Cassar S Focus Shift

[bvf]

This is a companion post to the recent one about El Nikkor 105mm focus shift, this time looking at the Cassar S 50mm lens.

 

What these graphs show is how much the point of focus (i.e. in front of the lens, at the subject) changes for different wavelengths and different distances from front of lens to subject. As pointed out in the other post, a better metric would be how the focal point (i.e. behind the lens) moves. This is a bit trickier to do with my limited resources, but I may get round to it. Anyway, you may find the information below of interest.

 

With the simple set-up I was using (observing how the point of focus moved along a 30 cm ruler at 45 degrees to the lens axis) I was limited to maximum subject distance of about 115 cms.

 

All the graphs below relate to the Cassar S - if you want to see the El Nikkor curves, they are in the other post.

 

UV

 

In terms of the distance by which the point of focus moves, the Cassar S seems a lot worse than the El Nikkor 105. But when you relate the focus shift to magnification, the 2 lenses look comparable at 380 and 345nm, but the Cassar seems to have more of a problem at 315nm.

 

 

 

Visible

 

In visible, the shift in point of focus is a lot more controlled as you would expect. The amount of shift is comparable to the El Nikkor, although when related to magnification the Cassar S is slightly better. But it looks like the Cassar S might be worse at greater subject distances than the ones I measured.

 

But to be honest I can't see any point in using either of these lenses for photography in the visible anyway (except for convenience when doing UV vs. Visible comparison shots) as modern lenses are far better.

 

 

 

IR

 

In the IR, the Cassar has significantly more shift in point of focus than the El Nikkor. However, again I can't see any role for the Cassar in IR photography - my Canon EF 50mm f/1.8 performs far better. But the El Nikkor I do use for IR, as my modern 105mm lenses (Sigma 24-105mm and Sigma 105mm Macro) have hot-spot problems.

 

 

 

Full Spectrum

 

This illustrates quite well how the focus shift is controlled within the visible spectrum for whch the lens was designed, although there are indications that the situation might not be so good at greater subject distances.

 

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

 

[OlDoinyo]

I am surprised you could get enough through to measure at 315 nm--that is outside the usual range of interest. So the UV and IR shifts are in the same direction for this lens, which is interesting. I wonder what infinity focus would show.

[bvf]

I am surprised you could get enough through to measure at 315 nm--that is outside the usual range of interest. So the UV and IR shifts are in the same direction for this lens, which is interesting. I wonder what infinity focus would show.

 

What I did was photograph a ruler to see where the point of focus was. I blasted the ruler with 4 flashguns up close to teach the 315nm filter who was boss.Trouble was it was a white plastic ruler, and all plastics becomne dark in UV as the wavelength decreases. But I was just about able to get a usable image.

 

Yes, I'm surprised the UV and IR shifts are in the same direction.

 

I'm doing some measurements of how the focal point (i.e. behind the lens) moves, as opposed to this information which was all about how the point of focus (i.e. in front of the lens) moves - in the focal point data I will include an infinity point.

 

BTW -I need to chack my data for the last graph above to see whether I allowed for the ruler being at 45 degrees. So I may need to change that graph, but the only effect will be to multiply the y-axis scale values by about 0.7 (i.e. sine of 45). The other graphs already include this factor.

[nfoto]

Textbooks tend to to suggest UV focus should be corrected more or less like the IR shift, i.e. in the same direction. Some lenses do depart from this general scheme, amongst them the UV-Nikkor for which the IR correction is "opposite" (farther away).