Why is there no 300 or 400mm macro lens?

Nic Planarchy said:

I don't think those distances give the same framing.

Maximum magnifcation (1:1) on a Sigma 50mm macro is at a distance
of 18.9cm when the DOF at f8 is 0.5cm.

Maximum magnifcation (1:1) on the 150mm macro is at a distance of
38cm when the DOF at f8 is 0.19cm.

Click to expand...

Those do not look like the correct numbers, to me: they appear to be film-plane to subject distances for these lenses, rather than the lens to subject distance required by the DOF program.

As hinted at by ljfinger, the DOF for macro is dependent on the f number and the magnification only. The thing that changes is the perspective.

However:

When a lens is internally focussing, the change in focal length modifies the f-number. A back of the envelope calculation using your numbers tells me that the internally focussing lens is operating at f/5 at 1:1, and will have an accordingly reduced depth of field.

The lens operating at a fixed focal length will have greater depth of field, but the exposure will change by about a stop between infinity and 1:1 magnification.

--
Kevin Moore

Hmm.. and now even more confused.

If the 150 is reducing the effecting f stop I am losing DOF aren't I? Or not?

When I switched to the 150mm (from the 105) I believed I observed a reduction in DOF. So I searched for an answer.

As ever with the internet I can find sites claiming both that focal length does affect DOF and ones that says it definetely doesn't (and some which say it doesn't other than at extreme magnifications without clarifying "extreme magnifications"). My physics is not good enough any more to decide which is correct. I would love to know.

Yes, I should go out and perform some tests.

Unfortunately not possible at the monent as I dropped my 150mm last week and it needs repairing Rest assured there'll be tests as soon as it returns if only for my own peace of mind.

Apologies to the OP for hijacking the thread and thanks to all who have sought to explain.

All the best .

--
NiC, Cannon 5D user.

Nic Planarchy said:

As ever with the internet I can find sites claiming both that focal
length does affect DOF and ones that says it definetely doesn't
(and some which say it doesn't other than at extreme magnifications
without clarifying "extreme magnifications").

Click to expand...

At the same magnification and effective f-stop, focal length has ZERO effect on defocus blur as measured at the blurred object . (The effective f-stop may differ from the indicated f-stop at extreme magnifications, though; this typically becomes significant around 1:4 magnification.)

But the defocus blur as measured on the image can change, because while subject magnification is held constant, the magnification of other objects in the frame is affected by perspective. A longer focal length forces a more distant perspective, which enlarges the background - blur and all. That makes it look blurrier, even though exactly the same background details are resolved.

What about DOF? It is not the same as background blur, as it only describes the extent of the sharp part of the image. And since DOF is very small in macro photography, it is hardly affected at all by perspective, as Lee Jay's numbers demonstrate. (This is not true when DOF is large.)

There is no contradiction here, once you are aware of the distinction between DOF and background blur!

--
Alan Martin

No I'm not talking of background blur, it's DOF I'm interested in... the DOF across a Dragonfly's eye for example. My perception was that I was getting lower DOF with a 150mm macro than a 105mm and I sought an explanation.

There has been a suggestion that the 150 is actually variable in focal length and effective f stop (if I've understood correctly).. does this explain the percieved difference in DOF (presumably not the focal length if we're still at 1:1)......and I now read here:

http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00BWkV

that the DOF is also dependent on "pupil magnification"... I would welcome anyone who understands the equations better than I to tell me whether this explains my perceived DOF differences?

Many thanks, all the best.

--
NiC, Cannon 5D user.

Nic Planarchy said:

that the DOF is also dependent on "pupil magnification"... I would
welcome anyone who understands the equations better than I to tell
me whether this explains my perceived DOF differences?

Click to expand...

Maybe this will help:

http://doug.kerr.home.att.net/pumpkin/Depth_of_Field.pdf

--
Lee Jay
(see profile for equipment)

maybe the best performer in Canons line up with 2X extender. Worth a try if you really want reach.

Nic Planarchy said:

Hmm.. and now even more confused.

If the 150 is reducing the effecting f stop I am losing DOF aren't
I? Or not?

When I switched to the 150mm (from the 105) I believed I observed a
reduction in DOF. So I searched for an answer.

Click to expand...

The short answer: Yes, you get shallower DOF at the same f-stop. But you can stop down farther to compensate, with no disadvantage either in diffraction softening or in shutter speed. What might surprise you is that at 1:1 magnification, your new "longer" lens may be effectively shorter than the old one!

Now for the long answer...

