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Most camera lenses are stopped down by partially closing the iris
diaphragm. That reduces the light coming through, and increases
the depth of field.

But what if instead you left the iris wide open, but stuck an
opaque black disk on the center of the lens front element?

It seems that should also cut down on the light coming through, but
would preserve a shallow depth of field. On the other hand, it
probably would increase diffraction effects, so might tend to
produce soft pictures.

Well it just seemed that the possibility of being able to stop down
without losing most of the bokeh might be useful in some cases.

Peabody <> wrote:
> Most camera lenses are stopped down by partially closing the iris
> diaphragm. That reduces the light coming through, and increases
> the depth of field.
>
> But what if instead you left the iris wide open, but stuck an
> opaque black disk on the center of the lens front element?

You need a larger disk to do this than you might think. An 86%
central obstruction is very nearly enough for two stops.

This will produce a number of effects you are not likely to want.
A large central obstruction will kill acutance (sharpness)
while leaving you with high resolution. A normal stop will kill
resolution, but will leave you with good acutance until you
get to really small stops. (Small central obstructions,
say the 30% typical of mirror lenses and telescopes, have little
effect on acutance, but obstructions of well over 50% are another
matter).

You are unlikely to like the look of the out of focus areas
with a lens with a large central obstruction, so the thinner
depth of field is not likely to be any kind of advantage.

It doesn't have to be a thought experiment. If you have a
spare normal lens with which you are willing to do the
experiment of attaching masks with water soluble glue than
you can actually stick a black disk 80-90% of the front
element diameter on the front element of the lens and
see for yourself.

(while you are at it you can make a mask with a pair of small
round holes and observe what happens to the images as you focus)

Peter.
--


>
>

Peter Irwin <> writes:

> It doesn't have to be a thought experiment. If you have a
> spare normal lens with which you are willing to do the
> experiment of attaching masks with water soluble glue than
> you can actually stick a black disk 80-90% of the front
> element diameter on the front element of the lens and
> see for yourself.

Spare UV filter, for that matter, so the lens isn't at risk at all.
--
David Dyer-Bennet, dd-; http://dd-b.net/
Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/
Photos: http://dd-b.net/photography/gallery/
Dragaera: http://dragaera.info

On 01/06/2012 04:27, Peabody wrote:
> Most camera lenses are stopped down by partially closing the iris
> diaphragm. That reduces the light coming through, and increases
> the depth of field.
>
> But what if instead you left the iris wide open, but stuck an
> opaque black disk on the center of the lens front element?

Provided you didn't obscure much more than 10% of the aperture width it
would be almost indistinguishable for in focus images, but out of focus
highlights would have the characteristic bokeh of a mirror lens.

The latter has exactly the geometry you describe as the black spot in
the centre of the lens is where the secondary sits.
>
> It seems that should also cut down on the light coming through, but
> would preserve a shallow depth of field. On the other hand, it
> probably would increase diffraction effects, so might tend to
> produce soft pictures.

It isn't the diffraction effects that hurt image quality here. Apertures
with modest central obstructions are well studied as Newtonian and
Catadioptric reflecting telescopes.

http://www.telescope-optics.net/obstruction.htm

It is the characteristic donut bokeh on out of focus highlights that you
will either love or loathe. This is a feature of classic geometrical
optics in the out of focus light cones.

Diffraction effects to first order initially sharpen the in focus image
psf very slightly when you obscure the lens centre until the obscuration
becomes too large and image quality degrades.
>
> Well it just seemed that the possibility of being able to stop down
> without losing most of the bokeh might be useful in some cases.

Sample psf and images affected should be on astronomy sites but for a
photographers perspective try
http://www.bobatkins.com/photography...cal/bokeh.html

The donut bokeh is particularly annoying on specular highlights.


--
Regards,
Martin Brown

isw <> wrote:

>In article <20120601->,
> Peabody <> wrote:
>
>> Most camera lenses are stopped down by partially closing the iris
>> diaphragm. That reduces the light coming through, and increases
>> the depth of field.
>>
>> But what if instead you left the iris wide open, but stuck an
>> opaque black disk on the center of the lens front element?
>>
>> It seems that should also cut down on the light coming through, but
>> would preserve a shallow depth of field. On the other hand, it
>> probably would increase diffraction effects, so might tend to
>> produce soft pictures.
>>
>> Well it just seemed that the possibility of being able to stop down
>> without losing most of the bokeh might be useful in some cases.
>>
>
>Why not just use a neutral-density filter?


