Long Telephoto Lens – fotosaurier

My Crazy Lenses – Topcor R 30cm f/2.8 and its Modern State-of-the-Art Counterparts – „Supertele-Lenses“

Travel on my time-machine
The known Facts – Topcor 30cm f/2.8
Topcor 30cm f/2.8 – Optical Performance
The Reference: Canon EF 300mm f/2.8 IS USM
Three more 300mm f/2.8-teles

DSCF2458_Alle300f2,8-5_blog — **Fig 1**: From left to right – Topcor R 30cm f/2.8, Arsat Yashma 300mm f/2.8, Tamron SP LD (IF) 300mm f/2.8, Minolta AF Apo-Tele 300mm f/2.8, Canon EF 300mm f/2.8 L IS USM

Attention: I part from my „crazy lenses“ due to my age: it takes too much time to take care for my optical baybies! If you are interested in the Topcor R 30cm f/2.8, please leave a price-proposal in the comment-section (this is seen only by me) or by mail to webmaster@fotosaurier.de

1. On my time-machine:

I own the Topcor R 30cm f/2.8, which I am looking at here, since a few years – but I have not used it too often. It is very heavy, long and dark, giving the impression of a tank-breaking weapon: you definitely will get trouble at any security check nowadays … and in the best case you will earn compassion instead of admiration! Too bad, because it is an ingenious piece of optical engineering.

Information about Topcor lenses today are rare and not always reliable. I will restrict myself to reliable information and I will try to verify legends … or destroy them.

So I entered my time machine and travelled back into the year 1958. I was 13 years old at my arrival there – and at the Topcon (Tokyo Kogaku) factory I met a team of innovative engineers, who were fanatically burning for the QUALITY of their products – and really proud of it! The year before (1957) they had introduced a new SLR-camera (Topcon R), which was designed in Bauhaus-style, i.e. with clear and modern lines – and they were ready to ignit a firework of innovations around the SLR-concept within the next few years (from first-in-industry TTL-exposure-metering to first electric winder).

And they had introduced a line of lenses for this SLR-system-camera, among which the Topcor 30cm f/2.8 peaked out. Another „first-in-industry“-innovation.

I looked around in the photo-stores and could not find any Canon- or Nikon-SLRs there: the dealers told me, that both companies were just bringing out SLRs. It seemed, that the Topcon-people had considered the German SLRs, which were already on the market, as their competition. Personally at that time I was already a SLR-user (of my father’s Contaflex – which means, that from time to time my father was still allowed to use it himself).

Everybody, who is acqainted with the rules of the market, would have expected, that shortly after an innovation like the Topcor R 30cm f/2.8, the major competitors would bring out a similar product.

But that did not happen – so I returned in my time-machine. Finally I found out, that it took the new japanese competitors more than a decade! And there was no comparable Lens in Europe, as far as I could see. 13 years later Nikon presented a prototype, to be tested during the Olympic Winter Games of Sapporo in 1972.

The real next step was taken by Canon with a 300mm f/2.8 Lens for their new FD-System, using a lens made of FLUORITE in 1973 (some say 75)! This was finally 16 years after the arrival of the Topcor-lens … and just in that year, when Topcon stopped the production of their supertele-lens.

2. The known facts:

This Topcor R 30cm f/2.8 monster-tele-lens with 300mm focal length was presented to the world in 1958 („Topcon Club“ says 1957!) – one year before Canon or Nikon started to produce any SLR – and 13-16 years before any other lens- or camera-maker presented such a fast 300mm tele-lens. Not only at the 1964 Olympic Games in Tokyo but all the time until 1972 it was without any competition. As a consequence, there even was produced quite a number of lenses with Nikon mounts! Next to Topcon, Canon brought out its Canon FD 300mm f/2.8 S.S.C. Fluorite lens in 1973 – setting the level for professional superlele-lenses for the next decades and until today. Just a few years later Topcon went completly out of the business with SLR-cameras and lenses. Sad, but even the extensive book „Topcon Story“ by Marco Antonetto and Claudio Russo (1) does not answer the question „why?“. Today Topcon is a market-leader in geodesic instruments.

Stephen Gandy (3) estimates – cameraquest.com – that 700-800 lenses have been produced in total during 18 years of production.

