Using Canon 100mm Macro with a range of extension tubes

[Joyful]

Compliments of the Season to you all !

When you are out in nature, strolling up a gentle incline, you are seldom aware of how far you have climbed. Only when you stop to look around do you see how high you have risen. In the same way, when I look back at my images this time last year, before I discovered this wonderful site, only now can I fully realise how far I have come.

THANKS TO YOU ALL WHO HAVE SO UNSELFISHLY HELPED ME.

I have a Canon 25mm ext tube as well as 3 x Phottix Ext tube sets (9, 16, 25mm). Using these in all possible combinations I can get extensions from 7mm all the way up to 173mm when I use them all.

My question for Rik or any other genius - What is the formula to allow me to calculate the magnification for the Canon 100mm Macro (non-L) on a Canon 40D Crop sensor size 22.2mm at all the possible intervals between 7 and 173 to save me having to test them all (about 67 combinations) ?

Many Thanks

Joyful

[ChrisR]

For a 100mm lens, (focused at infinity), if you put 100mm extension between camera and lens you get 1:1.
For each further extension of 100mm, Magnification goes up by 1
So Magnification is F/ext. (Bear in mind that this allows for the in-body distance, effectively 100mm, which would be needed to get the lens to focus at infinity, for M=0)

If you focus the lens close, that's not quite like normal extension because the lens alters the glass around to give Internal Focusing, which probably shortens the focal length as you set it to focus closer.

When you closer than 1:1, you may get better performance by reversing the lens but that's quite a hassle, so just put the lens on minimum focus, where it'll be best corrected.

That makes the actual magnification you'll get, hard to predict, so the best way is to use a ruled target to see what the field of view is. If you use a sheet of 1mm ruled graph paper, you could draw a rectangle for each extension with the details (DOF, steps etc) alongside.


You can work out what the lens' focal length becomes at minimum focus, by seeing what extension increases M by 1. If your lens will focus to 1:1, then at that position it has in internal extension equivalent to one FL. So if it has become (taking a number from thin air) say a 70mm lens, then you can treat it as a 70mm lens at a total distance (including the infinity part) of 140mm from the sensor. Each further 70mm of extension will increase M by 1.
This you can calculate and convert back to fields of view, or of course use the ruler/graph paper.

Field (width) of View, = 22.3/M .

Does that help?
Something to ponder for the soporific last few days of the year..

[orac]

while working on a project I came across this
http://photo.stackexchange.com/question ... be-will-be

its not ideal, and more info can be found here
http://www.photomacrography.net/forum/v ... hp?t=29569

in either case you need to know the magnification of the lens.
from the first link you can get what Intrinsic extension (Ie)

Ie = mag * focal

once that is known

NewMag = (Ie + Te) / focal

Where Te is tube extension

Idealy you should use the correct focal length which is

NewFocal = AddedExtension / (NewMag - OldMag)

but this would mean you have to measure the magnification of every combination. You could do this as you use each one and build a data base with the information you need

[rjlittlefield]

My question for Rik or any other genius - What is the formula to allow me to calculate the magnification for the Canon 100mm Macro (non-L) on a Canon 40D Crop sensor size 22.2mm at all the possible intervals between 7 and 173 to save me having to test them all (about 67 combinations) ?

Sorry, I missed this one earlier.

As ChrisR notes, the simplest approach is to set the lens ring at its 1:1 position when adding any extensions. In that position, the effective focal length of the lens is probably about 80 mm. (Yes, the focal length changes as you turn the focus ring.)

Using that number, you can expect magnification of about

NewMag = 1 + AddedExtension/80

In this simple formula, "1" is the magnification that we assume the lens gives at its 1:1 position, and "80" is the effective focal length that we assume the lens has when its focus ring is in the 1:1 position.

For higher accuracy, you can improve the estimate by measuring the actual magnification for two cases, and using those numbers to calculate the effective focal length. That calculation goes like this:

LensFocalLength = AddedExtension / (NewMagnification - OldMagnification)

So, if you measure (or assume) that OldMagnification=1.00X with no extension, and adding 25 mm of extension gets you 1.32X, then you would calculate

LensFocalLength = 25 / (1.32 - 1.00) = 78.125 mm

Then use that improved value of LensFocalLength in the earlier equation:

NewMagnification = OldMagnification + AddedExtension/LensFocalLength

from the first link you can get what Intrinsic extension (Ie)

The calculation of "intrinsic extension" as discussed at stackexchange.com is misguided, since it assumes that the lens does not change focal length when its ring is turned.

but this would mean you have to measure the magnification of every combination.

This is not correct. You only have to measure the magnification for one combination. From that you calculate the effective lens focal length, than use that number in the formula, with any amount of extension, as explained above.

--Rik

[Joyful]

Thanks Rik and others for your replies - I will get to work in the morning checking the formulas !