Hyperfocal question

NeilL

Both the Bob Atkins' and the Nikonians' dof calculators approximately agree, in rounded figures eg that for a 14mm lens, cof 19 microns, at aperture f13 and distance to subject 1 mile, H is 3 feet. They both give the far focus limit as infinity, however they both say that the near focus limit is 3 feet.

Now, it was my understanding that the near focus limit is half H, ie 18 inches. So how can I reconcile that with the near focus limit of 3 feet above, the same as the hyperfocal distance?

Thanks.

Neil

Ambrola

What the jmkjpj are you talking about!

ziggy53

The most convenient DOF calculator is the online version at:

http://www.dofmaster.com/dofjs.html

Assuming a 1.6x crop sensor, 13mm focal length lens, subject at 5280 feet, f13, COC of 0.019 mm:

Depth of field
Near limit 2.66 ft
Far limit Infinity

This does not take into account print sizes or viewing distances, so this is just an approximation and starting point. In actuality, lens design and specific lens performance will also play a part.

Experience and testing will demonstrate actual performance of a system so you must use these calculations as guidelines and not photographic gospel.

NeilL

ziggy53 wrote: »

The most convenient DOF calculator is the online version at:

http://www.dofmaster.com/dofjs.html

Assuming a 1.6x crop sensor, 13mm focal length lens, subject at 5280 feet, f13, COC of 0.019 mm:

Depth of field
Near limit 2.66 ft
Far limit Infinity

This does not take into account print sizes or viewing distances, so this is just an approximation and starting point. In actuality, lens design and specific lens performance will also play a part.

Experience and testing will demonstrate actual performance of a system so you must use these calculations as guidelines and not photographic gospel.

Thanks Ziggy.

What I don't get is how come near limit is given as = H, instead of 1/2H?

Neil

NeilL

Ambrola wrote: »

What the jmkjpj are you talking about!

"jmkjpj" ??? what's that???

Seriously, if you know what depth of field is you know what I am talking about. If you want everything in an image to be more-or-less in focus then everything must be in the dof range. Dof changes with a number of factors, so how can you be sure you will have everything in focus? You use a dof calculator, aka hyperfocal distance calculator.

Ok?

Neil

ziggy53

NeilL wrote: »

Thanks Ziggy.

What I don't get is how come near limit is given as = H, instead of 1/2H?

Neil

You are trying to apply a generality which works fairly well for only a narrow range of focal lengths. With a very short/wide-angle focal length (or very long focal lengths), the generality does not work well at all.

If you want a very complete set of formulas see:

http://www.normankoren.com/Tutorials/MTF6.html

NeilL

ziggy53 wrote: »

You are trying to apply a generality which works fairly well for only a narrow range of focal lengths. With a very short/wide-angle focal length (or very long focal lengths), the generality does not work well at all.

If you want a very complete set of formulas see:

http://www.normankoren.com/Tutorials/MTF6.html

That's all right then!:D I was concerned I was looking at it cockeyed.

Thanks again!

Neil

indefinitepronoun

ziggy53 wrote: »

You are trying to apply a generality which works fairly well for only a narrow range of focal lengths. With a very short/wide-angle focal length (or very long focal lengths), the generality does not work well at all.

If you want a very complete set of formulas see:

http://www.normankoren.com/Tutorials/MTF6.html

I'm afraid that this does not really answer NeilL's question. His choice of focal length is not outside of the range of validity of hyperfocal distance calculations. Rather, his confusion arises because he has set his subject distance incorrectly in his DOF calculator.

NeilL, when you enter the subject distance as 1 mile, you are telling the calculator that you are focusing at a distance of one mile. For the 14mm focal length you are using, this focal distance is basically infinity. And when you focus on infinity, the near limit of DOF is equal to the hyperfocal distance, not half the hyperfocal distance. This is what you observed in your first post. (In this case, objects at infinity, where you actually focused, are going to be sharper than those at the hyperfocal distance, which will be just at the limit of acceptable sharpness, as defined by the circle of confusion you chose.)

To cause the DOF to extend from half the hyperfocal distance to infinity, as is usually the goal in hyperfocal focusing, you must focus at the hyperfocal distance. Try it out in your DOF calculator: for your above setup, enter a subject distance of 3 feet. Then your DOF should extend from roughly 1.5 feet to infinity. In this case, objects at half the hyperfocal distance and also objects at infinity will be just at the limit of acceptable sharpness, as defined by your circle of confusion.

As ziggy53's link says, there are plenty of people who think that hyperfocal focusing is a bad idea, because objects at infinity will not be as sharp as if you just focused on infinity. Personally, I try to choose my DOF and point of focus depending on which elements are most important in my composition.

Hope this helps!

NeilL

indefinitepronoun wrote: »

I'm afraid that this does not really answer NeilL's question. His choice of focal length is not outside of the range of validity of hyperfocal distance calculations. Rather, his confusion arises because he has set his subject distance incorrectly in his DOF calculator.

NeilL, when you enter the subject distance as 1 mile, you are telling the calculator that you are focusing at a distance of one mile. For the 14mm focal length you are using, this focal distance is basically infinity. And when you focus on infinity, the near limit of DOF is equal to the hyperfocal distance, not half the hyperfocal distance. This is what you observed in your first post. (In this case, objects at infinity, where you actually focused, are going to be sharper than those at the hyperfocal distance, which will be just at the limit of acceptable sharpness, as defined by the circle of confusion you chose.)

To cause the DOF to extend from half the hyperfocal distance to infinity, as is usually the goal in hyperfocal focusing, you must focus at the hyperfocal distance. Try it out in your DOF calculator: for your above setup, enter a subject distance of 3 feet. Then your DOF should extend from roughly 1.5 feet to infinity. In this case, objects at half the hyperfocal distance and also objects at infinity will be just at the limit of acceptable sharpness, as defined by your circle of confusion.

As ziggy53's link says, there are plenty of people who think that hyperfocal focusing is a bad idea, because objects at infinity will not be as sharp as if you just focused on infinity. Personally, I try to choose my DOF and point of focus depending on which elements are most important in my composition.

Hope this helps!

Hi indefinitepronoun, thanks for your excellent information, and welcome to DGrin!

Your explanation cleared up my confusion, and also gave me a better understanding of hyperfocal focusing, and so more power to make better focusing decisions. Thank you.

Neil