Topcor-R-300f2,8_DSCF2335_blog — Fig. 2a: R.Topcor 1:2.8 f=300mm on Topcon SuperD- source: fotosaurier

Titel_DSCF2320 — Fig. 2b: R.Topcor 1:2.8 f=300mm – source: fotosaurier

R328cut — Fig. 2c: Lens scheme of Topcor 1:2.8 30cm – source: http://www.topgabacho.jp/Topconclub/lens3.htm

The lens is made of six single lenses in four groups – of which lens no. 6 (group 4) is the filter (diameter 39mm), which is, of course, part of the optical design! This filter is an early (maybe the first) example of a filter which is positioned in a slot in the rear part of the lens-body. In the book „Topcon Story“ (page 128) there is an error in the spreadsheet listing of the data of the R.Topcor-lenses: the data in the last line are the data of the „300mm 2.8“ and not of the f/5.6-lens. Here the no. of elements is „five“, which is correct, when you don’t count the filter as an active optical member …

The lens has a preset diaphragm and has a built-in sunshade (telescoping in two stages!). It is 383 mm long (from camera-flange to front-edge of the pulled-back sunshade – total length with shade pulled out is 477 mm) and weighs 3.1 kgs (without front and rear caps). Measured at my sample (ser. no. 34.1359). The initial sales-price was $ 1.125,–. (In the literature you will find: 415/412 mm length and 3.3 kgs weight).

It may be interesting to mention here, that right away from the introduction of the first Topcon-SLR, an extremely ambitious lens-program was planned – however, realized only partly. The Topcor R 13,5 cm f/2.0 (6 lenses) had also preset diaphragm and it was discontinued with the Topcon RE camera system – so it is said to be extremely rare. It has a yellowish color cast (due to rare-earth-glass?), not a big problem with todays digital cameras …

However, a 50mm f/0.7 lens, which is mentioned in „Topcon Club“ only, was never made for the SLR-camera market – maybe, this was one of the very early oscilloscope-registration-lenses, which are also known from Germany and GB even at WWII-times.

And a 1000mm f/7 catadioptric lens was only experimentally made in 1958.

„Topcon Club“ (2) writes about this:

„The interchangeable lenses which appeared with the appearance of TOPCON R are various kinds of the Auto Topcor of 35mm/100mm, and R TOPCOR (a preset diaphragm) of 90mm/135mm/200mm/300mm among these – although the bright thing and the dark thing were prepared about 135mm and 300mm – it should mention especially – it is the „high-speed lens“ of 135mm f2 and 300mm f2.8. 50mm f0.7 – such a bright lens was already completed during wartime by the Tokyo optics. Do you believe it ? Although possibly this grade was an easy thing, even so, the 300mm f2.8 lens will be an astonishment thing in 1957. I talked in detail on „the page of TOPCOR“ about this lens. We have to wait for marketing of the product of NIKON which is the next 300mm f2.8 lens at any rate till 1977. However, TOPCON did not build the super telephoto lens 500mm /800mm those days. Furthermore, the Refrector Topcor 1000mm f7 is appearing in the catalog in ’59. However, this was not launched regretfully.“

Later – from 1969 on – a RE Topcor 500mm f/5.6 telephoto-lens was even produced with automatic diaphragm and meter coupling!

Can such a fast long telephoto lens like this early 300mm f/2.8-design without Fluorite- or ED-lenses be any good – on the scale of professional photography? There are hints, that rare-earth glasses were used to make these lenses (also for the other famous 13,5cm f/2.0, also supplied since 1958). But I do not know details about this.

I will answer the question about the optical quality here – also comparing this lens with a modern top-notch tele-lenses like Canon EF 300mm f/2.8 L IS USM, which I personally classify as today’s state-of-the-art reference, supported by photo-friend Thomas, who borrowed his Canon lens to me.

Finally I will take a glance on a state-of-the-art modern astronomical refractor, which normally does perform at diffraction-limited resolution on stars!

Topcor 30cm f/2.8 – The Optical Performance on analog film (year 1969):

Stephen Gandy (3) wrote in his blog:

WIDE OPEN its resolution was 56lines/mm center and 34lines/mm at the edges. By f/8 it was 80 lines/mm center and 65 at the edges. Many normal lenses don’t achieve this sharpness — much less 300/2.8 leviathans ! Camera 35 summed it up by saying „INCREDIBLY FANTASTIC.“ I would have to agree. „

(In the original text in Stephen’s blog, the reported resolution values are noted as „56mm“ or „34mm“. I have taken the freedom, to correct this to what it should read: lines per mm, „lines/mm“!)

