Macro vs extension tube

Hi, Olaf,

Olaf Ulrich said:

You can design formulas for any set of input parameters.

Click to expand...

Of course, assuming that the wanted result is a function of them.

Indeed. As a matter of fact, such a formula would compute the effective aperture number internally from the input parameters.

Click to expand...

Of course. But that's not a "fundamental" input to the underlying DoF equations, so this would be a roundabout formulation. It's ultimately D and f that are needed (among other parameters).

But if for some reason we wanted to cater to a user who had available only the "effective f-number" applicable to the setup of interest, then such a formulation would be useful.

In case you're interested, here is the most accurate formula for depth-of-field that I am aware of. . . .

Click to expand...

Yes, that looks vaguely familiar.

In case you're wondering why the formula takes f as one of the input parameters even though we stated that, at large magnifications, depth-of-field does not depend on focal length then please note that f gets cancelled out of the equation at high magnification but not at low magnification.

Click to expand...

Yes, indeed. The same can be said for the familiar approximation that assumes p=1 (such as I use in my DoF calculator spreadsheet.

Thanks for your inputs. I'm glad to have contact with you, and appreciate your extensive grasp of the principles here.

Best regards,

Doug

Extension Tube / depth of field.

Extension Tube Question / depth of field. Extension Tubes are used with any lens and I even use a bellows with Extension Tubes to get closer then Macro going to Micro 10X even 100X on the film plane. (I have mounted a Schneider 65mm Super Angulon lens made for a 4x5 camera to my Canon Bellows). If you need more death of field use a 28mm over a 50mm or 70mm? Sure death of field is less with longer lenses and wider apertures, lenses with a maximum aperture of 3.5 or F8 will have much better depth of field than a F 1.8 or 1.4, Nonetheless use a tripod and or a powerful flash to get f22.

Kroma Artazen said:

If you need more depth-of-field use a 28 mm over a 50 mm or 70 mm?

Click to expand...

No. On a bellows unit, a 28 mm lens will yield less depth-of-field than a 50 mm or 70 mm lens, not more.

-- Olaf

Olaf Ulrich said:

No. On a bellows unit, a 28 mm lens will yield less depth-of-field than a 50 mm or 70 mm lens, not more.

-- Olaf

Click to expand...

Well, I say you're both wrong. At equivalent image magnifications, the DoF's will be, well, equivalent.

Of course we are assuming the same effective aperture per the earlier definition...

Hi, Jack

Jack_Flesher said:

Well, I say your both wrong. At equivalent image magnifications, the DoF's will be, well, equivalent.

Of course we are assuming the same effective aperture per the earlier definition...

Click to expand...

The pertinent parameter is the (actual) f-number.

But of course, for a given magnification, and a given (actual) f-number, we will have the same effective f-number, so what you say is true as well.

Best regards,

Doug

Another interesting relationship at relatively large magnifications is that for a given magnification (and again assuming a fixed choice for acceptable circle of confusion), the total depth of field varies almost directly with the f-number (actual, or effective, as one might for some reason want to use, since they have the same ratio at any magnification).

For example at a magnification of 1.0 (1:1), choosing an acceptable circle of confusion of 0.031 mm, for an f/number of 2.0, the total depth of field is very nearly 0.25 mm. At that same magnification, for an f/number of 4.0, the total depth of field is very nearly 0.50 mm; for an f/number of 6.0, the total depth of field is very nearly 0.75 mm.

Now if for some reason we want to operate with effective f/number:

At a magnification of 1.0 (1:1), and an acceptable circle of confusion of 0.031 mm, for an effective f/number of 4.0, the total depth of field is very nearly 0.25 mm. At that same magnification, for an effective f/number of 8.0, the total depth of field is very nearly 0.50 mm; for an effective f/number of 12.0, the total depth of field is very nearly 0.75 mm.

[Effective f-number here means from a geometrical standpoint; that is, does not take into effect lens transmission.]

We would see the same thing at other magnifications that are fairly large - for example, 0.5 (1:2)

Best regards

Doug

Jack_Flesher said:

Well, I say you're both wrong. At equivalent image magnifications, the DoF's will be, well, equivalent.

Click to expand...

