# Thread: Single lens optics & aperture placement

1. ## Single lens optics & aperture placement

Some time ago i made this basic magnifying glass lens, i installed a aperture in front of the lens, it's about 27mm from the lens element. Does this affect the way i should calulate F stops? The lens is 195mm in focal length, largest opening is 60mm. So 195 / 60 would be something like f3.2? But it started wondering if the gap between the iris & the element would screw this calculation up, as the lens is i bit larger than the largest iris opening..

2. ## Re: Single lens optics & aperture placement

I made one that fits my Sinar. I used the mechanical shutter behind the lens. The math matched the tests I shot. I would think that as long as the shutter doesn't vignette, time is time.

3. ## Re: Single lens optics & aperture placement

..I installed a aperture in front of the lens, it's about 27mm from the lens element. Does this affect the way i should calulate F stops?

In fact (see below) when the aperture is located in front of the single lens element, this probably does not affect the calculation of the f-number.

The lens is 195mm in focal length, largest opening is 60mm. So 195 / 60 would be something like f3.2?
Yes, exactly.
But when the iris is located elsewhere, you should first check from the film plane that the iris actually limits the effective aperture and not the lens mount. Most probably, this is the case, if the aperture is smaller in diameter than the lens mount diameter (60 mm) and if it is located less than 30 mm in front of the lens. But look from the corners of the image, you'll see that the iris will be obstructed by the lens mount when the iris is not located just against the glass, and the actual aperture combining both takes the shape of a cat's eye.

If the additional iris actually limits the lens aperture, when it is located in front, the actual diameter of the iris is the right one for computing the f-number.
If the iris is located behind the lens, the effective diameter to be taken into account is the diameter of the image of the iris seen from the front, through the lens. A positive lens often acts as a magnifying lens, hence the effective diameter will be bigger in this case.
Technically speaking, the effective diameter to be taken into account is named : diameter of the entrance pupil, the entrance pupil being the image of the iris as seen from the front, through the lens.
If the iris is located in front, its image is... the iris itself, hence the effective diameter is the iris diameter, cannot be simpler !!.
And the position of the iris in front does not change the f-number.
Hence if your lens has a focal length of about 200 mm, placing an aperture of 20mm will yield a f-number of f/10.
Small apertures are often required in order to get an image with acceptable sharpness.

You should place your iris as close as possible from the lens in order to avoid some vignetting ine the image : make tests and see what happens, when the iris is located too far in front, you'll loose many rays that will be obstructed by the lens mount, in the edges of the image.

But the proper placement of the iris has a very strong influence on the aberrations.
For example If your lens element is biconvex and symmetric, by placing the iris close to the lens you'll minimize distorsion. But other aberrations will remain.
One design that works fine with a single lens element is the Wollaston meniscus, the lens is a single convex-concave meniscus and strangely enough you should place the concave side in front and the iris at a certain distance in front. This is absolutely counter-intuitive !!
This design minimizes some aberrations and was known... to William Hyde Wollaston himself in the XVIII-st century....

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