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Protective filter on MP-E yes/no?
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augusthouse



Joined: 16 Sep 2006
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Location: New South Wales Australia

PostPosted: Fri Nov 20, 2009 1:36 am    Post subject: Protective filter on MP-E yes/no? Reply with quote

I don't think this question has been asked before with specific reference to the Canon MP-E 65mm.

Using a protective UV Filter yes/no?

With consideration of the 1x-5x capabilities of the MP-E, I believe the answer is no. Could someone confirm? Thanks.

Craig
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elf



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PostPosted: Fri Nov 20, 2009 2:48 am    Post subject: Re: Protective filter on MP-E yes/no? Reply with quote

augusthouse wrote:
I don't think this question has been asked before with specific reference to the Canon MP-E 65mm.

Using a protective UV Filter yes/no?

With consideration of the 1x-5x capabilities of the MP-E, I believe the answer is no. Could someone confirm? Thanks.

Craig


I'm of the opinion that protective UV filters will always degrade the image somewhat and should almost never be used (simply for protection) on any lens. The only exception is inclement weather related, for example: blowing sand or driving rain. A lens cap + lens hood will provide the same level of protection without degrading the image.

Another reason not to use one is the loss of working distance.

Joseph S. Wisniewski has written quite a bit about filters and has shown there are reasons to use a UV filter as a UV filter.
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Planapo



Joined: 07 Nov 2006
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Location: Germany, in the United States of Europe

PostPosted: Fri Nov 20, 2009 5:18 am    Post subject: Reply with quote

Interesting question!

I'd be interested to know, if people always put such filters on their lenses for protection as a standard procedure when shooting outside?

Elf, could you provide a link to Joseph's writing, please?

--Betty

P.S.: Good to see you're back posting, Craig! Very Happy
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Photographer



Joined: 28 Oct 2009
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PostPosted: Fri Nov 20, 2009 5:32 am    Post subject: Reply with quote

I reckon it would be pretty easy to scratch the front lens of a MP-E 65
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DaveW



Joined: 04 Aug 2006
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Location: Nottingham, UK

PostPosted: Fri Nov 20, 2009 5:42 am    Post subject: Reply with quote

Funny thing before I looked in here I had just posted the following on a general photographic site about using "transparent lens caps" or protective filters over lenses. Obviously there are some benefits for general photographers in adverse conditions, but not many I can see for most macro photographers, particularly in the studio:-

----------------------------------------------------------------------------------------------------------------------
To quote from an article on photographic optics:-

"Filters are optical elements that either attenuate or separate light through several means including absorption, obscuration, dispersion, and interference. Filters can also attenuate light by deflection using refraction and diffraction. A filter that is part of an image-forming system must be of high optical quality in terms of the flatness and thickness uniformity. Filters used in illumination systems such as over lamps or in enlargers can be of a lower quality."

Original reference here:-

http://encyclopedia.jrank.org/articles/pages/1236/Photographic-Optics.html

Also, though it will be of no interest to general photographers because it is so small, a filter does alter the focus of a lens slightly, but for people like macro photographers using focus stacking the lens should always be focused after the filter is on the lens.

This astronomical site talks about eyepiece filters, but it is just the same situation over a camera lens itself. I did read once the focus displacement was one third the thickness of the filter, but cannot find it now, probably in one of my macro books. But as some extreme photostacked images are working at sub millimeter depths of field per image this can matter.

http://www.bpccs.com/lcas/Articles/filtshift.htm

Note also the comments on plain glass in the first link above, which also applies to filters:-

"A glass window with plane parallel sides does not rotate an incident oblique ray, but the emergent ray is laterally displaced. When placed in a lens system, this can cause image aberrations or a focus shift."

DaveW
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Photographer



Joined: 28 Oct 2009
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PostPosted: Fri Nov 20, 2009 4:50 pm    Post subject: Reply with quote

Thank you Dave
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elf



Joined: 18 Nov 2007
Posts: 1283

PostPosted: Fri Nov 20, 2009 5:03 pm    Post subject: Reply with quote

Planapo wrote:
Interesting question!

I'd be interested to know, if people always put such filters on their lenses for protection as a standard procedure when shooting outside?

