# Lens Use Guide for Macro Photography

Started May 10, 2020 | Discussions thread
Re: Shutter speed and focal length and magnification

bclaff wrote:

...I know close up adapters reduce the focal length but I just assumed extension tubes maintain the original focal length. Correct?

Focal length doesn't change but magnification increases; see below.

Also, how do you calculate the shutter speed you should use with a macro (assuming no stabilization)? ...

I hope my answer won't be too technical. I'll leave out some details.

Using focal length is convenient but ultimately what matters is the angle between the center of the exit pupil and the diagonal of the sensor.
The distance from the sensor to the exit pupil is (1 + m * p) * f where m is magnification and f is the focal length.
Note that at infinity focus m is zero and the distance is simply f.
(It works out that f and the sensor diagonal give you angle of view.)
Also, until you focus pretty close m is close to zero and the distance is still very close to f.

But, close-up photography is different; magnification becomes significant.

We seldom know pupil magnification so we'll just assume it's 1.

So now the distance is (1 + m ) * f (and the sensor diagonal doesn't change)
(BTW, this is the same math as effective f#)

So, to apply the usual "rule of thumb", or to fool IBIS that makes focal length assumptions, I would think you should adjust focal length by a factor of (1 + m).

In other words, at 0.5x magnification I'd multiply the focal length by 1.5
(focal length doesn't change, this is just to apply the rule of thumb or perhaps to enter into the camera).

I plan to use auto extension tubes with a long lens (that has O.I.S.). So, the lens will be powered and communicating with the body. Unfortunately, I don’t believe there’s any way to change the focal length in the body unless it’s an unpowered, non-communicating lens.

The lens I’m using, the 45-175, will have extremely limited functionality unless it’s powered and communicating with the body, so there’s really no option. The body is only going to see the focal length communicated by the lens.

So, you’re essentially saying that the O.I.S./IBIS won’t function properly (i.e. as intended) when extension tubes are added to this lens?

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Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

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