Steve, concerning your quest for smaller apertures, I was wondering if the old idea of a pin-hole camera would work. By covering the lens with a thin black foil with a tiny hole in it, you should be able to get the desired DOF. Ansel Adams did it, Zorkendorfer from Germany still has an accessory for this purpose, but with a skylight filter and a peace of aluminum foil it should work. Ofcourse the hole is not were it should be, but the optical quality with very small apertures isn't optimal anyway. Maybe worth experimenting.

-- Peter Gooijer (rockrose@freeler.nl), February 02, 2002

Answers

I have done some of what you suggest. I have placed a bayonet lens cap on several different lenses with a hole to represent an f/45 and f/64 stop. The problem is that there is usually vignetting due to the stop being in the wrong place. Also, changing the stop location runs the risk of adding distortion. I tried putting a black paper with pin hole where the gelatin filter usually goes in the 45. That didn't work either. The solution may be to trade in the f/22 lenses for ones with smaller stops (135, 55-100, 90-180). I have done this except for the 90-180.

-- Steve Rasmussen (srasmuss@flash.net), February 02, 2002.

Michael, small film size does not lend itself to small apertures. If you look at the diffraction formula 1390/f# you will see that f/64 yields a maximum lp/mm of about 22. For 35mm film to have such a low resolution due to diffraction, greatly limits enlargement possibilities. It is not nearly so critical to have 15 to 20 lp/mm when using 4x5 or 8x10 inch sheet film. This is one reason why you will see such small stops on view lenses and not on 35mm. To have an f/64 stop on a 67 might be a stretch but I would love to try it.

-- Steve Rasmussen (srasmuss@flash.net), February 09, 2002.

This does work, as far as it goes. In fact, about 10 years ago in Camerart Magazine (Japan's English-language how-to photography magazine), there was a brief column on doing this with 35mm lenses. I believe they were mounting to the back of the lens though, not via a filter on the front. I could probably find the article if someone wanted the details or even just the bibliographic citation. Also, though I doubt I could locate this article, the old "Model Railraod Treasury" includes a chapter in which one author describes how he takes apart his Mamiya TLR, and insert a pinhole discs between the elements. This is the ideal method as the aperture is located where it should be. Essentially, you are making a minute waterhouse stop. Problem is, for SLR users anyway, it that the TLR cameras are easy to take apart, and you have another lens to view from. SLR lenses are much more complex, and you would always need to switch lenses to see what you are doing - or make a viewing frame of some sort. I am still waiting to find a dirt cheap P67 lens (dropped in a lake or something?) to try this waterhouse stop idea. This idea is not as uncommon or weird as it might first sound. I know that machinsts like S.K. Grimmes (sp?) and others have customized lenses for greater depth of field, so users do ask for it - and pay to have it done. if you want only one more stop, you can sometime file down parts to make the aperture close a little more. But, in general, the blades are usually about at their limit as made by the manufacturer (close them down too much more and they will bind).

-- Michael Tolan (mjtolan@kbjrmail.com), February 02, 2002.

Placing something, anything, in front of the first element -where a filter normally goes- would not have any effect on depth of field. When you do this, you simply reduce (greatly) the ability of the lens to gather light. What you want is to effect the "focusing" of the light the lens gathered. It would have to be at the other end of the lens or, ideally, as you found out, where the manufacturer places the aperture blades. The only way to do this would be to alter the barrel, and have inserts you could use - and this would also require some sort of protective barrier to keep dust and whatever else might find its way into your lens. The easiest, though perhaps not the most cost-effective thing, is to do as you say and go with the smaller-stop lenses (though why they didn't make their 135mm macro with another stop is sort of a mystery).

-- Michael Tolan (mjtolan@kbjrmail.com), February 04, 2002.

Michael,

Putting a stop in front of the lens most assuredly will affect your depth of field. The old Kodak portrait lens has the aperture in front of the lens, as do some of the old large format convertibles. The Schneider 150/265 convertible lens can be used with either the front or rear element removed; if you remove the front element, the aperture is in the front of the lens. I have personal experience using apertures in front of this and other lenses, like the goerz dagors.

Going smaller than f22 on any lens of a focal length shorter than 100 mm is going to give you some diffraction problems, though, and putting a stop in front of the lens does increase the aberrations that result in coma - a teardrop effect on the edges of the image. My advice is to stick with the smallest stops Pentax lets you use. If you really need more depth of field than you can get with careful focusing, get a view camera.

-- Erik Ryberg (ryberg@seanet.com), February 06, 2002.