Because that would be straightforward, and far too easy.

Thanks for all the responses. I hadn't thought about the donut
bokeh, which wouldn't be a desireable result. Well, it was just a
thought.

On 01/06/2012 13:30, Peabody wrote:
> Thanks for all the responses. I hadn't thought about the donut
> bokeh, which wouldn't be a desireable result. Well, it was just a
> thought.
>
Don't discount it completely on those grounds I do have a couple of
mirror lenses and they are very portable for 1000mm f10 wildlife
photography. A conventional unfolded lens would be very heavy and
cumbersome so I can live with the quirky donut bokeh.

The trick is not to have too much detail in the background.

You would not normally do it to a good lens though. You might use a
dense ND filter or if you really want to control the bokeh directly and
are prepared to sacrifice some resolution an apodising mask.
(expensive way of doing it though)

--
Regards,
Martin Brown

Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:
> On 01/06/2012 13:30, Peabody wrote:

>> Thanks for all the responses. I hadn't thought about the donut
>> bokeh, which wouldn't be a desireable result. Well, it was just a
>> thought.
>>
> Don't discount it completely on those grounds I do have a couple of
> mirror lenses and they are very portable for 1000mm f10 wildlife
> photography. A conventional unfolded lens would be very heavy and
> cumbersome so I can live with the quirky donut bokeh.

1000mm! I'm impressed! I have a 500mm mirror lens which I very much
like for wildlife for that reason (on a crop sensor DSLR), but a big
problem for me is simply aiming such a narrow field of view
quickly. Same problem as a powerful telescope or pair of
binoculars. But wildlife often moves, sometimes pretty quickly, so
it's often gone by the time I've got the lens aimed at where it
was. And forget birds in flight -- it's difficult enough to
find and track image filling aircraft in flight!

How do you approach this problem? I'm developing a lens aiming and
tracking sighter based on a gun sight which works very well, but I
haven't yet got a stable enough detachable mounting to avoid having to
calibrate the thing every time I set it up. Which takes a few
minutes if I want it accurate enough to locate the creature on the
central autofocus sensor, which is the only AF sensor the mirror lens
can use.

> The trick is not to have too much detail in the background.

> You would not normally do it to a good lens though. You might use a
> dense ND filter or if you really want to control the bokeh directly and
> are prepared to sacrifice some resolution an apodising mask.
> (expensive way of doing it though)

Or instead of the central disk iris reduction, which worsens bokeh,
you could use a graduated ring obstruction, a circular graduated ND
filter. That's how the old Minolta (now Sony) 135mm STF lens achieves
what many consider the best bokeh of any lens. At the cost of being a
purely manual lens.

--
Chris Malcolm

On 03/06/2012 09:34, Chris Malcolm wrote:
> Martin Brown<|||newspam|||@nezumi.demon.co.uk> wrote:
>> On 01/06/2012 13:30, Peabody wrote:
>
>>> Thanks for all the responses. I hadn't thought about the donut
>>> bokeh, which wouldn't be a desireable result. Well, it was just a
>>> thought.
>>>
>> Don't discount it completely on those grounds I do have a couple of
>> mirror lenses and they are very portable for 1000mm f10 wildlife
>> photography. A conventional unfolded lens would be very heavy and
>> cumbersome so I can live with the quirky donut bokeh.
>
> 1000mm! I'm impressed! I have a 500mm mirror lens which I very much
> like for wildlife for that reason (on a crop sensor DSLR), but a big
> problem for me is simply aiming such a narrow field of view
> quickly. Same problem as a powerful telescope or pair of
> binoculars. But wildlife often moves, sometimes pretty quickly, so
> it's often gone by the time I've got the lens aimed at where it
> was. And forget birds in flight -- it's difficult enough to
> find and track image filling aircraft in flight!

Birds in flight is pretty much a no hoper, but most of the big mirror
lenses I have used at least have a gunsight point at one end and a
sighting notch at the other. With a bit of practice and a good tripod
pan tilt it is possible to get the target in the FOV fairly quickly.

