From: Chris Malcolm on
Kennedy McEwen <rkm(a)nospam.demon.co.uk> wrote:
> In article <7hi05qF2u1crvU1(a)mid.individual.net>, Chris Malcolm
> <cam(a)holyrood.ed.ac.uk> writes
>>Kennedy McEwen <rkm(a)nospam.demon.co.uk> wrote:
>>
>>> 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.

> It certainly isn't quite as clear as you seem to think it is. All you
> have shown is that the transmission of an undefined zoom lens is less
> than that of the mirror lens, despite the obscuration. That is one
> reason why almost every astronomical telescope used for deep sky work,
> where every last photon counts, uses a mirror rather than refractor
> design: the transmission loss of refractors is often more than the loss
> of the central obstruction.

I went out this morning and repeated the experiment with three lenses,
and took photographs so that I could see small differences in exposure
from the histograms. Today was a better day for the experiment. The
sky was more heavily overcast, more evenly overcast, and varied in
brightness less with time. Yesterday I must have let some light
variations creep in, because today I found the differences in exposure
between a SAL 50mm f1.4 lens at f8, a SAL 18-250mm at 50mm and f8, the
18-250mm at 250mm and f8, and the SAL 500mm f8 (reflex) to be less
than 1/5th stop when matrix metered. The 18-250mm has no noticeable
vignetting, the 50mm slight vignetting, and the 500mm reflex very
noticeable vignetting, at rough guess half a stop. So if metered in
the centre it comes out brighter.

Note too that the camera is a Sony A350 with an APS-C sized sensor, so
the 500mm reflex, which is a full frame lens, would have more fall off
towards the edges and would therefore on matrix metering come out
darker. So my finding that all three lenses matrix metered come out
with pretty much the same transmission values is in part an artefact
of using a 1.5 crop sensor.

> Incidentally, I just dug out my old Tamron 500mm mirror lens from the
> back of the cupboard. I haven't used it in years but it is a similar
> spec to the Sony at 500mm f/8, although obviously not an AF model and I
> have to use an adapter to mount it on my Canon cameras.

> I measured the mirror diameter, which is what actually matters here, and
> it is 62.5mm +/- 0.25mm, exactly as it should be for the lens
> specification. The front aperture of that lens is pretty much the same
> size as the Sony at 80mm. The central obstruction however is 36mm
> diameter, so if the f/# was spec'd against area rather than diameter, it
> would be an f/9.8 - half a stop higher. But it is spec'd at f/8 and,
> from the measurements it certainly is a true f/8. Guess what? Using
> your same test against a 100-400 f/4.5-5.6 zoom lens at 400mm f/8 gives
> a similar result. That old 500mm f/8 mirror lens is almost 1/3 of a
> stop faster than the refractive zoom at 400mm, but I know it is a true
> f/8 design from the dimensions.

> The critical dimensions and specs of the Tamron and the Sony are so
> similar that is it just nonsense to suggest that the Sony can have a
> stop 5mm larger than the Tamron (which is what would be required to
> compensate for the area of 25mm diameter obscuration) or more, if it
> turns out that the central obstruction on the Sony is closer to Tamrons
> 36mm. It would need a front aperture closer to 90mm to permit that and
> the Sony just isn't that large.

> I suggest you make some attempt at measuring the diameter of the primary
> mirror on the Sony lens, it isn't difficult to do, rather than basing
> your claims on second order effects.

How can I measure the diameter of the stop? Note that in the
Sony/Minolta design the front glass face is slightly convex, which may
be what permits the size of the secondary mirror to be smaller. I can
see the stop at the back of the lens round the edges of the mirror,
but am not sure how to measure it, and how the front lens would affect
that measurement.

The best idea I could come up with was to place a very brightly lit
ground glass screen over the back of the lens, breathe on the front,
and try to measure the diameter of the projected bright circle. My
estimate is that it is 72-74mm diameter.

