{"id":26343,"date":"2024-06-29T22:30:20","date_gmt":"2024-06-29T22:30:20","guid":{"rendered":"https:\/\/garfnet.org.uk\/cms\/?p=26343"},"modified":"2026-01-20T18:26:40","modified_gmt":"2026-01-20T18:26:40","slug":"testing-my-cat","status":"publish","type":"post","link":"https:\/\/garfnet.org.uk\/cms\/2024\/06\/29\/testing-my-cat\/","title":{"rendered":"Testing my cat"},"content":{"rendered":"

By which I mean testing my Tamron 500mm Catadioptric lens. It was a nice warm reasonably clear day today, peaking 24\u00b0C. So about an hour before sunset I took my Tamron, fitted on the front of my \u00b54\/3 Lumix DMC-GX7 up to my favourite vantage point on Portsdown Hill, Hampshire. This is a wonderful place to take pictures because of its commanding view over Portsmouth, Gosport and the Isle of Wight. <\/strong><\/p>\n

I always loved Portsdown Hill, ever since my dad took me up there when I was just six years old, in his old red VW Beetle, Beethoven’s 6th playing on the Pye Cambridge AM car radiio. I still remember how it felt like we’d just driven to the top of the world. But I digress…<\/p>\n

ngg_shortcode_0_placeholderThese images are not quite SooC (straight out of camera). It was quite a hazy evening. So I stretched the contrast a bit to compensate for this. However all the other anomalies and defects are left as-is<\/em>.<\/p>\n

Using a Tamron 500mm Catadioptric<\/h2>\n

I know catadioptrics have issues and are certainly not everybody’s cup of tea<\/em>. One of the problems with most cats is that the aperture is fixed. In the case of this lens it’s f\/8. The hyperfocal<\/a> distance of a 500mm at F\/8 on a \u00b54\/3 sensor (17.3mm x 13mm) is around 2.2 km – about a mile and a third in “old money”. But it’s slightly more complex in this instance because Adaptall type lenses often focus slightly “beyond infinity<\/em>“. I’m guessing this was to allow for tolerances in adaptor design, to make sure that the lenses would actually focus to infinity regardless of the camera upon which they were mounted.<\/p>\n

However, this overshoot makes focussing be a tad tricky. You have to twist the lens to infinity position, then turn it back very slightly, Fortunately my \u00b54\/3 cameras have quite good focus highlighting. This seems to work moderately well with this lens, even if the squiggly lines are a little hard to see, at times. Thus making it possible to obtain a reasonably sharp focus even on quite a hazy day.<\/p>\n

Anyone for doughnuts?<\/h2>\n
\"Doughnuts<\/a>
Doughnuts – our stairwell safety LEDs severely defocussed. Unedited.<\/figcaption><\/figure>\n

Main problem with catadioptric lenses is that the front of the lens effectively has a massive hole in the middle. When one is focussed on an object, this is not an issue. However, strange artefacts can appear when the lens is defocussed. It often forms a sort of doughnut-shaped bokeh. You will notice that some areas in the images that are out of focus also have doughnut-shaped bokeh. I think there are some small specks of dust on my sensor, and they seem to have become doughnuts too.<\/p>\n

Despite their shortcomings, I love catadioptric lenses. They look cool and really appeal to my “inner six-year-old<\/em>“. The biggest advantage of catadioptric lens is that for a given focal length, it is significantly smaller and lighter than a conventional lens. This particular lens weighs just 646 grams. For a 500mm, that is incredibly lightweight. Fitted to one of my little Lumix \u00b54\/3, that’s a combined mass of just 1.15 kg. For a camera setup that can see things 20 km away, is not too bad really.<\/p>\n

Regular readers will also know that I have something of a penchant for vintage Tamron MF lenses. I have collected quite a few of them over the years. I reckon this was made probably early mid 1980’s and has become one of my favourites.<\/p>\n

\"Tamron<\/a>
Tamron 500mm f\/8 catadioptric lens fitted on Lumix GX80.<\/figcaption><\/figure>\n

Viewpoint<\/h2>\n

I was up on Portsdown Hill, Hampshire, England, facing mostly south, between bearings of approximately 135\u00b0 and 225\u00b0, at an elevation of roughly 100 metres. My approximate position is marked with a blue triangular flag in the OpenStreetMap screen grab below. Note, top of map is north and most of Portsmouth (to the south) is just a few metres above sea level.<\/p>\n

\"Map<\/a>
Map showing roughly my viewpoint and some of the places I could see using this 500mm catadioptric lens.<\/figcaption><\/figure>\n
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