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Guest nfoto

How to get rid of the hot spot of Coastal APO 60 mm f/4

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Guest nfoto

Those fortunate enough to own or use a Coastal APO 60 mm f/4 lens know this is a superb performer in nearly all areas. On its own the lens focuses to 1:1.5 (so is not quite 'macro') and it is virtually parfocal over the entire spectral range 300 to 1100 mm. The sharpness is mind-boggling and the clarity of its colour rendition has to be seen to be believed.

 

So, what's the caveat? It has been known for a while that this lens produces a strong hot spot when focused close. I believe NG member Andrea G.Blum was the first user to report this issue. I was quite puzzled at her findings, since my initial testing hadn't shown this issue,  but I was able to corroborate them later. It turned out I had largely avoided the hot spots by using added extension to get beyond 1:3 instead of using the built-in helicoid mechanism and had not stopped down beyond f/8-11, both of which measures mitigate the issue to some extent (according to Dr. Brian Caldwell, the optical designer).

 

A little later I moved the Coastal lens onto my broad-band modified Panasonic GH-2 cameras and largely forgot about the hot spot issue as it by some magic apparently had disappeared. The reasons for that will be clear later. Once in a while, when the lens saw use on some of my Nikons, the issue resurfaced though. So the magic had its limitations.

 

Fellow UV shooter Enrico Savazzi posted an interesting article http://savazzi.freehostia.com/photography/coastalopt_60_hotspot.html that indicated the hot spot issue could be tamed by using a specially crafted lens hood on the Coastal lens. He verified that the hot spots had gone on his Panasonic camera, but could not tell if the same solution would work on a larger sensor format. Now, this intrigued me as I rarely if ever encountered the hot spot problem on my own GH-2 camera and I hadn't done anything special as far as I could recall. Why was that?

 

This is one of my work-horse Panasonic GH-2 cameras with the standard setup for UV.

 

post-15-0-74985600-1376729737_thumb.jpg

 

To protect against the dangers of field work (rain etc) the lens had an improvised shade consisting of  K-4 + K-5 rings. The Baader U2" filter is inside the lens mount adapter.

 

post-15-0-00019400-1376729888_thumb.jpg

 

So, could this explain why no hot spots were observed at all?

 

Let us return to the Nikons. Here is another work-horse, a broad-band D600 with the Coastal lens attached. The Baader U2" sits in a step ring on the front. This setup does indeed produce a strong hot spot. On the Nikons I cannot use rear-mounted filters unless I aim for higher magnification using a bellows device or a focusing helicoid. So the filter(s) usually go  to the front of the lens.

 

post-15-0-51693200-1376730176_thumb.jpg

 

Thus we need to add some kind of shading to the lens. As the Baader U2" has the unusual and (to photographers) awkward 48 mm thread, the filter needs to be inserted in some kind of retainer. I use a K-4 ring and a small gasket to make the filter sit tight inside the K-4.

 

post-15-0-39707300-1376730464_thumb.jpg

 

However, something has to be put in front to prevent the filter from dropping out. Another K-4 ring could be used. but for now I used a spare lens hood for the Voigtländer 90 mm f/3.5 SL.2 lens. Anything in terms of step rings could be used, starting with 52 mm threads and ending with a clear aperture of approx. 40 mm (so, for example, 37.5 or 39 or 40.5 mm to 52 mm would do But the Voigtländer item was  next to me on the work table).

 

post-15-0-48962900-1376731418_thumb.jpg

 

Add this to the K-4 retaining the Baader filter, and the hot spot issue now vanishes for good. The basic configuration will not vignette towards infinity. If a better shading is required, add another K-4 or a K-5 or the small 39 mm ring for the Voigtländer. Be warned you now have vignetting before you reach 1:3. So remove the additional ring(s) for medium- to long-distance work and you'll be just fine, no vignetting occurs.

 

This is the final setup for the Nikons.

 

post-15-0-33870500-1376730920_thumb.jpg

 

Now we are in a position to understand why the Panasonics behaved differently. Firstly, the filter pack was by default shaded as it was in the rear not in the front, and secondly, the K-rings I added as a makeshift lens hood prevented off-axis stray light into the lens. For the same proactive measures to work on the Nikons with their larger sensor formats, you need a conical lens hood with a clear opening slightly larger than the front element of the lens. If the makeshift hood was removed from the Coastal when it was attached to a Panasonic, the hot spot reappears. This clearly indicates the underlying issue is inside the lens assembly and is not per se caused by the filter. Perhaps an edge of an optical cell is insufficiently coated or some reflective surfaces inside the casing remain. We are reminded that an optical design, no matter how good, ultimately is limited by the physical structure it is placed into.

 

I have tested these setup (Panasonic and Nikon D3200/D600) and even at f/45 there no longer is any hot spot.

 

Without any hood (D600) a strong hot spot is obvious. It starts to manifest itself from f/8 onwards and here at f/45 is pretty prominent.

 

post-15-0-74253700-1376732081_thumb.jpg

 

Now, without altering anything of the set up otherwise, add the lens hood. Poof - no more hot spot. Contrast is improved so all the dust on the filter is clearly visible.

