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Lens Mount Adapters FAQ

Stopped Down Metering

Lens diaphragms enable us to control the amount of light getting through our lens during the moment of exposure. A variety of mechanisms have been used over time, some of which we will explore below. Knowing about these options may help you pick lens options which will work with your camera in various modes. You can also save some serious amounts of money using preset telephotos and other tips described below.

Waterhouse Stops

The original lens diaphragms were a series of metal discs, with holes of varying sizes drilled in their centers. Named after their inventor, Waterhouse stops could simply be inserted into a slot in the lens to serve as a lens diaphragm.

Believe it or not, but I have a third party accura 12mm fisheye which uses a variation on this Waterhouse stop idea. Instead of a variable lens diaphragm, this lens has an off-center wheel in which a series of holes or stops were drilled. With these fisheyes, depth of field is so huge that there isn't even a focusing ring! I just leave it at the smallest and sharpest stop. The money saved by this trick went into the rugged all metal design and optics.

Manual Diaphragm Lenses

The next step in the evolution of lenses was the manual lens diaphragm. By rotating an aperture ring, a series of metal blades formed a nearly circular opening whose size could be varied. The higher quality aperture rings had more than a dozen thin metal blades to provide a nearly circular shape. Many modern lenses economize by using only a half-dozen or so lens blades. These differences known as bokeh can be most readily seen in the out-of-focus highlights in the film or print.

While manual aperture rings let you set the desired size of the lens openings, there are some problems with them. Being manual, they are slow to operate. Manual lens diaphragms are simple and cheap to build, so you may find them in some older or economy model lenses.

You may be surprised to also find them in some of the most expensive long telephoto lenses too. In this case, the tripod mount lenses are not really setup for grab shots, but rather for deliberate and slow operations. The cost and difficulty of extending automatic diaphragm coupling in longer lenses can be a limiting factor too. The simpler manual mechanism is more reliable too. Similarly, many bellows setups use manual lens operation.

The most common time you will see manual lenses on 35mm cameras is with certain adaptable lens mounts (see Lens Mount FAQ at this site). Many mount adapters permit choice of either automatic or manual operation modes (e.g., some YS mount adapters). This older lens mount adapter design was needed because some early 35mm cameras used manual diaphragm lenses (e.g., pentax screw thread). You had to manually close down the lens to the desired aperture for metering and photography. So these older mount adapters had to offer the option of both manual and automatic operation. More recent mount adapters (e.g., Tamron adaptall-2) are solely automatic in operation, and have dropped this option of choosing manual or automatic operation.

Preset 13.5cm f/4 Bellows Nikkor
Note Dual Aperture and Pre-set Rings!

Pre-set Diaphragms

Pre-set diaphragms are a variant of manual diaphragms. Manual lens users often found they didn't stop down correctly to the desired shooting aperture, but under or over-exposed the film in error. Just nudging some manual aperture rings could change their settings.

The solution was a pre-set control ring, in which the photographer moved the ring to pre-select the desired shooting aperture. Now he or she only had to twist the aperture ring, and it would stop at the pre-set setting. This trick made using manual lenses easier and less error prone.

With a pre-set lens, you can compose, focus, and shoot hand-held simply by twisting the lens aperture until it stops at the pre-set aperture. In practice, you often work in daylight (f/16 rule) or slowly varying lighting conditions. Once pre-set, you just twist and shoot. Simple! But in changing lighting conditions, you need something better...

Pre-set operation is also sometimes called "semi-automatic" operation in some older camera manuals and lens instruction sheets.

Fixed Aperture Lenses

Some lenses and optical instruments such as microscopes and telescopes have fixed apertures. There is simply no way to stop down or control the aperture of these optics.

A catadioptic mirror lens is probably the most familiar fixed aperture lens to most photographers. Consider the popular 500mm f/8 mirror lens. These lenses have a fixed aperture set by their design of f/8. Actually, my experience has been that most such lenses have an actual transmittance of light that is closer to f/11. You end up with an f/8 lens' depth of field and an f/11 lens level of light!