The resolution values, which I use in my digital IMATEST measurements, typically are given in „Line-pairs per picture-height“ = „LP/PH“. Picture-height being 24mm with 24×36-format, you have to divide the „lines/mm“-values by two to get to „line-pairs“ – and then multiply with 24 to achieve LP/PH.

The highest given value of 80 lines/mm corresponds to 960 LP/PH stopped down to f/8 in the center or 760 LP/PH at f/8 at the edge – the lowest value 34 lines/mm with open diaphragm at the edge corresponds to 408 LP/PH.

What does that mean?

In 1969 the test results for resolution were measured on film – „Modern Photography“ used Plus-X Pan with standardized development – and the reading of the „just resolved“ line-pattern was done with a standardized enlarging glass … I personally used the method myself at that time, too, and it is quite reproducible as long as the same person does the reading … It is somewhat sensitive to the vision-capabilities of the reading person! And of course the grain of the analog film material (negative b&w film!) is the limiting factor for the resolution-reading on film for really high resolutions.

Today’s modern 24 MP-sensors deliver resolutions of 2,000-2,400 LP/PH using MTF30 (30% contrast) as the parameter for reading out the resolution values from the MTF-curve. My Sony A7R4-Camera (62 MP), which I use for my measurements, has a Nyquist frequency of 3.168 LP/PH and delivers up to 3.800 LP/PH-readings with the best known lenses.

The following spreadsheet gives an overview on the physical data of the Topcor-lens and the other lens-monsters, analysed here:

300f2,8_physData — **Fig. 3**: Physical Data of the five 300mm f/3.8-Lenses – source: measured by fotosaurier

3. Topcor 30cm f/2.8 – Optical Performance

My IMATEST-Results of the optical properties of the Topcor R 30cm f/2.8 lens:

To exclude potential vibration-initiated degradation of resolution in my test-shots at these long focal-lengths I used my heavy (>10 kgs) and sturdy astronomical telescope-mount:

DSCF2537_Topcor_AufAstroMontierung_blog — **Fig. 4**: My massive astronomical lens mount – here with SonyA7R4 attached to Topcor 30cm f/2.8 – source: fotosaurier

DSCF2535_OnTargetTop — **Fig. 5**: The set-up keeps the lens and camera steady even at 0,4 seconds. – source: fotosaurier

Following you see the results of my IMATEST-measurements:

Topcor_R-300f2,8_Spreadsheet-23 — **Fig. 6:** Optical measurment-results for **Topcor R 300mm f/2.8** adapted to Sony A7R4 with 62 MP – resolution values given in LP/PH – source: fotosaurier

Topcor_R-300f2,8_Graph-23 — **Fig. 7**: Resolution measurement-results for **Topcor R 300mm f/2.8** as graph – source: fotosaurier

The lens is unique at that time regarding to „speed“ – an extremely ambitious piece of optical engineering. Remind, that the distortion is practically zero and the CA-area in the center 0,8-1,4 pixel – 1 pixel at Sony A7R4 is 3,8 microns on the sensor!

What is center, what is part way and what is corner? In the following graphs from IMATEST you see: „Part-Way“ is the large part of the picture extending close to the narrow side (left/right). „Corner“ is the narrow area outside the second dotted circle on the picture below.

DSC07122_Topcor300f2,8_8,0_Multi-ROI_2023-02-04_01-00-54 — **Fig. 8**: „**Cente**r“ resolution is calculated as mean from the values inside the inner circle (in my setting always two values), „**part way**“ is the mean of all values between the inner and outer circle, „**corner**“ is the mean of all values positioned outside the outer circle – source: fotosaurier

DSC07110_Topcor300f2,8_8,0_Lens_MTF_2022-11-29_22-55-46 — **Fig. 9:** Topcor R 30cm f/2.8 resolution plotted over radius of picture circle – source: fotosaurier

So, let’s compare the measurements to the value, that were given in analog times on film:

The comparison in the spreadsheet Fig. 10 shows: The lens „out-resolves“ normal analog films by far! Stopped down it reaches the limits of the analog medium even at the edges of the frame!