If you find it too tricky a small monocular scope might help or the
devices sold to astronomers as telrads (but dunno how good in sunlight)
>
> How do you approach this problem? I'm developing a lens aiming and
> tracking sighter based on a gun sight which works very well, but I
> haven't yet got a stable enough detachable mounting to avoid having to
> calibrate the thing every time I set it up. Which takes a few
> minutes if I want it accurate enough to locate the creature on the
> central autofocus sensor, which is the only AF sensor the mirror lens
> can use.

Mine has a primitive mechanical gunsight built in. Takes a bit of
getting used to but compared to an astronomical telescope where the
image is inverted it is a piece of cake. Good to about half a degree and
the FOV is about a degree on slide and 0.7d onto DSLR.

My conventional 500mm f4 lens is a lot easier to use and focus. Being
stuck at f10 in low light can be a real pain focussing and framing. It
also has a gunsight built in.
>
>> The trick is not to have too much detail in the background.
>
>> You would not normally do it to a good lens though. You might use a
>> dense ND filter or if you really want to control the bokeh directly and
>> are prepared to sacrifice some resolution an apodising mask.
>> (expensive way of doing it though)
>
> Or instead of the central disk iris reduction, which worsens bokeh,
> you could use a graduated ring obstruction, a circular graduated ND
> filter. That's how the old Minolta (now Sony) 135mm STF lens achieves
> what many consider the best bokeh of any lens. At the cost of being a
> purely manual lens.

Or rather the bokeh is only perfect at the fully open aperture.

--
Regards,
Martin Brown

Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:
> On 03/06/2012 09:34, Chris Malcolm wrote:
>> Martin Brown<|||newspam|||@nezumi.demon.co.uk> wrote:
>>> On 01/06/2012 13:30, Peabody wrote:
>>
>>>> Thanks for all the responses. I hadn't thought about the donut
>>>> bokeh, which wouldn't be a desireable result. Well, it was just a
>>>> thought.
>>>>
>>> Don't discount it completely on those grounds I do have a couple of
>>> mirror lenses and they are very portable for 1000mm f10 wildlife
>>> photography. A conventional unfolded lens would be very heavy and
>>> cumbersome so I can live with the quirky donut bokeh.
>>
>> 1000mm! I'm impressed! I have a 500mm mirror lens which I very much
>> like for wildlife for that reason (on a crop sensor DSLR), but a big
>> problem for me is simply aiming such a narrow field of view
>> quickly. Same problem as a powerful telescope or pair of
>> binoculars. But wildlife often moves, sometimes pretty quickly, so
>> it's often gone by the time I've got the lens aimed at where it
>> was. And forget birds in flight -- it's difficult enough to
>> find and track image filling aircraft in flight!

> Birds in flight is pretty much a no hoper, but most of the big mirror
> lenses I have used at least have a gunsight point at one end and a
> sighting notch at the other. With a bit of practice and a good tripod
> pan tilt it is possible to get the target in the FOV fairly quickly.

> If you find it too tricky a small monocular scope might help or the
> devices sold to astronomers as telrads (but dunno how good in sunlight)
>>
>> How do you approach this problem? I'm developing a lens aiming and
>> tracking sighter based on a gun sight which works very well, but I
>> haven't yet got a stable enough detachable mounting to avoid having to
>> calibrate the thing every time I set it up. Which takes a few
>> minutes if I want it accurate enough to locate the creature on the
>> central autofocus sensor, which is the only AF sensor the mirror lens
>> can use.

> Mine has a primitive mechanical gunsight built in. Takes a bit of
> getting used to but compared to an astronomical telescope where the
> image is inverted it is a piece of cake. Good to about half a degree and
> the FOV is about a degree on slide and 0.7d onto DSLR.

> My conventional 500mm f4 lens is a lot easier to use and focus. Being
> stuck at f10 in low light can be a real pain focussing and framing. It
> also has a gunsight built in.

With a "red dot" type of gunsight, much faster to use than lining up a
pin and notch, it's quite easy to track birds in flight with my 500mm
reflex, and hold them long enough on the central autofocus sensor to
catch a shot. I'm a complete novice at shooting birds in flight, but
here's three seagull shots to show that the gunsight device works well
enough to catch birds in flight with the 500mm reflex.

http://www.flickr.com/photos/chris_malcolm/7347060460/
http://www.flickr.com/photos/chris_malcolm/7161850959/
http://www.flickr.com/photos/chris_malcolm/7347059834/

--
Chris Malcolm