Working distanc and dof for forward and reverse mounted lenses

[baachitraka]

I don't own any macro lens, instead I reverse mount 50mm zuiko that more or less gives me 1:1 magnification and a working distance of 10cm and razor thin dof.

My question is what is the working distances and dof of forward mounted 50mm and 100mm macro lenses?

I thinking to purchase either the cheaper 50mm or expensive 100mm macro lens in the future. So some insight is useful in this regard.

 

[Dan Fromm]

Working distance depends on the lens' focal length, the lens' mechanical design (think deeply recessed front element) and magnification.

DoF depends on magnification and aperture.

If you want working distance, get the longer lens. Since DoF doesn't depend on focal length, stop worrying about it.

 

[wiltw]

The lens 'nodal point' is where (inside the lens) the optical rays cross (and change from rightside up to upside down.
When ANY lens is being used at 1:1 macro magnification, it is (by definition) 2*FL from node to subject AND 2*FL from node to focal plane. This is true of both forward mounted and of reverse mounted optics.

With a usual 'normal' 50mm lens, you can get it to focus and achieve 1:1 magnification simply by mounting a 50mm extension on the 50mm FL normal lens.
With a WA 25mm lens, you can get it to focus and achieve 1:1 magnification simply by mounting a 25mm extension on the 25mm FL normal lens.
With a short tele 100mm lens, you can get it to focus and achieve 1:1 magnification simply by mounting a 100mm extension on the 100mm FL normal lens.

So increase of working distance (between the front edge (exterior) of the lens and the subject is always with the longest FL, even at 1:1 magnification.
If 50mm FL lens, 2*50 is greater than 100mm FL lens' 2*100 (from lens node to subject).

 

[ic-racer]

At 1:1, lens orientation is inconsequential. Lens orientation would come into play once one is magnifying or reducing an object size. This doesn't contradict the fact that reverse mounting a conventional 35mm camera lens is like adding an extension tube.

 

[baachitraka]

I have understood most of the parts, but when I reverse mount my 85mm zuiko then nothing was in focus.

Are there any limits to the focal lengths when reverse mounting?

 

[Chan Tran]

I have understood most of the parts, but when I reverse mount my 85mm zuiko then nothing was in focus.

Are there any limits to the focal lengths when reverse mounting?

There is no limit to the focal length but you may need to find a way to get the lens away or closer to the film plane. The problem is we don't know where the nodal planes of the lens are.

 

[MattKing]

I have understood most of the parts, but when I reverse mount my 85mm zuiko then nothing was in focus.

Are there any limits to the focal lengths when reverse mounting?

The 85mm f/2 lens is, IIRC, a true telephoto.
Which means the nodal point is not between the middle lens elements. Instead, it is effectively either closer to the front element or, possibly, actually in front of the front element (when the lens is mounted normally).
So when you reverse it, the nodal point may be in a location that makes it unusable.
That lens works reasonably well on an extension tube.
It also works well on a 2X teleconverter - 170mm f/4 is actually quite handy, if you don't mind the loss of resolution arising from the teleconverter, and the close focus distance is quite convenient.

 

[wiltw]

There is no limit to the focal length but you may need to find a way to get the lens away or closer to the film plane. The problem is we don't know where the nodal planes of the lens are.

Ordinarily a 50mm OM lens reversed should be about 5-8" from rear element to subject. If the reverse mount puts the lens nodal point closer than 2*FL, it is actually going to give a magnification less than 1:1 (1.0x); this is analogous to putting 25mm extension tube on a 50mm FL lens, where the subject is far enough from the film plane that the subject magnification at the focal plane is bigger than 1/10 but less than 1/4.
A 25mm tube with 50mm lens ordinarily focuses about 4-5" between lens front and subject.