Micro lens question on DX Nikon

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Bill16

Senior Member
Hey you smart people! I have the 105mm lens listed in my signature and it's 1:2 and I'm using it on my D3100. After seeing a post about 1:1 macro I got curious and found my 105mm isn't 1:1 . So I bought the extension tube PN-11 as an article I read said would make it 1:1. But when I went to use it, my 105mm lens had to be practically sitting on top of the subject to focus on it.
So my question is this. Does the need of the extension tube only aply for FX? Does the fact that my D3100 is DX make my old lens 1:1 without the need for the extension tube? Or is my lens suppose to be just two inches from the subject?

Thank you for your help! :D
 
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WayneF

Senior Member
Bill16 said:
Hey you smart people! I have the 105mm lens listed in my signature and it's 1:2 and I'm using it on my D3100. After seeing a post about 1:1 macro I got curious and found my 105mm isn't 1:1 . So I bought the extension tube PN-11 as an article I read said would make it 1:1. But when I went to use it, my 105mm lens had to be practically sitting on top of the subject to focus on it.
So my question is this. Does the need of the extension tube only aply for FX? Does the fact that my D3100 is DX make my old lens 1:1 without the need for the extension tube? Or is my lens suppose to be just two inches from the subject?

Thank you for your help! :D
Click to expand...


All of that sounds very normal for macro. This 1:2 lens needs the added extension to become a 1:1 lens, on any camera, FX or DX.

Normally, when we photograph something, the subject is maybe 10 feet, or 100 feet, or maybe a mile, in front of lens. The focal length suggests the sensor is 105 mm behind the lens (at infinity focus, but there are approximations and details of where this point is measured, probably near the front element in this telephoto case, but you get the idea). And lets say 105mm is 1/3 foot.

The ratio of the focal length, say 1/3 of a foot, to the subject distance, say 100 feet, is the magnification of that image, how small the image is on the sensor or film (which is a great size reduction). 10 feet distance is 10 times larger than if at 100 feet.

If we get 1:1 size magnification, that means the subject is as close in front as the sensor is in back (necessarily equal, at 1:1 magnification). So yes, 2 to 6 inches in front of lens (called working distance) is reasonable for 1:1. Same as the focal length, which gets longer focused up close (but again, where these points are measured from is not likely same as we would guess).

The new 105mm f/2.8 VR macro lens is more like 6 inches (working distance, in front of lens) at 1:1. The lens focal length also becomes longer too, to focus that close.


Here is an excellent chart of the working distance of many Nikon macro lenses

Macro lens calculations

If I recognize your 105 f/4, it is near the bottom of the list, and says 272 mm, which is 10 inches in front of lens. However, the chart states that is for the lenses 1:2, not 1:1. 1:1 will be probably about half of that (twice the magnification).
 
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Bill16

Senior Member
Thank you Wayne! So I will likely get 5 inches working distance on a FX Nikon where as I get around two inches working distance on DX?
 
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WayneF

Senior Member
Nothing is simple Bill. :)

The lens will focus at 1:1 at the same working distance on either camera, FX or DX. The lens is the lens and it does what it does, no matter what.

The difference is this:

If you did this at distance for 1:1 on DX, the 1:1 image would be 1.5x larger than your sensor, i.e., cropped. But it is still 1:1, meaning same size on the sensor as in real life, what you can see of it.
This made more sense in film days, 1:1 is same size on the film as in real life. Say 1/2 inch tall either way for example, is 1:1. Same size projected onto digital sensor too, we just cannot take it out and look at it.

But the cropped view of DX is smaller, so for the same view (same field width) you have to back away 1.5x farther. Then you get same view, but it is not 1:1, it is more like 1:1.5 (called 67%, 1/1.5).

For example, consider copying a 35 mm slide with the macro lens.

On FX, you do 1:1, and the slide is the same size on the sensor as real life. A good copy.

On DX, you have to hold the slide further forward, so it becomes only 67% size (1.5x ratio) and then its image fits on the smaller DX sensor, and again, you get a good copy. But it is not 1:1.
 
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Bill16

Senior Member
Thank you so much for explaining it to me! I think I will stick to 1:2 when using my 105mm. But the extension helps on my 200mm 1:4 to get me some good close up shots, so the extension isn't a waste! Lol :D
 
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WayneF

Senior Member
Bill16 said:
Thank you so much for explaining it to me! I think I will stick to 1:2 when using my 105mm. But the extension helps on my 200mm 1:4 to get me some good close up shots, so the extension isn't a waste! Lol :D
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Yeah, if we can assume the DX 1/1.5 size is already (lets say) close to 1:2, there are two ways to think of it.

Maybe you wont miss too much without extension, only between 1/1.5 and 1:1 magnifications. 1:1 is really sort of a special case anyway, I'm probably back at two feet much of the time with my macro lenses.

This is actually nonsense though, because the 1:2 is actually x1.5 too, so it is 1:3 on DX. I mean, if for the same view.

But still there could be times you really wanted 1:1, knowing you only get the smaller cropped central area, but you still wanted close for magnification. Then 1.2 is not enough, at least not all that could be done with extension.

But you have the extension, so you can always do what you need to do at the time, use it or not.
 
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