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03-07-2011, 07:38 AM   #1
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What is the True Focal Length of your IF lens?

Just a curiosity point here, but also something that is missing from all the lens data that gets published.

At some level of conciousness, we all know that internal focusing on lenses is actually achieved by changing the len's focal length, and that the published focal lengths are only at infinity focus.

This has ton's of impacts from the determination of depth of field at any focusing distance, to selecting lenses for specific functions where you are shooting close in, to selecting the lens hood to avoid vignetting yet achieving "optimum" protection while shooting into the light.

For example, my Vivitar (aka samyang, ProOptic, Bower et. al.) 85mmF1.4 actually has a focal length of 39.3mm at the 1meter close focus limit.

I have not yet looked at how the lens moves from 85mm to 39mm as a function of distance , but I plan to.....

So, the question to all of the forum is, how do your lenses really compare, when it comes to shooting close.

I propose that people do some tests on their lenses, and post the results here (or in the lens review section) on a lens by lens basis.

You can either post the calculated flcal length at any distance or post only the field of view (either in inches or cm) and shooting distance.

To calculate the Focal length, simply use the following.

M = f / (f-d)

where:
M = magnification ratio
f = focal length
and d = the distance from the subject to the lens.

If you use the sensor widtrh of 23.5mm (for pentax ASP-C) and a ruler or identifyable distance for the width the magnification is pretty easy to calculate, then just plug in magnification and distance and solve for focal length.

03-07-2011, 08:30 AM   #2
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This seems a little strange to me...

Snatched from wiki:

QuoteOriginally posted by some bloke in the article about "Focal Length":
When a photographic lens is set to "infinity", its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane.

To render closer objects in sharp focus, the lens must be adjusted to increase the distance between the rear nodal point and the film, to put the film at the image plane. The focal length f, the distance from the front nodal point to the object to photograph S1, and the distance from the rear nodal point to the image plane S2 are then related by:

1/S1 + 1/S2 = 1/f

As S1 is decreased, S2 must be increased. For example, consider a normal lens for a 35 mm camera with a focal length of f = 50 mm. To focus a distant object (S_1\approx \infty), the rear nodal point of the lens must be located a distance S2 = 50 mm from the image plane. To focus an object 1 m away (S1 = 1000 mm), the lens must be moved 2.6 mm further away from the image plane, to S2 = 52.6 mm.

Which means that your focal length should increase when focusing closer.
03-07-2011, 08:42 AM   #3
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Jim

I do not dispute the formula below, but note that it applies to a simple lens first of all, and second only discusses the concept that for a simple lens the distance from the focal plane increases as you have the subject move closer than infinity, to achieve focus.

Note that as S1 approaches infinity, hence 1/S1 approaches zero, S2 approaches the focal length.

Also note that when S1=S2 (the condition for 1:1 macro) the distance is 2F.

This formula is used to calculate the focal length of a simple lens assuming you don't have a target at infinity.

Also don't confuse S2 with focal length. FOcus for a simple lens (or an externally focused lens where the entire group moves, is achieved by increasing S2. i.e. moving away from the focal plane when closer than infinity..
03-07-2011, 12:02 PM   #4
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tamron XR Di 28-75mmF2.8

Just to give an idea of how things work out, I checked my Tammy 28--75

at 28mm focal length,
36cm away from a target it measures 26.3mm focal length, and at 106cm away it measures 27.7mm

at 75mm focal lenght it is a little different
33 cm away from a target it measures 61.7 mm and 103 cm away from a target, it measures 68.9 mm

This is infinitely better than my 85mmF1.4 which is at less than 1/2 the focal length at min focus.

03-07-2011, 03:51 PM   #5
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QuoteOriginally posted by Lowell Goudge Quote
Just to give an idea of how things work out, I checked my Tammy 28--75

at 28mm focal length,
36cm away from a target it measures 26.3mm focal length, and at 106cm away it measures 27.7mm

at 75mm focal lenght it is a little different
33 cm away from a target it measures 61.7 mm and 103 cm away from a target, it measures 68.9 mm

This is infinitely better than my 85mmF1.4 which is at less than 1/2 the focal length at min focus.
I think you'll have to explain a little more how you have come to these conclusions. Where have you found the equation, why does it work that way? And are you sure about measuring the distance from the subject to the lens, and not to the sensor or some nodal point in the lens?

I've only read the most basic optical physics and I don't think too many of us are optical engineers so this might need some explaining to make sense.

