Mirror lens diameter

[iangreenhalgh1]

Hi folks

I have a project that requires a large diameter helicoid. So I thought a cheap mirror lens would be the place to find such a thing.

So, what I need to know is - what is the approx diameter of the helicoid on the typical cheap 500mm mirror lens?

[dan_]

What diameter you need it to be, Ian?
You may look at Bronica or Pentax 6X7 helicoids, too. Or at old Russian or German 300mm/4 or bigger broken lenses, as well.
Finding a very large diameter helicoid may be not so easy.
I needed a greater than 80mm (inner diameter) helicoid for my TT&H 12 Inch/4 aerial lens and I could only find a proper one after ~3 years of monitoring ebay.

[fuzzywuzzy]

[calvin83]

Samyang 500mm f/6.3 if you can find one cheap. It takes 95 mm filter.

[iangreenhalgh1]

Thanks guys.

I need one that is 77mm diameter or bigger.

4-5mm of extension sounds like much too little.

[dan_]

Before finding the big helicoid I've used an adapted Pentacon 6 macro bellows for focusing the big lens. It can be adapted for larger threads, it's pretty solid and it's not very big. It is relatively cheap, too. You can use one as a good temporary solution till you find a proper helicoid.
Pentacon 300/4 has the smallest outer diameter = 82mm (but I don't know its inner diameter...).

[calvin83]

If the the weight of the optical block is not too heavy and the distance of the rear element to the sensor is not very limited, you may consider adding a hood to the front of a lens/helicoid to make more room to hold the optical block...

[iangreenhalgh1]

Good suggestions, thanks. I've employed a set of P6 bellows to adapt my mamiya rz67 lenses to my a850.

The optics are quite heavy, I'm using the front cemented doublet from a broken Fujinon TV broadcast grade lens which is about 75mm in diameter and a Fujinon broadcast grade 0.8x converter which has 82mm threads. I want to vary the distance between them, hence the helicoid.