Lens behaviour gets complicated in macro photography:

• Field of view depends on more than just the focal length.
• Sensor illumination depends on more than just the f-stop.
• You need to know where on the camera to measure subject distances from!

For an external focus lens, focusing closer is achieved simply by moving the lens away from the sensor. This extension of the lens narrows the FOV, just like increasing the focal length. So the lens becomes effectively longer - and because its pupil size does not increase to compensate, it becomes effectively slower as well.

(Yet most macro lenses at 1:1 have a shorter working distance than expected from their focal length! That's not really a contradiction, because the subject distance for purposes of FOV and DOF is not measured from the front of the lens - instead, it is measured from the center of perspective, which is the entrance pupil inside the lens.)

Assuming a symmetrical lens, the focal length and f-stop are effectively multiplied by a factor of 1 + M, where M is the magnification. At 1:1 magnification, a 50mm f/2.8 macro lens becomes an effective 100mm f/5.6 - twice as long and two stops slower than it is at infinity focus. (At 1:5 magnification it loses only half a stop.)

For a non-symmetrical lens, the factor becoms 1 + M/P, where P is the pupil magnification (see http://www.vanwalree.com/optics/dofderivation.html ). So macro light losses (including extension tube losses) depend somewhat on lens design, even for externally focused lenses.

For an internal focus lens, things are very different. Instead of moving as a unit, the lens (ideally) remains in place and changes its focal length to focus closer. That means its FOV, and thus its effective focal length, doesn't change! The same applies to the effective f-stop if the pupil size remains the same - which is consistent with your DOF observations.

This can lead to surprising conclusions. Suppose you compare your 105mm and 150mm macro lenses at f/4 and 1:1 magnification, and let's assume that the 105 is symmetrical external focus while the 150 is purely internal focus - probably not too far from the truth. Then in effective terms, the old lens is a 210mm f/8 while the new one is a 150mm f/4. That explains your DOF observations; the surprise is that your new lens is effectively shorter!

A related bit of trivia: Nikon DSLRs actually calculate and report the effective f-stop for macro lenses, while Canon DSLRs (and probably most others) don't. But neither gives any indication of the effect on FOV.

--
Alan Martin

Nic Planarchy said:

No I'm not talking of background blur, it's DOF I'm interested
in... the DOF across a Dragonfly's eye for example. My perception
was that I was getting lower DOF with a 150mm macro than a 105mm
and I sought an explanation.

Click to expand...

Maybe my other post will help... I mentioned background blur vs. DOF just to explain the seemingly contradictory claims about DOF on the Internet. (Or sometimes actually contradictory, when the authors weren't aware of that distinction!)

--
Alan Martin

Thanks Alan and Lee Jay,

I think I'm begining to understand.... I'm glad at least that there is a possible explanation for my observations.

I do like to understand these things and at least I'm not running down the DOF = x/focal length type avenue anymore....

Thanks again, much appreciated.

--
NiC, Cannon 5D user.

Lee Jay said:

I think DOF is based on magnification, though I admit the DOF rules
at high magnifications aren't something I've derived or studied.

Click to expand...

A longer lens tends to give more magnification, unless you move in closer with the shorter one. Often that's not possible, which is why we associate shallow DOF with long lenses. But that's a little like saying the sun came up today because I had coffee yesterday; both are true, but it's not a cause-and-effect relationship.

Forrest said:

Forrest41926 said:

I think DOF is based on magnification, though I admit the DOF rules
at high magnifications aren't something I've derived or studied.

Click to expand...

A longer lens tends to give more magnification, unless you move in
closer with the shorter one. Often that's not possible, which is
why we associate shallow DOF with long lenses. But that's a
little like saying the sun came up today because I had coffee
yesterday; both are true, but it's not a cause-and-effect
relationship.

Click to expand...

I was talking about magnification, not focal length.

--
Lee Jay
(see profile for equipment)

Here are some ideas for what to test when you get your lens back - you could try it with the 50mm macro in the meantime. They are not actual DOF tests, but they measure the effects of extreme magnification, and the results should explain the DOF differences you have seen.

• Light losses: Focus at infinity, and meter the exposure in A mode for a white wall several feet away. Without moving the camera, focus down to 1:1. Does the metered shutter speed change, and how much? (If you move the camera to achieve focus, it shouldn't change the result; but you'd need a self-luminous subject like a CRT display to avoid shadows.)

• Field of view (this is really worth seeing for yourself): Focus at infinity, and take a picture of two small light sources at least several meters away and nicely spaced in the frame. Then focus to 1:1 and take another picture from the same position - never mind that it will be massively out of focus. (Stop down if necessary to keep the blur circles from overlapping.) Measure the distance between the bright spots in the first photo, and between the centers of the blur circles in the second photo. Does the distance change with focus - and which way? What does that say about FOV?

