Extension Tubes
John Prosper
>rsh...@mcube.com wrote:
>
>: Instead of extension tubes, consider the two-element Nikon diopters: 3T,
>: 4T, 5T, and 6T.
>
>: I've got the PN-11 and PK-13 tubes, and practically never use them.
>: Instead, I put a 5T or 6T on a zoom, and get sharp results without
>: losing light to extension. Plus, the zoom action works like a built-in
>: focusing rail.
>
>
>(snip)
>
>but what about quality???
>which is better, the tube or the diopter element??
>
>(I'm particularly thinking of getting the canon 200 2.8 with the 25mm
>extension tube)
Technically speaking, it all depends.
As far as the degradation of the image goes, the tube is better
since it is hollow and does not contain any optical elements to
degrade the image further. ALL optical elements degrade image
quality.
However, as the first (quoted) poster has observed, the tube also
reduces light. Thus, the ability of the photographer to focus
accurately in diminished lighting is also a technical
consideration.
In addition there are other factors to image quality not being
mentioned here (eg., the employment and PROPER use of a tripod
or flash to diminish or eliminate blur, how perfectionistic are
the techniques employed by the photographer--including
appropriate selection of the plane of focus to best maximize the
photographer's expression of the subject theme).
The two-element diopters can give very good results, indeed,
used properly. However, they WILL diminish quality slightly.
The critical question is: how much quality is one willing to
lose?
Given Jeffrey's equipment--a Canon 200mm F/2.8 prime optic--a
25mm extension tube would not affect image quality and would
lose only 1/4 of a stop of light, hardly a concern in normal
lighting conditions. So the lens reduces to a ~F/3.1 optic.
Under darker conditions where you may desire every smiggen of
light possible, the diopter starts looking very reasonable.
--
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| John A. Prosper | Internet: pro...@cs.fsu.edu |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Bo-Ming Tong
John Prosper <pro...@xi.cs.fsu.edu> wrote:
>Given Jeffrey's equipment--a Canon 200mm F/2.8 prime optic--a
>25mm extension tube would not affect image quality and would
>lose only 1/4 of a stop of light, hardly a concern in normal
>lighting conditions. So the lens reduces to a ~F/3.1 optic.
>Under darker conditions where you may desire every smiggen of
>light possible, the diopter starts looking very reasonable.
25mm will not take you much closer to your subject. You may need a
100mm tube (!) in order to get something near 1:1 (the exact length of
extension needed depends on a lots of things).
Have you ever heard of anyone who puts an extension tube to a 200mm
macro lens ? A 2x teleconverter is always used to turn a 200mm macro
lens from 1:2 to 1:1, and the reason is that an extension tube would
be too long.
--
Bo-Ming Tong ~{LF1&Cw~}, PhD student, 602-6219603, bmt...@cs.arizona.edu
Department of Computer Science, University of Arizona, Tucson, AZ 85721
Nikon FAQ maintainer, get from http://www.cs.arizona.edu/people/bmtong/
John Prosper
>John Prosper <pro...@xi.cs.fsu.edu> wrote:
>>Given Jeffrey's equipment--a Canon 200mm F/2.8 prime optic--a
>>25mm extension tube would not affect image quality and would
>>lose only 1/4 of a stop of light, hardly a concern in normal
>>lighting conditions. So the lens reduces to a ~F/3.1 optic.
>>Under darker conditions where you may desire every smiggen of
>>light possible, the diopter starts looking very reasonable.
>
>25mm will not take you much closer to your subject. You may need a
>100mm tube (!) in order to get something near 1:1 (the exact length of
>extension needed depends on a lots of things).
Who ever said he needed to get much closer?
>Have you ever heard of anyone who puts an extension tube to a 200mm
>macro lens ?
In the first place nobody ever said the 200mm F/2.8 optic was a
macro. The only macro optic close to the focal length that I am
aware of is the Sigma 180mm F/2.8 unit. If it is a macro, then
I stand corrected; there would be virtually no need for any
additional magnification.
Furthermore, there is nothing wrong with using a 25mm or even a
14mm extension tube to cut down on the minimum focusing distance.