A little change in focal length would be logical but cutting it in half that just seems strange. That should show in quite a drastic way regarding DOF and FOV, have you tried this practically or is it just a theoretical experiment? Forgive me for being sceptical but at the moment it just doesn't make sense.
03-07-2011, 05:41 PM   #6
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QuoteOriginally posted by Jimfear Quote
I think you'll have to explain a little more how you have come to these conclusions. Where have you found the equation, why does it work that way? And are you sure about measuring the distance from the subject to the lens, and not to the sensor or some nodal point in the lens?

I've only read the most basic optical physics and I don't think too many of us are optical engineers so this might need some explaining to make sense.

A little change in focal length would be logical but cutting it in half that just seems strange. That should show in quite a drastic way regarding DOF and FOV, have you tried this practically or is it just a theoretical experiment? Forgive me for being sceptical but at the moment it just doesn't make sense.
I've actually compared the image size of my vivitar against my super-tak 85/1.9. The change in focal length is real
03-07-2011, 09:27 PM   #7
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QuoteOriginally posted by Jimfear Quote
This seems a little strange to me...

Snatched from wiki:

Which means that your focal length should increase when focusing closer.
I don't think it means that. For an IF lens, S2 would be constant in that formula, because IF lenses don't extend or contract. Which means that when S1 gets smaller, f needs to get smaller as well for the equality to continue to hold. Which means that the focal length must decrease as you focus closer.
03-08-2011, 06:37 AM   #8
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QuoteOriginally posted by Jimfear Quote
I think you'll have to explain a little more how you have come to these conclusions. Where have you found the equation, why does it work that way? And are you sure about measuring the distance from the subject to the lens, and not to the sensor or some nodal point in the lens?

I've only read the most basic optical physics and I don't think too many of us are optical engineers so this might need some explaining to make sense.

A little change in focal length would be logical but cutting it in half that just seems strange. That should show in quite a drastic way regarding DOF and FOV, have you tried this practically or is it just a theoretical experiment? Forgive me for being sceptical but at the moment it just doesn't make sense.
QuoteOriginally posted by Laurentiu Cristofor Quote
I don't think it means that. For an IF lens, S2 would be constant in that formula, because IF lenses don't extend or contract. Which means that when S1 gets smaller, f needs to get smaller as well for the equality to continue to hold. Which means that the focal length must decrease as you focus closer.
correct. but it is even more complex because in an IF lens, usually the rear element is also fixed, so the equation really falls apart.

For the sake of measurement, I have elected the "nodal point" to be the frong element. While this may not be 100% accurate, for the purpose of estimating the focal length of a lens, based upon the measured magnification ratio, and this distance it is good enough.

As to the source of the formula,

M = f / (f-d) where D is the distance to the lens (S2) this is from the same wikipedia page, just a little further down than the formula 1/S1 + 1/S2 = f/F

The issue was discussed, but not really properly answered both in magazines and on the forum over a year ago with issues and questions about super zooms having different field of view than primes at any equal focal length. The same issue holds true there, the lens makers have found it easier to modify focal length to achieve focus when not at infinity.

The issue was also discussed with respect to 1:1 macro using newer IF macro lenses, and the fact that they do not achieve 1:1 at a working distance of 2 x focal length, and the pros and con's of this. The pro is since focal length changes and reduces, the aperture may be increasing such that with magnification the image remains brighter, as opposed to using extension tubes and a true externally focused (entire lens group moves) lens, where light fall off at extreme magnification is an issue

All I am doing is proposing , through this thread, that we attempt to characterize our Internally focused lenses, to get a better understanding of lens performance.

Also if focal length loss at close focus can be as severe as my tests with the vivitar suggest, this can change one's approach in a studio. It is very clear that in the 1-2 meter working distance, I can do things with my vivitar 85 that I can't do with my super tak 85. On the other hand, if magnification is what I am looking for, my super tak 85 would be the lens to select.

03-08-2011, 06:54 AM   #9
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Confused

I have to admit, I am confused.

Went back and re-checked my 85F1.4, and can't now re-produce what I measured before. I must have in the first quick check, made a data entry.

In fact, unlike my Tammy which reduces focal length, to focus closer, the 85F1.4 actually seems to go the other way.

At minimum focus, it's focal length is 94mm, and tapers down rather quickly to 89 m by 1.9 meters distance

my first calculation was based upon believing the focus distance scale, which may not have been a good idea.
03-08-2011, 07:02 AM   #10
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Some lens designs appear to suffer from "breathing" more than others In a recent (unscientific) test I compared my Sigma 180mm f/3.5 APO EX DG [IF] to my FA*200mm f/4 ED [IF] Macro. As far as I can tell neither lens appears to be losing much focal length, I could easily attribute any loss of focal length to my testing method. One of the easiest ways to observe this effect is by racking the lens from infinity to closest focus distance with a k7/k5 with live view with the grid display turned on.
03-08-2011, 08:22 AM   #11
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Lowell is correct as far as I know.