Erik, Just because a unit can be used with the aperture in front of the front or only lens element doesn't mean it increases depth of field, does it? It obviously controls exposure, but depth of field is another matter. Are you sure the front-mount aperture alters depth of field? I am not saying you are wrong, but somehow it doesn't sound correct. Any kind of portrait lens might be a bad test case, as depth of field is not of much importance, and to a great many photographers doing portraits, it is often looked on as a drawback (the older the portrait lens, the less depth of field it will have - as a general rule, and there is no shortage of people looking for AN old wollensak Verito, for just that reason). As for the convertible lenses, I don't know much about them, but I realize the aperture can be in front of the remaining element, depending on which element is removed (and I also realize the lpm changes at different apertures - but this, too, is separate from depth of field). I guess I would have to test one but, intuitively, having the aperture in front just sounds like a bad -if not the worst- way to do it; I must be missing something, as I don't see how a converted lens with front aperture at f11 is going to have more depth of field than the same at f8 (though it would not be the first time I am wrong!). Yes, what you say about small stops is true, but I have never found slight diffraction to be the ultimate evil so many other people seem to view it as. In 35mm, I often shoot 21mm and 24mm lenses at f22, because of the result. I think longer lenses could almost always benefit by another stop or more. Years ago, I remember reading that Sigma was designing a 100mm or 135mm (Panthel 64?) lens that went to a whopping f64. Of course, I notice it isn't in production anymore and no other company followed them down this road, so maybe I am alone or almost alone out there in this insane quest.

-- Michael Tolan (mjtolan@kbjrmail.com), February 07, 2002.

Michael,

I know it seems funny, but yep, the smaller the aperture in front of the lens, the greater the depth of field. The portrait lens I mentioned and the old Schneider Symmar convertibles all use a variable sized stop in front of the lens. You can actually do this experiment with your own eyeballs and a piece of paper, though it takes some care. Make a small hole in some dark paper and hold it against your eye so most of your view is blocked. Hold a ruler or some other object in front of your eye about a foot away. Position yourself so that another object with writing on it (about 4 or 5 feet away) will just come into enough focus to barely read it while peering through the small aperture and focusing on the close object. Now remove the paper, and continue focusing on the object you are holding in your hands. The distant object will go out of focus - you won't be able to read it.

Of course, you can run the same experiment with your SLR. Use a fast lens (like the 2.4 105mm for the P67) and cut out a hole a half inch in diameter or so in a piece of cardboard. Point at a book title or something and set your focus so you just barely cannot read the title. Now stick the cardboard in front of the lens. Presto! You can read the title. There is more depth of field.

There is a really great book on focus called "The Ins and Outs of Focus" by Harold Merklinger. It is definitely worth reading. He did some very persuasive, simple experiments with his Leica and has some interesting stuff to say about hyperfocal distances.

Good luck with your pictures.

-- Erik Ryberg (ryberg@seanet.com), February 08, 2002.

Yes, you can increase depth of field by placing an aperture in front of the lens, but as Michael indicated and Steve found out, it is the worst way to do it, especially if done with a lens not designed to have such a stop. The idea that lens manufacturers always make lenses with the smallest acceptable aperture, is simply not true - not even close. You can find years, throughout 1965-1980, where various Japanese companies all issued the same focal length lens in the same mount, with aperture minimuns varying at f16, f22, and f32 (maybe even f45!). And, some of these lenses were made by the ame companies; same glass and barrel, just different aperture insertions. Obviously there are other factors at work - namely what will the average user do, handheld, and how long will this lens last if we make it? The longer the focal length, the easier the lens is to adapt as the actual size of the hole you are making shrinks with focal length (I have done it many times). Often, the reason a wide angle lens has a min of f16 or perhaps f22 is NOT beause of diffraction or any optical concern, but because they can't make the blades meet up and form a solid circle at such tiny points. The problem, hinted at above, is that the blades might bind and no lens company will risk that - hence the watered-down lens you buy at the store. Discs are the way to go and several well known lens/shutter gurus have done this for people. The key is to make drop-in aperures the way companies make drop-in filters for mirror lenses and what not. Of course, it all depends on what you want and whether or not you want to alter a several hundred dollar lens. But, there is no optical reason why Pentax 6x7 lenses couldn't go another stop or two - especially if done with the aperture where the lens designer intended it to be. Even if you reached the point of losing corner quality, with a negative that big, you can often adjust for it accordingly. The trade-offs can be worth it, depending on what your goal is.

-- marcus singer (MSingerPhoto@aol.com), February 12, 2002.