Analog-digital-resolution — **Fig.10:** „Camera35’s“ resolution measurements for Topcor R 30cm f(2.8 of 1969 on film compared with digital IMATEST values (at 30% MTF = „MTF30) with Sony A7R4 – source: fotosaurier

I found no real technical explanation, how Topcon-engineers managed to generate this phantastic lens at that time without ED/LD/AD/Fluorite-glass. There is a second tele-lens – the 13,5cm f/2.0, also introduced 1958, with first-in-industry potential – and finally the Topcor 2,5cm f/3.5 super-wide, which surprises with best-in-class resolution values (see my blog-article on historical 24/25mm-lenses!).

If somebody knows the secret: please, tell us!

Look at a sample picture taken with the Topcor at the end of this article at 65% enlargement size (see Fig. 24).

Now, let’s have a glance on some other historical Superteles:

Alle_300er_2,8_DSCF2573 — **Fig. 11**: From left to right: Topcor R 300mm f/2.8, Canon EF 300mm f/2.8 IS USM, Minolta AF Apo-Tele 400mm f/2.8, Tamron SP (60B) 300mm f/2.8 LD (IF), Arsat Yashma-4H MC 300mm f/2.8

4. The Reference: Canon EF 300mm f/2.8 IS USM

Canon-EF_300f2,8_DSCF2451_blog — **Fig. 12**: **Canon EF 300mm f/2.8 IS USM – source: fotosaurier**

Canon EF 300mm f/2.8 IS USM is rated as the reference of this class of lenses. In this case it is not the latest „Mk II“-version of it, which came out 2011 – but the first version of 1999, which is tested here. It represents nevertheless already the top-class of the super-teles (as all its predecessors since 1973!)

Here are the IMATEST results of its optical properties:

Canon-EF_300f2,8-L-IS-USM_AF_Spreadsheet — **Fig.13**: Optical properties of **Canon EF 300mm f/2.8 IS USM** from my IMATEST-measurements, with autofocus – source: fotosaurier

And here the Graphs of resolutions center, part way and corner:

Canon-EF_300f2,8-L-IS-USM_AF_Graph — **Fig. 14**: IMATEST-Resolution (LP/PH) of **Canon EF 300mm f/2.8 IS USM –** center – part way and corners – source: fotosaurier

Not may comments necessary to this – the figures and graphs should speak for itself … Just to mention: the distortion at the Topcor-lens is even lower than that of the Canon – but both are neglectable for a supertele!

Canons leadership in this class of professional supertele-lenses was generated by the policy, not to drop a product into the market, which was „just possible“ at present, but to persue a consequent plan for the future: to solve the „secondary spectrum“-problem of long tele-lenses, which means to use extreme „anormal dispersion“ lens-materials, which do the job without optical compromising.

So in 1975 – 2 years after Nikons first presentation of its first 300mm f/2.8 ED-lens (which was not very convincing and had to be replaced four years later by the ED-IF-version) – Canon introduced their FD 300mm f/2.8 Fluorite-Supertele, in which they used a front-lens made of fluorite-monocrystal material (no glass!) and a UD-glass-lens. This lens was already praised close to perfect (absence of chromatic aberrrations). Canon accepted for this a compromise, which made the lens longer and heavier: to protect the soft and sensitive fluorite-crystal-material in the front lens, there was a fixed additional plane protection element of glass in front!

Finally new fluorite-glass-formulations became available, which allowed to drop the sensitive crystal-lens. Over the introduction of Autofocus (EOS – 1987) and still more glass-elements, Canon finally introduced the legenday lens EF 300mm f/2.8L IS USM in 1999 with very fast AF and image-stabiliser, which is tested here.

Enjoy the results!

5. Finally – three more 300mm f/2.8-teles:

Minolta AF APO-Tele 300mm f/2.8 (1985)
Tamron SP LD (IF) 300mm f/2.8 (60H) (1984)
ARSAT MC Yashma-4H 300mm f/2.8 (1990?)

For these three lenses I also have to thank foto-friend Thomas, who borrowed them to me!

5a. Minolta AF APO-Tele 300mm f/2.8 (1985)

Minolta-Apo_300f2,8_DSCF2460_blog — **Fig. 15**: **Minolta AF APO-Tele 300mm f/2.8 – source: fotosaurier**

This lens had a mechanical defect: the diaphragm could not be closed below f/5,6. However: in these lenses principally mainly the open aperture is really significant – why should you carry around such a weight, to make pictures with f/11?

