Why does a loose 135mm lens act like a magnifier while a 50mm lens shows an inverted image, and can this be used for macro?

Asked 1/25/2013

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I have a Nikkor 135mm f/2.8 non-AI lens that normally focuses no closer than about 5 feet on a camera. But if I hold the lens by itself a few inches from an object and look through it with my eye, I can get a sharp, upright, magnified view, almost like a loupe. By changing the object distance I can change the magnification.

If I try the same thing with my 50mm f/1.8 AI lens, the image behaves differently and appears inverted.

Why do these two lenses act differently when used this way? And is there a practical way to use this kind of very close focusing on a camera for macro photography?

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

13y ago

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When you have the lens mounted on the camera and you look through the viewfinder you are viewing an image projected onto the camera's focussing screen. This is like taking a picture of a picture (with your eye being the second camera).

When taking a picture of a picture in this way the focusing ability of the second camera cannot affect the image itself i.e. if you photograph a non-macro image with a macro lens you can't make the image be focussed any closer.

Now when you hold the lens up to your eye you are producing a compound lens system with the camera being the first lens, and the lens in your eye being the second. When chaining lenses in this way both lenses affect the focusing properties of the whole system, thus you are able to focus much closer than when the lens is on the camera.

However, there is no way to exploit this particular effect to take macro photos without removing the lens from your eye and installing it in front of the camera sensor. This is not recommended for safety reasons, however you can buy add on lenses to enable you to take close up photos see What is the use of these Hoya +1 and +2 filters?. There are other ways to get closer focusing from your lens, see: How can I take a macro shot without a macro lens?

Whether you get an upside down image when looking through a lens depends on subject distance and where you eye is in relation to the lens. With a 50mm lens I can get a right way up image (that my eye is unable to quite bring into focus) when I hold the lens right up to my eye, and a well focussed upside down image when I hold the lens at arms length. With a longer focal length (and different construction, the 135 is a telephoto) these distances change.

Originally by user1375. Source · Licensed CC BY-SA 4.0

user1375

13y ago

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AI Answer

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When you look through a loose lens with your eye, you are no longer using the lens the way a camera does. Your eye becomes the second lens in a compound optical system, so both the camera lens and your eye affect focus and magnification. That’s why a lens can seem to “focus” much closer in your hand than it can when mounted normally.

Mounted on a camera, the lens must project an image onto the sensor or focusing screen at a fixed distance, so its normal minimum focus distance still applies.

The upright vs inverted view is likely due to the lens focal length and the distances involved. Depending on object distance relative to the lens, the lens can act more like a magnifier in one setup and form a real inverted image in another.

For actual macro on a camera, the practical approach is to increase lens-to-sensor distance: use extension tubes, or try freelensing. Reversing a 50mm lens is also a common macro technique. Those methods let the lens focus much closer than normal.

UniqueBot

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13y ago

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