Keep in mind that a chip or other large defect on the front element
can't transmit any image badness as long as it's blacked-in. Since no
light can pass through it the only effect is a small decrease in
contrast due to a small increase in diffraction around the edges of the
defect. But since all edges in the optical path (like lens cells and
aperture blades) contribute to diffraction the percentage increase due
to a small defect is quite small. The situation is no different than
that of a mirror lens which places a central obstruction in the front
element that may be as large as 30% or more of the total glass area.
The faster the mirror lens the larger the obstruction. This is one
reason why mirror lenses may have slightly reduced contrast relative to
other designs. And, unless the obstruction is very large, there will
also be a modest loss of light. Remember, you'd have to block 50% of
the total area to lose one stop.
I'm sure I've shown this link here before but I'll repeat it. There are
two images shown here both taken with my Mynolta A1. Also shown are
100% crops from both images. One of the images has had a 3/4" (19mm)
square of black electrical tape stuck onto the middle of the front
element. The area is 361 square mm. The A1's front element is about
40mm in diameter for an area of 1257 square mm. The blockage is 29% or
a bit over 1/2 stop. See if you can tell the difference.
<http://www.chucknorcutt.com/dust%20spec%20test/>
A sizable scratch is also best handled by blacking it in but, if you
compute the area of the scratch you'll see that it's trivial in a size
comparison to the unaffected area.
Not that we shouldn't treat our optics well and also keep them clean but
I think most folks obsess too much over small defects. Fine if you're a
collector but as a shooter it takes a very large defect to make a
noticeable difference to an image.
Chuck Norcutt
Piers Hemy wrote:
> I remember the lens, but don't remember the chip, Brian.
>
> In my experience, the bigger the chip and the shorter the focal length, the
> greater the problem - I had an 18-28 zoom with a 1/8 inch (3mm) chip which
> was clearly visible in images at the 18mm setting. And once you knew where
> to look, it could be found in images at the 28mm setting.
>
> But we are not talking about cleaning marks, which I do agree are not likely
> to be of practical impact if they don't form an overall "mist".
>
> Piers
>
> -----Original Message-----
> From: Brian Swale [mailto:bj@xxxxxxxxxxxxxxx]
> Sent: 05 June 2009 09:56
> To: olympus@xxxxxxxxxxxxxxxxx
> Subject: Re: [OM] ( OM ) Missed a nice lens :-((
>
> --snip
>
> The importance of such marks can be over-estimated, as far as I know. I
> believe the seller's notes on this were probably fair comment.
>
> How many of you remember the 24mm Zuiko shift lens grafted onto a 6x6 camera
> body, that hit eVilBay? That lens had a large chip on one one side, fixed
> with a dab of silicone sealant as far I could tell. Made NO apparent
> difference, to judge by the large test image posted..
> I have read other reports that chips etc especially on the front element
> make little or no difference (unless hit by direct sunlight ! )
>
> Brian Swale.
> --
>
--
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