Minolta-AF-Apo-Spreadsheet — **Fig. 16:** Optical properties of **Minolta AF 300mm f/2.8 Apo – source: fotosaurier**

Minolta-AF-Apo-Graph — **Fig. 17:** IMATEST-Resolution (LP/PH) of **Minolta AF 300mm f/2.8 Apo –** center – part way and corners – source: fotosaurier

This Minolta lens comes closer to the Canon-legend than any of the others – but with quite some distance in resolution in the corners open aperture.

Excelent lens!

5b. Tamron SP LD (IF) 300mm f/2.8 (60B) (1984-1992):

Tamron-SP_300f2,8_DSCF2475_blog — **Fig. 18**: **Tamron SP LD (IF) 300mm f/2.8 (60B) – source – fotosaurier**

This is the shortest and lightest lens of the quintuple, which arrived even one year before the Minolta – containing two low-dispersion (LD) lenses – with manual focusing:

Tamron-SP_300f2,8_Spreadsheet — Fig. 19: Optical properties of **Tamron SP 300mm f/2.8 LD (IF) 60B – source: fotosaurier**

Tamron-SP_300f2,8_Graph — **Fig. 20**: Imatest resolution graphs of **Tamron SP 300mm f/2.8 LD (IF) 60B – source: fotosaurier**

Tamron – third party winner: Great Lens!

5c. ARSAT MC Yashma-4H (1990?):

Yashma_300f2,8_DSCF2478_blog — **Fig 21**: **ARSAT MC Yashma-4H – source: fotosaurier**

I do not know much about this lens. Funny about it is to me, that in most cases, when it is offered as a used lens, it is given the addendum „sovjet lens„! In 1990, when it was delivered first (I saw other sources with the date 2007 …) the Sovjet Union no longer existed – which means that, ARSAT being located in KIEW, the lens has UKRAINIAN roots.

As far as I know, it was generally produced in Nikon-mount.

ARSAT_Yashma_300f2,8_Spreadsheet — **Fig. 22**: Optical performance of **Arsat MC Yashma-4H 300mm f/2.8** – source: fotosaurier

ARSAT_Yashma_300f2,8_Graph — **Fig. 23**: Resolution graphs of **ARSAT MC Yashma 300mm f/2.8** – source: fotosaurier

Open aperture and stopped down the lens is convincing in the center – about 10-15% below the other superteles – but with still very good CA in the center.

From f/4.0 it is also very good in the large part of the frame – just 10% below the Topcor.

In the corners it is on par with the Topcor open aperture – but it does not improve so much while stopping down. For analog film use it was also a good lens – with exception of the softer corners with typical CA-values of non-apochromatic lenses … and a much higher distortion than all the other superteles.

What about apochromatic correction in supertele-lenses?

Lenses of 300mm f/2.8 need apochromatic correction to be really sharp. The chromatic aberrations („secondary spectrum“) are the major restictions in sharpnes for these long focal lengths all over the frame! All these lenses, tested in this report, have apochromatic correction – in varying degrees of perfection! In the ARSAT Yashma the apo-correction is only partly successful.

Herbert Börger

fotosaurier, Berlin 13.02.2023

Literature:

1- „Topcon Story – Topcon Enigma“ by Marco Antonetto and Claudio Russo, by Nassa Watch Gallery, Collectors Camera Publishing, CH 6907 Lugano, Switzerland – 1997

2- Web site „http://www.topgabacho.jp/Topconclub/FPslr1.htm“

This, the first super fast long telephoto lens produced for any camera system world wide, came to the market in 1957. This was a large and heavy lens, with a 130mm maximum diameter, a length of 412 mm and a weight of 3.3 kg. The optical design was one of 6 elements in 4 groups. The selling price, at the time, was 135,000 Yen making it the most expensive lens on the market. Special filters slide into a slot at the rear of the lens barrel and this lens was probably the first to use this method. Unlike the 135mm f2 R Topcor, this lens was listed in catalogues into the later half of the 1970s. Because of it’s large aperture it was chosen as the official lens of record for the Tokyo Olympics. An odd thing concerning this lens is that many of those remaining have been modified for the Nikon mount, while those with the original Topcon mount are very scarce. The early lens case was made of leather but later on Topcon began supplying a hard case with the TOPCON emblem promontory displayed. The R Topcor 300mm f2.8 lens still compares favorable, with regards to regards to sharpness and contrast, to modern lenses with fluorite elements. Today this lens is almost forgotten but was highly praised in former times.

