How do you define MFD

I am a complete newcomer into the world of macro photography. I wanted to understand what is the precise definition of minimum focusing distance. Is it calculated from the front element of the lens or from the sensor plane? Could someone please help clarify this confusion in my mind?

If it is from the sensor plane it makes sense in my mind, if it is from the front element then I have a few questions about extension tubes.

The distance is calculated from the focal plane (sensor) to the object.

On DSLRs the focal plane is often marked by a lone on the body.
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Chris R

Chris R-UK said:

The distance is calculated from the focal plane (sensor) to the object.

On DSLRs the focal plane is often marked by a lone on the body.

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Or even a 'line' on the body. ;-) The line often looks like this ---o---
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Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

Thanks to both of you. This makes eminent sense and clears a confusion in my mind. So if I keep my sensor at the minimum focusing distance from my subject and then attach a few extension tubes to make the front element of my lens reach much closer to my subject, I get a much larger magnfied image. Is that right?

And thanks for clarifying the word "lone". I was going to ask what on earth does it mean ???

I don't know the details right now, but the effect of extension tubes is highly dependent on which lens you use. This is because there are different ways a lens can be constructed to focus and this has an impact on how extension tubes affect the image. You should post what lenses you intend to use, and also do some research on your own.

As a general rules, I have heard that internally focusing lenses don't work well with extension tubes. I have not verified this in any way, so consider this to be a rumor, it may be wrong.

Sudipto Roy said:

So if I keep my sensor at the minimum focusing distance from my subject and then attach a few extension tubes to make the front element of my lens reach much closer to my subject, I get a much larger magnfied image. Is that right?

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Yes. There is a calculator on this page that will give you a rough idea of about how much tubes increase magnification: http://dpanswers.com/content/prod_macro.php#etub

(I use the appreviation "CFD" rather than "MFD" - it means "Close Focus Distance" and it is the same as "MFD".)

As quadrox says: simple symmetrical lenses work better with extension tubes than complex zooms and IF designs. The Nikon 50 mm lenses (all of them) are excellent.
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– gisle [ See profile/plan for equipment list ]

quadrox said:

As a general rules, I have heard that internally focusing lenses don't work well with extension tubes. I have not verified this in any way, so consider this to be a rumor, it may be wrong.

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I've not heard such a claim and can't think of any reason it might be true. A lens does not know or care where it is in space & all a tube does is to move the lens to a different position in space.

A tube has no optical influence on a lens.

Dave Martin said:

Dave Martin said:

As a general rules, I have heard that internally focusing lenses don't work well with extension tubes. I have not verified this in any way, so consider this to be a rumor, it may be wrong.

Click to expand...

I've not heard such a claim and can't think of any reason it might be true.

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I can. You should read a bit about different lens designs and you would probably understand. I do not remember enough to explain it to you, but the way the focus system is constructed definitely has impact on how extension tubes work.

Dave Martin said:

A lens does not know or care where it is in space & all a tube does is to move the lens to a different position in space.

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When reading the above statement literally there is nothing wrong with it. However, you are implying that the position in space does not affect the image that is projected onto the sensor plane. And that is just plain wrong.

Dave Martin said:

A tube has no optical influence on a lens.

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Not the lens itself, but definitely on the image being projected onto the sensor.

quadrox said:

Dave Martin said:

quadrox said:

As a general rules, I have heard that internally focusing lenses don't work well with extension tubes. I have not verified this in any way, so consider this to be a rumor, it may be wrong.

Click to expand...

I've not heard such a claim and can't think of any reason it might be true.

Click to expand...

I can. You should read a bit about different lens designs and you would probably understand. I do not remember enough to explain it to you, but the way the focus system is constructed definitely has impact on how extension tubes work.

Click to expand...

I have read about and understand different lens designs.

If you simply move a zoom or IF lens in space but do not move the zoom or focus mechanisms the lens' optics do not change in any way - ie. the relationship between the internal elements does not change in any way - it is equivalent to a non-zoom or non-IF lens with that particular arrangement of elements.

quadrox said:

quadrox said:

A lens does not know or care where it is in space & all a tube does is to move the lens to a different position in space.

Click to expand...

When reading the above statement literally there is nothing wrong with it. However, you are implying that the position in space does not affect the image that is projected onto the sensor plane. And that is just plain wrong.

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Why is that "just plain wrong"?

quadrox said:

quadrox said:

A tube has no optical influence on a lens.

Click to expand...

Not the lens itself, but definitely on the image being projected onto the sensor.

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How can the tube have an influence on the image? It does not interact with the light in any way.

