[Leica] DOF Tables, or a tiny bit of physics

[kanner at acm.org (Herbert Kanner)]

1. The depth of field values depend on a decision as to an acceptable circle 
of confusion. This is a given.

2. The size of the circle of confusion is geometrically calculated from two 
numbers: 1) the focal length, which is the distance from the real principal 
plane of the lens to the exact focal point (zero diameter for circle of 
confusion) for parallel rays entering the lens e.g. subject at infinity; 2) 
the f stop, which is the ratio of the focal length to the diameter of the 
ray bundle AS MEASURED AT THE REAL PRINCIPAL PLANE.

So how can different lenses of the same focal length have different DOF 
tables? Two possible explanations; I prefer the second.

1. The f number is a bit nominal and may not be all that accurate. 2. The 
focal length is probably not exactly as stated. For instance, if one lens 
came out 74.7 mm and another 75.3 mm, wouldn't they both be called "75"? I 
suspect that all the complex calculations leading to corrections for all 
those abberations: coma, spherical, astigmatism, chromatic--I think that's 
all of them, would make it impractical to aim for an exact focal length.

Just my guess.
 
Herbert Kanner
kanner at acm.org
650-326-8204

Question authority and the authorities will question you.




On Jan 29, 2013, at 12:02 PM, Cedric Agie wrote:

> I have (plasticized) cards with DOF charts for all the lenses I've
> got. As well for Leica as for Hasselblad. When in doubt, for example
> with a tele or a macro set-up, I always have them at hand and I check
> them before taking some pictures. Be it portraits, flowers or insects
> etc. It's too bad to discover that the nose, the eyes of the girl, the
> bird or the insect etc are out of focus when they are gone. In
> practice forget MFT arguments, because for you they are useless. It's
> good for the optical engineers to conceive and check the lenses they
> produce...... check what the others are doing or for the people of the
> sales department who don't always know what to say to sell their so
> called new and better products. Also forget arguments such as cit;
> ".... small residual optical faults corrected in the camera
> software....".
> 
> It takes some thinking and time to calculate or check these DOF charts
> for the lenses you've got. But do it, it's a good exercise. Sometimes
> you'll discover strange things or funny phenomenon's and even mistakes
> that went unnoticed for 10's of years. Even in Leica (Leitz) or Zeiss
> letterature. You'll know your lenses much better. And by the way, when
> you're reading articles or hear arguments, you'll soon discover if the
> guy really knows what he is talking about. In the end it will stay
> somewhere in the back of your mind. That makes the difference between
> an  even talentfull amateur and a very good professional photographer.
> The pro will take one or two pictures. Some amateurs will show you the
> one picture he or she took almost by chance and not the others.
> 
> Is it really necessary to pay almost two times the price of a lens for
> an increase of just one stop and a DOF that is suddenly reduced to
> centimetres or millimetres? For macro follow this rule: fully close
> the opening of your (best) lens and then open it one stop. And check
> it's DOF. To the contrary of what some say, digital photography hasn't
> changed these basic rules.
> 
> Are your really going to walk around in daylight with a Noctilux 0.95
> and a speed of 400 ISO or more? Or is it just for fun? When you do
> these calculations you really start thinking. And do take notes of
> your new tests.
> 
> On the other hand I know (a very few) pro's who can walk around at
> night in a city, in a bar, around an orchestra or a theatre, inside
> it's buildings and take pictures in dim light, going unnoticed and
> come back with marvellous pictures.
> 
> cedric.agie at gmail.com
> 
> 
> 
> 
> 
> 
> 2013/1/28 Marty Deveney <benedenia at gmail.com>:
>> On Tuesday, 29 January 2013, Frank Filippone wrote:
>> 
>>> True, but note that the 90, at F2 , has the same limited DOF as the 75 at
>>> F1.4.
>>> 
>>> We do not hear too much about difficulty with focusing the 90.....  I
>>> wonder
>>> why not?
>>> 
>> 
>> My principal difficulties with the 75 Summilux occur closer than the 90
>> focuses, in the extremely shallow dof range under 1 m.
>> 
>> Marty
>> 
>> _______________________________________________
>> Leica Users Group.
>> See http://leica-users.org/mailman/listinfo/lug for more information
> 
> _______________________________________________
> Leica Users Group.
> See http://leica-users.org/mailman/listinfo/lug for more information