Talk:Diaphragm (optics)

Diaphragm versus Aperture

This article, translated from the Dutch original, was very confused about what a diaphragm is. The diaphragm is the thin opaque stop, not the hole in it. Most of what was said about the diaphragm was more appropriate to the aperture. So I tried to fix it. Dicklyon 03:45, 27 April 2006 (UTC)

Some more diaphragm confusion...

I've recently been expanding the diaphragm disambiguation page, and I wanted to ask here to see if anyone can help clear up some confusion about the optical meaning of diaphragm.

Firstly, we have:

• diaphragm (optics)
• iris diaphragm (redirect to above)
• diaphragm shutter
• shutter (photography)

1) Should diaphragm shutter be merged or redirected to diaphragm (optics) and/or shutter (photography), or should they all be merged/redirected to a single article?

merge it with shutter. It doesn't fit here so well. Dicklyon

2) There has been some discussion of the etymology of diaphragm here (may need to search archives). The OED gives a date for the telescope term as 1829. Does anyone know whether this telescope diaphragm is (as I suspect) the same as a camera diaphragm.

Removed Image

Hi all,

As per the "leanup" request tag I have removed 1 image (file code below), as I see no real reason to have two "Iris" images in the body. However I see no point to remove any more as there is not so much "cluttering" of the page and even find it hard to understand why it even had the tag in the first place.

Anyhow, kindest regards, Mr. NiceGuy

Six-blade iris diaphragm

(MrNiceGuy1113 (talk) 08:55, 7 October 2012 (UTC))

Yes, certainly the same. Google books only finds one book with diaphragm and optics before 1829; see below.

3) Why is this equipment called a diaphragm? I suspect (but don't have any sources for this) that the meaning came from the iris of the eye, which in all probability was the inspiration for the iris diaphragm - though quite why the iris is considered a diaphragm I'm not entirely clear about.

Yes and no; in optics, the diaphragm pre-dates photography, and in photography the diaphragm pre-dates the iris diaphragm; but in 1826 (An Experimental Treatise on Optics: Comprehending the Leading Principles of the Science, and ...), Jean-Baptiste Biot and John Farrar describe the diaphragm in the "organ of vision" thus: "The diaphragm, when seen from without, exhibits the appearance of a colored ring, of a variable tint, even in man, and called the iris. The circular aperture through its center, by which rays enter into the interior of the eye, is called the pupil." A diaphragm is a thin layer of material for separating regions; in optics, it separates by blocking light, but has a hole (aperture) in it to admit a controlled amount of light. Dicklyon 04:43, 19 May 2006 (UTC)

Anyway - anything about the etymology would be great, but what I am mainly after is clarification of the optical use of the term: are telescope and camera diaphragms the same thing, and do they act like the iris of an eye? Carcharoth 23:03, 18 May 2006 (UTC)

yes and yes. But the diaphragm shutter is obscure and only marginally related. I'm going to revert that merge tag. Dicklyon 04:43, 19 May 2006 (UTC)

As to the OED's 1829 funny use of diaphragm in a telescope, I guess I don't understand it; but google books finds an 1828 diaphragm in a telescope eyepiece, which is certainly the optics definition: "We took particular care, for example, always to adjust the diaphragm which is placed in the focus of the eye-piece of the telescope, so that its edges..." and since the title is "Extracts from a Journal: Written on the Coasts of Chili, Peru, and Mexico, in the Years 1820", I'd presume the term was being used optically in Europe before 1820. When google gets more books scanned, maybe we'll learn more... Dicklyon 06:12, 19 May 2006 (UTC)

I was reading your Glossographia Anglicana Nova quote at Aperture#History, and I think something there might explain the OED's "assemblage of lines of reference in a telescope" quote concerning diaphragm. Specifically, "Aperture, in Geometry, is the Inclination of Lines which meet in a Point." Or maybe not. Just a thought. Carcharoth 22:03, 19 May 2006 (UTC)

I read some of the google books hits on diaphragm and telescope before 1830, and it seems that what they're talking about is a diaphragm at a focal plane that carries some cross hairs or calibration markers of some sort. An "assemblage of lines of reference" probably comes from those measurement reticles. Dicklyon 00:31, 20 May 2006 (UTC)

Shutter or diaphragm?

Thanks for the etymological comments above. Putting that aside for a moment, I want to clear up some confusion. I'm still not clear on whether a shutter (photography) is different from a diaphragm (optics). Is it the case that some sorts of shutters can be called diaphragms, but also that something can be an optical diaphragm without being a shutter (eg. the iris of the eye)? I'd draw a Venn diagram to show what I'm trying to say, but I don't know how to here. Carcharoth 10:52, 19 May 2006 (UTC)

Yes, an iris is a diaphragm but not a shutter, and many shutters are not diaphragm. As a long time photographer and professional photography technologist, I had never heard of a diaphragm shutter. So the overlap in your Venn diagram would be very small, in my opinion. Dicklyon 14:08, 19 May 2006 (UTC)

Thanks for that. Just a few more points and questions.

