From: Robert Spanjaard on
On Wed, 16 Sep 2009 18:04:18 +0000, Chris Malcolm wrote:

>> The difference is very slight amongst lenses, and plays within less
>> than 1/3 of a stop of light. Given metering differences, technique and
>> so on, it is quite unimportant. (I do wonder if the manufacturers bias
>> the aperture slightly to account for light transmission - perhaps some
>> do).
>
> I wonder about the Sony/Minolta 500mm f8 catadioptric. For f8 a 500mm
> lens should have a 62.5mm objective lens, but it appears to be about
> 80mm diameter, and it doesn't look as though there are large stops
> inside, although there's probably some.

To capture the same amount of light as a f/8 lens, this 500mm reflex has
to be slightly bigger to compensate for the mirror sitting in the middle.

--
Regards, Robert http://www.arumes.com
From: David J. Littleboy on

"Kennedy McEwen" <rkm(a)nospam.demon.co.uk> wrote:
> In article <7hcnl2F2t08vsU1(a)mid.individual.net>, Chris Malcolm
> <cam(a)holyrood.ed.ac.uk> writes
>>
>>I wonder about the Sony/Minolta 500mm f8 catadioptric. For f8 a 500mm
>>lens should have a 62.5mm objective lens,
>
> No it shouldn't. It should have a 62.5mm stop. This may or may not be
> positioned at the front objective, but if the stop is not on the front
> element then that will indeed be physically larger than the stop diameter.

For a miror lens, it doesn't work that way. For a mirror lens to be "f/8",
the area of the opening in the "stop" needs to be pi*((500/8)/2)^2, but
since the shape of the stop is a donut, you've got

pi*(r1/2)^2 - pi*(r2/2)^2 = pi*((500/8)/2)^2.

Or something like that. (There's been a lot of forgetting that mirror lenses
borrow the center section of the aperture for the folded part of the path in
this thread.)

> Few photographic have the stop at, or imaged onto, the front element.

Yes. I object to the Canon 24-105/4.0, since a 105/4.0 lens "ought" to have
a front element around 30mm in diameter, but the actual front element is
over 70mm. The heroic effort of making the wide end 24mm makes the lens
rather enormous. I'd bet Canon could make a 35-105/2.8 that womps the
24-105/4.0 in optical performance and weighs about 2/3 as much. Heck, they
could probably make a 55-110/2.0 that womps the 24-105/4.0 in optical
performance and weighs about 2/3 as much. If the 17-40/4.0 weren't such a
dog, I could be real happy with either of those (imaginary) lenses and an
equally imaginary 17-40/4.0 II L.

--
David J. Littleboy
Tokyo, Japan


From: David J. Littleboy on

"Kennedy McEwen" <rkm(a)nospam.demon.co.uk> wrote in message
news:nMw2BgEygjsKFwHP(a)kennedym.demon.co.uk...
> In article <zr-dnbmRxv41DCzXnZ2dnVY3goidnZ2d(a)giganews.com>, David J.
> Littleboy <davidjl(a)gol.com> writes
>>
>>"Kennedy McEwen" <rkm(a)nospam.demon.co.uk> wrote:
>>> In article <7hcnl2F2t08vsU1(a)mid.individual.net>, Chris Malcolm
>>> <cam(a)holyrood.ed.ac.uk> writes
>>>>
>>>>I wonder about the Sony/Minolta 500mm f8 catadioptric. For f8 a 500mm
>>>>lens should have a 62.5mm objective lens,
>>>
>>> No it shouldn't. It should have a 62.5mm stop. This may or may not be
>>> positioned at the front objective, but if the stop is not on the front
>>> element then that will indeed be physically larger than the stop
>>> diameter.
>>
>>For a miror lens, it doesn't work that way. For a mirror lens to be "f/8",
>>the area of the opening in the "stop" needs to be pi*((500/8)/2)^2,
>
> Not so, David, that is a very common misconception. Mirror lenses are
> exactly the same as any other lens: the f/# is defined by the ratio of the
> diameter of the stop, not its area, to the focal length.