Jack, you simply don't get what's being talked about.

On a bellows unit or on a set of extension tubes---i. e. at more or less the same extension---a 28 mm lens will yield more magnification than a 70 mm lens and thus, less depth-of-field.

At the same magnification, depth-of-field of course will be the same---I kept repeating that mantra often enough, didn't I? However you'll have a hard time trying to achieve the same magnification with a 70 mm lens and with a 28 mm on a bellows.

-- Olaf

Olaf Ulrich said:

On a bellows unit or on a set of extension tubes---i. e. at more or less the same extension---a 28 mm lens will yield more magnification than a 70 mm lens and thus, less depth-of-field.

Click to expand...

With equal *extensions*, of course.

At the same magnification, depth-of-field of course will be the same

Click to expand...

Of course, and why I suggested magnification formulae for macro DoF calculations originally.

However you'll have a hard time trying to achieve the same magnification with a 70 mm lens and with a 28 mm on a bellows.

Click to expand...

Depends on how long the bellows is ;-)... And let's not forget that since the typical 28mm lens for 35mm SLR's is going to be a retrofocus design, it is actually possible to add enough extension with an un-reversed 28 to require the subject plane be positioned *behind* the front element of the lens-group (a physical impossibility) to be in focus at the image plane! And since this will not happen with a 70mm lens designed for 35mm cameras at any extension, it is likely we can actually achieve more magnification from the 70mm lens.

Cheers,

Hi, Jack,

Jack_Flesher said:

With equal *extensions*, of course.

Click to expand...

Indeed.

Note that in the case of lenses with other than "all group" focusing, the precise formal concept of "extension" (the distance the lens is forward of the position providing focus at infinity) becomes rather scrambled. But perhaps you are referring to the amount of "additional extension" provided by an extension tube.

And let's not forget that since the typical 28mm lens for 35mm SLR's is going to be a retrofocus design, it is actually possible to add enough extension with an un-reversed 28 to require the subject plane be positioned *behind* the front element of the lens-group (a physical impossibility) to be in focus at the image plane!

Click to expand...

True.

And since this will not happen with a 70mm lens designed for 35mm cameras at any extension . . .

Click to expand...

I'm not sure that''s true.

If the first principal point is behind the front vertex of the lens (certainly possible even with a "non-retrofocus" design), then with sufficient "extension" we can still require the object plane to be behind the front vertex (thus imposing a limit on magnification).

It seems that for my Canon EF 24-105 mm f/4 L IS, at 70 mm, (with the focusing ring set to either infinity or closest focus) the first principal point seems to be well "inside" (based on a little bush-league optical bench work).

What am I missing here?

Best regards,

Doug

Doug_Kerr said:

I'm not sure that''s true.

If the first principal point is behind the front vertex of the lens (certainly possible even with a "non-retrofocus" design), then with sufficient "extension" we can still require the object plane to be behind the front vertex (thus imposing a limit on magnification).

Click to expand...

Hi Doug:

Good point, and I have never personally tried it with a conventional lens at more than about 3:1 magnifications. But I have stacked two bellows units together plus tubes with a conventional 50 and it worked. This was a long time ago and I do not specifically recall, but it is entirely possible I also had the 50 reversed as well which would alter things...

In concert with the above, I have a friend who built a custom 2.5 meter long bellows for a specific client's project. It is basically 4 Cambo 4x5 view cameras -- meaning 8 view camera standards -- mounted on a long optical bench and inter-connected at each standard by their bellows, in this case a total of 7. Anyway, he mounts his Canon DSLR at the rear and usually mounts an enlarging lens up front for massive, over 20:1 magnifications and that works. Of course copy cameras operated on this same principal...

It seems that for my Canon EF 24-105 mm f/4 L IS, at 70 mm, (with the focusing ring set to either infinity or closest focus) the first principal point seems to be well "inside" (based on a little bush-league optical bench work).

What am I missing here?

Click to expand...

I think we can dispense with the zoom --- a zoom that goes beyond 70mm will be much longer physically than a 70mm non-telephoto prime, so indeed I agree it's entirely possible for the first nodal point (FNP) to fall inside the front element with moderate extension when set to less than the maximum focal length.

