Velocity Reviews > So, why *do* mirror lenses have such narrow DOF?

# So, why *do* mirror lenses have such narrow DOF?

Paul Ciszek
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Posts: n/a

 07-14-2012
The two big complaints I hear about mirror lenses are:

1) The depth-of-field is frustratingly narrow

2) The f number is too high

Those two would seem to conflict with each other. Although, since the
light-accepting orifice of a mirror lens is a ring rather than a circle,
it has a larger diameter than it should for the amount of light gathering
area that it has. Is it a matter of "This thing focuses like a f/5 but
exposes like a f/8"?

Or is there something funky about the Maksutov-Cassegrain optics that
makes it follow completely different rules than lens-based optics do?

--
Please reply to: | "We establish no religion in this country, we
pciszek at panix dot com | command no worship, we mandate no belief, nor
Autoreply is disabled | will we ever. Church and state are, and must
| remain, separate." --Ronald Reagan, 10/26/1984

Wally
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Posts: n/a

 07-14-2012
On Sat, 14 Jul 2012 15:35:49 +0000 (UTC), http://www.velocityreviews.com/forums/(E-Mail Removed) (Paul
Ciszek) wrote:

>The two big complaints I hear about mirror lenses are:
>
>1) The depth-of-field is frustratingly narrow
>
>2) The f number is too high
>
>Those two would seem to conflict with each other. Although, since the
>light-accepting orifice of a mirror lens is a ring rather than a circle,
>it has a larger diameter than it should for the amount of light gathering
>area that it has. Is it a matter of "This thing focuses like a f/5 but
>exposes like a f/8"?
>
>Or is there something funky about the Maksutov-Cassegrain optics that
>makes it follow completely different rules than lens-based optics do?

Depth of field is due to simple and basic geometry. (That does not
mean the calculations are simple.)

Each pixel on the sensor subtends a cone of light that comes from the
lens. The base of the cone is at the lens and is roughly a circle.
Actually it's the shape of the iris if the lens is not fully open, for
a refracting lens. With a mirror lens, the base is a circle with the
obstructing concentric circle missing.

Imagine a point of light being focused on the sensor of the camera. It
is a point because the sensor intersects the cone at the apex. If you
change the lens-sensor distance, then the light won't be in sharp
focus any more -- because now the sensor plane intersects the cone a
bit away from the apex.

So now, instead of a point, you have a small circle on the sensor. The
size of that circle depends on simple geometry -- the size of the base
of the cone (ie the effective aperture of the lens) and the distances
along the axis from the aperture to the sensor and to the focal point.
None of this has anything to do with the mirrors in the mirror lens.
DOF only depends on that cone and the axial distances.

Calculation of DOF is fairly tricky because most of the time we don't
know the size of the base of the cone (the exit pupil). The standard
published DOF formulas usually assume it is equal to the size of the
aperture. But in modern complex lenses, that's rarely the case.
Generally in telephoto lenses, the exit pupil is smaller than the
aperture. That would help to increase DOF. For mirror lenses, as you
mention, the diameter is greater to allow for the presence of the
obstruction, and that would decrease the DOF.

W

RichA
Guest
Posts: n/a

 07-15-2012
On Jul 14, 11:35*am, (E-Mail Removed) (Paul Ciszek) wrote:
> The two big complaints I hear about mirror lenses are:
>
> 1) The depth-of-field is frustratingly narrow
>
> 2) The f number is too high
>
> Those two would seem to conflict with each other. *Although, since the
> light-accepting orifice of a mirror lens is a ring rather than a circle,
> it has a larger diameter than it should for the amount of light gathering
> area that it has. *Is it a matter of "This thing focuses like a f/5 but
> exposes like a f/8"?
>
> Or is there something funky about the Maksutov-Cassegrain optics that
> makes it follow completely different rules than lens-based optics do?
>
> --
> Please reply to: * * * * | "We establish no religion in this country, we
> pciszek at panix dot com | *command no worship, we mandate no belief, nor
> Autoreply is disabled * *| *will we ever. *Church and state are, and must
> * * * * * * * * * * * * *| *remain, separate." --Ronald Reagan, 10/26/1984

Part of it is also a bit of an illusion. Mirror lenses tend to focus
a lot closer than refractive lenses of the same focal length, which
means the DOF is shallower in some mirror lens images leading to the
"feeling" they have very shallow DOF.

David Dyer-Bennet
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Posts: n/a

 07-15-2012
(E-Mail Removed) (Paul Ciszek) writes:

> The two big complaints I hear about mirror lenses are:
>
> 1) The depth-of-field is frustratingly narrow

Never heard that complaint before.

And so far as I know, it's the same as for a refractive lens of the same
focal length and aperture.

> 2) The f number is too high

> Those two would seem to conflict with each other. Although, since the
> light-accepting orifice of a mirror lens is a ring rather than a circle,
> it has a larger diameter than it should for the amount of light gathering
> area that it has. Is it a matter of "This thing focuses like a f/5 but
> exposes like a f/8"?

There *is* some of that. The light transmission of a lens, measured in
f/stop units, is called the t/stop. Most 500mm f/8 mirror lenses are

> Or is there something funky about the Maksutov-Cassegrain optics that
> makes it follow completely different rules than lens-based optics do?

I don't believe so.

The two big complaints I hear about mirror lenses are:

1) The bokeh is full of donuts

2) The aperture is fixed.

--
David Dyer-Bennet, (E-Mail Removed); http://dd-b.net/
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Chris Malcolm
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Posts: n/a

 07-15-2012
David Dyer-Bennet <(E-Mail Removed)> wrote:
> (E-Mail Removed) (Paul Ciszek) writes:

>> The two big complaints I hear about mirror lenses are:
>>
>> 1) The depth-of-field is frustratingly narrow

> Never heard that complaint before.

> And so far as I know, it's the same as for a refractive lens of the same
> focal length and aperture.

They're often the first really long lens bought as an experimental toe
in the long lens water. Long lenses have a number of oddities and
problems which make them hard to get good quality images from. Mirror
lenses have a few extra ones, so I think the "mirror factor" often
gets blamed for problems that are in fact generic long lens problems.

But if you were a salesman selling long expensive refractor lenses
would you say that to a frustrated customer who came in complaining of
problems with his mirror lens?

>> 2) The f number is too high

>> Those two would seem to conflict with each other. Although, since the
>> light-accepting orifice of a mirror lens is a ring rather than a circle,
>> it has a larger diameter than it should for the amount of light gathering
>> area that it has. Is it a matter of "This thing focuses like a f/5 but
>> exposes like a f/8"?

> There *is* some of that. The light transmission of a lens, measured in
> f/stop units, is called the t/stop. Most 500mm f/8 mirror lenses are

>> Or is there something funky about the Maksutov-Cassegrain optics that
>> makes it follow completely different rules than lens-based optics do?

> I don't believe so.

> The two big complaints I hear about mirror lenses are:

> 1) The bokeh is full of donuts

> 2) The aperture is fixed.

Even when it's not producing doughnuts the harsh hard edged bokeh
makes it more obvious when things are slightly out of focus. So the
depth of sharp focus looks shallower than the same depth of focus on a
lens with smooth soft edged bokeh.

--
Chris Malcolm