Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
07-30-2011, 02:32 PM   #1
Site Supporter




Join Date: Dec 2010
Location: San Diego, CA
Posts: 29
Extension tube vs Close-up lens

Would a close-up lens give narrower depth-of-field than a comparable extension tube giving the same magnification increase? I'm assuming no, since they both essentially decrease your minimum focus distance. But I'm wondering if the correction of the added close-up lens affects the amount of bokeh.

I know from a disadvantege standpoint, the close-up lens also decreases your maximum focus distance (usually losing your capability to focus to infinity), while the extension tube decreases the amount of light hitting the sensor and sometimes even decreasing autofocus reliability. Does the extension tube decrease your max focus distance as well?

07-30-2011, 02:46 PM   #2
Inactive Account




Join Date: Aug 2008
Location: Michigan, USA
Photos: Gallery | Albums
Posts: 7,485
QuoteOriginally posted by wutsurstyle Quote
Would a close-up lens give narrower depth-of-field than a comparable extension tube giving the same magnification increase? I'm assuming no, since they both essentially decrease your minimum focus distance. But I'm wondering if the correction of the added close-up lens affects the amount of bokeh.

I know from a disadvantege standpoint, the close-up lens also decreases your maximum focus distance (usually losing your capability to focus to infinity), while the extension tube decreases the amount of light hitting the sensor and sometimes even decreasing autofocus reliability. Does the extension tube decrease your max focus distance as well?
What both will do is change your lens to subject distance for any focus at all. That is what more than anything else will affect your DOF which is a function of Distance vs Aperture. So the closer you are to your subject, the narrower the DOF at any given aperture. Personally, if I'm going to choose between the two, I'm going to choose the tubes to avoid putting any other glass in the optical path. This Will change the exposure required from a regular mounted lens whereas with the closeup lenses, it may not change.

07-30-2011, 02:52 PM   #3
Veteran Member
Docrwm's Avatar

Join Date: Sep 2010
Location: Somewhere in the Southern US
Photos: Gallery | Albums
Posts: 11,275
Tubes over another layer of (usually) lesser quality glass for me too. I prefer a bellows over tubes because it gives more precise control - I found my M42 Pentax Bellows unit for $9 in its original box and am really enjoying it immensely over my set of tubes.
07-30-2011, 03:04 PM   #4
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
Would a close-up lens give narrower depth-of-field than a comparable extension tube giving the same magnification increase? I'm assuming no, since they both essentially decrease your minimum focus distance.

The quick answer is DOF is the same at the same actual f-stop and magnification.

If a close-up lens is used the actual fstop is that shown on the lens. while if an extension tube is used the f-stop set on the lens is increased by a factor of 1+m.


But I'm wondering if the correction of the added close-up lens affects the amount of bokeh.

Yes; what the close-up lens actually does is to decrease the lens focal length a lot - by a factor of 1/(1+m) - therefore you'll see much more background; If background can be seen there'll be more of it with the close-up lens.


I know from a disadvantege standpoint, the close-up lens also decreases your maximum focus distance (usually losing your capability to focus to infinity), while the extension tube decreases the amount of light hitting the sensor and sometimes even decreasing autofocus reliability. Does the extension tube decrease your max focus distance as well?

yes - but the actual working distance will depend on details like the original lens focal length.

07-30-2011, 04:50 PM - 1 Like   #5
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
A closeup lens/adapter changes the focal length of the optical system, and reduces the working distance. Exact magnification depends on the focal length of the host lens. Here is a table of dioptres vs working distances (inches and metric):

+1 >> 20-38" (500-950mm)
+2 >> 13-20" (330-500mm)
+3 >> 10-13" (250-330mm)
+4 >> 8--10" (205-250mm)
+5 >> 6.5-8" (165-205mm)
+6 >> 6-6.5" (153-165mm)
+8 >> 5" --- (127mm)
+10 > 4" --- (102mm)

The common Raynox DCR-150 is +4.8dpt; the DCR-250 is +8dpt.

Calculating focal lengths and dioptres is simple: 1000/dpt= FL, and 1000/FL= dpt. A host lens of 100mm is thus 1000/100= +10dpt. Dioptres are additive. Thus that 100mm lens plus a DCR-250 (+8dpt) together are 10+8= +18dpt, and the new focal length is 1000/18= 55.55mm.

