Question About Extension Tubes

[Jon Ruyle]

Do you mean the image plane to the ccd? (ie, a + b above) Are you sure?


In that case, mfd = a + b above, so according to the lensmakers equation, I get mfd = f[(m+1)^2]/m, where f is the focal length and m is the max magnification. This formula seems to overestimate the mdf reported in lens specs.


If mfd is the distance between the "ideal optic" and the focal plane (a above) I think the formula would be mfd = f(m+1)/m, which agrees much more closely with specs in Bryan's reviews for lenses like the 500 f/4.


Possibly I've made a mistake in my algebra, but I don't think so


Anyway, it doesn't really matter what mfd is. Bob- if you want the distance from the front element to the subject, use the second formula. If you want distance from the subject to the ccd, use the first.


Just know that real lenses differ from ideal, so neither formula will be exact.

[Fast Glass]

Yes I am sure, just google "how is minimum focus distance is measured" and you can confirm it too. That screws your math a bit[:P]


Cheers,


John.

[Jon Ruyle]

It doesn

[bob williams]

@John and John, Thanks for the detailed explanation and formula's. Sorry I didn't get back to you earlier but this weekend has been a little crazy. It's only been 30 years since I took Algebra 101, but I think I can fight my way through this [:^)]. I do understand that focus distance is measured from the CCD and not the front element on the lens----Canon was even kind enough to put a little mark on my camera so I knew exactly where the ccd is.


I'll wrestle with this tomorrow and let you know what I discover, i.e. Calculation vs reality.


Thanks again,


Bob

[bob williams]

Ok Guys----I suppose I should just get out the tape measure and find out for myself what the MFD is with the 500 and 24 mm ext tube---but I am stubborn and hard headed and really want to understand this; I thought I almost had it until I looked at Bryan's Lens Specs which are:
<table align="center" cellpadding="3" cellspacing="0" class="SpecsTable" style="width: 625px;"]
<tbody>
<tr>
<td class="SpecsTableCell"]Specified Min Focus Distance</td>
<td class="SpecsTableCell"]177.2&rdquo;</td>
<td class="SpecsTableCell"]</td>
</tr>
<tr>
<td class="SpecsTableCell"]Tested Min Focus Distance (Wide / Long)</td>
<td class="SpecsTableCell"]163.86&rdquo;</td>
<td class="SpecsTableCell"]</td>
</tr>
<tr>
<td class="SpecsTableCell"]Maximum Magnification (MM)</td>
<td class="SpecsTableCell"]0.12x</td>
<td class="SpecsTableCell"]</td>
</tr>
<tr>
<td class="SpecsTableCell"]MM with 12mm Extension Tube</td>
<td class="SpecsTableCell"]0.15-0.03x</td>
<td class="SpecsTableCell"]</td>
</tr>
<tr>
<td class="SpecsTableCell"]MM with 25mm Extension Tube</td>
<td class="SpecsTableCell"]


0.18-0.05x
</td>
</tr>
</tbody>
</table>





So, as I ponder the last line----where did the .18 come from? If the native magnification is .12 x and the added magnification with the 25 mm ext tube is .05 x---shouldn't the end result be .17x?---Is this just a rounding error or adjustment, or is there something I am missing?


Thanks for your patience, and instructional wizdom,


Bob

[neuroanatomist]

So, as I ponder the last line----where did the .18 come from? If the native magnification is .12 x and the added magnification with the 25 mm ext tube is .05 x---shouldn't the end result be .17x?---Is this just a rounding error or adjustment, or is there something I am missing?

The 0.05x addition assumes that the 25mm extension tube is actually adding 25mm of extension. In fact, the EF25 is nominally 25mm in length, but actually provides 27.25mm of extension, meaning 0.1745x final calculated magnifiction (still not quite to 0.18x even with rounding). But, that calculation is based on the specified magnification, which is associated with a specified MFD of 177.2". Most lenses can manually focus closer than the specified MFD, so perhaps Bryan is actually empirically determining the magnifications (his tested MFD is closer than the specified MFD, meaning a slightly higher magnification).

[bob williams]

The 0.05x addition assumes that the 25mm extension tube is actually adding 25mm of extension. In fact, the EF25 is nominally 25mm in length, but actually provides 27.25mm of extension, meaning 0.1745x final calculated magnifiction (still not quite to 0.18x even with rounding). But, that calculation is based on the specified magnification, which is associated with a specified MFD of 177.2". Most lenses can manually focus closer than the specified MFD, so perhaps Bryan is actually empirically determining the magnifications (his tested MFD is closer than the specified MFD, meaning a slightly higher magnification).

Thanks John, That makes sense to me. I just wanted to make sure I wasn't missing something here.


Thanks


Bob