--
Chris Malcolm
From: OldBoy on
"Chris Malcolm" <cam(a)holyrood.ed.ac.uk> wrote in message
news:7hjq6bF2qbhq8U1(a)mid.individual.net...
> Kennedy McEwen <rkm(a)nospam.demon.co.uk> wrote:
>> In article <7hi05qF2u1crvU1(a)mid.individual.net>, Chris Malcolm
>> <cam(a)holyrood.ed.ac.uk> writes
>>>Kennedy McEwen <rkm(a)nospam.demon.co.uk> wrote:
>>>
>>>> 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.
>
>> It certainly isn't quite as clear as you seem to think it is. All you
>> have shown is that the transmission of an undefined zoom lens is less
>> than that of the mirror lens, despite the obscuration. That is one
>> reason why almost every astronomical telescope used for deep sky work,
>> where every last photon counts, uses a mirror rather than refractor
>> design: the transmission loss of refractors is often more than the loss
>> of the central obstruction.
>
> I went out this morning and repeated the experiment with three lenses,
> and took photographs so that I could see small differences in exposure
> from the histograms. Today was a better day for the experiment. The
> sky was more heavily overcast, more evenly overcast, and varied in
> brightness less with time. Yesterday I must have let some light
> variations creep in, because today I found the differences in exposure
> between a SAL 50mm f1.4 lens at f8, a SAL 18-250mm at 50mm and f8, the
> 18-250mm at 250mm and f8, and the SAL 500mm f8 (reflex) to be less
> than 1/5th stop when matrix metered. The 18-250mm has no noticeable
> vignetting, the 50mm slight vignetting, and the 500mm reflex very
> noticeable vignetting, at rough guess half a stop. So if metered in
> the centre it comes out brighter.
>
> Note too that the camera is a Sony A350 with an APS-C sized sensor, so
> the 500mm reflex, which is a full frame lens, would have more fall off
> towards the edges and would therefore on matrix metering come out
> darker. So my finding that all three lenses matrix metered come out
> with pretty much the same transmission values is in part an artefact
> of using a 1.5 crop sensor.

Light measurement is not dependent on image sensor size;
A specfic sensor is used.

>> Incidentally, I just dug out my old Tamron 500mm mirror lens from the
>> back of the cupboard. I haven't used it in years but it is a similar
>> spec to the Sony at 500mm f/8, although obviously not an AF model and I
>> have to use an adapter to mount it on my Canon cameras.
>
>> I measured the mirror diameter, which is what actually matters here, and
>> it is 62.5mm +/- 0.25mm, exactly as it should be for the lens
>> specification. The front aperture of that lens is pretty much the same
>> size as the Sony at 80mm. The central obstruction however is 36mm
>> diameter, so if the f/# was spec'd against area rather than diameter, it
>> would be an f/9.8 - half a stop higher. But it is spec'd at f/8 and,
>> from the measurements it certainly is a true f/8. Guess what? Using
>> your same test against a 100-400 f/4.5-5.6 zoom lens at 400mm f/8 gives
>> a similar result. That old 500mm f/8 mirror lens is almost 1/3 of a
>> stop faster than the refractive zoom at 400mm, but I know it is a true
>> f/8 design from the dimensions.
>
>> The critical dimensions and specs of the Tamron and the Sony are so
>> similar that is it just nonsense to suggest that the Sony can have a
>> stop 5mm larger than the Tamron (which is what would be required to
>> compensate for the area of 25mm diameter obscuration) or more, if it
>> turns out that the central obstruction on the Sony is closer to Tamrons
>> 36mm. It would need a front aperture closer to 90mm to permit that and
>> the Sony just isn't that large.
>
>> I suggest you make some attempt at measuring the diameter of the primary
>> mirror on the Sony lens, it isn't difficult to do, rather than basing
>> your claims on second order effects.
>
> How can I measure the diameter of the stop? Note that in the
> Sony/Minolta design the front glass face is slightly convex, which may
> be what permits the size of the secondary mirror to be smaller. I can
> see the stop at the back of the lens round the edges of the mirror,
> but am not sure how to measure it, and how the front lens would affect
> that measurement.
>
> The best idea I could come up with was to place a very brightly lit
> ground glass screen over the back of the lens, breathe on the front,
> and try to measure the diameter of the projected bright circle. My
> estimate is that it is 72-74mm diameter.
>
> --
> Chris Malcolm


From: Chris Malcolm on
Kennedy McEwen <rkm(a)nospam.demon.co.uk> wrote:
> In article <7hjq6bF2qbhq8U1(a)mid.individual.net>, Chris Malcolm
> <cam(a)holyrood.ed.ac.uk> writes

>>How can I measure the diameter of the stop? Note that in the
>>Sony/Minolta design the front glass face is slightly convex, which may
>>be what permits the size of the secondary mirror to be smaller.

> It is slightly convex on the Tamron too, but that can't change the
> physics - the effective stop is the diameter that a bundle of parallel
> image forming rays have as they enter the lens. ie. the apparent optical
> diameter of the lens as viewed from infinity. It doesn't matter what
> sits in front of or behind the stop.

In other words it's the entry pupil rather than the physical size of
the stop I need to measure.

>> I can
>>see the stop at the back of the lens round the edges of the mirror,
>>but am not sure how to measure it, and how the front lens would affect
>>that measurement.
>>
> You really need a collimator to do that properly but, assuming that you
> don't have access to one, there are a couple of alternatives which give
> good approximations.

How about a laser pointer?

[snip advice about doing it visually]

Thanks for that. I'll return to this interesting investigation later.

> As I said at the start of this, if it really was faster than f/8 then
> Minolta would not have called it f/8 - that would amount to underselling
> their product.