AFAIK a +dioptre lens doesn't noticeably affect light transmission -- no light loss, unlike extension (tubes and/or bellows). (newarts might correct me here.) A closeup adapter DOES put more glass between subject and frame, and thus can reduce contrast; whether this is bothersome is up to you. A corrected Raynox adapter has much less distortion and aberrations away from the image center than do cheap adapters. A simple cheap +dioptre meniscus *will* introduce such flaws, but depending on your subject and technique and PP, this might not be serious. Such adapters are cheap and fun; neither the Raynoxi nor the cheap-o's interfere with lens-camera automation.
_________________________________________

Extension (tubes and/or bellows) change the focus distance and diminish the light reaching the frame.

* Magnification is calculated as M= (TE-FL)/FL where TE is Total Extension and FL is focal length. A 100mm lens at infinity focus has 100mm of extension built into it; add another 100mm of extension, and magnification is (200-100)/100= 1:1.

* Light loss is simple: (M+1) where M is magnification. Thus that 100mm lens on 100mm extension has light loss of (1+1)= 2 f-stops. Yes, MAGNIFICATION EATS LIGHT! Long extension often demands flash or other external light.

Cheap M-type macro tubes or bellows kill lens-camera automation. AF tubes are available, hard to find, and costly. (And you don't need AF for macro!) A-type tubes are a bit cheaper and more common. A-type teleconverters are commoner and cheaper and are easily de-glassed to become tubes. Then there are macro-focusing teleconverters; I have one, don't use it, and thus can't talk about it now. Maybe later...
_________________________________________

Oops, I didn't address DOF. Well, it's like this: At macro distances, DOF is hair-thin no matter how you get there. Knowing the focal length and aperture and focus distance, you can consult online DOF calculators to tell you just how microns of sharpness you have. Plug in the numbers; read'em and weep.

Hope this helps!

Last edited by RioRico; 07-30-2011 at 04:57 PM.
07-30-2011, 10:17 PM   #6
Site Supporter




Join Date: Dec 2010
Location: San Diego, CA
Posts: 29
Original Poster
Thanks for the explanation guys! I guess the underlying reason for my DOF inquiry is that I had a feeling that the light loss inccured when using an extension tube effectively increases the f-stop (depending on the extension distance). Doesn't this change in f-stop change your DOF? All the while, a comparible close-ups lens at the same mag ratio does not change your f-stop, hence a difference in bokeh between the two setups? Correct me if I'm wrong.
07-30-2011, 11:02 PM   #7
Veteran Member
RioRico's Avatar

Join Date: Jun 2008
Location: Limbo, California
Posts: 11,264
OK, here's the correct line: The aperture number is the ratio of the iris opening to the focal length. So if the iris diameter is 1/11 the focal length, the aperture is f/11. Adding extension changes the lens-to-frame and lens-to-subject distances. Adding extension does NOT change the iris size, nor the focal length. Thus the aperture remains the same. You may see references to EFFECTIVE APERTURE -- and I almost wrote the formula above like that, but it's misleading, like the crap.factor EFFECTIVE FOCAL LENGTH, because the ratio hasn't changed.

But an adapter... hmmm, that DOES change the focal length of the optical system, and the lens-to-subject distance -- but not the iris size nor lens-to-frame distance. In the example I gave above, a 100mm lens plus a DCR-250 make an +18dpt = 55.55mm lens. If the lens was set to f/4, its iris would be 100/4= 25mm. But with the adapter, the effective aperture (the term is valid here) is 55.55/25= f/2.2. That DOES affect DOF -- thins it out! (Well, I *think* that's right. newarts, does this scan?)

The question then is: Given the razor-thin DOF at macro scales, does this make a difference? Is it noticeable? What's the difference in DOF -- nine or twelve microns? If I were teaching, I guess this is where I'd say: The solution is left as an exercise. Meaning, y'all should find an online DOF calculator and plug in the numbers. And maybe do some test shoots. Learn by doing, eh? Not me -- I'm going to bed now. G'night.
07-31-2011, 08:57 AM   #8
Inactive Account




Join Date: Dec 2008
Location: Ames, Iowa, USA
Photos: Albums
Posts: 2,965
@wutsurstyle & RioRico...

Here I use N for f-number to avoid confusion with f for focal length.

The reason for confusion surrounding "effective fnumber" & how/when to use it is that most of the common equations like DOF, exposure, diffraction, shake reduction, etc tacitly assume the lens is one focal length away from the sensor - but for macros it isn't one focal length away - its actual distance is always f(1+m) - meanwhile the diameter of the actual physical aperture has not changed.

image.distance = focal.length(1+m) where m=sensor.width/scene.width

The m term is usually very small - for a meter wide portrait, m is about 1/40 - that's why it is seldom included in equations intended for normal use..

Unless an equation is specifically written for macro conditions one should substitute f(1+m) for f, and N(1+m) for N for application in macro conditions.