Since you can't vary the aperture (as it is fixed), you usually vary the shutter speed or choice of film (and film speed) instead. In bright sunlight, some photographers find neutral density filters helpful in reducing the amount of light in sandy beach or snow scenes. By contrast, a 500mm f/8 glass lens could be adjusted in aperture, unlike the fixed mirror lens.

Other optics such as telescopes, microscopes, and some macro (bellows) lenses may be fixed optics, depending on their design. As with the mirror lens, you are stuck with one depth-of-field setting (''wide open''). A few more complex mirror and lens designs permit some leeway, but introduce greater costs and other problems.

Automatic Diaphragm Lenses

Today, most photographers have never seen anything but an auto-diaphragm lens. These lenses stay fully open for composing and metering the shot. When you press the shutter, a connecting pin or rod moves to automatically close down the lens diaphragm to the desired shooting aperture. After the exposure, the lens diaphragm instantly and automatically opens up for bright open-aperture viewing again.

T-mounts

T-mounts are used to mechanically mount manual or preset lenses to a wide variety of camera bodies. There is no connection of aperture pins or other automation, so auto-diaphragm and auto-focus options are all out. But there are lots of older T-mount lenses available, as our table of third party lenses shows.

I have several 400mm and 500mm T-mount telephoto lenses, including a Tamron 400mm purchased in the last year for $15 and a Cambron multicoated 500mm f/8 glass lens bought for $50. A T-mount adapter can be bought used for under $10 US on up. So $75 + s/h enables me to make some fun telephoto shots when atmospheric haze and turbulence permit (i.e., early morning). Since these lenses need tripod mounting to use, the slight delay in stopping down manually with a preset control is a minor inconvenience.

At the other end of the scale, I have that 12mm Accura fisheye lens in a T-mount (circa $120 US). As with my long telephotos, I simply have to swap out the T-mount adapter to use it on any of my Nikons, Minolta SRT, Topcon, Pentax M42, or K-mount cameras. This approach lets me avoid buying a rare and expensive 12mm fisheye when I just want to have some fun.

Summary

You are likely to encounter mostly automatic diaphragm lenses which will act just like your OEM lenses with the same mount (e.g., AI adapter acting like AI lenses).

Manual and pre-set lenses are mostly restricted to older, low cost models of prime lenses and even some preset zooms.

You will find some very expensive specialty lenses, such as fast telephotos (e.g., Century Precision Optics), which have a pre-set option.

Finally, most mirror lenses will have fixed apertures, as will most microscopes, telescopes, and other optics.

Let's look at some low cost telephoto lenses and lens test data interpretation [see also table at top of postings pages]. We will add two Spiratone telephoto lenses, one a relatively low cost $60 300mm f/5.5 lens, and the other a more costly ($155) 300mm f/4.5 lens. However, be aware that this 300mm f/4.5 lens is a preset lens.

Finding a preset telephoto lens is another tipoff to the astute lens buyer that this lens might offer more optical and mechanical quality for the same dollars. The reason is that a good bit of the cost of a telephoto lens is in the long linkages needed to actuate the (larger) lens aperture mechanism to stop down automatically. Get rid of that mechanism, and you have more money to spend on optical quality.

A second tipoff is the range of f/stops. The 300mm f/5.5 Spiratone lens only stops down from f/5.5 to f/16, about 5 stops. The 300mm f/4.5 stops down from f/4.5 to f/32. You not only get a bit more wide open coverage, but also f/22 and f/32. Such extended f/stop ranges usually suggest that the lens is higher quality and cost, and optically good enough that such smaller f/stops are justified and useful quality. We can infer that the 300mm f/5.5 was already only acceptable in the center at f/16, and probably not acceptable beyond that point.

The 300mm f/4.5 Spiratone preset lens did surprisingly well for a low cost import telephoto. Very few lenses, and even fewer telephotos, merit "excellent" ratings in the entire f/stop range in the corners, as with this lens. Generally, the standards for corner sharpness are less than in the center of the lens, as it is harder to get them sharp. So when you see a pattern in which the edges are higher than the center in ratings, it is not likely that the edges are really better or higher resolution than the lens center. The optics rarely work like that. What it means is that the overall resolution is better balanced, and there is not as much dropoff in the edges as you might typically expect. So a center rating of very good, and an edge rating of excellent, might very well correspond to both center and edge scoring in the 50 to 55 lpmm range (say,).

