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10-07-2009, 12:07 PM   #1
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DOF: Macro lens vs Ext. tube.

I recently started using my F-135/2.8 with a 25mm ext. tube and am very happy with it. Dof is always tricky and I have to stop down quite a bit to get decent dof. I was wondering whether a dedicateed macro will have a wider dof?

Is the dof different when used in an ext. tube, or is it only dependent on the aperture, focal length and the subject distance.


Last edited by pcarfan; 10-08-2009 at 02:52 AM.
10-07-2009, 12:48 PM   #2
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QuoteOriginally posted by pcarfan Quote
Is the dof different when used in an ext. tube, or is it only dependent on the aperture, focal length and the subject distance.
It's the latter.
10-07-2009, 12:48 PM   #3
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Depth of field is controlled by aperture and reproduction ratio on the sensor/film. The focal length of the lens and the camera to subject distance are not part of the equation.
I'm certain the circle of confusion brigade will have a field day with this post.
10-07-2009, 01:21 PM   #4
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I've read somewhere that also the design of the lens can have a small impact on the DOF. For instance, a 200mm macro lens may in fact have more DOF than a 100mm lens when both are at the same magnification and f-number. And more blurry background. Awesomeness ensues. Crazy.

I'm no expert on the matter, but here is a link to the macro section of an article that has everything you need to know about DOF. You should read the entire article if you really want to grasp how aperture, format, viewing size, focal length, etc etc all affect apparent DOF.

Enjoy! I did.

Depth of field

10-07-2009, 03:55 PM   #5
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Thanks.....................
10-07-2009, 04:17 PM   #6
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QuoteOriginally posted by arpaagent Quote
I've read somewhere that also the design of the lens can have a small impact on the DOF. For instance, a 200mm macro lens may in fact have more DOF than a 100mm lens when both are at the same magnification and f-number. And more blurry background. Awesomeness ensues. Crazy.
There's those Nikkor DF or whatever they are voodoo lenses that can shift the DOF back and forth somewhat, but I don't think they are actually altering the amount of DOF. Lens design will have a small effect, but this is kinda splitting hairs, since the effect will be VERY slight.
10-07-2009, 05:32 PM   #7
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to get back to the OP's question, DOF is going to be independant of macr lens vs standard lens at any equal focal length because to put it simply a macro lens is just a lens that has the focus extension of the extension tube built into the focusing mechanism.
10-07-2009, 07:12 PM   #8
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QuoteOriginally posted by Lowell Goudge Quote
to get back to the OP's question, DOF is going to be independant of macr lens vs standard lens at any equal focal length because to put it simply a macro lens is just a lens that has the focus extension of the extension tube built into the focusing mechanism.
Not necessarily true. Taking a look at my 100mm macro, the rear element does not move when focusing. I think this is called a FREE design (Fixed Rear Element somthing), and that may have some effect. that article mentioned the relative size of "pupil" of the aperture when viewed from front/back. DOF characteristics can change depending on this, and I think a lot of the more modern macro lenses are not simple helicoid with a single lens assembly.

Just food for thought

Also more food for thought, I think the F135 also has a fixed rear element. Since it's internal focus and doesn't extend when focusing, something inside is moving independent of the front and rear lens elements. Not sure what that says for DOF either.

10-07-2009, 10:04 PM   #9
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QuoteOriginally posted by arpaagent Quote
Not necessarily true. Taking a look at my 100mm macro, the rear element does not move when focusing. I think this is called a FREE design (Fixed Rear Element somthing), and that may have some effect. that article mentioned the relative size of "pupil" of the aperture when viewed from front/back. DOF characteristics can change depending on this, and I think a lot of the more modern macro lenses are not simple helicoid with a single lens assembly.

We had this discussion a few years ago on the PDML. One of the techno wonks did some number crunching and came to the conclusion that the A 100/2.8 macro is, in fact, a zoom lens of sorts. By the time you get it down to 1:1, it is closer to being an 85mm lens.
It is one of the FREE design lenses. I suspect they all share that characteristic.
10-24-2009, 09:31 AM   #10
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As a returning to photography novice, I seem to recall that most real macro lenses were 'flat field of focus' or close to it as opposed to a more curved critical focus field for conventional lenses. This is usually apparant when shooting flat subjects like documents or paintings. Perhaps someone with more expertise can add to this if it is accurate. This might explain fussy corners with some lenses using extension tubes. This also might be more apparant using full frame.
Thanks
10-24-2009, 10:06 AM   #11
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pcarfan, this might give you some idea. >>> https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/69616-whats-go...acro-lens.html
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