Best way to test for effect of scratch on fisheye lens

[Bill De Jager]

After years of desiring one I finally purchased a Nikon 7.5mm f/5.6 fisheye lens from a major and trusted vendor.  Unfortunately, the front surface of the lens has a small scratch.  There also may be a small nick - I'll have to look at it more carefully in better light in the morning.

I'm wondering what the best test is to determine whether this scratch has a noticeable effect on images.  I have an F2 and could shoot some fine-grained B&W film, or I could try to use this lens on any of several Nikon digital bodies including a Df.  Any suggestions for equipment, settings, and subject to provide the best assessment of whether I should return this lens? 

[longzoom]

Be extremally caution trying to set this lens on newest digital bodies. Dark chambers of some are too tight to let long back part of the lens  go in. If the small nick is reflecting, simply cover it with matt black paint - will be no  problem. Some of the bodies will try to return mirror-up back in working position, after shatter release. Check it. In my experience, this lens is good for B/W, coze color/contrast ratio is very low. One more thing - metal upper VF may, in some cases, create problems by short-circuit upper contacts, so, isolate them. Do not let it happened. Good luck!  LZ

[Seapy]

after shatter release.

I hope not!!! 

If the small nick is reflecting, simply cover it with matt black paint - will be no  problem.  LZ

Good tip, thanks. 

I have one lens I use around the workshop which has gouges and scrapes in the front element, they only show up if I point the camera at a bright defined light source, like the sun?  Even then nothing objectionable.

[Birna Rørslett]

Agree with most of the above. The 7.5mm works fine on all Nikon bodies where the mirror can be raised in some way to allow the lens to be mounted. Just ignore the sound emanating from the camera when you release the shutter, or if that bothers you, cover the long rear protruding section of the fisheye with some black tape.

The 7.5 even does UV, if you can arrange suitable filtration.

Here it is on my humble D40X, set up for UV.

[Bill De Jager]

Thank you, longzoon, Seapy, and Birna.  I appreciate your advice.  Fortunately the scratch is short and shallow.

I'm looking forward to using this lens, eventually on an FTZ.  It'll be interesting to compare it to the non-AI 8mm/2.8 fisheye lens that I have.

[Birna Rørslett]

The 7.5 is tiny compared to the 8/2.8, however the latter can be focused.

[Birna Rørslett]

Two examples with the 7.5mm f/5.6 (both on Nikon D40X). One is"Hotel California" (UV), the second is how winter comes unexpectedly (IR).

As both were shot hand-held, they are no direct documentation of image quality as such. But it's a useful lens, at that.

[pluton]

Good luck with your new fisheye, Bill.
To test for visibility of the scratch, I would attempt to arrange 2 shots:
One where the the scratch can be made to appear as a bright, light-catching spot in a dark field, and another shot where the scratch would be imaged as a dark spot against a bright field.
Both done at the smallest practical stop.
If I were to place an ink or paint dot over scratch, I'd make sure that whatever ink or paint I used was easily and fully removable.

[Chip Chipowski]

As both were shot hand-held, they are no direct documentation of image quality as such. But it's a useful lens, at that.

Nice lens for a self portrait

[Bill De Jager]

Good luck with your new fisheye, Bill.
To test for visibility of the scratch, I would attempt to arrange 2 shots:
One where the the scratch can be made to appear as a bright, light-catching spot in a dark field, and another shot where the scratch would be imaged as a dark spot against a bright field.
Both done at the smallest practical stop.
If I were to place an ink or paint dot over scratch, I'd make sure that whatever ink or paint I used was easily and fully removable.

Thanks, Keith.  I think I can do the former by photographing a large lawn and the latter by photographing cloud cover in the sky when it's relatively bright, making sure I orient the lens to put the scratch in the right place. I may need to wait a day or two depending on the weather, as it's finally started raining here after 219 consecutive days of no rain.  This is not the way our climate here is supposed to work, despite the reputation of California as never getting rain. OTOH, there may be some nice sun-and-shadow shooting weather once the front passes through.  Everything outside is now freshly washed and (once the sun comes out) the low sun angle produces some nice lighting.

[Bill De Jager]

Regrettably there are small specks inside the front element at its apex that I hadn't seen before. I only saw them once I took the lens into different lighting to take sample photos.  These aren't dust, they're slightly brownish and almost like flyspecks.  Not good. They will not come off with cleaning and they are not quite on the outer surface.  The outer element is very thin at the apex so I believe the specks are on the inner surface.

How did I not see this before? The light fixture over the dining room table has many small bulbs, producing many points of light reflecting off the front element of any lens. I've learned to ignore these, and this time I ended up fooling myself because the specks were camouflaged among these reflections. 

I'll return the lens.  Thanks again for the information and advice.  I'll keep my eyes peeled for another sample from a reputable dealer.