John Shaw occassionally employs a 14mm tube on his 180mm for that
very reason. Given the right subject, a couple of feet closer
can make a tremendous difference in composition.
> A 2x teleconverter is always used to turn a 200mm macro
>lens from 1:2 to 1:1, and the reason is that an extension tube would
>be too long.
>--
>Bo-Ming Tong ~{LF1&Cw~}, PhD student, 602-6219603, bmt...@cs.arizona.edu
>Department of Computer Science, University of Arizona, Tucson, AZ 85721
>Nikon FAQ maintainer, get from http://www.cs.arizona.edu/people/bmtong/
Bo-Ming Tong
John Prosper <pro...@xi.cs.fsu.edu> wrote:
>Furthermore, there is nothing wrong with using a 25mm or even a
>14mm extension tube to cut down on the minimum focusing distance.
>John Shaw occassionally employs a 14mm tube on his 180mm for that
>very reason. Given the right subject, a couple of feet closer
>can make a tremendous difference in composition.
Yeah, right. A lot of bird photographers use short extension tubes on
800mm lenses too. Just to get a bit closer.
John Prosper
Jeffrey Albro <jal...@emerald.tufts.edu> wrote:
>thanks for the info, John.
>
>However, what is the formula for finding the actual magnification with
>the extension tube??
>
>I looked in the lens FAQ's but couldn't find it.
>
>the canon 200 2.8 close focuses to 1.5 m/ 4.9 ft and has a magnification
>of .16 at that focus.
>
>-jeff
>
>
>John Prosper (pro...@xi.cs.fsu.edu) wrote:
>: >
>: >but what about quality???
John Prosper
Subject: Re: Extension Tubes
Summary:
Expires:
References: <7769-16...@mcube.com> <3j04rj$g...@emerald.tufts.edu> <3jadb6$k...@mailer.fsu.edu> <3jd766$p...@emerald.tufts.edu>
Sender:
Followup-To:
Distribution:
Organization: Florida State University Computer Science Dept.
Keywords:
Cc:
{ Sorry about the empty post earlier. Let's try it again.
John A. Prosper }
Jeffrey Albro <jal...@emerald.tufts.edu> wrote:
>thanks for the info, John.
>
>However, what is the formula for finding the actual magnification with
>the extension tube??
>l
>I looked in the lens FAQ's but couldn't find it.
>
>the canon 200 2.8 close focuses to 1.5 m/ 4.9 ft and has a magnification
>of .16 at that focus.
Generally speaking,
(1) Magnification = Total extension used/Focal length used
or
1/8 = 25mm/200mm,
so you can figure on 1/8X (0.125X) magnification.
Bear in mind that the close focus resolution of many conventional
lenses is well below professional quality (exceptions being macro
optics) since they are optimized for use at infinity, especially
those with a so-called macro mode (which involves big-time!
compromises in quality). Isn't the Canon 200 F/2.8 an APO optic?
Differences in quality really show at wide apertures so one should
stop down for better quality.
To increase power one can opt for a larger tube--say, a 50mm--to
get 1/4X magnification, although the big tube can be somewhat
unweildy unless it has a rotating tripod collar. You would
probably find it more convenient to use dual-element diopters (eg.,
Nikon's 3T-->6T). (I understand that Canon has their own dual-
element diopters although I know nothing about them.)
The Nikon diopters come in two filter sizes (52mm, 3T & 4T; 62mm,
5T & 6T) and two diopter strengths (+1.5, 3T & 5T; +2.9, 4T & 6T).
I own the 5T and 6T myself. Diopters modify the focal length (FL)
of the prime lens according to the following formula:
(2) Combined FL = 1000/((1000/FL prime lens) + diopter power)
With a +2.9 diopter and a 200mm lens, the modified FL becomes
126.58228.
Diopters have their own FL given by
(3) FL = 1 meter/Diopter power
So a +2.9 diopter has a FL of about 345 (1000mm/2.9 or 344.82759).