3- Web site of Steven Gandy: „https://www.cameraquest.com/top30028.htm“

**Fig. 24**: Mathäuskirche in Hambühl, seen **from 1,2 km distance** with Topcor 300f2,8 (taken at f/5,6 with Sony A7R2 at iso800) – narrow vertical crop of nearly full frame, which you see here at about 65% enlargement – „**ooc“** – no post-treatment of the picture) – source: fotosaurier

Long Telephoto-Lenses and Temperature

Would you expect, that the optical performance of your photographic lenses can be seriously influenced by the operating temperature? Have you ever realized lack of sharpness in extreme environmental temperature conditions?

The simple answer is, of course, that within the specifications for use, given by the makers, there should be no such concern. But it is not that simple.

For amateur astronomers with their mostly very long telescope-focal-length optics (mirror or lens) this fact is very common:

before using the instrument in the clear and mostly cold winter-nights, you have to put the telescope early enough outside (shielded against due) to bring it into a thermal equilibrium with the ambient air at the time you start your observations. The reason: during essential temperature-changes of the optical components (mirrors, lenses) and their mounting devices, their surface-shapes and adjustment change and destroy the extremly precise optical alignment – until the thermal equilibrium is restored. The refractor-lenses may be mounted to allow for some thermal differences, but large mirrors have to be mounted and adjusted extremely precise, so that the cooling-down of the mount, that holds the mirror, may even generate mechanical tension on the mirror – and that generates optical distortions! So we should remind: the absolute temperatures are not the problem – but the thermal transition stages from warm to cold or opposite way!

This fact is also an important design aspect for telescopes: the preferred structure is „as open as possible“ to allow the air to circulate and to generate a good heat-exchange with the internal telescope structure to speed up this process. While the air gets colder during the night, the instrument’s optics can follow close enough to keep the temperature difference low.

There is an impressive document in the archives of the Mt. Wilson Observatory (near L.A., USA) describing the „first-light“-moment of the new 2,5 meter mirror telescope (Hooker-Telescope) on November 1, 1917 – use this link to the adventurous story! („First light“ is the moment, when somebody looks through the finished instrument for the first time.) Here the first-light moment at Mt. Wilson is described near the end of the long text in this link and shows, what a three hour cool-down time made to the optical properties of the 2.5 meter mirror, (which was made by George Willis Ritchey – and allowed for the detection of the expansion of the Universe by Edwin Hubble shortly after taking this telescope into service.).

Picture 1: 2,5 m (100 inch) Hooker-telescope on Mt. Wilson: just struts hold the mirrors to ease the circulation of air for for a fast achievement of temperature equilibrium – source: Ken Spencer, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Many instruments in astronomy are closed assemblies, using a corrector-plate (Schmidt-system) or meniscus-lens (Maksutov-System) in the entry of the tube and the mirror at the rear-end (catadioptric telescope – see also my specific blog-article here.) The big disadvantage of these closed systems is the „inertia“ in cooling down due to the closed volume in the telescope tube. Therefore often slits around correctors and mirrors are placed, which allow for sufficient circulation of air through the tube – and even active ventilation is used to shorten the period to reach equilibrium. In some big modern telescopes, the mirror may even be actively temperature-controlled.

Picture 2: „Closed“-tube optical system Maksutov-Cassegrain-Teleskop – source: Wikipedia – Author: Halfblue – http://creativecommons.org/licenses/by-sa/3.0/.

Long telephoto-lenses for normal photography can not be open systems, because the lens-barrels definitely have to be tightly sealed to avoid the invasion of dust, humidity or corrosive gases.

This means, that you have to plan and prepare carefully to bring your equipment to ambient temperatueres in time to avoid these thermal problems. For photographic equipment this would equally refer to the situation, when you come from climate-controlled environment (e.g. hotels) into wery hot (and humid) areas. There is an additional problem, that in bringing cold equipment into hot-humid environment, there might be condensation of humidity on the lenses/mirrors.

This problem is even more delicate with catadioptric lenses (mirror/lens-systems often called just „mirror-lenses“ – in German „Spiegel-Objektive“). In these the surface-shape of the mirrors and the adjustment from mirror to mirror is extremely sensitive for the optical performance of the lens-systems.

I have to-date not realized this with focal lengths of up to 350 mm (though it might be also there to a certain dergree) – but this is definitely an important aspect for focal lengths between 500 mm and 1,000 mm or longer.