 

post-15-0-86014500-1376732115_thumb.jpg

 

By the way, the Coastal 60 APO is amazingly sharp even at f/45.

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Guest JohnBrew

A terrific write-up, Bjorn. I'm just starting to play with IR, but I'm using a Leica M8.2. 

 

BTW, have you used the Coastal for landscape? and if so could you post an example? 

Thanks, John

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Guest nfoto

Most of the landscapes have been done by multispectral approaches to make efficient use of the parfocal feature of the Coastal lens.

 

Not sure how web-sized images are going to show the tremendous quality of this lens anyway.

 

I found a landscape (Mouth of Geiranger Fjord on the UNESCO World Heritage list) for you, though. Late evening sun and very high contrasts in the landscape.

 

post-15-0-56079800-1376744215_thumb.jpg

 

Here is a less comprehensive view (of a veteran railway locomotive) to show the vibrancy of the colour rendition.

 

post-15-0-80681700-1376744328_thumb.jpg

 

 

Do note that hot spots never were an issue for the kind of photography shown above. It strictly was a problem in close-up photography, which unfortunately may have included most tasks the lens was put to.

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Guest JohnBrew

Thanks for the examples, Bjorn. They give me fodder for the mind.

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Guest nfoto

Watch where you're going - we are talking about a $4.500 lens here :)

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Let's remind the readers that using extension tubes can cause coma. IIRC, Brian Caldwell said this goes away if you stop down enough - f/8 or f/16? Thus the lens shade is the better solution for the Coastal 60/4.0. FWIW, I was never completely successful at stopping hotspots on the Coastal 60/4.0 in all ranges using extension tubes, although they mostly worked.

 

The front element of the Coastal 60/4.0 is CaF2, so the lens hood serves an additional protective purpose.

 

On 19 June 2008, Bjørn, you wrote about the hotspotting Nikkor 50/1.8D AF lens:

Putting on a long hood seems to help.

So the lens shade/hood idea for mitigating hotspots was there in your head all along. :D

 

On 10 June 2009, I wrote about the Coastal 60/4.0:

For the record, absence or presence of a lens hood plays no role in the Coastal hotspot problem
as I tested both with and without lens hoods.

 

Clearly, I did not test with the "right" lens hood, so I never followed up on the lens hood idea.

 

To mitigate the Coastal 60/4.0, you must use the "right" lens hood.

Not every lens hood will work properly.

See Bjørn's solution above.

See also Enrico Savazzi: http://savazzi.freehostia.com/photography/coastalopt_60_hotspot.html

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I'm going to write Brian Caldwell and see if he can give us some specs

on what to look for in a lens hood based on the design of the Coastal 60/4.0.

 

From Savazzi: 

Long & narrow lens hoods prevent off-axis rays from entering the lens

but cut down on illumination and may cause vignetting depending on sensor size.

And, at higher magnifications you would need a narrower lens hood.

 

Interior finish of a lens hood for the CO/60 is important too.

It must not be UV, Visible or IR reflective.

 

So there may not be one 'perfect' lens hood solution, but 2-3 lens hoods needed to use with the CO60.

 

Remember, many of us have seen hotspots more extensive than what is stated on the Jenoptic website.

This is probably due to real-world use versus lab-testing use.

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Bjørn: Are there any difference to the hot spot issue when using the Precision U instead of the Baader U2 filter?

Are the K4 and K5 rings "standard" or did you apply any special paint on the inside of them or the Voigtländer domed hood?

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Guest nfoto

Both filters can give hot spot. Or more accurately, as long as they are used on the Coastal lens and the lens itself can make hot spots so can the filters ....

 

The K-rings were just retrieved from the normal jumble floating around on my work table. Nothing special done to them. Same applies to the Voigtländer hood.

 

If you substitute with step rings these tend to be quite glossy so some kind of flat paint or black flocking might be required.

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I wonder if lens hoods would cure hotspots on hosttopping IR lenses ?

 

I'll experiment with my 60/2.8G AFS Micro-Nikkor on the D5100-IR.

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I wonder if lens hoods would cure hotspots on hosttopping IR lenses ?

 

I'll experiment with my 60/2.8G AFS Micro-Nikkor on the D5100-IR.

If it can work iwith UV, why shouldn't it work with IR instead? Though a negative answer for the AF-S 60/2.6 won't mean it won't work for other combinations of lenses and hoods.

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Guest nfoto

Only further investigation will answer that question but I fear the solutions aren't as easy. Firstly because most problematic lenses for IR are zoom designs and secondly, it'll be a formidable task to find a suitable hood for each individual lens since each may need to be different. Suffice it for the time being to have found a viable solution for the Coastal 60 APO.

 

Besides, hot spots in UV apparently are much less common than in IR so the underlying mechanisms might be different as well. In fact, the only other lens I can remember had a UV hot spot was the vintage 135 mm f/3.5 for Nikon S mount.

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Bjørn,

 

I really enjoyed your article, How to get rid of the hot spot of Coastal APO 60 mm f/4.  I try to keep current on Savazzzi's site and so had already read his article.  However, knowing he was testing on the µ4/3 format I was wondering how well his solution would work on full frame sensor.  You have clearly evaluated that application, most interesting.

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