Oops. So mirror lenses are a great example of where you really need to think
about the t stop.

>> but
>>since the shape of the stop is a donut, you've got
>>
>>pi*(r1/2)^2 - pi*(r2/2)^2 = pi*((500/8)/2)^2.
>>
> The central circular obstruction on a mirror lens makes the t/# notably
> higher than the f/# (by the second term on the LHS of your equation) but
> it does not change the f/#, which has a specific definition.

Come to think of it, that makes sense, since the DoF for an f/8 mirror lens
(using the correct definition) will then be the same as for an f/8 generic
lens of the same focal length. Despite the other part of thread discussing
that.

--
David J. Littleboy
Tokyo, Japan


From: Chris Malcolm on
Kennedy McEwen <rkm(a)nospam.demon.co.uk> wrote:
> In article <zr-dnbmRxv41DCzXnZ2dnVY3goidnZ2d(a)giganews.com>, David J.
> Littleboy <davidjl(a)gol.com> writes
>>"Kennedy McEwen" <rkm(a)nospam.demon.co.uk> wrote:
>>> In article <7hcnl2F2t08vsU1(a)mid.individual.net>, Chris Malcolm
>>> <cam(a)holyrood.ed.ac.uk> writes
>>>>
>>>>I wonder about the Sony/Minolta 500mm f8 catadioptric. For f8 a 500mm
>>>>lens should have a 62.5mm objective lens,
>>>
>>> No it shouldn't. It should have a 62.5mm stop. This may or may not be
>>> positioned at the front objective, but if the stop is not on the front
>>> element then that will indeed be physically larger than the stop diameter.
>>
>>For a miror lens, it doesn't work that way. For a mirror lens to be "f/8",
>>the area of the opening in the "stop" needs to be pi*((500/8)/2)^2,

> Not so, David, that is a very common misconception. Mirror lenses are
> exactly the same as any other lens: the f/# is defined by the ratio of
> the diameter of the stop, not its area, to the focal length.

>> but
>>since the shape of the stop is a donut, you've got
>>
>>pi*(r1/2)^2 - pi*(r2/2)^2 = pi*((500/8)/2)^2.
>>
> The central circular obstruction on a mirror lens makes the t/# notably
> higher than the f/# (by the second term on the LHS of your equation) but
> it does not change the f/#, which has a specific definition.
>
>>> Few photographic have the stop at, or imaged onto, the front element.
>>
> In fact, the stop on most mirror lenses falls at the back of the lens
> barrel, close to or actually on the primary mirror - ensuring that
> defects or dust on the mirror do not image on the focal plane. With the
> stop right at the back of the lens, some over-sizing of the front
> aperture is necessary to avoid vignetting of the image at the corners.

> In the case of the Sony, the mirror is approximately 100mm behind the
> front aperture. By similar triangles, the radius of the front aperture
> must be at least sqrt(12^2+18^2) * 100 / 500 larger than the radius of
> the stop to prevent the corners of an unstabilised 35mm frame from
> vignetting at infinity - a little more at the closest focus.

> So the minimum diameter of the front aperture of this lens must be at
> least 62.5 + 2 * sqrt(12^2+18^2)/5, which is ~72mm. Given some
> practical tolerance space, allowances for in-body sensor stabilisation
> and minimum focus distance, it is fairly easy to see why this lens has
> an 80mm front aperture, and it would still need that even without the
> central obstruction, eg. if it was an off-axis parabaloid.

> The central obstruction is around 25mm diameter (estimated by eye - not
> measured). To accommodate that and achieve a t/8 solution, the lens
> stop would require to be increased to 67.3mm, which would make an 80mm
> front aperture very marginal indeed, with no allowance for tolerances
> and other effects at all.

Yet that appears to be what Sony/Minolta have done. I went outside and
aimed a refracting zoom lens set at 250mm focal length and f8 up into
a featureless white sky. No matter how I waved it around, and whether
I set it to spot metering, centre weighted, or matrix, it nearly
always gave 1/100th sec as the appropriate shutter speed, and
sometimes 1/80th sec.