If you mount a 100mm lens in space (like on an optical bench) and put a subject 10m in front of the lens an image will be formed in space a certain distance behind the lens in the image plane; if you move the subject closer to the lens the image moves further from the lens. Whether or not the light goes through a tube or the image plane is inside a camera has nothing to do with the quality of the image.

Now consider what happens if the light were shined backwards through the lens - what was the image becomes the subject an what becomes the image was the subject ie. the lens is acting as a strong macro lens - but nothing has been done to the lens itself - only to where it sits in space regarding the image and object.

I believe there is nothing inherent in the optics of a zoom or IF lens to make it worse than any other fixed element lens when the subject gets closer than a certain distance to the lens.

To be as open as I can I will grant that lenses are designed expecting the subject to be a certain distance in front of the lens (that's because the curvature of the wavefront striking the lens depends on the subject distance.) This in turn affects the quality of the image when the subject is at some other distance. But there is no apparent reason that the image quality should depend whether the lens has internally moveable elements.

Now I grant you that Zoom and IF lenses involve more compromises in design than fixed lenses - and are often a lot softer at certain settings but I don't think that implies they should be especially bad when used for macros.

A particular IF or Zoom lens at a particular focal length might be worse than a particular fixed element lens when used for macros, but there's no reason that it must be worse.

Barrie Davis said:

Chris R-UK said:

The distance is calculated from the focal plane (sensor) to the object.

On DSLRs the focal plane is often marked by a lone on the body.

Click to expand...

Or even a 'line' on the body. ;-) The line often looks like this ---o---
--
Regards,
Baz

Click to expand...

Thanks Baz, haven't woken up yet!
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Chris R

Let me be clear on what I mean.

Take a 50mm IF lens and focus it at infinity - now it truly is a 50mm lens. Now secure the focus ring so the internal elements can't move - now it is a fixed element 50mm lens.

Now use it on extension tubes or bellows; why should it behave any different than any other fixed element 50mm lens?

Dave Martin said:

quadrox said:

As a general rules, I have heard that internally focusing lenses don't work well with extension tubes. I have not verified this in any way, so consider this to be a rumor, it may be wrong.

Click to expand...

Click to expand...

Dave Martin said:

I've not heard such a claim and can't think of any reason it might be true. A lens does not know or care where it is in space & all a tube does is to move the lens to a different position in space.
A tube has no optical influence on a lens.

Click to expand...

Have you ever wondered why some fixed focal length lenses is designed with complex internals where elements move in complex patterns in relation to each other?

If real objectives behaved like thin lenses, and you could just cement all the elements inside a tube and focus them by moving the entire tube back and forth along the optical axis with no loss of quality, good objectives could be made a lot cheaper than they are.

Part of the answer is that objectives do suffer from various distortions and aberrations. Unlike thin lenses, the designer can correct these distortions and aberrrations by making them cancel each other. With symmetrical lenses, like Zeiss Planar and derivatives, the cancellation of aberrations works well at 1:1 magnification (with extension tubes), and equally well at the much lesser magnifications you get when the lens is not used with tubes. When you put tubes on a symmetrical lens to get to 1:1, it performs just as well as without tubes.

With other optical formulas, things are less simple. The optical formula for a non-symmetrical lens may do a very good job cancelling aberrations when the lens is used within the magnifications it was designed for, but when you add tubes to get towards 1:1, the correction formula no longer works well, and optical performance is visibly degraded.

For example, the Nikon 200mm f/2.0 is one of Nikon's finest lenses, but if you try to use it with extension tubes, you will notice much more lateral CA. On the other hand, the modest Nikon 50 mm f/1.8 will not suffer any degradation if you use it with tubes.
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– gisle [ See profile/plan for equipment list ]

Sudipto Roy said:

Thanks to both of you. This makes eminent sense and clears a confusion in my mind. So if I keep my sensor at the minimum focusing distance from my subject and then attach a few extension tubes to make the front element of my lens reach much closer to my subject, I get a much larger magnfied image. Is that right?

Click to expand...

No. What tubes do is allow the lens to focus closer than its minimum focus distance. So you put the lens on tubes and you move the camera closer to the subject.

Then likewise, let me be clear what I mean:

When you attach a lens that is set to 50 mm to a camera without extension tube and focus on a given subject, the image will be focused/sharp at exactly the sensor plane, but not behind or in front of it.

Now assume that you have indeed focused the lens at a given subject. When you remove the lens, add an extension tube, and put the lens onto the extension tube, the image at the sensor plane is no longer in focus. This is what I meant when I said you were wrong about extension tubes not affecting the optical image.

