Small aperture

[peter gooijer photography]

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.

[michael_tolan]

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).

[steve_rasmussen]

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.

[michael_tolan]

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).

[erik_ryberg]

Michael,

 

<p>

 

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.

 

<p>

 

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.

[michael_tolan]

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.

[erik_ryberg]

Michael,

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

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.

 

<p>

 

Good luck with your pictures.

[steve_rasmussen]

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.

[marcus_singer]

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.