However, I assumed Olaf was discussing prime lenses, and most 70mm primes would be conventional designs on 35mm, or possibly slight telephoto (and physically shorter with an even further forward FNP still), and thus the FNP should remain outside the front element, similar to my 50 result above. By the same token, I'd predict that when your zoom is set on the maximum 105mm setting, the FNP should also remain in front of the front element -- but admittedly I do not know either of these for certain at very long extensions and would need to do some research before commenting definitively. However, that is an entirely academic concern to me as I never shoot that type of macro, so I'll leave the attendant research and math to others that might be interested --- you perhaps? ;-)

Cheers,

Jack_Flesher said:

But I have stacked two bellows units together plus tubes with a conventional 50 and it worked.

Click to expand...

Why did you do that? Didn't you own a wide-angle lens?

Jack_Flesher said:

In concert with the above, I have a friend who built a custom 2.5 meter long bellows for a specific client's project. It is basically four Cambo 4x5 view cameras -- meaning eight view camera standards -- mounted on a long optical bench and inter-connected at each standard by their bellows, in this case a total of seven. Anyway, he mounts his Canon DSLR at the rear and usually mounts an enlarging lens up front for massive, over 20:1 magnifications and that works.

Click to expand...

What an entirely stupid thing to! To achieve a magnification of 20:1, only fools would construct 2.5 meters (8 ft) of extension from seven view-camera bellows when you can achieve the same goal with just one 35-mm-format bellows plus a set of extension tubes, i. e. an extension of 30 cm (1 ft) or thereabouts. That's way cheaper to acquire and way easier and quicker to operate. And image quality will be as good or better.

And for magnifications higher than 25:1 or 30:1, you'd simply shoot through a microscope rather than fiddling with camera lenses and endless extension.

-- Olaf

Olaf Ulrich said:

Why did you do that? Didn't you own a wide-angle lens?

Click to expand...

Sure, but the when the focus point moved behind the front element before I got to the magnification I wanted, I had to come up with a alternate solution.

And for magnifications higher than 25:1 or 30:1, you'd simply shoot through a microscope rather than fiddling with camera lenses and endless extension.

Click to expand...

It would have been tough to get the large hunks of delicate, intricately-carved Qing Dynasty ivory on the focusing stage of a microscope without breaking it up first -- and I don't think the client would have been too happy with that if we had tried...

Just curious Olaf, what type of photography do you regularly do? I am guessing you've never done much real-world commercial product work...

Olaf Ulrich said:

What an entirely stupid thing to! To achieve a magnification of 20:1, only fools would construct 2.5 meters (8 ft) of extension from seven view-camera bellows when you can achieve the same goal with just one 35-mm-format bellows plus a set of extension tubes, i. e. an extension of 30 cm (1 ft) or thereabouts. That's way cheaper to acquire and way easier and quicker to operate. And image quality will be as good or better.

Click to expand...

Hi Olaf,

You're quick to pass judgement on the choices of others, but perhaps the intended reproduction had to cover a larger area than 1.2 x 1.8 mm of the object? I wasn't there, so I don't know the compromises that had to be made, do you?

Anyway, there is more than one way to skin a cat, in that sense you are right.

Bart

Bart_van_der_Wolf said:

You're quick to pass judgement on the choices of others ...

Click to expand...

I don't think so. But if so then I'd appreciate an explanation as to why the photographer chose such a---usually pointless---detour. In the meantime, Jack gave an explanation: The reason simply was because the photographer didn't understand the basic techniques of photography.

Bart_van_der_Wolf said:

... but perhaps the intended reproduction had to cover a larger area than 1.2 x 1.8 mm of the object?

Click to expand...

Not if the camera used behind all those excessive extension was just a Canon DSLR. Anyway, for any given magnification, a larger image format would require a wider bellows, not a longer one.

Jack_Flesher said:

Why did you do that? Didn't you own a wide-angle lens?

Click to expand...

Sure, but the when the focus point moved behind the front element before I got to the magnification I wanted, I had to come up with a alternate solution.

Click to expand...

Umm ... and the solution to simply reverse the wide-angle lens would have been too easy, or what? Or did you want to preserve the poor image quality of a non-reversed lens?

-- Olaf