Front Mounting Barrel Lens ahead of Aperture

[Kevin Crisp]

For you optical types...what are the drawbacks to front mounting a lens in front of the aperture blades? In other words, the lens is front mounted on a shutter, the existing barrel has no aperture control, and the intent is to use the shutter's diaphram for stopping it down. The entire complete lens, front and rear elements are stuck out front of the diaphram still in the barrel mount. How will this affect lens performance versus having the aperture blades in between front and rear elements? I've looked at the Grimes explanation of this, but their example is using the diaphram blades of the front mounted lens, with the shutter behind, which isn't exactly what I have in mind. Other than possible loss of image circle and maximum aperture, what are the draw backs of this? Thanks, as always.

[Dan Fromm]

I'm not an optical type, but I have and use a heap of front-mounted lenses.

Short answer is, a stop behind the lens vignettes, i.e., cuts off the edges of the image, but doesn't change the lens' working aperture. So you can't accomplish what you want.

I hope you haven't bought the diaphragm-less lens yet.

To answer the obvious next question, yes I have a 200/4 MicroNikkor AIS and yes its diaphragm is behind the rear element and yes it does the expected. But that lens' nodes are reversed, and the diaphragm is between them.

Cheers,

Dan

[David du Busc]

The only drawback is that it doesn't work. The aperture needs to between the lens elements. If mounted behind the lens, your rig becomes a fixed aperture [wide open] lens with a vignetting device and then a shutter. I'm sure someone can offer a more technical opinion>>> but I don't think you can't get there form here... or is it you can't get there from there?

[Kevin Crisp]

I am not understanding these responses. A single element protar works quite well with the entire lens in front of the aperture, probably somewhat better with the aperture in front of the lens. It does not become a fixed aperture lens. I would be able to adjust apertures by closing down the blades behind the front mounted shutter. I don't understand how stopping down the aperture behind the lens would NOT change the lens' working aperture, as suggested above. (Wouldn't this be how the human eye works?) I guess the question is whether the aperture has to be in the middle of the lens -- does moving its position behind somehow degrade the lens performance, and if so, why?

[Michael S. Briggs]

The position of the aperture is a critical design parameter of a lens. Most lenses are designed with the aperture more or less in the middle. Moving the aperture to behind the lens will not result in the same performance -- a different set of rays will go through the lens. If the optimum set of rays for minimum abberations for a given f-number were obtained by placing the aperture behind the lens, the lens designer probably would have done so.

Using an small hole behind the lens will reduce the light passing through the system and thus will reduce the f-number of the system. Whether this aperture will result in good performance is another issue.

There are a few lenses designed to have the aperture behind or in front of all of the optical elements. Convertible lenses are used in this configuration. Typically the performance of a converted lens is less than that of the full lens, but this is probably mostly because the absence of the other half of the lens means that some abberations aren't being corrected.

Front mounting of lenses is usually done as an inexpensive way of adding a shutter to the lens. The shutter behind the lens is thought of as either being fully opened or closed -- it either allows all light rays through, or none. The aperture originally provided with the lens, which is almost always in the middle of the lens, will be defining which rays pass through the system. For this to work without additional rays being blocked and creating vignetting, the shutter opening must be large enough.

[N Dhananjay]

Think about it this way. Light from some point on the subject strikes all parts on the surface of the lens. The lens bends this light to form a cone of light starting from the front of the lens and continuing into the lens. All the light rays making up this cone come to a point at the centre, cross and continue on to form another cone of light on the other side of the centre. This is the reason the aperture is typically located at the centre of the lens. When the aperture is located at the centre, it basically adjusts the light gathering power of the lens. As you close it down, light from the periphery of the lens is cut out. However, information is not lost - light from any part of the scene hits every point on the surface of the lens. So if you throw away information from the edges, that information is still there since light from that part of the subject also hit other parts of the glass. If the aperture is located behind the lens, as you close the aperture, what you are doing is actually cutting into the cone of light i.e., adjusting the light gathering power as well as vignetting. This is a loss of information i.e., light from one part of the subject has been prevented from reaching the ground glass/film. Whether the vignetting is bad enough depends. There is a page on SK Grimes site that has a sketch of the vignetting that could occur with a front mounted lens. Having said all this, I use a front mounting system wherein all my barrel lenses are front mounted on a large Betax 5 shutter. However, each lens has its own aperture. The location of the stop is important in correcting certain aberrations. Cheers, DJ

[John Banyai Jr.]

In most lenses the stop is used to correct defects. Single element achromat lenses had the stop (obviously) behind the lens. The serious errors of such a lens were eliminated to a large extent by working at a small stop, typically f14. The speed was then increased by using multi element designs. The rapid rectilinear was a symmetrical doublet with the stop centered between the elements, each element cancelling the distortion in the other. The symmetrical doublet eliminated pincushion and barrel distortion,coma and lateral color shift. Astigmatism was still present. This problem was overcome in the 1880s with special glasses. Dr. E. von Hough designed a symmetrical cemented triplet in 1893 called the Goerz Dagor, aperture 7.7 later increased to f6.8. The Ziess protar is an extension of the same principle. It uses a quadruple cemented configuration. A single protar was said to give the highest possible correction with a single lens. Protars were made to work at f6.3 when complete,or at f12.5 when the symmetrical components were separated and used singly. Convertible lenses are usually of these types and are so highly corrected that they can be suitably used as individual components with the stop either before or after the lens. In any case these lenses (I own examples of both) produce excellent images at full aperture...wide open. Naturally you lose control of depth of field if you cannot stop down but I don't see any other problem occuring.What kind of barrel lens are we talking about here? I have a large Ross portrait lens that doesn't have an adjustable diaphram, it used separate inserts with different sized holes to vary the f-stop. This is a petzeval type of portrait lens and the stop is closer to the front element than the rear. This lens has a lot of spherical aberration(nice effect for portraiture) when used wide open but beyond f11 the image becomes quite sharp. In this lens the position of the stop does correct the defects inherent in the design. However, once again the lens can be used wide open with the resulting shallow depth of field and spherical abberation. A stop behind this lens would just reduce the amount of light reaching the film plane.

[Arne Croell]

In optics one makes a distinction between the "aperture stop" (which is what you want) and the "field stop" which limits (vignettes) the image area. The latter is in the image or the object plane - a film gate of a camera is a field stop. An explanation is found here:

<a href="http://electron9.phys.utk.edu/optics421/modules/m3/Stops.htm"> http://electron9.phys.utk.edu/optics421/modules/m3/Stops.htm <a>

Both Dan and David pointed out that by having the stop behind the lens you go from an aperture stop to a field stop - how much depends on the distance from the lens. The aperture stop is between the nodal points of the lens, so it can be outside the physical lens depending on the construction. In the case of convertible single elements the physical stop is close enough to them to act as an aperture stop.

Again, a pure field stop would be in the image or the object plane, so your setup would be somewhere in between aperture and field stop with some vignetting and some stopping down effect combined.

[onnect17]

Link updated:
http://electron6.phys.utk.edu/optics421/modules/m3/Stops.htm

[Mark Sawyer]

For you optical types...what are the drawbacks to front mounting a lens in front of the aperture blades?

For a simple lens design, like a Landscape lens, more curved field and increased coma. Whatever lens you're using may have enough corrections designed in to compensate, at least to a degree.