hyperfocal length and "optical infinity"

[kavan]

Hello Forum,
does anyone understand how the hyperfocal distance really works?

I know that if we focus at H, everything from H/2 to infinity will be in focus (acceptable). The first step is to choose an acceptable, tolerable blur circle size....

But how is it different from focusing at infinity? Objects are considered at infinity if they are 20X,30X the focal length away from the lens....

Focusing at the hyperfocal length means that the what is at H is in perfect focus and everything else is slighly out of focus but acceptable...How do we achieve such huge depth of field?

thanks
kavan

[Donald]

Have a go at reading the tutorial here on CiC. That might answer some of the question.

[ajohnw]

You should have asked about this in the main forum not here.

I think the web is causing a lot of confusion on this subject. The whole idea is simple really. Focal depth to some circle of confusion increases with distance so there will always be more depth of field past the actual focal point which would ideally have a circle of confusion determined by diffraction. That's 1.36xFratio in um (0.001mm units) for a perfect lens. Varies with colour but there isn't much point in worrying about that. Distance has nothing to do with it other than the maths are based on items at infinity. It works in practice to a 1st approximation anyway so it doesn't matter if the lens is focused at twice it's focal length or several thousand times in principle the minimum circle of confusion in theory at least doesn't change. I practice no lens is perfect so there will be ??? variation. This touches on why some macro photographers shoot at the lowest F number and take many shots at different depths and stack them to obtain a greater depth of field. They get maximum resolution that way. It more complicated than that in practice.

The achievable depth of field then comes down to the final size the image is going to be viewed at and the viewing distance. This sets the maximum circle of confusion that can be allowed on the sensor as it will be usually magnified when the final image is viewed. There is some odd things on the web in this area. My understanding is that the usual 0.030mm or so quoted for 35mm actually relates to the final image if it's to be viewed at 250mm (10in) not the circle of confusion on the sensor. 0.030mm on the sensor would come out at 0.21 mms on an A4 print which with good eyesight would be easily visible from 250mm. Fortunately it's not that critical really and often just like an artists painting the main subject is well defined, jpg compression and the cameras colour interpolation etc helps too and a number of other things. Some depth of field calculators list circles of confusion sizes after the format sizes. Sometimes it's important. You might have a subject for instance that needs detail at the pixel level of the camera and that really does reduce the depth of field and the highest F number that can be used. Conversely you may have a subject that the set up just can't resolve with sufficient clarity and finish up with a mess. You will have to google and read up on MTF and MTF50 to get into that. In many ways it's best to bear it all in mind as it may explain why some shots just don't work out on a screen or printed. In the extreme for 0.030 on a print it's about 800dpi and if I remember correctly a pc screen can only achieve less than 10% of that. Reduction to a jpg and sharpening also tends to masks these problems.

John

[kavan]

Hello John,

so we focus at some particular distance D, the image is perfectly crisp. If the image plane is fixed in position,
as we move forward or backward from that distance D, the objects will appear more blurred (due to nonzero circle of confusion).
The depth of field is usually of finite size and symmetric about D. Can the depth of field be asymmetric too? Does that happen in optical systems with multiple lenses?

Now, where can we pick up from here to explain how the hyperfocal distance H works? Instead of D, we are focusing at H (determined by f# and blur circle have been set).....

thanks
kavan

You should have asked about this in the main forum not here.

I think the web is causing a lot of confusion on this subject. The whole idea is simple really. Focal depth to some circle of confusion increases with distance so there will always be more depth of field past the actual focal point which would ideally have a circle of confusion determined by diffraction. That's 1.36xFratio in um (0.001mm units) for a perfect lens. Varies with colour but there isn't much point in worrying about that. Distance has nothing to do with it other than the maths are based on items at infinity. It works in practice to a 1st approximation anyway so it doesn't matter if the lens is focused at twice it's focal length or several thousand times in principle the minimum circle of confusion in theory at least doesn't change. I practice no lens is perfect so there will be ??? variation. This touches on why some macro photographers shoot at the lowest F number and take many shots at different depths and stack them to obtain a greater depth of field. They get maximum resolution that way. It more complicated than that in practice.

The achievable depth of field then comes down to the final size the image is going to be viewed at and the viewing distance. This sets the maximum circle of confusion that can be allowed on the sensor as it will be usually magnified when the final image is viewed. There is some odd things on the web in this area. My understanding is that the usual 0.030mm or so quoted for 35mm actually relates to the final image if it's to be viewed at 250mm (10in) not the circle of confusion on the sensor. 0.030mm on the sensor would come out at 0.21 mms on an A4 print which with good eyesight would be easily visible from 250mm. Fortunately it's not that critical really and often just like an artists painting the main subject is well defined, jpg compression and the cameras colour interpolation etc helps too and a number of other things. Some depth of field calculators list circles of confusion sizes after the format sizes. Sometimes it's important. You might have a subject for instance that needs detail at the pixel level of the camera and that really does reduce the depth of field and the highest F number that can be used. Conversely you may have a subject that the set up just can't resolve with sufficient clarity and finish up with a mess. You will have to google and read up on MTF and MTF50 to get into that. In many ways it's best to bear it all in mind as it may explain why some shots just don't work out on a screen or printed. In the extreme for 0.030 on a print it's about 800dpi and if I remember correctly a pc screen can only achieve less than 10% of that. Reduction to a jpg and sharpening also tends to masks these problems.

John

[ajohnw]

Optics are optics so in principle a system with one lens behaves the same as one with 10 as far as diffraction and circles of confusion go. The extra lenses are hopefully there to make the system better optically.

What you need to get your head round is the fact that for some certain circle of confusion depth of field increases with distance so for instance say you focused to infinity to shoot a scene you would have less total depth of field than if you focused until the circle of confusion meets what ever you are after at the infinity point. The focus distance scale if there was one would be showing something less than infinity and you would have more depth of field. The minimum circle of confusion would still be at the distance on the scale.

To me this area is a lot of hype. It can be used with care but it's difficult to ensure that the distant items is at the required focal point for what ever circle of confusion is required. Where circles of confusions do come into photography is in portraiture and advertising shots, fashion photgraphy etc etc and when pins sharp shots are needed at full frame on digital cameras. As far as the last point is concerned it will vary according to the lens - quality in many respects and also diffraction. The other areas needs a tape measure and sums. The general idea is a sharp subject and a very slightly blurred background. How much blur is debatable but it has the effect of making the subject stand out. Artists often use the same technique for the same or a slightly different reason. Some part of the painting will be done with elaborate care. That's what draws the eye. The rest may not be anywhere near as clearly defined sometimes and more often in that case to save time. The odd thing is that the brain doesn't notice until people look at the painting piece by piece. With photography generally the blur should be such that it is still possible to see what the back ground is but not so much so that it detracts from the subject and draws the eye - that's how it makes a subject stand out. If you are going to worry about this area that is a much better thing to worry about. It's useful.

John