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Thought experiment - oddball iris
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Peabody
May 31, 2012, 11:27:31 PM5/31/12
to
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.
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.
Darrell Larose
May 31, 2012, 11:52:21 PM5/31/12
to
light falloff at the edges. Mirror lenses do exhibit a hotspot in the
center caused by the dark disk of the secondary mirror. So I would think
a expanding dark disk iris would decrease resolution and sharpness more
than blade diffraction does in current designs.
--
Darrell Larose
_____________________________________________
web: http://DarrellLarose.ca
blog: http://DarrellLarose.wordpress.com
_____________________________________________
Peter Irwin
Jun 1, 2012, 12:53:48 AM6/1/12
to
Peabody <waybackNO...@yahoo.com> 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%
> 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?
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.
--
pir...@ktb.net
>
>
isw
Jun 1, 2012, 1:02:45 AM6/1/12
to
David Dyer-Bennet
Jun 1, 2012, 1:39:11 AM6/1/12
to
Peter Irwin <pir...@ktb.net> 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.
> 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.
--
David Dyer-Bennet, dd...@dd-b.net; http://dd-b.net/
Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/
Photos: http://dd-b.net/photography/gallery/
Dragaera: http://dragaera.info
Martin Brown
Jun 1, 2012, 2:25:45 AM6/1/12
to
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
> 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?
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
> 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.
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
> 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.
photographers perspective try
http://www.bobatkins.com/photography/technical/bokeh.html
The donut bokeh is particularly annoying on specular highlights.
--
Regards,
Martin Brown
Bruce
Jun 1, 2012, 2:39:31 AM6/1/12
to
Peabody
Jun 1, 2012, 8:30:44 AM6/1/12
to
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.
bokeh, which wouldn't be a desireable result. Well, it was just a
thought.
Martin Brown
Jun 2, 2012, 3:47:46 AM6/2/12
to
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
Chris Malcolm
Jun 3, 2012, 4:34:19 AM6/3/12
to
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
> 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.
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)
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
Martin Brown
Jun 6, 2012, 3:35:39 AM6/6/12
to
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
> 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!
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
> 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.
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.
>> 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.
--
Regards,
Martin Brown
Chris Malcolm
Jun 7, 2012, 2:28:37 AM6/7/12
to
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
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