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Archive 2016 · Printing Nikkor 150mm With x 1.5 TC & Gold Nugget

  
 
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p.1 #1 · p.1 #1 · Printing Nikkor 150mm With x 1.5 TC & Gold Nugget


My Printing Nikkor 150mm has a minimum aperture of f11. A further stop’s worth of DOF would be useful.

A couple of days ago I had a weird experience with this lens. Sometimes for macro, I will put a high quality teleconverter behind the prime lens, to gain magnification and DOF at the same working distance.

I tried this with this lens and was surprised at an effect I have never seen before in decades of photography. With the TC placed in the conventional position, just behind the lens, I found the opposite of what I was expecting. I had to move away* from the subject to regain focus and the magnification was down, such that the field of view was increased by the ratio in which I wanted a decrease. It was a x1.5 TC and the field of view had increased by linear 50%.

* I didn't measure the two distances.

A TC can be used in various places in the extension, which in this case is about 220mm. This morning I will put in as close to the camera as the mount will allow. I have done this before, with other lenses. The magnification factor will always be different in the two positions for a given lens. If I recall correctly, the difference is in proportion to the focal length. That supposes the above effect is not still present.

There was no inversion of the images, just the decrease in magnification where I expected an increase.

There was some chromatic aberration. In this context, weeks earlier I obtained some quiet strange light effects. I was just playing around, seeing if a very short extension might get infinity focus. I can't remember exactly what I saw but I have vague memories of a white, diffuse disc of light with bluish edges, no image resolved. You might say "what do you expect in a lens designed for copying and optimised at 1:1?".

Further experimentation has established two things:

1) The expected function of the TC is obtained by inserting it between the adapter and the extension.

2) The chromatic aberration was, as I predicted, largely due to the TC's position behind the lens, with a contribution from the test chart and sensor not being very parallel.

For enthusiasts, here are the working details. (Measurements, other than FOV, to nearest 5mm).

Because I am using these on m4/3 camera, with a crop factor of 2, the FOV at 1:1 (full frame, effectively 2:1 from the crop factor) is 17.5mm. The extension was such as to give this. Insertion of the TC should not, effectively, increase the extension, although it does physically. The adjustable position of the lens diaphragm was set at 1:1 throughout.
For simplicity, the starting magnification will be taken as x1.

Without the TC: FOV was 18mm, working distance (no hood used) 245mm, subject to sensor distance 590mm, magnification x1

x1.5 TC behind lens: FOV 30mm, working distance 650mm, subject to sensor distance 1035mm, magnification x0.6

x1.5 TC behind extension: FOV 13mm, working distance 270mm, subject to sensor distance 650mm, magnification x1.4

The distance of the mount of the TC from the sensor was 170mm when behind the lens, 86mm when attached to the adapter.

Please note that insertion of a TC would normally leave the working distance the same, so this may give a different magnification when the lens is set for other values by varying the extension. (I am not about to investigate that!).

Turning to the question of chromatic aberration:

The target was a test chart ISO 12233 resolution chart for digital lenses, the recognised standard.

I set my printer to give the best quality and the best blacks with maximum DPI selected and printed onto A4 card. Even so, at 1:1 coloured specks can be seen throughout. The bottom line is that, on brief but careful viewing, there in no difference with the TC in place.

It looks promising that I can use the TC to gain a stop's worth of additional DOF. We shall see. I have to look closer at the resolution of detail and I am not sure if I have enough images for this purpose. The aperture of particular interest is the lens's minimum one of f11
I should point out that this is a fixed focus lens and getting the very sharpest detail requires very tiny movements of the lens. With the camera, extension and lens on a heavy Benbo tripod, this is very tedious, requiring slight nudges of one or more tripod legs and/or adjustments in height of the central column and/or fine rotation of the ball joint. Also, pressure on the shutter release button can cause visible changes in focus. Flash was essential, with the camera in manual mode.

After finding my DIY test chart not fully satisfactory (see above) I dug out a Photography Magazine glossy one, with some very fine detail. This is the one I have used for other lens tests.

For the resolution, very slight differences in focus were more significant than any difference in fine detail resolved with or without the TC. Examination of several images, from the lens alone and from the TC between the extension and the adapter, showed the resolution to be the same for all practical purposes.

With the TC, there was some chromatic aberration where some large blocks of lack edges touched some edges of white blocks along straight edges. These, very thin blue or orange, lines could easily be edited out if no other problems were found in macro use. Everything photographed was a mixture of black and white, down to extremely tiny black dots. No so aberration was seen where smaller black shapes met their white surroundings.

Here are some images shot today, of a jelly fungus, at a little under 1:1. The lens was set at f11.. With the m4/3 crop factor this is equivalent to f16 on full frame. The TC takes that to f22. ISO 200, triple flash, hand-held, with some support

These preliminary results are quite good. I used some micro contrast adjustment to allow for diffraction. The stereo I cross-eye. More images, of lichens, are to be processed.

Harold




© Harold Gough 2016





© Harold Gough 2016





© Harold Gough 2016




Jan 27, 2016 at 11:43 AM





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