Iris Diaphragm & Light Simulations

If you have no clue as to what the iris diaphragm does, you will on average get the best image if you keep it open, i.e. in a position which gives you the brightest image. At least you will fare better than you would if you ignored it completely.
Now - the iris diaphragm is there for a reason. Knowing what it does will allow you to see some things that would be difficult to see without it.

Iris Simulation - Compact Bone
This section of ground, unstained compact bone is one of the sections where the iris diaphragm is quite useful. The tissue is unstained. What you see are "natural" colours and structural features which become visible because different parts of the bone scatter or diffract light to different degrees. If the iris diaphragm is gradually closed, the relative amount of scattered and diffracted light increases in proportion to light that just passes straight through the section. What we see in this section depends on precisely this type of light to begin with and - no surprise - if we gradually close the iris diaphragm it becomes easier to see the histological features of this preparation. The total amount of light reaching the eye decreases as we close the iris diaphragm. You will have to adjust light levels to take full advantage of the function of the iris diaphragm.
Iris Simulation - Skeletal Muscle
This H&E stained human skeletal muscle sections illustrates both the advantage of using the iris diaphragm and its disadvantages. You should be able to see the striations of the muscle with the iris diaphragm fully opened. If you look closely, you can even see the Z-lines, A-bands and I-bands. If you close the iris diaphragm it becomes more obvious that the muscle is striated but the detail of the contractile apparatus is gradually obscured. If you look at other parts of the image while closing the iris diaphragm, you will find that also other features become either visible or obscured. Neither the opened nor the closed iris diaphragm reveal all that there is to see in this section, and you should use different diaphragm settings when you look at this tissue.