figuring f-stop with step down rings

[Rigel003]

I'm interested in trying to stop down the 77m aperture of my Canon 70 - 200 f2.4 using step down rings rather than the internal iris diaphragm. Like these (http://www.ebay.com.au/itm/77-72-67-62-58-55-52-49-mm-Step-Down-Rings-SET-7pcs-/220429429682?hash=item33529e2bb2:g: KWoAAOSw7ThUgWCV). Trying to avoid having bright stars show the diffraction pattern caused by the iris.

Is there an easy way of working out what diameter ring you'd need the aperture to be to create the various f-stops? For example what diameter ring would create the equivalent of f/4 if I'm using the lens at 70mm or at 200mm? You'd think it would be just a simple matter of division of aperture by focal length, however 200 divided by 2.8 is not 77.

[RickS]

The f-ratio is focal length divided by aperture, so it's pretty easy to calculate. For f/4 at 70mm focal length you'd want a 17.5mm aperture. At 200mm focal length the aperture would be 50mm to give you f/4.

Cheers,
Rick.

[Ryderscope]

I've puzzled over this question myself at times.

With my Canon 200mm F2.8 lens, the filter diameter at the front of the lens is 72mm. At F2.8 the lens will be wide open. Calculating this = 200/F2.8=71.43 so that's pretty darn close to 72mm. This being the case, calculating the size of the aperture mask to place on the front of the lens to achieve a particular f stop is straight forward.

However, for some lenses it doesn't seem to be as straight forward as that. With my Canon EOS 100mm Macro F2.8 lens the filter size on the front of the lens is 58mm diameter.Calculating the required aperture withe lens wide open gives 100/2.8 = 35.7mm which is less than the diameter of the front of the lens.

I have the same issue with my 50mm f1.2 lens which at 50/1.2 gives me 41.6mm which again is less than the diameter of the front of the lens (58mm again).

On this basis it seems that the diameter of the aperture at the front of the lens must be less that I would have normally calculated.

rw

[pdalek]

You will find the required size for shorter lens focal length settings will be smaller than available rings.
Use a gel filter holder with squares of metal, platic, cardboard with holes.
The hole size is (focal length)/(required F#)

With a zoom lens you need several apertures to cover the full zoom range in convenient steps.

Alternatively a high quality adjustable iris could be used. These are expensive.

A field stop of this type will give greater vignetting and aberrations than a correctly placed stop.

Note that the size of the front element in a camera lens is always larger than the F# would imply. This is to give a wide angle of view with reduced vignetting.

[Rigel003]

Thanks for the helpful responses. I guess the bottom line is that the actual diameter of the filter thread on the outside lens is in fact always larger than the real aperture (diameter) of the lens and this explains why the figures don't work out as expected. Yes Patrick. Good Point. I'm clearly not going to be able to use step down rings to get the 70 mm down to f4.