So now your subject is no longer in focus. What do you do? You turn the focus ring until your subject is in focus again. But what happens now is that while you focus, the focus lens elemnts will move around. Exactly how they move around depends on the construction of that particular lens and this has an direct effect on what magnification and abberations you will see after adding the extension tube. Some lenses will add strong abberrations, because the focus system is used in a way that it was not supposed to be used when the lens was designed. Other lenses will simply not give much of an magnification effect.

Now, I don't know exactly what types of lenses result in what types of effects when adding extension tubes. But I know that different lenses will have different effects.

Use a simple symmetrical design lens, mount your extension tube and set the lens for minimum focus. This alligns the elements of the lens for the best correction for close focusing.

You focus by moving the whole camera. Focus rails help on a tripod.
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Cheers, Craig

Equipment in Plan via Profile

Guidenet said:

Use a simple symmetrical design lens, mount your extension tube and set the lens for minimum focus. This aligns the elements of the lens for the best correction for close focusing.

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In the case of lenses not specifically designed for macro photography, it is also beneficial to mount the lens the wrong way round using a special reverse adaptor into the filter thread of the lens. This improves close-up image quality, whether or not tubes are in use....

..... it means the approach angles for entering rays more closely match the intended angles for rays exiting the lens in its normal orientation, which itself means the original distance type corrections of optical aberrations are somewhat reorientated to the business of focusing close.

The reverse adaptor also adds some lens extension in its own right, making it possible to do a good deal of close-up photography without any other tubes... it is acting as a short extension ring, and, with short focus lenses like a 50mm, it is the short extensions that are used most often.

Considering how cheap they are, I think everyone with a 50mm should have a reverse adaptor kicking around in their kit, even if it is their only concession to equipping for ultra-close shooting.

http://www.amazon.com/Fotodiox-Filter-Thread-Reverse-Adapter/dp/B001G4NBSC
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Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"

quadrox wrote:....

quadrox said:

Now assume that you have indeed focused the lens at a given subject. When you remove the lens, add an extension tube, and put the lens onto the extension tube, the image at the sensor plane is no longer in focus. This is what I meant when I said you were wrong about extension tubes not affecting the optical image.

So now your subject is no longer in focus. What do you do? You turn the focus ring until your subject is in focus again .....

Click to expand...

Ahhh! There's the difference...

You don't have to move the focus ring - you move the image plane with respect to the lens . That's what tubes and bellows do - the lens elements don't move with respect to each other; the lens stays constant and the object and image planes move.

l grant you that an IF lens focused at infinity was probably designed with the notion that the object was in fact at infinity; therefore when the object is not at infinity the image will likely not be as good - but that's true for a fixed element lens as well (although the fixed element lens was probably designed expecting the object was closer than infinity.

In fact an IF lens on a tube or bellows might give the best macro image when adjusted to its minimum focus distance because that's when the designers expected the object to be close to the lens...

I'm not disputing that the IF lens might be different than a non-IF lens when used for macro purpose only that difference might well be small or even better when used at the short focal distance end.

I feared your original post gave the impression that one would expect IF lenses on tubes to fail.

You wisely counseled that one might be cautious & research the situation; but researching is hard to do for the general case because not much is published along this line. Therefore the casual user won't even try.

I suggest simply buying a cheap tube and trying the lens one has; it might well be ok when used on tubes or bellows.

Thanks a tonne for this lively discussion. Sorry that I was a little late to respond as I got caught up with some work. And my geographical location doesn't make matters easier.

I think it was foolish of me not to mention my camera/lens set up. I have a Canon 7D and now a Tamron 90 mm macro lens. I haven't got the extension tubes yet but intend to get the Kenko set, if I feel the flies will look larger when I shoot them with the extenders.

As I understand, I cannot take the camera - rather the sensor plane - nearer than 11 inches. So I add an extension tube to shorten the distance and increase the size of the image. Is that it?

Sudipto Roy said:

As I understand, I cannot take the camera - rather the sensor plane - nearer than 11 inches. So I add an extension tube to shorten the distance and increase the size of the image. Is that it?

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The 11" distance is a limit imposed by the focusing mechanism of that particular lens. To get the front of the lens closer to the object the lens must be moved further from the sensor & that lens just won't crank out any further.

A tube or bellows moves the lens away from the sensor and thereby allows the lens to move closer to the object.

The equations are:
Sensor.lens.distance = focal.length(1+magnification)
object.lens.distance = focal.length(1+1/magnification)
Focusing.distance = focal.length(1+magnification)(1+1/magnification)

when magnification gets greater than 1 the distance between object and sensor increases!