Are dof tables wrong?

[adwb]

 

according to dof tables from several sources, 
if 
sony A7 full frame.
lens focal length is 35mm
aperture is f8 
focus distance to subject is 10 feet
then:-
closest focus point is 6.3 -6.2 feet
furthest point in focus is 18.1 ft

So images taken at 10 foot are pin sharp. 
With out altering any thing on the camera or its position I then move the object to 6 foot and the image is soft,
move it to 7 ft its still soft
move it to 18 foot its sharp.

why?

I am wanting to pick a focus at f8 and be able to let the subject move around not need to refocus every shot but with this discrepancy it does not work
what am I missing??

[christer]

Well, the dof depends on how you define it. Dof tables were first  commonly used when Leica cameras came on the market. Leica used dof to assure buyers that they could get results they were used to from their 6x9 cm negatives if they enlarged their 24x36mm negatives to 6x9 and kept within the dof limits as set by Leica. The word to look for if you want a definition is "Zersteuungskreis"; could it be "circle of con...." in English?. Definitions vary depending on where you read them....

 

I assume you know that there is only one shooting distance that will give maximum sharpness. The rest will me more or less sharp. YMMV with regard to where sharpness ends.....

 

I have the Leica tables and I compared them to the values that my Batis 85 shows in the display. Obviously Zeiss use dof definitions that are much stricter than Leica used to publish for analog use.

 

I do not use dof tables. I have done a few dof test myself and based on that experience I set the aperture as long as shutter speed and ISO are within my limits.

[addicted2light]

1) first and foremost the circle of confusion the DOF table you're using is based on - what christer was telling you in the previous answer -, needs to be way more strict if you need pixel-level sharpness. A few apps let you set the circle of confusion by yourself

 

Incidentally, the larger the sensor (or the film) and the higher the megapixel count, the smaller the DOF is gonna be. With the A7r (36Mp) most of the times I have to recur to focus stacking for the same images that I can shoot with a Fuji (only 16Mp) in one shot

 

2) DOF generally extends 1/3 in front of the focus point, 2/3 behind. That's probably why your 18ft shot looks fine

 

3) This one is often ignored: DOF depends as well from the specific optical scheme of the lens you're using, so two lenses of the same focal length - say two 35mm - can exhibit quite a bit of a difference both in terms of "depth" of the DOF and in its "front / back" distribution. DOF tables take into account only a "generic" lens

 

Just take a look at the picture in this old post of mine to see how much of a difference the optical scheme can make:

 

Battle of the 50s: an update on the Pentax Takumar “war”

 

4) lastly, the closer you are to your subject the shorter the extent of your DOF, all other things equal

[adwb]

thank you both, for the help full replies, i'm now wiser than before, 

[Golem]

All the CoC info is "erudite", and anyone can

search online for "white papers", tutorials, etc  

if you want more details, equations, and such.

  

However, the very specific complaint detailed

in the OP sounds like either a focus error or,

maybe more likely, a case of believing in the

focus scale reading. Nearly all modern lenses

have huuuuugely sloppy focus scales.

 

A rather slight focus error, easily hidden from

close inspection if using a 35mm lens at f:8.0,

combined with naively believing the lens focus

scale readout, could very well explain what is

described in the OP. 

  

Considering all that, I estimate that the actual

focus distance stated as 10ft was more likely

14ft [+/-].  The DoF depth, of 6.5 to 18ft @10ft

true focus, would then shift, further away from

the camera, to become about 8.5 to 26ft when

the true focus becomes approximately 14ft.  

  

_____________________________________ 

   

  

Altho the OP seems very detailed, sufficient info

is not there. HOW was critical focus obtained ?

HOW was the 10ft distance determined ? WHAT  

method and WHAT targets are used in changing  

the subject distance from 10ft to 6 and 7ft ?

[adwb]

Altho the OP seems very detailed, sufficient info is not there. 

HOW was critical focus obtained ? by eye using focus magnification on rear screen

HOW was the 10ft distance determined ? by using a tape measure, stretched out across the floor 

WHAT method and WHAT targets are used in changing  the subject distance from 10ft to 6 and 7ft ?  the target was was porcelain model head with eyelashes and a wig that was put on the tape at each distance

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[Golem]

 

Altho the OP seems very detailed, sufficient info is not there. 

HOW was critical focus obtained ? by eye using focus magnification on rear screen

HOW was the 10ft distance determined ? by using a tape measure, stretched out across the floor 

WHAT method and WHAT targets are used in changing  the subject distance from 10ft to 6 and 7ft ?  

the target was was porcelain model head with eyelashes and a wig that was put on the tape at each distance

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Certainly does look like you've covered all the variables. Surely

you focused wide open before stopping down, and it's unlikely

your lens would have such extreeeeme focus shift on stopping

down. While it is true [as posted] about DoF tables being based

on a "generic" lens design, it's also hard to believe that you have

a 35mm lens whose design places nearly all the DOF behind the

true focus point [with almost none toward the foreground]. Did

you also bracket focus ? Unless your lens has the unusual DoF

just described, then a focusing error is the only option. If you did  

bracket focus and chose the correct image, then it seems that

you do indeed have a very weird lens. I'm just having trouble

believing that ....

[Guest all8]

Might be worth reading up on Hyperfocal Distance.

 

http://digital-photography-school.com/getting-landscapes-sharp-hyperfocal-distances-and-aperture-selection/

[LCT]

DoF tables are based on average CoC values (typically 0.03mm) taking into account a viewing distance of about 30cm without magnification. For more demanding purposes, like viewing magnified crops, smaller CoC values must be chosen which could explain your results with a pinch of salt. Here for 0.03 and 0.02mm CoC:

 

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