If you compare the preset Spiratone 300mm f/4.5 with the Samigon 300mm f/5 lens, which cost about the same ($155 vs. $145), you can see the benefits of the lower cost preset design. Granted, you have to manually stop down before using the preset lens, a good bit of an inconvenience (but since it should be tripod mounted anyway, not that big a deal). But the 300mm f/4.5 lens is 1/3rd of a stop faster than the f/5 lens. It is only a third of a stop slower than the pro class 300mm f/4 lens, yet takes more readily available and cheaper filters. As noted above, the preset lens also stops down to f/32, providing an extra stop and more Depth of Field coverage than the Samigon auto diaphragm 300mm f/5 lens.

The big difference lies in the lens performance, with the preset lens clearly outperforming the auto Samigon 300mm lens. As noted, the 300mm preset lens scored all excellents in the corners, while the Samigon lens dropped off past f/8 to f/22. The center resolution of the preset lens is also much better than the Samigon auto lens, being all excellents, very goods, or good ratings. By contrast, the Samigon was only "acceptable" for the stops from f/11 through f/22. So the 300mm f/4.5 preset lens is at least as sharp as the Samigon wide open through f/8, and rather sharper in the center and especially the corners throughout the rest of its more extended f/stop range.

In short, if you can live with the preset limitations and filter size and weight of the Spiratone 300mm f/4.5, it is the better lens optically both in terms of resolution and capabilities (e.g., f/stop range), based on these tests. The Samigon 300mm is nicely optimized for wide open use, but falls down past f/8. If you don't need auto-diaphragm operation, then a preset lens in a fixed long telephoto may be a good way to gain optical quality at low cost....

rec.photo.marketplace
From: Bruce & Amy Hansen [email protected]
[1] Zoomar 400/600/800/1200mm lens-HELP
Date: Fri Feb 05 1999

Does anyone have any idea what a Zoomar convertible lens is worth? It screws together in sections, and makes two different focal lengths; 400/4, and 600/5.6. It comes with a Zoomar 2x teleconverter. The lens is a pre-set, and uses a "T" mount. Have you seen one for sale anywhere?

Thanks for any help.
bruce hansen

Date: Tue, 23 Mar 1999
From: Robert Monaghan [email protected]
To: [email protected]
Subject: Re: Diaphram nomenclature question

Hi Jeff,

auto refers to the fact that when you take the photo, you just press the shutter - the lens is automatically stopped down to the taking aperture (e.g., f/8).

On manual cameras, you set the f/8 on the lens ring, but the lens remains wide open for viewing and focusing (not darker). When you click the shutter, the lens has a lever that gets hit by a bar that causes the lens to stop down automatically - hence "auto". This is the fastest and most convenient setup, short of autofocus ;-)

On auto-aperture cameras like Nikon FE, the camera may select and set the aperture with compatible auto-diaphragm lenses (controls how far the lever moves and hence f/stop) - just as if it were a Nikon made lens which makes sense as these third party have copied the nikon lens' autodiaphragm and mount setup. Otherwise, you may have to use stopped down metering and set aperture in manual mode(s) if your camera is not auto-aperture capable.

on purely manual lenses, you set f/8 and the lens stops down and stays down to meter at f/8 and view at f/8 - image is darker as stopped down

preset lenses - you select f/8, based on stopped down metering or handheld light meter. When ready to take photo, you twist preset ring to set lens to f/8 and click shutter to take photo. Faster than manual use, no fumbling as preset setting lets you stop at the right place without any problems, so you don't have to look at lens when stopping down being major advantage, yet the viewfinder is bright and used wide open for focusing until the last second when you rotate the preset ring.

hope this makes more sense - it is a bit hard to visualize until you have handled them - yet despite the funky sound of all this, it works fine and can be done with many cameras and lenses, saving major $$ - and putting most of the dollars spent on the lens on the optics rather than mechanics...

regards bobm

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