Maximum Focus Distance with EF25 and EF12 Tubes?

[sheldonnalos]

I've searched a lot and can't find an easy answer to this question...

 

What would be the *maximum* focus distance with the EF12 or EF25

extension tubes when using a 50mm f/1.4 or 85mm f/1.8? I'm looking

to try them out for macro, but am also curious how useful they might

be in a closer portrait setting (i.e. 12mm extension on the 85mm for

very tight headshots). I know I'll lose infinity focus, but will I

still be able to focus 5-10 feet out?

 

Thanks!

[oceanphysics]

f+f^2/e where f is the focal length and e is the extension tube length. Answer is in millimeters, so divide by 1000 to get meters.

 

85 + 85^2/12 = 687mm = 0.687m.

[fourfa]

bookmark

 

I've been thinking of this to bring in the close focus on my 70-200IS

[mark u]

All the info you need is here:

 

http://bobatkins.photo.net/photography/eosfaq/closeup.htm

[lester_wareham]

Note the usual extension formula only applies to lenses focused with overall linear extension and will not be accurate with internally focussed lenses.

[mtwhite]

I have a similar question about this formula. The maximum focusing

distance one gets from this is the distance from the subject to the

film/sensor plane, is that right?

[mark u]

Matthew, that is correct. However, if you look at the link I provided it lists not only the focus distance, but also the working distance (between the front of the lens and the subject) for all EF lenses in production by some time in the late 1990s. It would be nice if the table could be brought up to date with more recent lenses, and also perhaps extended to cover some of the more popular good quality third party lenses too, but that requires someone to actually test and measure the data, since as was pointed out above, the simple optical formula essentially only applies to a theoretical "thin lens" and in any case can't handle the working distance information.

[eosdoc]

I've searched a lot and couldn't find an easy answer...<BR>

so I made a calculator that anyone can use to find the solution.<P>

All the info you need is here:<BR>

  <a target=loser title="Julian's

Lens

Calculator" href=

http://ca.geocities.com/lokejul/jlcalc.htm

>http://ca.geocities.com/lokejul/jlcalc.htm</a>

<P>

For internally focussed lenses, you can easily reverse-calculate the ACTUAL

<b>Focal length</b> by plugging either the published (<i>v.s.</i>) or measured

numbers for:<BR>

    1. <B>Closest focus</b>, and<br>

    2. <B><i>Mag_Max</i></b>

<br>

The order for entering numbers is important for this calculation. Once you

click <b>Calculate</b>, the value for <b>Focal length</b> is updated.

<P>

Once you have calculated the ACTUAL <b>Focal length</b>, you can apply the

usual extension formula as it applies to lenses focused with overall linear

extension. To make use of this accurate approximation for internally focussed

lenses, simply choose a value from the <b>Extension</b> listbox, or type a

number in the box for <b><i>Total extension (mm)</i></b>.

<P>

You may have a concern about the formula my calculator uses. Note that

<b>Focus distance</b> is the working distance measured from the front

(nodal point) of the lens to the subject. Finding the front nodal point is

a trivial exercise if you can line up two pencils on a table!

<P>

Ignoring any link rot, refer to the Lens FAQ and Lens Tutorial at:<BR>

  <a target=_top href=

http://www.photo.net/optics/

>http://www.photo.net/optics/</a>

<P>

The real-time "Slide-rule" approach to calculation is much easier to perform

than to explain. If you are unsure of your calculations, post some results for

me to check your working.

<BR>

<b><i>Q.E.D.</i></b>

[eosdoc]

<blockquote><small><i>

It would be nice if the table could be brought up to date with more recent lenses, and also

perhaps extended to cover some of the more popular good quality third party lenses too,

but that requires someone to actually test and measure the data...

<br></small></i></blockquote>

Hi Mark,<P>

Great idea. Are you volunteering to write a page for <a target=eosdoc title="EOS

Documentation

Project" href=http://eosdoc.com

>eosdoc</a> :-)? I could code a simple javascript calculator just to calculate

<b>Actual focal length</b> given <b><i>Mag_Max</i></b>

and <b>Focus distance</b>. New data can be added using the visitor feedback

mechanism.

[oceanphysics]

<i>I have a similar question about this formula. The maximum focusing distance one gets from this is the distance from the subject to the film/sensor plane, is that right?</i>

<p>

Assuming you're talking about the formula I derived above, or Julian's calculator, which gives the same answer, no. It's the distance from the lens' nodal point. Bob's lens-specific table gives slightly higher answers, so I assume they're from the film plane.

<p>

<i>

Note the usual extension formula only applies to lenses focused with overall linear extension and will not be accurate with internally focussed lenses.

</i>

<p>

Irrelevant to the question asked. The question was about the maximum distance, so the lens will be focused at infinity and the focal length as advertised.

[sheldonnalos]

Thanks all, Here's what I'm coming up with...