To calculate magnification (M) using diopters, one can use use
either of two formulas. The first is given by:
(4) M = FL prime lens/FL of diopter
With your 200mm and a +2.9 diopter, M =0.58X (200/344.82759). An
easier way to compute M is given by:
(5) M = Diopter power * FL prime lens (in meters)
Again with the 200mm and our +2.9 diopter we have
(6) M = 2.9 * 0.2 = 0.58X
For higher M's, we can add a teleconverter to our diopter/prime
lens combo. Our new M is given by:
New M = Old M * teleconverter strength
Using a 1.4X (2X) converter to our 200mm/+2.9 diopter combo yields
an M of 0.812 (1.16X).
You may want to pick up a copy of John Shaw's "Closeups in Nature"
for other ideas (eg., stacked lenses).
Teun Hendriks
Bo-Ming Tong <bmt...@CS.Arizona.EDU> wrote:
>In article <3jadb6$k...@mailer.fsu.edu>,
>John Prosper <pro...@xi.cs.fsu.edu> wrote:
>>Given Jeffrey's equipment--a Canon 200mm F/2.8 prime optic--a
>>25mm extension tube would not affect image quality and would
>>lose only 1/4 of a stop of light, hardly a concern in normal
>>lighting conditions. So the lens reduces to a ~F/3.1 optic.
>>Under darker conditions where you may desire every smiggen of
>>light possible, the diopter starts looking very reasonable.
>
>100mm tube (!) in order to get something near 1:1 (the exact length of
>extension needed depends on a lots of things).
From experience, 27.5mm extension (a PK13) on a 180mm changes the
focusing range from (1.5m-infinity) to ~(1m -1.5m). This is a very useful
range (1/6X -1/3X).
I like to use this combination in the field. Carrying the extra weight
of a PK13 has not bothered me yet:-)
- Teun
--
Teun Hendriks tel: (914) 945-6408
Philips Laboratories fax: (914) 945-6141
345 Scarborough Road
Briarcliff Manor, NY 10510, USA email: t...@philabs.philips.com
Robert M Atkins
Jeffrey Albro <jal...@emerald.tufts.edu> wrote:
>thanks for the info, John.
>
>However, what is the formula for finding the actual magnification with
>the extension tube??
>
>
>the canon 200 2.8 close focuses to 1.5 m/ 4.9 ft and has a magnification
>of .16 at that focus.
>
As I've said before, the FAQ is for stuff Canon *doesn't* tell you!
The Canon EF Lenses brochure gives the Magnification of the 200/2.8 + EF25
as 0.31x
You can get rought estimates of magnification using the standard high
school physics formula:
1/object_distance + 1/image_distance = 1/focal_length
The trouble comes in defining exactly what object and image distance are
at close range (distance to lens? distance to film plane? - actually
distance to nodal points)
You caluculate how much effective extension is built into the lens
at it's closest focus (about 30mm in this case), then add the extension
distance, then calculate the point the lens will focus on, the find
magnification by (object_distance)/(image_distance)
This suggests that the close focus distance with the EF25 will be about
3ft.
Bob Atkins
AT&T BEll Labs, Murray Hill, NJ Email: r...@clockwise.att.com
Get Canon EOS FAQ from moink.nmsu.edu (/rec.photo/canon) via anonymous ftp
Jeffrey Albro
However, what is the formula for finding the actual magnification with
the extension tube??
I looked in the lens FAQ's but couldn't find it.
the canon 200 2.8 close focuses to 1.5 m/ 4.9 ft and has a magnification
of .16 at that focus.
-jeff
John Prosper (pro...@xi.cs.fsu.edu) wrote:
: >
: >but what about quality???
131G30000-D.GALENSKY(MT4743)1019MT
Teun Hendriks <t...@philabs.philips.com> wrote:
>In article <3jffe4$g...@lectura.cs.arizona.edu>,
>Bo-Ming Tong <bmt...@CS.Arizona.EDU> wrote:
>>In article <3jadb6$k...@mailer.fsu.edu>,
>>
>>100mm tube (!) in order to get something near 1:1 (the exact length of
>>extension needed depends on a lots of things).
>
>From experience, 27.5mm extension (a PK13) on a 180mm changes the
>focusing range from (1.5m-infinity) to ~(1m -1.5m). This is a very useful
>range (1/6X -1/3X).
i use a 52.5mm tube (PN11) with tripod collar to
get to about half life size with my 180/2.8.
it works great...plus the tripod collar is very
convenient.
duane