Golden Oldies: Pre-Set Lenses and T-mounts

Over the many years that I have been dealing with camera equipment, it seems that the least desirable (from a selling point) have been pre-set lenses and lenses that have a T-mount.  First of all, let me tell you what I am talking about here.  

Two pre-set T-mount lenses, M42 mount

When you photograph, you set your aperture on the lens and/or adjust your shutter speed until you get the light meter indicator to where it suggests the correct exposure. That’s pretty much how it works with any camera, including digital, unless you have it in complete auto mode, and the camera makes those decisions for you. If you are using an SLR, everything looks the same to you through the viewfinder, unless you push a button or lever to observe the depth of field, which stops down the lens to the chosen aperture. We do it so automatically, that it’s easy to not think about what is actually happening inside the camera.  That’s called open-aperture metering, and even though the lens is wide-open the mechanical or electronic linkages relay all that information to the metering system. When the shutter is fired, the lens then stops down to the selected aperture (assuming that you are not shooting wide-open, of course). It’s been with us since 1963, and adopted by most camera manufacturers by 1971. While cameras have evolved to do just about anything via some electronic adjustment, the basic system of aperture/shutter control is really all you need to control the exposure of an image.

Aperture pin, (indicated by red circle) on a M42 lens

Before the days of open-aperture metering, we had stop-down metering, which means that you set your aperture and pushed a button or lever on the camera to actuate the diaphragm and the light meter.  The problem with that system is that the smaller the aperture, the dimmer the image, making focusing more difficult. If you have used a Pentax Spotmatic, Canon FT QL, Praktica, Zenit, Minolta SR, and any number of M-42 mount cameras, it’s likely that you have used stop-down metering. It’s why many lenses in the early days have an M and an A at the base of the lens. M=Manual, and the aperture is wherever you set it at all times, and the A = Automatic, meaning the aperture changes from wide open to the selected aperture when the shutter is released. Often such lenses have the word "Auto" on them, at least before it became standard on all cameras and lenses.

Three different lenses with A and M control

To actuate the diaphragm in the lens, there needs to be a small pin at the base that is pressed against by an arm or movable plate inside the camera at the moment the shutter button is pressed. Usually, that lever is in front of the mirror. It’s not needed for rangefinder cameras, or twin-lens reflex cameras, because you are not looking directly through the taking lens to focus and compose.  However, on an SLR, you want to have the view as bright as possible, so the lens only stops down when the shutter is released.  

Pre-set T-mount lens has no aperture pin

But wait, what if there is no aperture actuator in the camera or on the lens?  You could still do the manual route and suffer with the dimmer image at smaller apertures, or you could use a pre-set lens. Pre-set lenses have two rings for setting the aperture(usually towards the front of the lens, not at the base). The foremost ring sets your final aperture, and the following ring closes the aperture when you are ready to shoot.  You can view your subject at the full aperture of your lens, and since your shooting aperture is been pre-set, you just close the second ring to shoot. I’ll tell you more about this later on why this is a good thing today. 

Simplified system - shooting aperture at top and
C to close down, O to open up

Final aperture set at f/22, but open to view at f/2.8

Final aperture at f/22

135mm Spiratone lens

T-mount on lens, adapter at right

Now, obviously, the pre-set system is rarely used on modern lenses, but back in the 1950s and 60s, it was pretty common.  It was also used a lot on T-mount lenses, which are third-party lenses made by a variety of manufacturers into the 1970s. The T-mount is a 42mm thread which has a different pitch than the M-42 mount that it is sometimes confused with.  The usual T-mount is a screw mount using a male 42×0.75 (42 mm diameter, 0.75 mm thread pitch) metric thread on the lens with a flange focal distance of 55 mm and a mating female 42mm thread on a camera adapter or other optical component.  The T mount is a mechanical-only mount, meaning it does nothing beyond physically connect the lens to the camera. It does not transfer electrical signals, auto-focus information, or aperture settings. So, the beauty of the T-mount is that you can use one lens with different SLR camera mounts, so long as you have the right T-mount adapter.  The other part of this is that you don’t have to worry about the linkage to an aperture pin, because there isn’t any.  The image below shows a Spiratone 135mm f/3.5 T-mount lens that I have used on M-42, Pentax K, Nikon F, Konica, Leica R, and Minolta SR/MD mount cameras.  You may ask why I would bother.  The aperture in this lens is completely circular due to the many-bladed diaphragm, and it gives very pleasing image. I have used it on digital SLRs, as well.  Since it’s also a pre-set lens. I can easily focus and stop the lens down to any aperture, not pre-determined click stops. 

T-mount lens with a variety of adapters

So, here we are with some lenses from a time when NOTHING was automatic, yet they can still be used today, with the proper adapter, on your film SLR or your DSLR or mirrorless camera.  The beauty of the pre-set lens becomes more apparent when you see the image on your LCD, and adjust the aperture to get exactly what you want in the image.  Since there are no electronic connections, you may have to do all the exposures in Manual mode with some cameras, and Aperture-Priority in others.

I should add that not all T-mount lenses are vintage. Some T-mounts date from the 1980s, and are often seen on mirror telephoto lenses, oddball art lenses, slide copying units, and telescope and microscope adapters. You can also buy new 500mm telephoto lenses that use the T-mount, AND have pre-set apertures. So, the T-mount and pre-set lenses have never really gone away after all these years. The beauty is that you can usually pick up a used T-mount lens or a pre-set lens quite cheaply, and the only factor will be if you can find a T-mount adapter for your SLR. T-mount adapters are easily found for M-42 Pentax screw mount, Pentax K-mount, Nikon F-mount, Olympus OM, Minolta SR, and Canon FL. I think I have seen most pre-set lenses/T-mounts for M-42, as it was a very popular mount before the mid-1970s.