[collinj]

With a 25mm extension tube, my K5 and a SMC A 28mm f2.8 lens with a native magnification of 0.13x I have calculated an approx. magnification of:

0.13x + (25mm/28mm) = 1.02x

I have noticed that the subject size changes dependent on focal distance (for example infinity vs. 1 foot). This makes sense because the camera is closer to the subject with the focus set at 1 foot vs infinity.

My question:
What focal distance would be utilized to realize the 1.02 magnification?

Thanks.
Jeff

[FilmORbitz]

With a 25mm extension tube, my K5 and a SMC A 28mm f2.8 lens with a native magnification of 0.13x I have calculated an approx. magnification of:

0.13x + (25mm/28mm) = 1.02x

I have noticed that the subject size changes dependent on focal distance (for example infinity vs. 1 foot). This makes sense because the camera is closer to the subject with the focus set at 1 foot vs infinity.

My question:
What focal distance would be utilized to realize the 1.02 magnification?

Thanks.
Jeff

Could I instead do the one where one train leaves Chicago eastbound at 40mph, while a second leaves NYC westbound at an unknown time and velocity. They cross paths in Akron. Which train has the most passengers? Show your work

[pathdoc]

I don't agonise over what magnification I'm getting. I have an idea, grab a camera, fit the macro technology, compose the shot and push the button. If it looks good, I keep it. If not, I don't.

Unless you are doing this in a scientific or technical context, don't agonise. Put the math away and take the shot.

[Pentax Syntax]

With a 25mm extension tube, my K5 and a SMC A 28mm f2.8 lens with a native magnification of 0.13x I have calculated an approx. magnification of:

0.13x + (25mm/28mm) = 1.02x

I have noticed that the subject size changes dependent on focal distance (for example infinity vs. 1 foot). This makes sense because the camera is closer to the subject with the focus set at 1 foot vs infinity.

My question:
What focal distance would be utilized to realize the 1.02 magnification?

Thanks.
Jeff

Hi Jeff

Intruiging question. I remember some threads on the forum and DPREVIEWS dealing with extension tubes that discussed issues like this. As you know, the wide angle lenses require short working distances so my guess would be very little working distance (what you call focal distance). I know that macro focal considerations differ significantly from photographic norms but I look forward to seeing what the optical gurus on the forum come up with in response.

[photoptimist]

If your lens of focal length, f, is a simple one then the formulas for a so-called "thin-lens" can be used to get the distance estimates. The distances at magnification, m are:

subject-to-lens = f*(1+1/m)

lens-to-sensor = f*(1+m)

For f=28mm and m=1.02, there will be about 55mm from subject to the center of the lens and about 57mm from the center of the lens to the sensor.

Focal distance -- which controls the projection of the scene on to the sensor and sets the relative magnification of different parts of the subject -- is the lens-to-sensor distance so that number would be 55mm in this example.

The distance scale on the barrel of the lens typically shows the total sensor-to-subject distance which would be 112mm in this case.

P.S. The "subject-to-lens" distance is to a certain optical point INSIDE the lens. The distances from the filter ring of the lens to the subject will be much smaller.

[Not a Number]

There are apps and spreadsheets to save you all the grunt work:

Lens Magnification and Depth of Field Calculator
Macro lens calculations

[swanlefitte]

My guess is its calculated for at infinity focus. Shoot a ruler and see. Then come back and tell us how much difference there is.

[BrianR]

My question:
What focal distance would be utilized to realize the 1.02 magnification?

The same focal distance that gives your lens 0.13x magnification, i.e. the minimum in this case. Think of your lens as having a built-in variable extension tube, this extension is what you're adjusting as you focus (nothing internally complicated happens with the older 'inert' lenses - all lens elements move as one group*), and this built-in extension is what the '0.13x' part of your magnification equation is accounting for.

If your lens is set to a different focal length, use the native magnification for that setting in place of 0.13x.


*Note things can get wonky and not follow the thin lens approximations if your lens is not inert. This is the case with most/many modern lenses, where optical elements shift around independently of one another as you turn the focusing ring

[collinj]

Thanks for all your replies (with the possible exception of one individual). I am not interested in the math other than to determine the approximate magnification for any given lens/extension tube setup. The "variance in magnification" that I noticed (focus at infinity vs. minimal focus for example) spiked my interest.

It makes sense that at minimal focus (forces the camera toward the subject) one realizes the largest magnification, therefore at minimal focus one obtains the mathematically derived "approximate magnification" (thanks BrianR).

Again, thanks for all your replies.
Jeff