• Pupil size: If you point the viewfinder toward a light source and look into the front of the lens, you can see the entrance pupil as a bright circle. For a constant-aperture zoom lens, the pupil size changes as you zoom. What happens with your macro lens as you focus from infinity to 1:1?

I'd appreciate if you could post the results!

--
Alan Martin

Lee Jay said:

Lee Jay said:

Lee Jay said:

I think DOF is based on magnification, though I admit the DOF rules
at high magnifications aren't something I've derived or studied.

Click to expand...

Click to expand...

Click to expand...

Lee Jay said:

Lee Jay said:

A longer lens tends to give more magnification, unless you move in
closer with the shorter one. Often that's not possible, which is
why we associate shallow DOF with long lenses. But that's a
little like saying the sun came up today because I had coffee
yesterday; both are true, but it's not a cause-and-effect
relationship.

Click to expand...

Click to expand...

Lee Jay said:

I was talking about magnification, not focal length.

Click to expand...

I was under the impression you were talking about DOF. Which, like both of us are saying, is based on magnification, not focal length.

Forrest said:

I was under the impression you were talking about DOF. Which, like
both of us are saying, is based on magnification, not focal length.

Click to expand...

Yes. So why did you say "focal length has nothing to do with it"? I don't understand the point of your subject.

--
Lee Jay
(see profile for equipment)

Will try this and post results.

Although the 50mm was a loan and I don't have access to that at the moment (I'll see what I can do though) but I do have a 105 again and will shortly (hopefully) have the 150 back.

thanks again.
--
NiC, Cannon 5D user.

Lee Jay said:

Lee Jay said:

I was under the impression you were talking about DOF. Which, like
both of us are saying, is based on magnification, not focal length.

Click to expand...

Click to expand...

Lee Jay said:

Yes. So why did you say "focal length has nothing to do with it"?
I don't understand the point of your subject.

Click to expand...

Because there's a great deal of confusion over the subject ... from this thread it seems like more people believe that focal length controls DOF than, well, the truth. My subject line "FL has nothing to do with it" was in respnose to how DOF works, and clearing up the confusion.

You explained the truth of the matter, and people seem to be doubting you ... so I'm trying to lend support here. You can get the same amount of DOF - shallow or deep - with a 180 mm lens as you can with a 100, so long as the magnification and aperture are the same.

Nic Planarchy said:

Hmm.. and now even more confused.

If the 150 is reducing the effecting f stop I am losing DOF aren't
I? Or not?

Click to expand...

You are losing depth of field, yes, but you are reducing the physical f/#, not the effective f/#.

So, to answer your question: "does depth of field depend on the focal length of the lens?" No.

A better question would be, "for given camera settings, does the depth of field depend on how the lens is focussed?" Yes.

To explain a bit further:

The f/number is the focal length of the lens, divided by the diameter of the aperture.

An internally focussing macro lens does so by changing the focal length. It does this without modifying the size of the aperture (at least not much), so the f/number is reduced when the focal length is reduced for close focus.

In macro photography, you will also hear about something called the "effective f/number", which is the image distance divided by the diameter of the aperture. Personally, I'd rather it was called "image ratio", or even "i/number", so I will, from here on.

The image distance is the distance from the exit pupil of the lens to the film plane. The focal length and the image distance are related:

i = (1+M) f (where M is magnification)

So,
i/# = (1+M) f/#

The i/# (effective f/#) determines the exposure compensation required. It also determines the amount of diffraction you get.

The f/# determines depth of field (together with magnification).

A fixed focal length lens focusses by changing the image distance, whereas an internally focusing lens focusses by changing the focal length, keeping the image distance (pretty much) constant.

The aperture setting on your camera sets the diameter of the aperture, so for an internally focussing macro, the i/# is constant, and the f/# changes as you focus. In other words, the f/stop you set on your camera is the i/# for an internally focussing macro.

Conversely for the fixed focal length lens, the f/# is constant and the i/# changes as you focus. In this case, the f/stop you set on your camera is the f/#.

Now, a small wager to go with your testing: if you use the same aperture setting and lighting for the two lenses, I'll bet you'll need four times (two stops) more exposure time for your 50mm lens, at 1:1.

--
Kevin Moore

Thanks.... and I was wondering about the differing amounts of light required if the f stop was being changed. The test results will be errr... illuminating... (sorry)

--
NiC, Cannon 5D user.