reversed lens - Macro - question

[Ikinaa]

Hi,

I got the adapter ring (58mm, male/male) to fix a lens reversed to my G3.
To try it out, I fixed my old Canon 35-105 to the cam.
Now, when I zoom to 35mm, the objects on the image are larger than when zooming to 105mm.
Anyone has an explanation? (Maths and physics welcome)

[cprevost]

I think it is because the lens is reversed.

[Ikinaa]

cprevost wrote:
I think it is because the lens is reversed.

That's my guess too, like when you look the wrong way in binoculars.
But why? What's the physics behind it?

[Ikinaa]

Ikinaa wrote:
cprevost wrote:
I think it is because the lens is reversed.

That's my guess too, like when you look the wrong way in binoculars.
But why? What's the physics behind it?

Hey! No science guy around here?

[Bruce]

I found that the magnification you can obtain with the reverse lens method can be calculated as:

Focal lenght of prime lens / focal lenght of reversed lens

So, for example if you are reversing a 50mm lens on a 100mm prime you'll obtain 2X (twice as much as lifesize) image.
The prime lens of G3 is a zoom lens, so magnification will change accordingly as you zoom in & out.

[Ikinaa]

bruce wrote:
I found that the magnification you can obtain with the reverse lens method can be calculated as:

Focal lenght of prime lens / focal lenght of reversed lens

So, for example if you are reversing a 50mm lens on a 100mm prime you'll obtain 2X (twice as much as lifesize) image.
The prime lens of G3 is a zoom lens, so magnification will change accordingly as you zoom in & out.

Thanks Bruce,

so if I understand correctly, the shorter the focal of the reversed lens, the greater the magnification, if I use a 28mm, I get an image twice as large as the one with the 50 mm?

[Bruce]

Ikinaa wrote:
Thanks Bruce,

so if I understand correctly, the shorter the focal of the reversed lens, the greater the magnification, if I use a 28mm, I get an image twice as large as the one with the 50 mm?

Yes, but I don't know if there will be issues with distortion or other optical problems (perhaps vignetting?). I see that most people that are doing reverse lens macro technique are using a 50mm as the reversed lens.

[Ikinaa]

bruce wrote:
Ikinaa wrote:
Thanks Bruce,

so if I understand correctly, the shorter the focal of the reversed lens, the greater the magnification, if I use a 28mm, I get an image twice as large as the one with the 50 mm?

Yes, but I don't know if there will be issues with distortion or other optical problems (perhaps vignetting?). I see that most people that are doing reverse lens macro technique are using a 50mm as the reversed lens.


I noticed that too. It would be interesting if people using different focals could post their experiences here.
As my 35-105 has a max aperture of f/3.5, all my pictures have a :( black circular frame around them. I still hope to find a second-hand 50mm f/1.4 (or less if it gives good results)

[mcolella]

I use a Ziess Planar 50 mm F1.4. Very sharp, no vignetting.

Note: to get maximum magnification, set G3 focus to manual at infinite end.

mcolella

[Bruce]

After some research I have found also that the reversed lens alone equals in magnification power to a powerful diopter, which can roughly be calculated as:

1000/focal of reversed lens

For example, a 50mm lens equals to a 20+ diopter.
Actual magnification depends also on the focal lenght of prime lens (our G3 zoom lens in this case), the longer the better.
Typical settings for reverse macro stated in books of photograpy are 50mm reversed on a 100mm focal prime. G3's zoom equals to a 140mm when fully extended, so as I stated in my previous post, a reversed 50mm will give a 140/50 = 2,8:1 image size on G3's sensor. Which is quite a lot :)