Quote:

Archived from groups: rec.photo.digital (

More info?)

How can I determine at what distance the Canon 300D kit 18-55 lense (or any

lens for that matter) starts to focus at infinity.

I am setting up a table of hyperfocal distances and I want to ignore

distances beyond which the lens will be focusing at infinity.

Is it 63 metres or 200metres or 1 kilometre??? Is there an equation that

will calculate this?

regards

PeterH

===========================================

Hi Peter,

Bart's answer is pretty good, although I'm guessing you'd like an actual formula and numbers. So I'll try.

I think Bart is correct about Circle of Confusion size being twice the pixel pitch. If dot of light smaller than the size of one pixel hit one pixel in the center, then the sensor would see that dot as the size of one pixel, as it cannot determine that it is smaller. If that dot does not strike the sensor on exactly one pixel (it crosses the border between 2 pixels and therefore strikes 2 pixels), the sensor will see it being the size of 2 pixels. So for all practical purposes, the smallest detectible dot size reportable by the sensor is 2 pixels in diameter.

First I will give an extreme case current example.

The Nikon D800 sensor is 35.9 mm wide, with 7360 pixels across. So pixel size (pitch) of Nikon D800 sensor is

35.9mm / 7360 = 0.004878mm

So our determining circle of confusion size is .009755mm (twice the pixel pitch)

Therefore any dot of light that is smaller than .009755mm will be seen by the sensor as being 0.009755mm.

Let's round it up to 0.01

Next we have the focal length of the lens. Let's use 50mm.

Then there is the aperture of the lens. The fastest lens reasonably available is f1.2, so we'll use that.

Now we can plug it into the Hyperfocal distance formula:

H= Hyperfocal distance

F= focal length = 50mm

f= f stop = 1.2

C= circle of confusion = 0.01

i= effective "infinity"

F^2 / C*f + F

2500 / .01*1.2 + 50 = 208000mm = 208 meters

The hyperfocal principle shows that when the lens is focused at the hyperfocal distance, everything from one-half the hyperfocal distance, to infinity, is in effectively equal "perfect" focus.

So with our f1.2 50mm lens (wide open) on a Nikon D800, infinity effectively starts at 104 meters when the lens is focused perfectly at 208 meters.

Realistically, no lens, that we might consider, performs critically sharply at the center until f2 or 2.8

For our case, if we use f2, we get a realistic H of 125 meters and i starts at 63 meters.

Now let's look at your 300D with your kit lens set to 35mm and f5.6 (opened wider it would be too blurry to get realistic results).

H= Hyperfocal distance

F= focal length = 35mm

f= f stop = 5.6

C= circle of confusion for 300D = 0.015

i= effective "infinity"

F^2 / C*f + F

H= 1225 / 0.015*5.6 + 35 = 14.8 meters

So we would focus at 14.8 meters and "infinity" would start at 7.4 meters.

300D, 18-55mm lens, f5.6:

18mm H= 4 meters, i= 2 meters

20mm H= 4.8 meters, i= 2.4 meters

24mm H= 7 meters, i= 3.5 meters

28mm H= 9.5 meters, i= 4.7 meters

35mm H= 15 meters, i= 7.5 meters

40mm H= 20 meters, i= 10 meters

55mm H= 36 meters, i= 18 meters

I hope this answers your question.