El Nikkor 105mm f/5.6 (Metal) Focus Shift

[bvf]

In another post there was a discussion about focus shift of the El Nikkor 105mm in the UV. I thought I'd spend some of my lockdown time in getting a feel for how significant this is. Graphs below.

 

I used my tri-colour UV filters (380BP20, 345BP25, 315BP25) and compared how the point of focus moved from the point of focus with visible light for subjects in the range of 24-105 cm from the front of the lens. What surprises me is:

• how significant the focus shift is.
  
• that the 380 and 315nm filters both focus further away, but the in-between 345nm filter focusses closer.
  
• the odd shape of the 315nm curve at the 102 and 145cm distances. Initially I thought I'd screwed up the measurements, but I was able to repeat the results.
  

[ulf]

Very interesting!!

Thank you very much for testing this.

 

I have always suspected that the distance to the subject affect the shift.

 

Roughly what magnifications do you have for these points?

 

Maybe you could add a fourth graph for light a bit into VIS too?

That will be interesting for "Bird's-View" and "Bee's-View" images.

[nfoto]

A neatly conducted experiment and a nice documentation of why Live View focusing is required for UV and other "invisible" ranges.

 

The results also indicate why one easily gets chromatic aberrations even within the narrow UV range unless the optical design is computed to mitigate this issue.

[bvf]

Very interesting!!

 

Roughly What magnifications do you have for these points?

 

Maybe you could add a fourth graph for light a bit into VIS too?

That will be interesting for "Bird's-View" and "Bee's-View" images.

 

That was a good question, Ulf. I should have done a graph on magnification. At a distance of 24cm from subject to front of lens, the subject width captured was 3.75 cm. This is on a Sony APS-C sensor with a width of 2.35 cm, so the magnification would be about 0.63. And if I've got my trigonometry right, magnification should be inversely proportional to subject-lens distance. So here is a graph of focus shift vs. Magnification. It would be interesting to look at the 315nm curve at smaller magnifications / greater distances.

 

 

I might spend a bit of lockdown time on looking at focus shift within visible and IR. But I'm not sure that I have the filters for the visible measurements. I have my tri-colour IR filters to do the same experiment in IR (at 750, 850, 1000nm), but to be honest that's not so interesting for me because the IR images are bright enough to focus using LiveView (whereas in UV the 315nm image is too dim to focus with).

[dabateman]

 

 

That was a good question, Ulf. I should have done a graph on magnification. At a distance of 24cm from subject to front of lens, the subject width captured was 3.75 cm. This is on a Sony APS-C sensor with a width of 2.35 cm, so the magnification would be about 0.63. And if I've got my trigonometry right, magnification should be inversely proportional to subject-lens distance. So here is a graph of focus shift vs. Magnification. It would be interesting to look at the 315nm curve at smaller magnifications / greater distances.

 

 

I might spend a bit of lockdown time on looking at focus shift within visible and IR. But I'm not sure that I have the filters for the visible measurements. I have my tri-colour IR filters to do the same experiment in IR (at 750, 850, 1000nm), but to be honest that's not so interesting for me because the IR images are bright enough to focus using LiveView (whereas in UV the 315nm image is too dim to focus with).

 

This graph I find most interesting. I think I zee the reverse for my UAT. The closer and higher the magnification, then the more focus shift I see.

Further away for large flowers, I don't need to adjust focus at all from Lp720 to 313nm filters.

[ulf]

This graph I find most interesting. I think I zee the reverse for my UAT. The closer and higher the magnification, then the more focus shift I see.

Further away for large flowers, I don't need to adjust focus at all from Lp720 to 313nm filters.

That might be due to that the focus depth decreases with increased magnification.

I think that change is faster than the shift for the higher magnification-end, assuming fixed aperture setting on the lens.

 

My usual magnification range is within 0.3x -1.5x

[bvf]

Repeated the experiment in Visible and IR.

 

Visible

 

Filters:

Blue - MidWest Optical BP470

Green - Hoya X1

Red - Hoya R24A

 

These filters were stacked with a UV/IR cut filter, as I was using a full-spectrum camera.

 

As you'd expect, the focus shift across the visible spectrum was small.

 

 

 

Infrared

 

Filters:

MidWest Optical BP735 + Hoya R72

MidWest Optical BN850

MidWest Optical LP1000

 

Focus Shift is greater than in the visible range, but not as extreme as for UV.

 

 

 

I guess my next task will be to do all this for the Cassar S.

[bvf]

This graph I find most interesting. I think I zee the reverse for my UAT. The closer and higher the magnification, then the more focus shift I see.

 

As Ulf says, it's probably because of depth of field - which is paper-thin in the macro range. So the relatively small focus shift at larger magnifications is more noticeable than the larger focus shift when the subject is further away.

[bvf]

One last graph on this - and probably the most interesting.

 

Here I have combined the UV, Vis, and IR data to see how the El Nik performa across the 315-1000nm range, and at different subject differences.

 

You can see the focus shift is well controlled in the visible range with subject distances up to about 1 metre - which fits in well with it being an enlarging lens.

 

(NOTE: this graph has been corrrected. The original version had incorrect values on the y-axis.)

 

[dabateman]

 

 

As Ulf says, it's probably because of depth of field - which is paper-thin in the macro range. So the relatively small focus shift at larger magnifications is more noticeable than the larger focus shift when the subject is further away.

 

But see your data for your lens says the opposite. As magnification is 0.1 there is high shift. At 1:1 your data says you wouldn't have any focus shift at all. Or am I reading your magnification plot incorrectly. This I find most interesting.

Thus your lens seems to be designed for Macro accross all wavelengths. If you were at 2x magnification I wonder if that holds.

[bvf]

But see your data for your lens says the opposite. As magnification is 0.1 there is high shift. At 1:1 your data says you wouldn't have any focus shift at all. Or am I reading your magnification plot incorrectly. This I find most interesting.

Thus your lens seems to be designed for Macro accross all wavelengths. If you were at 2x magnification I wonder if that holds.

 

At 1:1 there would still be a focus shift. It would be very small in terms of subject distance. But at 1:1 that very small shift would have a major effect, because your depth of field is sub-millimetre. On the other hand, a focus shift of 1,000 miles at true infinity doesn't have any impact.

 

Actually the shift in point of focus is not a very good metric. If I focus in visible at infinity, then filters like the 315nm would never focus on anything. So the shift in point of focus is meaningless.

 

What would have been a better measurement is how much the focal distance (i.e. behind the lens) changed rather than how much the the subject distance (in front of the lens) changed. I may have a go at this when i can summon up the energy.

[dabateman]

Ok that makes sense.

For the lens I made, the uncoated 39mm focal length. At 1:1 magnification I have a focus shift of about 100mm from 708nm to 380nm. In that holding the aperture constant. I go from one end of my rail to the other between 380nm (closest to subject) and 708nm (furthest from subject). I can change the element to sensor distance, which helps. All this changes the field of view.

At 365nm I fall out side my rail. Funny doing this yesterday. I may have actually been more than 1:1 now that I think about the flower size. Maybe 1.5x.