getting closer to the subject, the focal length decreases. It's the same as Tom wrote

most Internet foca discussions state that focal distances of macro lens decrease at 1:1. Expensive 180mm macro lenses may even reduce to 120-130mm and 100mm lenses usually have 70mm at 1:1

The formules in your link are unreadable on my screen. But maby you mean this

None of my three older Sigma macro lenses conform exactly to the thin lens formula, the one that gives 2X the focal length at 1:1:

105mm:- 313mm
70mm:- 257mm
50mm:- 189mm

I don't kno what you measured.


For some reason I have a smaller edit-window when I'm already logged in. And no preview bottum.

George

[Black Pearl]

The focal length does increase (not decrease as George has suggested) and from what I can find at 1:1 it doubles:

https://www.cambridgeincolour.com/tu...cro-lenses.htm

LENS EXTENSION & EFFECTIVE F-STOP

In order for a camera lens to focus progressively closer, the lens apparatus has to move further from the camera's sensor (called "extension"). For low magnifications, the extension is tiny, so the lens is always at the expected distance of roughly one focal length away from the sensor. However, once one approaches 0.25-0.5X or greater magnifications, the lens becomes so far from the sensor that it actually behaves as if it had a longer focal length. At 1:1 magnification, the lens moves all the way out to twice the focal length from the camera's sensor

[george013]

The focal length does increase (not decrease as George has suggested) and from what I can find at 1:1 it doubles:

https://www.cambridgeincolour.com/tu...cro-lenses.htm

LENS EXTENSION & EFFECTIVE F-STOP

In order for a camera lens to focus progressively closer, the lens apparatus has to move further from the camera's sensor (called "extension"). For low magnifications, the extension is tiny, so the lens is always at the expected distance of roughly one focal length away from the sensor. However, once one approaches 0.25-0.5X or greater magnifications, the lens becomes so far from the sensor that it actually behaves as if it had a longer focal length. At 1:1 magnification, the lens moves all the way out to twice the focal length from the camera's sensor

Time for the definitions.
The focal length of a 100mm lens is 100mm. It.s the distance optical center to the focal point.
The image distance is the distance from the optical center to the sensor.
The object distance is the distance from the optical center to the object.

So what here is called the "effective focal length" is the image distance.
A lens doesn't behave

as if it had a longer focal length.

, A 100mm stays a 100mm.

George

[xpatUSA]

None of my three older Sigma macro lenses conform exactly to the thin lens formula, the one that gives 2X the focal length at 1:1:

105mm:- 313mm
70mm:- 257mm
50mm:- 189mm

I don't know what you measured.
George

I should have explained: The values quoted are the minimum focusing distances (not focal lengths) in the lens specifications found in http://slrgear.com/reviews/showcat.php/cat/30. All of them are true 1:1 macro lenses.

Now, thin-lens theory indicates that the distance from the subject to the film plane is 4X the lens nominal focal length at 1:1 magnification. Hopefully we can agree on that.

All three distances fall short of that by a certain amount. I have already stated that, in my opinion, that has to do with entrance and exit pupils and such stuff - about which I know little.

[george013]

I didn't measure anything. The values quoted are the minimum focusing distances (not focal lengths) in the lens specifications found in http://slrgear.com/reviews/showcat.php/cat/30.

Now, thin-lens theory indicates that the distance from the subject to the film plane is 4X the marked focal length at 1:1 magnification. Hopefully we can agree on that.

All three distances fall short of that by a certain amount. I have already stated that, in my opinion, that has to do with entrance and exit pupils and such stuff - about which I know little.

I don't know from where it's measured. From the sensor, a mark on the camera, then that distance is the image distance + the object distance. Is it measured from the optical center, that's unknown. Or is it measured from the front of the lens.

I think it's the camera-mark. In that case the 105mm has become a78.25mm, the 70mm a 64.25mm and the 50mm a 47.25mm.

George

[Inkanyezi]

The confusion seems to arise from different ways of achieving a close distance in a macro lens, as well as some lenses being asymmetric, why they do not behave as an ideal thin lens.

Basically there are two ways of focusing. Either by extending the distance from the image plane, or by decreasing focal length. Call the latter floating elements or "zooming" if you will, but effectively, an inner focusing lens alters its focal length.

Let's disregard asymmetry for a moment, and look at these two principles, making believe that they are both thin lenses. Then a "normal" macro lens, as the old Micro Nikkor 55 mm, or a Makro Kilar, extends one full focal length for focusing to 1:1 reproduction. It follows the assumption rather closely. On the other hand, an inner focusing macro lens of say 100 mm, that does not extend when focusing, supposing its optical centre remains in place while focusing, will decrease its focal length to 50 mm, so instead of a 100 mm f/2,8 it will become a 50 mm f/1,4.

In practice, no inner focusing lens is symmetric, so the above is not entirely true. Figures will slide into something else, but the principle is still that the IF lens has a focal length substantially shorter when close.

The "old" way of achieving close focus has disadvantages for an AF system, as the distance in front of the lens will change as well, when the lens extends. Therefore, AF macro lenses are preferred to be inner focusing, as the object to lens distance remains the same while the lens focuses. An extending lens has substantial difficulty to find focus when close to the 1:1 ratio, while the inner focusing lens will work better.

So - as long as the lens is (near) symmetric and extends with focusing, the simple formula for lens extension works, but with a lens that inner focuses, it is accomplished by decreasing the focal length of the system.

Asymmetric lens designs will not perform perfectly after the formula, and the entrance and exit pupils will not be the same when the lens is extended. Both their virtual position and size may change.