Lenses always follow the thin lens optics rule:
1/f = 1/image.distance + 1/object.distance even for complex lenses.

The other simple thin lens optics rule always followed is:
magnification = image.distance/object.distance =image.size/object.size even for complex lenses.

This is because the various parameters of real lenses (principal points, etc.) are defined so the above rules are followed (image distance is measured from the "rear principal point" and object distance is measured from the "front principal point").

It follows from these basic rules that:
object.distance=f(1+1/magnification) or equivalently (as Lowell put it) m = f/(d-f) (edited for sign error)

Dave


PS There's often an error introduced in DIY measurements and application of these relationships because a real lens has thickness (the front and rear principal planes do not coincide.) However, such measurements can give one a good idea of what's going on with internal focus lenses as they are zoomed and focused.

Last edited by newarts; 03-10-2011 at 04:21 AM.
03-08-2011, 08:38 AM   #12
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QuoteOriginally posted by Digitalis Quote
Some lens designs appear to suffer from "breathing" more than others In a recent (unscientific) test I compared my Sigma 180mm f/3.5 APO EX DG [IF] to my FA*200mm f/4 ED [IF] Macro. As far as I can tell neither lens appears to be losing much focal length, I could easily attribute any loss of focal length to my testing method. One of the easiest ways to observe this effect is by racking the lens from infinity to closest focus distance with a k7/k5 with live view with the grid display turned on.
breathing is usually caused by lens extension when zoomed, and best demonstrated with super zooms (i.e. lenses with >about 5 x zoom range)

the bigma is a great example of this,
03-08-2011, 06:04 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote
breathing is usually caused by lens extension when zoomed, and best demonstrated with super zooms (i.e. lenses with >about 5 x zoom range)
no it effects primes as well especially internally focusing ones, cinematographers go to great lengths to choose fast, high quality lenses designed not to exhibit this effect. Ever wondered why cinematographic lenses can cost upwards of $20,000 - when an equivalent lens for a still camera will cost substantially less?. Occasionally in movies you do see the "breathing" effect, and in my opinion it completely breaks the fourth wall.
03-09-2011, 04:29 PM   #14
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QuoteOriginally posted by newarts Quote
Lowell is correct as far as I know.

Lenses always follow the thin lens optics rule:
1/f = 1/image.distance + 1/object.distance even for complex lenses.

The other simple thin lens optics rule always followed is:
magnification = image.distance/object.distance =image.size/object.size even for complex lenses.

This is because the various parameters of real lenses (principal points, etc.) are defined so the above rules are followed (image distance is measured from the "rear principal point" and object distance is measured from the "front principal point").

It follows from these basic rules that:
object.distance=f(1+1/magnification) or equivalently (as Lowell put it) m = f/(f-d)

Dave
Dave,

I think the correct formula is: m = f/(d-f).

A good example of an IF lens is the smc PENTAX D FA 100mm F2.8 WR Macro.
According to the specifications its maximum magnification = 1.00x at its minimum focusing distance = 0.303 m.
At magnification = 1x the total distance from the focal plane to the object plane = 4f (object distance = 2f + image distance = 2f).
So for this lens at magnification = 1x 4f = 303 mm (0.303 m) which results in f = 75.75 mm at magnification = 1x.
But at infinity (magnification = 0x) the focal length was 100 mm.

Sigurd.

Last edited by sigurdhu; 03-10-2011 at 11:11 AM.
03-09-2011, 06:07 PM   #15
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QuoteOriginally posted by sigurdhu Quote
Dave,

I think the correct formula is: m = f/(d-f).

A good example of an IF lens is the smc PENTAX D FA 100mm F2.8 WR Macro.
According to the specifications its maximum magnification = 1.00x at its minimum focusing distance = 0.303 m.
At magnification = 1x the total distance from the focal plane to the object plane = 4f (object distance = 2f + image distance = 2f).
So for this lens at magnification = 1x 4f = 303 mm (0.303 m) which results in f = 75.75 mm at magnification = 1x.
At infinity (magnification = 0x) the focal length = 100 mm, remember.

Sigurd.
yes, you are right, I made a sign error. The first equation is correct:

object.distance=f(1+1/magnification)
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