Common equations including macro conditions:
Diffraction.spot.diameter = 2.44 N(1+m) Wavelength
DoF = 2C N(1+m) /m^2
Relative.light.intensity.at.image = 1/[N(1+m)]^2
Shake.reduction.fnumber = N(1+m)
etc....

The case for a close-up lens added to a normal lens on a camera is interesting: the close-up lens reduces the focal length of the original lens by a factor of 1/(1+m) and it also increases its fnumber by a factor of (1+m) - these two factors exactly compensate so there is no illumination change when you put on a close-up lens.

Try it! exposure for a particular scene will be the same with and without a close-up lens!

Hope this helps... Sorry for the math but there is no other way.

Dave

PS RioRico said...Thus the aperture remains the same. You may see references to EFFECTIVE APERTURE -- and I almost wrote the formula above like that, but it's misleading, like the crap.factor EFFECTIVE FOCAL LENGTH, because the ratio hasn't changed.
The underlying problem is that focal length is normally used to approximate the distance from lens to image which changes with m.

The question then is: Given the razor-thin DOF at macro scales, does this make a difference? Is it noticeable?
It turns out the DOF for close-up lens and extension tube cases are the same for the same light intensity on the sensor.


Last edited by newarts; 08-02-2011 at 02:26 PM.
07-31-2011, 12:04 PM   #9
Site Supporter




Join Date: Jun 2011
Location: Texas
Photos: Gallery
Posts: 1,116
I've carried extension tubes and table-top tripod on hikes for 45 years, as you can get very good results with minimal equipment and weight. However, a macro lens (not a close-up lens attachment) can be even more convenient and give better results in the close-up range. A 50 or 60 mm F2.8 macro will focus to infinity and can also be a great "normal" lens. If I don't use that I may carry a 100 F4.0 macro in place of a short tele. The 100 give a bit more distance from the subject, and nice shallow DOF, even with the F4 max aperture.
(As you may guess I don't like zoom lenses!)
08-02-2011, 08:33 AM - 1 Like   #10
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,312
A couple of points here, which I think may help in clarifying the understanding of how diopter lenses and extension tubes work,

RioRico gave a pretty good explanation, with some slight calculation errors. specifically since a +1 diopter lens has a focal length of 1meter, the working distance should be 1000 mm not 950 since at a distance of 1000 mm from a point source, the light exiting a +1 diopter lens will be parallel, and that is the fdefinition of infinity focus,

essentually, the real difference between add on diopters and extension tubes is that add on diopters as correctly stated by Rio, actually modify the focal length of the lens.

Additionally, and due to the modification, the new combined lens is (when set at infinity) effectively on an extension tube, because the lens is now too far away from the sensor to focus at infinity. If you consider, for example a 100mm lens. when set to infinity the lens is effectively placed 100mm from the focusing plane (sensor) but when you add a +10 diopter lens to this, the net result is that the combined focal length is 50mm, but with the lens mounted 100mm away from the sensor, you end up with the same result (and magnification) as a 50mm lens on a 50mm extension tube.

Where the difference between close up lenses and extension tubes becomes noticible is that the 100mm +10 diopter lens has the same physical aperture therefore the F number changes. so the net result is that a 100mmF2.8 with a 10 diopter close up becomes a 50mm F1.4

from that point forward all else is the same. therefore a correct comparison would be for example a 100/2.8 lens wiht +10 diopter compared to a 50/1.4 with 50mm extension tube.


what you give up potentially with the 100mm solution is the potential distortion and imperfections of the additional glass in front of the lens, compared to the lens on a tube.
08-02-2011, 11:48 AM   #11
Site Supporter




Join Date: Dec 2010
Location: San Diego, CA
Posts: 29
Original Poster
These posts are pure awesomeness. Thanks for the info guys.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
amount, camera, close-up, decreases, distance, extension, focus, lens, pentax help, photography, tube
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
why use an extension tube? dh4412 Troubleshooting and Beginner Help 12 03-02-2011 07:30 PM
Made my own extension tube photolady95 Pentax Camera and Field Accessories 9 11-28-2010 10:05 AM
For Sale - Sold: Free - lens, lens case, m42 extension tube heliphoto Sold Items 3 11-30-2009 05:55 PM
For Sale - Sold: M 75-150mm lens and PK extension tube set kibipod Sold Items 9 06-23-2009 10:08 AM
DA lens & extension tube kibipod Troubleshooting and Beginner Help 2 04-17-2009 11:35 AM



All times are GMT -7. The time now is 02:27 AM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top