Elf, could you provide a link to Joseph's writing, please?

--Betty

P.S.: Good to see you're back posting, Craig! Very Happy


I thought posting this would lure him out of seclusion Smile

http://www.photos-of-the-year.com/articles/photography-filters/
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augusthouse



Joined: 16 Sep 2006
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PostPosted: Fri Nov 20, 2009 6:17 pm    Post subject: Reply with quote

elf wrote:
Quote:
I thought posting this would lure him out of seclusion Smile

Maybe Joseph has been busy with his book?

I recall the quote below from another thread which led me to ask regarding UV or 80A protective filter for the MP-E.

Joseph S. Wisniewski wrote:
Quote:
For higher magnifications and fast lenses, a piece of glass as thick as a standard photographic filter (2mm thick glass) will play hob with your resolution.

A lot of us here play with microscope objectives. Those are typically marked as to whether they're compensated for best performance through a 0.16mm thick slide cover glass. Imagine the effect of a piece of glass 12x as thick. I'd think it could affect even an enlarger lens...



Craig
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DaveW



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PostPosted: Sat Nov 21, 2009 2:56 am    Post subject: Reply with quote

I am getting a bit of text corruption with Elf's link, maybe it's Windows Explorer but I found it here without, though a condensed version:-

http://forums.dpreview.com/forums/read.asp?forum=1018&message=29592761

DaveW
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rjlittlefield
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Joined: 01 Aug 2006
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PostPosted: Sat Nov 21, 2009 3:24 am    Post subject: Reply with quote

To give a couple of relevant examples...

Here is what Joseph is talking about. This is what happens if you ask a Nikon 10X NA 0.30 objective to look through a Canon UV/Haze filter. The resolution is 50% of actual pixels on a Canon T1i, shot at 4752x3168 pixels. The two-frame animation is with and without the filter. I don't think you'll have much trouble figuring out which is which.



On the other hand, here is what happens when you look through that same filter using an Olympus 38 mm f/2.8 at f/2.8. The magnification is about 7X on sensor, and the resolution shown here is actual pixels on a Canon T1i, shot at 4752x3168 pixels. The two-frame animation is with and without the filter. This time it's rather more difficult to tell which is which.



I don't have an MP-E 65 to test with. But based on these results at comparable aperture and magnification, I wouldn't predict any dire results from looking through a filter.

It's always best to test with your own lenses and filters, of course.

--Rik
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g4lab



Joined: 23 May 2008
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PostPosted: Sat Nov 21, 2009 2:56 pm    Post subject: Reply with quote

This is a question I always used to fret and wonder about, being a compulsive photographer and lab rat.

It is clear that with microscope objectives you want filters much later in the optical chain.

A few years ago there was an article in Shutterbug done by Roger Hicks who was someone whose work I always respected. He wondered the same thing and decided to do some testing.

The short answer was that he found no observable optical difference in negatives shot with or without quality filters. He used quite a variety as I recall. Fine German filters , less expensive glass filters and Cokin plastic filters. Even the latter did not degrade the image. These results were counterintuitive but he presented lots of images. I really enjoyed that article because it put to rest something I had been wondering about since I almost always have had a UV filter "transparent dust cap" on my camera lenses. I will see if I can google up the article.
Edit: Does not seem available online.

http://www.rogerandfrances.com
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DQE



Joined: 08 Jul 2008
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PostPosted: Mon Nov 23, 2009 11:51 am    Post subject: Assorted UV and other filter tests (coatings, flare tests) Reply with quote

I use a very high quality multi-coated filter for my MPE-65 lens - I mostly do hand-held field macro photography and I am clumsy. Thus I need to have something between the front lens surface with its fragile anti-reflection coatings and bush limbs, etc. When looking through the viewfinder I become unaware of where the projecting bush limbs are.