Fantastic. I never turn down a chance to pontification. Can't do that on the article page.

Is an iris a diaphragm in a different sense to the way a diaphragm shutter is a diaphragm? Maybe another way to ask this is to ask whether an iris diaphragm is a diaphragm or a shutter? Also, would it be correct to say that some (optical) diaphragms can be adjusted to change the size of the aperture (the classic example being the iris (diaphragm) controlling the size of the pupil (aperture)). And would it also be correct to say that some diaphragms have a fixed aperture size and are just swapped around to get different aperture sizes?

An iris diaphragm is a diaphragm, and not a shutter; similarly, the iris of the eye is a diaphragm, and is the aperture stop of that organ. Yes, in the old days, before the iris diaphragm was invented, diaphragms had fixed-size apertures. In many devices, that's still true, that there will be one or a few fixed apertures. Look up a Waterhouse stop -- probably we should write about that here.

Oh, I'll leave that to those who know their stuff! I work in what could be called a support role in the photographic industry. It doesn't involve actual photography, but I deal with digital photos every day. But I've never really tried to find out more than the basics about how cameras work.

Going back to shutters, would it be correct to say that, like optical diaphragms, these are another way of controlling the amount of light that passes through the lens? But that they usually act at the point of taking the photo, as opposed to a fixed-aperture diaphragm where the aperture remains the same size as the shutter opens and closes?

Sort of. They control the duration of exposure as opposed to the brightness of exposure. In most SLR cameras, the iris diaphragm actually stays wide open until the point of taking the picture, then stops down to the preset aperture before the shutter opens.

So in a camera the diaphragm (helps) controls the brightness of exposure? That would be a helpful thing to add to the article.

So, if I have this right, an optical diaphragm is the thing that has the aperture at its centre, and (even if adjustable) does not move while the photo is being taken. While the shutter is a separate device that opens and closes to take the photo. The overlap in terminology coming when an optical diaphragm may be adjustable (like the iris of an eye), in the same way that a diaphragm shutter opens and closes, also like the iris of an eye. The difference being that an adjustable optical diaphragm would be adjusted to change the aperture before taking the picture, but a shutter diaphragm would open and close (also changing the aperture) during the process of taking the photo, but there would be a static diaphragm (with aperture) behind the diaphragm shutter?

Except I'm not convinced that any of us know what a diaphragm shutter is. In the only reference I can find, it's a stationary slit in the focal plane of a high-speed moving-film--moving-lens camera. You can't make a shutter out of an iris diaphragm, because it can't close all the way.

I found something here. That page is part of a sequence starting here. It seems that diaphragm shutter is synonymous with leaf shutter.

So the final point might be - is it possible for a diaphragm shutter to combine the role of a shutter and a diaphragm in a single object?

There may be single assemblies that do this combination, but I don't think the term diaphragm shutter applies to them.

Incidentially, part of my confusion may have been caused by the first few sentences at aperture, and the fact that aperture stop redirects to aperture, and the terminology there seems a little bit confused as well. Would you be able to clarify that as well? Sorry to go on at such length, but the articles on Wikipedia have really confused me and I would like to see them improved. The best way would probably be to get some clear diagrams of the equipment being described, and how they work. Carcharoth 15:40, 19 May 2006 (UTC)

Yes, that one is still confusing, and in my opinion not correct, in spite of my efforts. I'm about half way through trying to unconfuse diaphragms, apertures, stops, f-stop, f-numbers, etc. You should have seen it before... Dicklyon 18:44, 19 May 2006 (UTC)

Good luck! I'll try and read some of the pages and kick up a fuss if I can't understand anything! :-) Carcharoth 22:18, 19 May 2006 (UTC)

Analogy with the eye

Given that the iris is the eye's diaphragm, and that the iris reduces the size of the pupil (the eye's aperture) in bright light, and does the reverse in dim light, would it be correct and useful to make the analogy that the iris helps control the brightness of exposure of the retina in the same way that a diaphragm does in a camera for the photographic film? Carcharoth 10:44, 21 May 2006 (UTC)

It's not just an analogy; it's the truth. Are you saying that needs to be made explicit some place? Dicklyon 22:02, 21 May 2006 (UTC)

Yes. I think I've found a few places to put this, but maybe it should only go in one place? Carcharoth 14:48, 23 May 2006 (UTC)

It always says in the second paragraph: "A natural optical system that has a diaphragm and an aperture is the human eye. The iris is the diaphragm, and the opening in the iris of the eye (the pupil) is the aperture." If you think it needs to say more about exposure control, one place should be plenty. Dicklyon 23:21, 23 May 2006 (UTC)