Manual Focus Distance Window On Lens

[table1349]

In practical terms the difference in the circle of confusion between a FF Canon at 0.030mm and a 1.6 crop sensor camera at 0.19mm is slight. Do the math.

Hyperfocal distance, near distance of acceptable sharpness, and far distance of acceptable sharpness are calculated using the following equations (from Greenleaf, Allen R., Photographic Optics, The MacMillan Company, New York, 1950, pp. 25-27):

Hyperfocal distance:

Near distance of acceptable sharpness:

Far distance of acceptable sharpness:

where:

H
is the hyperfocal distance, mm
f
is the lens focal length, mm
s
is the focus distance
Dn
is the near distance for acceptable sharpness
Df
is the far distance for acceptable sharpness
N
is the f-number
c
is the circle of confusion, mm

f-number is calculated by the definition N = 2i/2 , where i = 1, 2, 3,... for f/1.4, f/2, f/2.8,...


Calculations using these equations must use consistent units. When focal length and circle of confusion have units of millimeters, the calculated hyperfocal distance will have units of millimeters. To convert to feet, divide H by 304.8. To convert to meters, divide H by 1000.

 

[Garbz]

Well there you have it. Take graphics calculators with you folks

Maybe I'm misreading the post but Garbz you seem to suggest that everything might not be tack sharp at f22? So if I focus to infinity and f22 will I have more tack sharp depth of field then at f2.8 focused on infinity?

Of when you focus on infinity do you give up the focus before infinity on your lens?

The easiest way to visualise this would be a series of bell curves. Which look like this: http://willohroots.files.wordpress.com/2009/09/bell_curve.jpg incase you don't know what I'm talking about.

With the horizontal axis being the distance, the middle of the bell curve the focus point, and the vertical axis sharpness. Now imagine at f/2.8 that bell curve is very very thin. Sharpness drops off quickly from the focal point. At f/22 the bell curve is very very fat. Sharpness will drop off slowly, but it it will drop off, i.e. It WILL have a peak sharpness as before.

The hyperfocal distance defines an "acceptable" sharpness whereby a normal person with normal vision using a 35mm film camera looking at a 6x4 print from a normal distance of arms length (correct me, i'm not sure on this) can not notice a visible difference between the focus point and the infinity point.

This all gets tossed out the window if you decide to print say a 40" wide print. The person now probably standing pretty close will see much more detail (meaning the circle of confusion is smaller). Thus even if it's the same image as the 6x4 they will now see that the background may not be quite as sharp as the focal point.

A work around for this is to find the hyperfocal distance and then focus a bit behind it. By pushing the focus point back you're moving your bell curve closer to to the back meaning that sudden cut-off at infinity would be at a higher point on the curve (sharper). But one thing you can not do is take a picture, and then zoom in as far as possible on the digital camera. Again the CoF would be different than when viewing the print, or when viewing it on your monitor.

Summary: You're right, I'm suggesting everything may not be tac sharp at f/22. There will still be one single point of ultimate sharpness. Everything else will just appear sharper than at f/2.8

 

[Vautrin]

So let's say I stop down to f 32. Yes, there's diffraction so the center of the bell curve will be less peaked, but will the tails be fatter and longer?

 

[Overread]

Essentially yes they will be however the following two images might give you an idea of diffraction

f5point6 at 5 times on Flickr - Photo Sharing!

f16 at 5 times on Flickr - Photo Sharing!

The aperture values on the listed shots are a little obscure because of the nature of the lens in question, but it gives a good showing of the sort of thing you are expecting to see. The "f16" shot has a very high depth whilst the f5.6 lacks this - however you can clearly see that the f16 shot is soft all over dispite being taken exactly the same way as the far sharper f5.6 shot.

Sadly this is macro so the 5.6 does not show the hyperfocal distance aspect, but as an (extreme) diffraction display it works rather well.

 

[Vautrin]

Very useful...Thank you...