 

With the 85mm f/1.8 (normally focuses to as close as .85 meters) and 12mm of extention, Bob's article lists the focus range as .49 to .72 meters, Julian's calculator lists .42 to .68 meters, and Ocean Physics answer matches with Julian's calculator. The 50mm f/1.4 (normally focuses as close as .45 meters) with 12mm of extension focuses from .24 to .32 meters (Bob's chart) or .18 to .25 meters (Julian).

 

With the 85mm f/1.8 and 25mm of extension, the numbers are from .28 to .35 meters (Julian) or from .38 to .43 meters (Bob's chart). For the 50mm f/1.4 with 25mm of extension, the focus range is a razor thin .125 to .142 meters (Julian) or .20 to .22 meters (Bob).

 

So, I guess the answer to my question is that once you start adding extension tubes, both the minimum and maximum focus distances are now closer than the original minimum focus distance of the lens without tubes. I gues my hope of being able to focus out to 5 or 10 feet isn't really realistic with the 85mm or 50mm lenses and extension tubes.

 

Thanks for all the help!

 

Sheldon

[eosdoc]

Right conclusion, but Sheldon forgot to allow for the length of the lens and the

register distance between the lens mount and the film plane.<P>

<a style="color: #FF0000;">WARNING!</a> The following might be a bit painful

for some.<BR>

It reminds me of the constipated mathematician... who had to work it

all out with a pencil!

<P>

<table border=1 cellpadding=2>

<TR>

<td>For EF 85mm f/1.8 USM<br>

Given the Closest focus distance (from film plane) = 850mm<BR>

and lens length = 71.5mm

</td><td><a target=loser href=

http://tinyurl.com/buapj

>Canon Camera Museum</a></td></tr>

<tr>

<td>

add the film/sensor plane-to-flange distance of 44mm

</td><td>

<a target=loser href=

http://www.a1.nl/phomepag/markerink/mounts.htm

>http://www.a1.nl/phomepag/markerink/mounts.htm</a>

</td></tr>

<tr>

<td>yields 115.5mm from film/sensor plane to front-of-lens.<BR>

Now, assuming that the front nodal point is very close to the front

element, the working distance should be 850mm less 115.5mm

</td><td><table><tr><td align=right><code><small>

850.0<br>

- 115.5<BR>

----------<BR>

738.5<br></small></code></td></tr></table>

</td></tr>

<tr>

<td>

Enter 0.7385 for<br>

<b>Focus distance (m)</b></td><td><a target=loser href=

http://ca.geocities.com/lokejul/jlcalc.htm

>Julian's Lens calculator</a><br></td></tr>

<tr><td>then enter 0.13 for<br>

<b>Mag_Max</b>

</td><td><a target=loser href=

http://tinyurl.com/buapj

>Canon Camera Museum</a>

</td></tr>

<tr><td>click <b>Calculate</b> for the answer<br>

85.1mm

</td><td> </td></tr>

<tr><td>

This value is consistent with the assumption that this particular lens maintains

the same 85mm focal length when focussed up close. So, for subsequent

calculations, we use<BR>

85mm for <b>Focal length</b>, and<BR>

0.7385m for <b>Closest focus</b>

</td><td> </td></tr>

<tr><td>

When you apply a 12mm extension tube, the focus (working) range is

calculated to be:<BR>

0.4m to 0.69m

</td><td><table><tr><td align=right><code><small>0.4<br>

+ 0.1155<br>

--------<br>

0.5155<br></small></code></td></tr></table>

</td></tr>

<tr><td>

Adding the 115.5mm we obtained earlier, this corresponds to a film/sensor

plane-to-subject range of:<BR>

0.5155m to 0.8055m

</td><td><table><tr><td align=right><code><small>0.69<br>

+ 0.1155<br>

--------<br>

0.8055<br></small></code></td></tr></table>

</td></tr>

</table>

<P>

Calculations for the EF 50mm f/1.4, or for any zoom lens with internal focus

are left to the reader as an exercise. :-)

[jim r]

Sheldon,

 

Although it probably won't get you out 5-10 ft., a teleconverter with extension (configured as a multiplier, i.e., body, tele, ext, then lens)can increase close focussing distance to give you more room to work. John Shaw's Close-up in Nature describes this well.

[eosdoc]

Plug in the values and see that by <a title=

Julian's

Lens

Calculator" href=

http://ca.geocities.com/lokejul/jlcalc.htm

>calculation</a>, the combination of:<BR>

    <code><small>subject -> lens -> extension tube ->

teleconverter -> body

</small></code>

<BR>

does not (theoretically) alter the minimum and maximum focus range, just the

magnification.

<P>

I have to admit that the Teleconverter math might be a bit dodgy, though. I will

gladly fix the formulae if someone shows me the better math.

<P>

I have lusted after the Vivitar Macro-Focussing Teleconverter, which is in one

device exactly what Jim proposes. However, as far as I know, this jewel was never

made for Canon EF mount. Pity.