Lester Wareham's post has a lot of info and links:

http://photography-on-the.net/forum/showpost.php?p=3797997&postcount=1


Some basic flare tests, showing the important of filter anti-reflection coatings:

http://www.kenandchristine.com/gallery/1054387


Assorted photographic test images and some discussion:

http://www.pbase.com/lightrules/uvtest1 comparison tests with photos

http://www.pbase.com/lightrules/uvtest2 comparison tests with photos
-------------
One unfortunate aspect of using high-quality filters with very good anti-reflectance coatings is that the camera + lens + filter system becomes VERY sensitive to finger prints and smudges. With a poorly coated, high-flare filter, the relative flare increase from a finger print on the filter is small, but with an almost invisible, high-quality anti-reflection coated filter, even the smallest smudge becomes visible to one's eyes and probably causes a potentially significant increase in flare. I find the same problem with my anti-reflection coated eyeglasses. Although the lenses don't reflect much light and thus are better (esp at night, when driving), as soon as I smudge the lenses in some way, they become hard to use due to flare.

I don't recall seeing any good controlled experiments demonstrating a visible resolution or detail loss in macro work with the MPE lens.

I hope these links and a few comments are helpful. If I were only doing studio macro work, I would probably get rid of the protective filter since the hazards to the macro lens coating(s) would be greatly reduced. Also, if I were able to take better macro photos I would worry more about small resolution losses due to a high-quality protective filter.
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Charles Krebs



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PostPosted: Mon Nov 23, 2009 4:58 pm    Post subject: Reply with quote

I'll typically use a "UV/protective" filters only when working in a situation where there is a real possibility of "foreign" material contacting the front element. Obviously when there is a good chance of direct damage ( i.e. sticking the front of a macro lens into the vegetation to focus on a subject buried within), but also when it will be necessary to regularly "clean" the lens quickly between shots in a "field" situation... salt-water spray, blowing sand/dust.

As mentioned, increased flare is always a concern. And there can be a huge difference in filter quality. Also, a filter that seems OK on a normal lens may prove to be a problem on a long (telephoto) lens. It's always a good idea to do a test on the actual lens you will be using. Over the years the most filter problems I've had were with polarizing filters and graduated ND filters used on focal lengths over 300mm. Polarizers need good manufacture... two pieces of glass (four glass surfaces) sandwiching the polarizing film. The "optical resin" graduated ND's were a nightmare on long focal length lenses.

Rik's animated GIFs drive home the point that it's all a matter of "degree". With microscope objectives, as numerical apertures increase (think lower f-number), the "thickness" of the glass coverslip becomes absolutely critical. I've posted this chart somewhere once before... interesting to look at (although this is the extreme case)...



Keep in mind that this is solely due to the "thickness", not plano-parallel surface errors.


I doubt a noticeable resolution loss would be experienced with a 65mm MP-E and a good UV-type filter. Certainly you would probably get more and better images if using the filter made you less hyper-cautious about damaging the lens while working outside.
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ChrisR
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PostPosted: Mon Nov 23, 2009 5:42 pm    Post subject: Reply with quote

Down with cover glass thicknesses and high NA short WD objectives, the rays are doing different things than from further out. It's not just a lateral displacement.
That leads to a query in my mind to which I don't yet have an answer. Is the thickness of the cover glass as critical for each of these:
Mplan 60x NA 0.7 WD 4.9mm (ELWD) (for no cover glass)
CFN__40x NA 0.7 WD 0.65mm (for 0.17 cg)

If the angles of incidence of the rays are different, (which I think they are), then I would expect different aberrations caused by absence/presence of glass. And of course the issue of contact, removing one of the interfaces.

When I arrived in the forum I was asking about a 32x LWD (14mm??) objective whose descrption indicated that it could be used through a glass stage, or not. That appears to follow.

The optics of cover glasses are described here, from Leitz booklet:
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Charles Krebs



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PostPosted: Mon Nov 23, 2009 6:13 pm    Post subject: Reply with quote

Chris,

Your Leitz excerpt (and the Nikon excerpt above) appear to address your questions in good part.

As to the 60/0.70 M Plan...


And the CF N 40/0.7 would loose over 90% with no cover slip in place, and probably about 10% with either a 0.15 or 0.19 cover slip (where subject is in contact with slip).

Granted, they are both 0.70 NA with quite different working distance, and the ray "geometry" may be different. But the designers knew the parameters and therefore the aberrations they needed to address from the beginning. I don't know about the specifics of objective design, but everything I've read (and experienced) says to stick increasingly close to those design parameters as the NA moves above 0.45.
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