Discussion of merits of hyperfocusing and infinity focusing

Harold Merklinger's object field depth of field concept and its application to Minox photography

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-- martin tai (martin.tai@capcanada.com), December 21, 1998

Answers

Many photographers, no matter what format they use, 8x11, 35mm, or medium format, when taking landscape photographs like to set camera lens on hyperfocal distance, to maximize depth of field.

But many photographers are not happy with the lack of sharpness of distant objects in their pictures; they feel these distant objects look too blurry in their pictures. Some try to stop down (for 35mm camera ), or double up the hyperfocal distance.

Harold Merklinger, in his book "THE INs and OUTs OF FOCUS"

presented an alternative depth of field theory, and provides a good alternative solution.

H.Merlinkger's analysis was based on object field instead of image field. His argument was that instead of looking at circle of confusion on the image plane, why not ask the question "what is the smallest resolvable object in the object plane "

In landscape or street photograph, accordingly to Merklinger, if you focus at hyperfocal point, the circle of confusion of distant object remains within preset limit, but the DISK OF CONFUSION grows rapidly without bound.

That explains why distant objects are fuzzy in the picture.

On the other hand, if the lens is focused at infinity, the disk of confusion is always equals to the diameter of lens opening, which is DOC= focal_length/ fstop.

For example for 35mm lens, DOC= 4.4mm at F8, DOC= 3.2 mm

Disk of Confusion is the smallest diameter the lens can resolve at a selected aperture.

One can choose how small the detail necessary in picture by simply looking at the size of opening of the lens, and turning the aperture ring to dial in the proper size.

For example if the thinnest tree twig has a diameter about 5 mm then you pick F11 for your Minoxar lens, with DOC = 3.2 mm, then the tree twig will be resolved in your picture. ( Up to 100 M, beyond that the actual DOC will grow without limit again, due to diffraction)

Same for Minox 8x11 camera, which has a DOC = 15mm/3.5=4.3 mm. When focus Minox 8x11 at infinity, any feature greater then 4.3 meter will be resolved up to 30 meter (due to diffraction, which is a different topic ).

In 35mm photography and Medium format circles, Merklinger has many followers (myself included ).

I apply his method to Minox ultramiature photography as well. During travel, I simply set my Minox at infinty and shoot.

I don't remember this topic ever came out at subminiature list discussions.

There are some discussion of Merklinger DOF theory at USENET GROUP

and Leica list

Is small lens opening-- large f stop better ?

If disk of confusion governs the resolution of objects, then it seems the smaller the lens openning the higher the resolution.

It is not-- because of diffraction.

The effect of diffraction on DISK OF CONFUSION and what is the optimum aperture at certain distance is a topic for another technical article

-- martin tai (martin.tai@capcanada.com), January 08, 1999.

Disk of Confusion for Minox 35

For 35mm camera, (Minox GTE/GTX/GTS, GL, EL,MDC etc etc ), to apply Harold Merklinger's depth of field theory for landscape photography is quite simple: take a look at the opening of you Minoxar/Minotar lens. For instance if you select F8, then the diameter of your len's opening is 35mm/8 = 4.4 mm.That 4.4 mm is the smallest DISK OF CONFUSION for a Minox 35mm camera set at F8. In other words object smaller than 4.4 mm will not be resolved.

Choosing aperture becomes very simple, DOF becomes very simple: Set lens at infinity, set aperture to match the smallest size you want to resolve, that is all.

Disk of Cofusion for Minox 8x11, focusing at Infinity

when focus Minox A/B/C/LX/TLX at infinity , the DISK OF CONFUSION is 15mm/3.5 = 4.3mm regardless of distance. In another world, 4.3 mm is the smallest distant object you ever expect to resolve with Minox 8x11 camera.

Of course, diffraction effect put a limit on how far out this 4.3 mm holds true.

-- martin tai (martin.tai@capcanada.com), January 08, 1999.

Dr.Harold Merklinger has a website, in which he introduced his alternative theory of DOF. But his book "THE INS and OUTS of FOCUS" is still by far the best source, the presentation in the book is far more complete and through. I higly recommend it as a must have book on every photographer's book shelf

Merklinger URL

-- martin tai (martin.tai@capcanada.com), January 08, 1999.

Disk of Confusion of 8x11 when hyperfocusing

When COMPLAN/MINOX lens is focus at hyperfocal distance H the DOC ( DISK OF CONFUSION ) =0;and object at half of H has a DOC = 4.3mm/2 = 2.2 mm; at twice of H, the DOC = 4.3 mm, beyound this, DOC increases linearly with distance, wihout bound ! For example, if Minox 8x11 camera is focused at its hyperfocal distance 4 meter, then at 2 meter point, DOC = 2.2; at 8 meter point DOC = 4.3 mm; at 12 meter, DOC = 8.6 mm, at 16 meter, DOC = 13mm, at 50 meter, DOC = 49 mm.

That is why, if you only look image plane, when a lens is focused at hyperfocal point, even though circle of confusion still within 1/60mm limit, and objects from 2 meter to infinity are supposed to be "sharp", but as a matter of fact the Disk of confusion grow geometrically, keep getting bigger and bigger. That is why far object becomes quite blurry.

-- martin tai (martin.tai@capcanada.com), January 08, 1999.

What happens to disk of confusion when you focus you Minox( be it 8x11 or 35mm ) ? When you focus your Minox at infinity, the light rays are parallel to optical axis, forming a cylinder of with diamter of cross section equals to the lens aperture opening. In other words, when you focus Minox 8x11 camera at infinity, the DOC is always 15/3.5= 4.3 mm; for Minox 35 camera, DOC =35MM/fstop, for example when fstop =f8, DOC for Minox GT-E = 35/8= 4.4 mm. When focus at infinity, from geometrical optics point of view, Disk of Confusion, ie the smallest object which can be resolved, never increase, in contrast to focusing at hyperfocal distance H, DOC increases without bound beyond 2*H. Hence, for landscape pictures, where distant mountaitns, cathedrals, street scences are your main object of interest, you should focus at infinity instead of at hyerfocal distance H; On the other hand, if your near object is more important in your composition, but you want to retain distant background to play secondary roles, then focus at hyperfocal distance.

-- martin tai (martin.tai@capcanada.com), January 10, 1999.