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[LENS] How to avoid a hot spot for the Coastal 60 APO lens

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#1 nfoto

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Posted 17 August 2013 - 11:32

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.freeh...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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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



[Also posted on fotozones.com http://fotozones.com...-apo-60-mm-f4/]

#2 Nico

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Posted 19 August 2013 - 10:00

Hi Bjørn,

You seem to really like to Coastal 60. Since I don't own any lens so far, that is really designed for UV-photography, I have constantly an eye on the market:
The Coastal 60 can be purchased brand new for around $4600,- so a used one should be (significantly) less. The only UV-Nikkor lenses that I have seen on Ebay over the last months are above $5000,-. I guess with some luck and patience one could buy one for less, but certainly not very often. I know that there is also the still manufactured Nikon Rayfact which is said to be identical with the UV-Nikkor.

So, as long a there is no really good deal on a used UV-105, the Costal 60 seems to be the best value for money in the class of UV-lenses on the market. I haven't found much information about the Costal 105 except that the build quality is not so great especially at that price-point.

Would you back the statement that the Costal 60 is the best UV-lens currently available at the $4600 price-level?

Best, Nico

#3 nfoto

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Posted 19 August 2013 - 10:12

Yes, no problem with that. It really is an exceptional lens. Too bad it acquired a reputation for making hot spots at high magnification in particular as the solution apparently is so simple.

The reason I put it on my GH-2 bodies was to have a hand-holdable kit with about the same angle of view as the 105 UV-Nikkor on my Nikons. It also matched pretty well using the 120 mm f/4 Medical-Nikkor on my D300 (used for obtaining reference pictures).

#4 nfoto

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Posted 19 August 2013 - 13:11

Just to clarify: the UV-Nikkor also is a remarkable and exceptional lens. Either of these two will provide all the image quality you require for making high-quality UV captures.

The Coastal 60 has the added benefit a CPU is already installed. So one won't miss the proper EXIF data which can be important for optimising the shooting setups (assuming a Nikon, Kodak or Fuji camera with F mount is used).

(my own UV-Nikkor lenses have CPUs too - but then I aim to have a CPU in any lens into which the CPU can be fitted. Almost there now).

#5 Andrea B.

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Posted 19 August 2013 - 16:55

I am less enamoured of the Coastal 60/4.0. For that kind of money it shouldn't need extra equipment (hoods & extensions) to shoot it properly. This is not to say that it isn't a good lens. It is a good lens, just not a good value, IMHO.

A good lens is only 1/2 of the UV kit required for good image quality. You must have also a good sensor over which to use a good lens. If either is missing, image quality is lessened.

Both the lens (CO 60/4 and UV-Nikkor) are overkill on a Panasonic Lumix where you will lose some amount of the benefits of the their handling of light, contrast and production of detail. These lenses need to be used over a very good sensor to see them at their best. This means one of the Nikon/Sony/Pentax DSLRs with a very good sensor. I rather hate to say that, but numerous tests prove it out - both my own and others more rigorous.

I worry that someone will spend in the thousands for one of these lenses and then be disappointed because they don't have a good sensor.

****

BTW, just to re-state the record, I DID try lens hoods with my CO 60/4 when I first found the hotspot problem. Interestingly, whatever lens hood it was that I tried then did NOT cure the hotspot problem. So while in theory the solution to the CO 60/4 hotspot problem is "simple", it actual practice it has taken 4 years to find it. Perhaps, not so simple after all.

Jenoptik should be selling a properly sized lens hood with this lens if indeed a lens hood is the ultimate "cure".
Andrea G. Blum
Often found hanging out with flowers & bees.

#6 nfoto

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Posted 19 August 2013 - 17:03

I'm not buying all your argumens Andrea.

Whilst it can be said the better sensor the better image, the notion of "how good" also needs to be questioned. For many purposes using the Nikons would be an overkill and the Panasonics or similar have their own advantages - need I say UV video properly balanced?

Even on a humble Panasonic the quality hike brought about by using a better lens is noticeable.

No $4.500+ lens is ever going to provide "good value" for the money.

#7 Andrea B.

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Posted 20 August 2013 - 03:22

Bjørn, I might grant you Lumix Video, but not Stills.

I've never seen the same detail/sharpness using the UV-Nikkor or UV-Planar on my GH1 or D300
like I have using them on the on the D600.
If a CO 60/4 improves a UV photo by amount X on a Lumix,
then why would someone stop there knowing they could get an improvement of X+Y over a better sensor ?
If they've spent $4600 + VAT on a CO 60/4, then what is another 2K for the Full Monty with a D600 ?

We've both slid down that slightly irrational, UV Slippery Slope long ago. :D

Oh well.
Maybe the UV flower signatures will be useful someday for some nice Botany student or researcher.

Added Later:
It occurs to me as an interesting irony that not much of the nuanced differences being discussed in the preceding posts actually show up fully in the resized, compressed photos we post online. We could make the argument that no-one but the lens owner is ever going to actually see what a CO 60/4 can really do - and that only when pixel-peeping.
So, why not use that CO 60/4 money for traveling to some interesting place instead ?
Andrea G. Blum
Often found hanging out with flowers & bees.

#8 nfoto

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Posted 20 August 2013 - 08:04

A comparison between different formats is never easy to conduct and unwittingly a bias can be easily introduced. Plus, extrapolating experiences from an old model (G1 or D300) to the entire range available later is not recommended.

Thus, you have a point if one compares using the same angle of view (or field coverage) which puts a disadvantage to any smaller format. However, if the comparison is done using similar detail of magnification, the results may well come out the opposite. My D3 was no match to the GH-2 in the latter case. I haven't done a strict comparison GH-2 to D600 so uncertain what way the coin will drop here. Suffice it to say that both cameras used sensible can deliver great UV images and who would want more?

It is only human nature wanting to use better tools when these are available. I see no harm in that if one can afford them. It is true that web-sized images only are a faint impression of the inherent quality of the originals but still we would want to have the best results. After all, if the mediocre way of thinking should rule why have Michelin-approved restaurants when junk-food and MacDonalds would be enough? I trust we aim higher than so.

Finally, even better than travelling is travelling with a CO 60/4 ...

#9 nfoto

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Posted 20 August 2013 - 08:43

Further thinking as a spin-off from the above lines of discussion: seems we should consider SCALE and the [indirect] costs of attaining a given scale of reproduction of our subject.

I'll post a few examples to illustrate this later.