Vignetting with Reversed 50

[Kevin Childress]

I wanted to share an experience I've had using a reversed Nikon 50mm f/1.8 mounted to a Nikon 105mm f/2.8 micro. I've always thought that using a reversed 50 was one of the more common alternatives to increasing reproduction ratio but I've never heard this issue mentioned using a setup like this.

I've attached an image that shows a very heavy vignette with the reversed lens. I get sharper images by stopping down the reversed lens and leaving the main lens wide open. But I still see the vignetting by stopping down the main lens and shooting the reversed lens wide open. Both scenarios are identified on the image here - the vignette looks about the same to me. As for the reversed lens being at f/8: The aperture range on that lens is 1.8 to 16. I really don't know how people refer to the f/stop of a reversed lens, but I get the sharpest images with the lens stopped down to about 1/2 of the aperture range so I just refer to this as f/8.

I've just always assumed the vignetting is a result of having the extra diaphragm hanging out in front of the main lens but haven't researched what can be done about it. I just find it odd that I've never seen the problem mentioned on any website or forum where reversed 50s are used.

Is there any possible solution for this? Would a 50mm f/1.4 help (I've been wanting to buy one anyway)?


Thanks!


Kevin

[enricosavazzi]

The general suggestion is stopping down the front (reversed 50 mm) lens and leaving the rear (105 mm, normally mounted and usually focused at infinity) lens fully open. From your description it sounds like you are doing the opposite. If you stop down the rear lens, you are more likely to get vignetting.

As a first test, I would suggest you only stop down the reversed 50 mm lens.

If you still get vignetting, there might be one or more of the following things to try:

- too much distance between front and rear lenses. Reduce the distance if possible (use just a reversing ring, no long stack of adapters, no extension tube between the two lenses). The old-style 105 mm Micro Nikkor has a deeply recessed front element when focused at infinity, which might also be a problem. More modern models have the front element close to the filter thread at all focus settings.

- rear lens is not focused at infinity. Focus it at infinity and test, then focus it at a close distance and test again (the latter is unlikely to cause vignetting, but it costs nothing to try and see). The front element of the old-style 105 mm comes closer to the filter thread when focusing closer, which might have an effect on vignetting. Image quality may be worse at either focus position, but that is another thing.

- front pupil of the rear lens is located too deep within the lens (often a problem with zooms, less common with telephoto primes). No solution, other than using a different rear lens.

It is also possible that both lenses give less vignetting when fully open, and an aperture is added between the two lenses. You could test with a round aperture (10 mm for example) cut out from a black thin cardboard sheet and mounted in the filter thread of either lens, held in place by the reversing ring that connects the two lenses.

[johan]

This 105mm lens seems to be a bit of a problem child with vignetting - I've had someone ask about it on my site too in the past and I've seen the vignetting on other shots using that lens. I've seen it be more successful though with both lenses wide open, maybe give that a try.

[rjlittlefield]

I've just always assumed the vignetting is a result of having the extra diaphragm hanging out in front of the main lens but haven't researched what can be done about it.

The vignetting is due to having two apertures that "fight" with each other, so that incoming light rays that would otherwise be bent so as to strike the corners of the sensor actually end up getting blocked by one aperture or the other.

Ideally the rear lens will have a large entrance pupil, far forward, so that all the light that gets through the front lens can also get through the rear lens.

Unfortunately it sounds like that particular model of lens has its entrance pupil far backward. That would be what causes it to be "a problem child".

As Enrico says, the best bet is usually to have the rear lens wide open and focused at infinity. That usually pushes the entrance pupil as far forward as it can go. But ultimately there's no way to tell besides trying it.

Even with both apertures wide open, rear focus may still matter because that changes the entrance pupil position.

--Rik

[NikonUser]

Maybe off topic but one way to get >1:1 with the Nikon 105 is to use extension.
117 mm exension tubes behind the Nikon 105 gives >2:1 magn. with min. vignetting
Q&D single shot:

[Kevin Childress]

Enrico,

Thanks for all the input. A few ideas below ...