From which focal length on these problems may occur, will mainly depend of the type of optical system – and of course the resolution of your cameras sensor!

Here I want to show you this effect with an example of a catadioptric lens of 800 mm focal length: the Vivitar Series 1 Solid Catadioptric 800mm f/11, used on the Sony A7Rm4 (60,3 MP, 35mm format – 3.77 µm pixel-pitch).

Picture 3: Vivitar Series 1 Solid Catadioptric 800mm f/11 – source: fotosaurier

It was the first day this year with just sligtly above zero outside temperature (+2 degree Celsius) and very clear air. At ca. 1:15 p.m.I set out the 800mm f/11 lens on the tripod on the balcony and tried to focus on my favorite landscape test target: a roof-top at about 40 m distance.

The advantage of this target is, that it has large AND fine details, low contrast AND high contrast areas and – most important – a sufficient depth, so that I can detect focusing errors very well!

Picture 4: Overview picture – complete field of view of the „roof-top“ landscape target in ca. 40 m distance taken with Sony A7Rm4 and Vivitar Series 1 Solid Cat 800mm f/11 – this is the „sharp“ picture after the cool-down period of the lens – source: fotosaurier

It was nearly impossible to meet the positive focus position – so I did the best guess and made the photo – and here is the 100%-crop around the focus-position, which is the first steel spring at the right side of the roof edge:

Picture 5: The 67% detail of the focus-area (clamp and spiral-spring!) made 15 minutes after setting the lens outside. Best guess of focus, however, you will find no sharper point in front or behind – the distance scale on the lens says 50 meters in this non-equilibrium temperature situation – source: fotosaurier

At this point of time the lens internally is still on room temperature of about 21 degrees … starting to cool down for about 15 minutes, which it took me to set everything up and focus carefully – but desperately, becaus no really sharp focus was seen in high viewing-magnification.

I had focused using the maximum viewfinder enlagement in the Sony camera and was sure: this is not a really sharp picture. But I could not find a better focus. Picture 5 is a 67% crop of the image taken. And as the subject has some depth: no – there is no better focus to be seen on this picture in front or behind the plane of the spring.

I left the lens with camera in this position for three hours and refocused the lens: now I experienced a quite snappy focus – and you can see the same crop-area here:

Picture 6: The 67% detail of the focus-area (refocused!) after additional 3 hours of the lens outside – source: fotosaurier

The gain in sharpness is damatical – and it exists over the whole field of view, not only in the plane of focus! Also out-of-focus areas show higher contrast now.

However, it connot be ignored, that this catadioptric lens in this picture does by far not use the potential 3,168 Line-Pairs per Picture Height Nyquist frequency of the cameras sensor. My estimate is, that we have here an MTF30 of about 1,100-1,200 LP/PH. So either the three hours of cool-down time were not yet sufficient – or the lens may be not better than this.

(The 1,200 LP/PH MTF30-resolution would correspond to 100 Lines/mm in older „analog“ data. Very good CATs in the 1970s had center-resolutions (measured on film) between 50 and 60 Lines/mm. This relation makes sense, as the difference (factor 0.6 lower for film!) may be owed to the effect of grain and the thickness of the emulsion.)

The „Solid Cat“ 800mm f/11 is a massiv piece of optics – the lens barrel is nearly completely filled with glass, as you see in the lens-scheme:

Picture 7: Lens-scheme of the Vivitar Series1 Solid Cat – source: Perkin Elmer Patent application

It is an absolutly unusual mass of glass – so I would not exclude, that the cooling time should even be longer to reach the thermal equilibrium. My plan is, to make a sequence of photos taken in shorter intervals and over a longer time – as soon as the outside temperatures go down again.

I am not so happy with the fact, that I had to use landscape-scene-shots to demonstrate the performance of the lens, however, for 800mm focal length my IMATEST testing-arena is too short. Maybe I will make a parallel IMATEST-trial then with a 500mm CAT.

So, please, consider this as a first teaser for the topic which has shown clearly, that photographic lens performance may seriously suffer during the time, a lens is undergoing strong temperature-change and before equilibrium is reached.

I promise to come back with a more elaborate research-plan soon.

Herbert Börger

Berlin, December 4th, 2020

Aphorism of the day: Scientific research is most successfull, when it brings up more new questions than it has answered. (fotosaurier)