I would like to add an iris to a standard 5 element lens.
My intuition says that adding this close to the front or backside will not only reduce light but also the image size.
Is this correct?

Does someone has a link explaining the effects or the right way to do this?

Rgds,
Frank

Your intuition is correct.  Likewise, your intuition should tell you that you ideally need to place the iris at a point where all points of the image share the entire aperture, e.g., someplace like the entrance aperture or any point in the train where the entrance aperture is re-imaged.  

TTFN

If you put the aperture in the wrong place, you will have vignetting. Vignetting means that the aperture blocks more light from one part of the image than another.

There is a single position somewhere in the lens called the "stop." The stop is the limiting aperture for all points in the image. You want to put the aperture at that position. If  your lens was sold without an aperture, it is likely that the stop is located at one of the faces of an optical element. If your lens came with an aperture and it was since removed and lost, the stop location could be anywhere (even in front of or behind the lens).

I don't know anything about your lens system (I also don't know if you are IR or Visible). If you can tinker with the innards without hurting anything, here is how to find the stop:

1. Place your lens in an imaging configuration which mimics the real-world usage. Your object should be a diffuse, brightly-illuminated surface. Place a piece of white cardboard or paper at the image. If your field-of-view (FOV) is large, shield the image plane from light passing around the lens. Turn out the room lights.

1. For each element in the lens, slide a thin blocker (covered in lens tissue, hopefully) across the element starting from the top. Do this on both the front and back of the element.

2. In most cases, you will notice that a blurry edge creeps across the image as you move your blocker across the element.

3. At one position, the image will dim uniformly as you move the blocker across. There will be no blurry edge in your image. This position is the stop. Put your aperture as close to this position as possible.

Note: despite your intuition, the stop can be located at the back of the last element in the lens. This is not so likely for visible lenses, but is often true for IR lenses.

CEV