Focus point & DOF

[Martyn]

I picked up an old Pentax 50mm (K mount) with the idea of using it with a reversing ring for macro. However, I was intrigued by the aperture ring and the markings that surround it. After some research, I believe I'm right in concluding that if this lens was fitted to the correct camera, if it was set to f22, I could set the focus ring with the infinity mark aligned with the nearest DOF focus point for f22. This would maximize the DOF to focus on things from a few feet to infinity.

 

Now here is the question; with a lens with no aperture ring or markings on the focus ring, ie, Nikon AFS DX G series, how can I maximize the DOF to make sure I get everything from infinity to the closest point in focus?

 

That question sounds as clear as mud, but I don't know the correct terminology for some of these things. Hope some of you may understand what I'm asking.

 

 

[Black Pearl]

You can get Hyper Focus (thats the term you're referring to) Apps for smart phones that will tell you what distance to focus for a given lens/aperture/camera combination, there are charts you can download from the net and a trawl around the manufacturers sites will garner further info for a given lens - but - you either need a lens with focus distance on it or you'll have to guesstimate. 

 

I have a Nikon t-shirt with it printed on....upside down so I can read it when I look down at my chest. You have no idea the conversations I've had while wearing it.

[Guest DaveW]

See:-

 

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

 

http://www.cambridgeincolour.com/tutorials/hyperfocal-distance.htm

[Martyn]

Thank you for the info, looks like I've opened up another can of worms!   This is all very interesting stuff because I'm finding on some of my pics that the foreground or background is not crisp. I've been pushing to extremes the closeness of some of the foreground items, results have been interesting, a few happy accidents and a few failures. I'm off now to do some more reading, hopefully I won't end up in a "circle of confusion"!

 

Here is an example of a happy accident, it was all supposed to be in focus

 

 

 

 

[Guest]

I picked up an old Pentax 50mm (K mount) with the idea of using it with a reversing ring for macro. However, I was intrigued by the aperture ring and the markings that surround it. After some research, I believe I'm right in concluding that if this lens was fitted to the correct camera, if it was set to f22, I could set the focus ring with the infinity mark aligned with the nearest DOF focus point for f22. This would maximize the DOF to focus on things from a few feet to infinity.

 

Now here is the question; with a lens with no aperture ring or markings on the focus ring, ie, Nikon AFS DX G series, how can I maximize the DOF to make sure I get everything from infinity to the closest point in focus?

 

That question sounds as clear as mud, but I don't know the correct terminology for some of these things. Hope some of you may understand what I'm asking.

It's not as easy as it seems. There are plenty of dof guides and this http://www.dofmaster.com/dofjs.html is the best I have found but they all seem to be based around an excessively large circle of confusion ( or simply only wanting to print to 6x8") if you want to print at A3 or A2 the figures suggested do not work. Accept that whatever the table tell you you will probably need 2 stops smaller at least and learn to focus correctly to the hyperfocal distance see my blog http://www.billallsopp.co.uk/correct-focus-and-hyperfocal-distance for the method.

[Martyn]

That's a great tip Bill! Thanks!

[Guest DaveW]

As Bill says it depends on the size of circle of confusion you adopt (the human eye cannot see the difference between a fuzzy dot and a sharp point under a certain size). But what apparent depth of field you get then depends the size of circle of confusion chosen at the taking stage and how much you enlarge the image so the eye can then see it is a blur and not a sharp point, plus how far away you then stand to view it, just as oil paintings improve with distance.

 

As an example a photo blown up large on an advertising hoarding may look sharp from across the street, but looks fuzzy close up since the circle of confusion has been enlarged enough to appear as a fuzzy blob that close and at that enlargement.  Also lower resolution can hide the difference, if everything is less sharp the depth of field appears more. For instance an image on your computer screen or TV would have apparently greater depth of field than if printed out on a reasonably large print since the prints resolution is far greater than TV or computer screens so would show differences in sharpness of points in front and behind of the point focused on far more.

 

One of the simplest explanations of the circle of confusion I have found is here:-