Thanks for the educational discussion. What we seem to have discovered
so far is that despite the conventional wisdom that catadioptric
lenses have an unusually large difference between f# and t#, at least
in the case of the popular Sony/Minolta and Tamron 500mm versions, the
operation of of the law of swings and roundabouts seems to have
arranged that they're very like our everyday refracting lenses in the
relationship betweem f# and t#. In other words, they're f8 lenses
which pass the amount of light you'd expect from an f8 lens. No
special exposure allowances for their catadioptric nature need be
made.

--
Chris Malcolm
From: Miles Bader on
Chris Malcolm <cam(a)holyrood.ed.ac.uk> writes:
> Thanks for the educational discussion. What we seem to have discovered
> so far is that despite the conventional wisdom that catadioptric
> lenses have an unusually large difference between f# and t#, at least
> in the case of the popular Sony/Minolta and Tamron 500mm versions, the
> operation of of the law of swings and roundabouts seems to have
> arranged that they're very like our everyday refracting lenses in the
> relationship betweem f# and t#.

One thing that comes to mind is while mirror lenses obviously have the
big honkin' obstruction, they should also have less loss due to actual
hunks of glass and glass interfaces. So who knows...

-Miles

--
Friendless, adj. Having no favors to bestow. Destitute of fortune. Addicted to
utterance of truth and common sense.
From: Paul Furman on
Kennedy McEwen wrote:
> In article <7hjq6bF2qbhq8U1(a)mid.individual.net>, Chris Malcolm
> <cam(a)holyrood.ed.ac.uk> writes
>> I went out this morning and repeated the experiment with three lenses,
>> and took photographs so that I could see small differences in exposure
>> from the histograms.
>
> Whilst I could half expect Sony to "do a Microsoft" and redefine f/stops
> to their own proprietary standard, this lens is an old Minolta design
> and, as a traditional photographic company, they simply wouldn't do
> that. If the lens was over-apertured to compensate for the central
> obstruction, say to f/6.5, then that is what Minolta would have called
> it: not least because there were a host of independent 500mm f/8 designs
> on the market at the time. Offering a faster alternative would have
> been a Minolta marketing man's dream. No, Chris, it is called an f/8
> lens because it IS an f/8 lens.
>>
>> How can I measure the diameter of the stop? Note that in the
>> Sony/Minolta design the front glass face is slightly convex, which may
>> be what permits the size of the secondary mirror to be smaller.
>
> It is slightly convex on the Tamron too, but that can't change the
> physics - the effective stop is the diameter that a bundle of parallel
> image forming rays have as they enter the lens. ie. the apparent optical
> diameter of the lens as viewed from infinity. It doesn't matter what
> sits in front of or behind the stop.
>
>> I can
>> see the stop at the back of the lens round the edges of the mirror,
>> but am not sure how to measure it, and how the front lens would affect
>> that measurement.
>>
> You really need a collimator to do that properly but, assuming that you
> don't have access to one, there are a couple of alternatives which give
> good approximations.
>
> The simplest is just to put the lens and camera onto a tripod with a
> rule vertically across the front aperture.

I'm not following all the details here but I tried the ruler idea on a
35mm f/1.4 and it measured a perfect 25mm. Hmm, well only at closest
focus, with the ruler on the back, looking through the front element.


> What you need to do is set
> your viewing point at infinity but that would be impractical and
> somewhat difficult to resolve the measurements on the ruler. ;-)
> However provided that you view from a distance large enough that the
> parallax between the rule and the mirror is insignificant, that should
> give you a good enough estimate. The mirror sits approximately 100mm
> behind the ruler and, since you are trying to differentiate better than
> 5mm in 65mm, ie. better than 7.5%, you need to be viewing from at least
> 1.3m from the lens, but further is better.
>
> If move your viewpoint by 65mm or so towards the side of the mirror that
> you are measuring on the rule that will also partially compensate for
> the parallax and should enable you to get the stop size to better than
> +/-1mm without too much effort.
>
> One difficulty that you will have is that the primary mirror will become
> obscured by the image of the secondary and its surround when viewing
> from a distance, for fairly obvious reasons: the secondary has to be
> large enough to capture all of the primary's reflected light. However
> if you shift your viewing position slightly to one side, ie. off axis,
> and the rule is vertical then the mirror should be clearly visible.
>
>> The best idea I could come up with was to place a very brightly lit
>> ground glass screen over the back of the lens, breathe on the front,
>> and try to measure the diameter of the projected bright circle.
>
> No, that would only work if it was a point source at exactly infinity
> focus. Your extended screen only enlarges the apparent size by the
> field of view and the projected distance.
>
>> My
>> estimate is that it is 72-74mm diameter.
>
> It certainly can't be that large as you only have a front aperture of
> 82mm (the original Minolta version had an 82mm front thread), and there
> simply isn't room to get the field of view through that to a mirror of
> that size. The front aperture is always oversized for this reason.
>
> As I said at the start of this, if it really was faster than f/8 then
> Minolta would not have called it f/8 - that would amount to underselling
> their product.


--
Paul Furman
www.edgehill.net
www.baynatives.com

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