Lens Reversing


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yannh

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Dec 10, 2007
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#1
Another newbie question here.

I understand that reversing some lens can bring great magnification, some use this setting for macro.

Anybody can have simple explanation why is this so?
Isn't reversing a lens still give the same focal length? Is minimum focusing distance the only difference?
 

zac08

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Feb 21, 2005
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#2
Another newbie question here.

I understand that reversing some lens can bring great magnification, some use this setting for macro.

Anybody can have simple explanation why is this so?
Isn't reversing a lens still give the same focal length? Is minimum focusing distance the only difference?
Think about it...

when light enters the front of the lens, wat is being done by the lens inside? It directs it such that it all focuses at a spot where the image plane is (either film or digital sensor).

So now, you reverse it. The lights direction is still similar but in a different direction. So the focusing distance is MUCH closer and allows a higher magnification.
 

zac08

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Feb 21, 2005
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#4
hi,

reverse a lens?

how?
Either holding the lens reversed by hand or using an adaptor.

Nikon has a BR-2A for 52mm filter thread to f mount. With that, there is a secure connection to the camera.

Another way is to use a male to male coupler. Reverse one lens against another. ;)
 

megaweb

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Jan 17, 2002
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#5
Reversed lens works like a close-up filter but with a high diopter value.
As you know standard close-up filters come with diopter value of +1, +2, +3, +4 or +10. With reversed lens e.g. 50mm, you can get diopter value of +20 or higher.
Focusing distance is depend on diopter value, formula = 1000/(diopter value) mm
e.g. a +2 close-up filter will give 1000/2 = 500mm = 50cm = 0.5m focusing distance
note : focusing distance is the distance from the camera sensor to the object
Thus, with reverse lens of +20 diopter values, you will get very close focusing distance
as 1000/20 = 50mm which is 5cm

How do you mount a lens reverse ?
Setup 1 : camera > lens > coupling ring > reverse lens
Setup 2 : camera > reverse lens adapter > reverse lens
 

yannh

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Dec 10, 2007
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#7
Thanks all for the info provided.

While consolidating some of my understanding, including physics definition, pls allow me to ask slightly more before I conclude anything.

Yes, in practice, we can see focusing distance is shorter after lens reversing. But is that because of effectively that's moving the lens further from the body (sensor)? Does the focal length change?

I understand diopter is nothing but just another way of representing focal length. Hence it's very meaningful for describing a close-up filter focal length. Since it's convex lens, by adding a cu filter onto a lens, it allow focusing at nearer distance. Now I don't understand how does that apply to reverse lens, since no additional optics has been introduced. Isn't a 50mm lens is always a +20 diopter, no matter forward or reverse?
 

megaweb

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#8
Yes, in practice, we can see focusing distance is shorter after lens reversing. But is that because of effectively that's moving the lens further from the body (sensor)? Does the focal length change?
Lens away from the body, focal length remains but focusing distance become closer. Extension tube (hollow) will give this effect.

I understand diopter is nothing but just another way of representing focal length. Hence it's very meaningful for describing a close-up filter focal length. Since it's convex lens, by adding a cu filter onto a lens, it allow focusing at nearer distance. Now I don't understand how does that apply to reverse lens, since no additional optics has been introduced. Isn't a 50mm lens is always a +20 diopter, no matter forward or reverse?
50mm lens forward is not for close-up photography. Only when it is reversed. Yes, always a +20 diopter. Try it yourself to understand more about the technique.
 

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