I then did the same experiment with the Sony 500mm reflex lens. It
nearly always gave 1/125th sec as the appropriate shutter speed, and
sometimes 1/160th sec. Of course we have a digitisation error, since
those are the smallest step changes in shutter speed the camera will
make. But the fact that the zoom sometimes dropped down to 1/80th
suggests that the exposure value was close to the lower limit of
1/100th sec, and the fact that the reflex sometimes moved up to
1/160th sec suggests that the exposure value was close to the upper
limit of 1/125th sec. So it's clear that the nominal aperture of this
lens at f8 must have been adjusted to give a transmission factor
compensated stop, rather than the technically accurate stop in terms
of focal length divided by entry pupil diameter. In other words the
aperture has been calculated in terms of entry pupil area, with the
secondary mirror obstruction subtracted, rather than the outside
diameter.

So why would the zoom have had a dimmer f8 than the reflex? Because
its aperture was calculated the technically correct way, and it then
suffered some transmission losses because of the many glass-air
interfaces in the lens? Or maybe just manufacturing tolerances in the
way the iris is operated. TTL metering means apertures don't have to
be precise.

If calculating the diameter of an internal stop in the reflex, by the
way, note that while the front glass element of the 500mm reflex looks
flat to casual inspection, it is in fact a slightly convex converging
lens, which will reduce the size the stop needs to be.

--
Chris Malcolm
From: Chris Malcolm on
David J. Littleboy <davidjl(a)gol.com> wrote:
> "Kennedy McEwen" <rkm(a)nospam.demon.co.uk> wrote in message
> news:nMw2BgEygjsKFwHP(a)kennedym.demon.co.uk...
>> In article <zr-dnbmRxv41DCzXnZ2dnVY3goidnZ2d(a)giganews.com>, David J.
>> Littleboy <davidjl(a)gol.com> writes
>>>
>>>"Kennedy McEwen" <rkm(a)nospam.demon.co.uk> wrote:
>>>> In article <7hcnl2F2t08vsU1(a)mid.individual.net>, Chris Malcolm
>>>> <cam(a)holyrood.ed.ac.uk> writes
>>>>>
>>>>>I wonder about the Sony/Minolta 500mm f8 catadioptric. For f8 a 500mm
>>>>>lens should have a 62.5mm objective lens,
>>>>
>>>> No it shouldn't. It should have a 62.5mm stop. This may or may not be
>>>> positioned at the front objective, but if the stop is not on the front
>>>> element then that will indeed be physically larger than the stop
>>>> diameter.
>>>
>>>For a miror lens, it doesn't work that way. For a mirror lens to be "f/8",
>>>the area of the opening in the "stop" needs to be pi*((500/8)/2)^2,
>>
>> Not so, David, that is a very common misconception. Mirror lenses are
>> exactly the same as any other lens: the f/# is defined by the ratio of the
>> diameter of the stop, not its area, to the focal length.

> Oops. So mirror lenses are a great example of where you really need to think
> about the t stop.

Unless as I suggest in my previous reply Sony/Minolta decided to do
the thinking for us, and gave the lens a nomimal aperture derived from
its entry pupil area rather than diameter.

>>> but
>>>since the shape of the stop is a donut, you've got
>>>
>>>pi*(r1/2)^2 - pi*(r2/2)^2 = pi*((500/8)/2)^2.
>>>
>> The central circular obstruction on a mirror lens makes the t/# notably
>> higher than the f/# (by the second term on the LHS of your equation) but
>> it does not change the f/#, which has a specific definition.

> Come to think of it, that makes sense, since the DoF for an f/8 mirror lens
> (using the correct definition) will then be the same as for an f/8 generic
> lens of the same focal length. Despite the other part of thread discussing
> that.

Unless of course the nominal f8 of the Sony/Minolta reflex has been
derived from area rather than diameter, as the exposure experiment I
report in my other reply suggests.

--
Chris Malcolm