So the simple formulae are useful when using a symmetric lens construction with extensions (bellows or other), but fail when the lens is not symmetric, and inner focusing lenses follow their own rules, regarding both focal length and effective aperture.

[TonyW]

I think I understand what Urban said but I suspect we are not discussing the quantity of most practical importance. The effect of varying the focal length with macro photography is in the relation between sizes of background objects and sizes of the main subject in the image This has an effect on the quality of the (generally out of focus) background. What we should be concerned about is the distance of the subject plane from the first nodal point of the lens, if I understand things correctly.

The question of effective aperture I haven't got my head around at all.

[xpatUSA]

I think it's the camera-mark. In that case the 105mm has become a 78.25mm, the 70mm a 64.25mm and the 50mm a 47.25mm.

but then . .

Time for the definitions.

The focal length of a 100mm lens is 100mm . . . . A 100mm stays a 100mm.

but a 105mm does not, apparently . . . just kidding, George, couldn't resist it, sorry . . .

[Glenn NK]

I have a semi-serious question - are we over-thinking this?

The focal length changes - but how much does it really matter when one is actually doing photography.

I'm sure it makes a difference, but (in the real world) when I do macro-photography, I don't worry about it - in fact I don't even think about it - I just take a picture of the flower, make sure I have enough DOF, that the important element(s) is/are in focus, and the framing is correct. And if I'm stacking, I curse the wind, but what happens to the FL hasn't been my top priority to date.

Maybe I should be concerned?

G

[DanK]

I have a semi-serious question - are we over-thinking this?

The focal length changes - but how much does it really matter when one is actually doing photography.

I'm sure it makes a difference, but (in the real world) when I do macro-photography, I don't worry about it - in fact I don't even think about it - I just take a picture of the flower, make sure I have enough DOF, that the important element(s) is/are in focus, and the framing is correct. And if I'm stacking, I curse the wind, but what happens to the FL hasn't been my top priority to date.

Maybe I should be concerned?

G

I'm with you. I do more macro than anything else, and this has never been an issue for me. I never think about it when actually taking pictures. What matters is to me for work at near 1:1 is the minimum working distance. I do, however, think about the effective f-stop, which is considerably higher than the nominal aperture, and therefore relevant to think about diffraction.

[george013]

but then . .



but a 105mm does not, apparently . . . just kidding, George, couldn't resist it, sorry . . .

It's the construction of a macro-lens. The focal length is changing physical, like in a zoom-lens. There is a group of floating lenses inside, Nikon calls them CRC, Close Range Correction. Canon and others will have them to but with another name. What I've understood that there primary function is to remain quality in the very short distance but also changes the focal length.

http://www.mir.com.my/rb/photography...sery_nikon.htm
http://imaging.nikon.com/lineup/lens/glossary.htm#crc
and more.

Beside that it seems to me advisable to use the same definitions so we can understand each other.

George

[george013]

I'm with you. I do more macro than anything else, and this has never been an issue for me. I never think about it when actually taking pictures. What matters is to me for work at near 1:1 is the minimum working distance. I do, however, think about the effective f-stop, which is considerably higher than the nominal aperture, and therefore relevant to think about diffraction.

What I've learned that the F-stop comes from astrology. There instrument are allways looking at infinity.
The formules of diffraction are based on the ratio diameter diaphragm and image distance. For the simplicity that has been changed to diaphragm diameter and focal distance. Normally the differneces will be little but not with macro. So instead of using the right formule, we are changing the value of the f-number. The result will be the same.

Brings to me the often heard question that if diffraction is related to the small diameter of the aperture, why doesn't it change when you use the same f-number and a longer focal length? Thus also a bigger diaphragm diameter. Well, because it's not related to the diaphragm diameter but to the ratio diaphragm diameter and image distance. It's a way to express the top-angel of the light-cone formed by diaphragm diameter en image distance.

If you want to work 1:1 the distance sensor to object would be theoretical 420mm with a 105mm. But with the here mentioned 105mm it's 313, a difference of 107mm.

George

[Inkanyezi]

I think it would be a Good Thing not to mess up terminology too much. Focal length in a lens that only extends the distance between optics and image plane is unchanged, because no change is done to the lens itself, only to the distance from the image plane. This is the situation when we use bellows or exension rings with a virtually symmetrical construction.

The change of angle of view depends on the extension, not the focal length. It is evident if we think of the pinhole camera and do the same thing, extend the distance from image plane to pinhole. It is evident that any object captured will be larger, as the pinhole effectively does alter its "focal length" - although the pinhole doesn't have one.

However, many macro lenses indeed do alter their focal length, but always to a shorter focal length when they are focusing closer. Therefore, they won't change the angle of view as dramatically as a lens that focuses only by extension. The virtues of altering the lens for getting closer are many. The lens may be better corrected for the close distance, and there isn't the "loss of light" experienced with the simpler design, and also, the distance in front of the lens is not changing as the lens focuses, which aids the function of AF.

With an inner focusing lens, it is easier to find focus when close to 1:1 reproduction, and it can snap in just as at longer distances, while the extending lens may become doubtful when it is close, due to the simultaneously changing distance between lens and subject. At 1:1 ratio, AF is useless with such a lens, and sharpness has to be "rocked" in by the user. This is however a standard procedure among most macro shooters, so it is not a serious drawback for the experienced user.

[travelfinn]

Have bookmarked this page on lenses, some pages in english. Read only a part of them but should answer most questions about different lenses from Nikon and how they work. www.pierretoscani.com Happy exploring the pages are awesome.