The general suggestion is stopping down the front (reversed 50 mm) lens and leaving the rear (105 mm, normally mounted and usually focused at infinity) lens fully open. From your description it sounds like you are doing the opposite. If you stop down the rear lens, you are more likely to get vignetting.

Actually my original post described both scenarios. As I said, I get the sharpest images from stopping down the reversed lens and leaving the main lens wide, but I can duplicate the vignetting with lens scenarios reversed.

If you still get vignetting, there might be one or more of the following things to try:

- ... Reduce the distance if possible (use just a reversing ring, no long stack of adapters, no extension tube between the two lenses).

I only use a male-to-male threaded adapter ring to join the lenses. It created ~5mm of extension but its the thinest ring I can find.

- rear lens is not focused at infinity. Focus it at infinity and test, then focus it at a close distance and test again...

Having the main lens at infinity always leads to worst case scenarios. I get the best working distance (about 32mm) with the main lens at infinity but the worst vignetting. The only way to eliminate the vignetting is to have the reversed lens wide open and the main lens focused to 1:1 in order to "focus beyond" the vignetting. Putting the main lens at 1:1 forces me to f/4.8 so I would still get soft images with that setup and the working distance is unmanageable at ~10mm.

It is also possible that both lenses give less vignetting when fully open, and an aperture is added between the two lenses. You could test with a round aperture (10 mm for example) cut out from a black thin cardboard sheet and mounted in the filter thread of either lens, held in place by the reversing ring that connects the two lenses.

I like the sound of this and I'll definitely give it a try. I expect some vignetting to remain but I'm interested in seeing where this leads.

[Kevin Childress]

This 105mm lens seems to be a bit of a problem child with vignetting - I've had someone ask about it on my site too in the past and I've seen the vignetting on other shots using that lens. I've seen it be more successful though with both lenses wide open, maybe give that a try.

Thanks for the reply, Johan. I've only had the 105mm f/2.8 for a few months. Prior to that I was shooting an DX lens on this body (a Nikon 85mm micro) that was a hold-over from my DX kit. I had the exact same problem with that lens. I've definitely tried shooting both lenses wide but there is still heavy vignetting and I can't live with that image quality.

[Kevin Childress]

Unfortunately it sounds like that particular model of lens has its entrance pupil far backward.

Indeed, the diaphragm is set back a ways from the front of the lens.

As Enrico says, the best bet is usually to have the rear lens wide open and focused at infinity. That usually pushes the entrance pupil as far forward as it can go.

Thanks again, Rik. I agree with this. If you can, take a moment to see my reply above to Enrico.

[Kevin Childress]

117 mm exension tubes behind the Nikon 105 gives >2:1 magn. with min. vignetting

NikonUser, right on. I've been testing recently with 71mm of extension and that's getting me 1.9x and my vignetting looks practically just like on your grid paper.

[rjlittlefield]

Thanks again, Rik. I agree with this. If you can, take a moment to see my reply above to Enrico.

Got it, thanks.

It is also possible that both lenses give less vignetting when fully open, and an aperture is added between the two lenses. You could test with a round aperture (10 mm for example) cut out from a black thin cardboard sheet and mounted in the filter thread of either lens, held in place by the reversing ring that connects the two lenses.

I like the sound of this and I'll definitely give it a try. I expect some vignetting to remain but I'm interested in seeing where this leads.

Adding yet another restriction will never let more light get through to the sensor, so if those corners are completely black, they'll stay that way.

On the other hand, adding an aperture as Enrico suggests may even out areas that are not completely black, by darkening the center area while leaving already darker more peripheral areas unchanged. Then with increased exposure to compensate for the darkening, you may get a wider usable field.

I'll be interested to hear how this works out for this particular lens.

--Rik