Telescopes: Depth of Field

If you do calculations it appears that plus lenses and telescopes have the same depth of field, but px feel it to be less in telescopes. This could be because the dept of focus for the telescope represents a smaller portion of the working space. Anyway here are some more lovely calculations

Ok, the px has 0.5D depth of focus. What change in incident vergence or object distance could the px tolerate:

a. With a plus lens mag of 10D

f = 10cm if obj at focal point

let emergent vergence = +0.5D
L' = L+F = 0.5-10 = -9.5cm

let emergence vergence = -0.5D
L' = L+F = -0.5-10 = -10.5cm

So overall there's a 1cm range

b. With a 2.5x scope with a +4.00

Emergent vergence Ltel = L'tel/M2 = +/-0.5/2.52 = +/- 0.08m

The incident vergence at the reading cap is of course -4D to match up with it so

delta l = (1 / -4D+0.08D) - (1 / -4D + 0.08D) = (1 / -3.92) - (1 / -4.08) = 1cm

NB Aberrations may also have an effect on the depth of field.

NB2 Changing the obj posn can alter the vergence of light entering the eye. This could create a refractive correction for spherical ametropia

• Move obj closer to magnifier: light divergent leaving magnifier so uncorrected myopes need to hold material closer
• V.V. ie uncorrected hyperopes need to hold material further away
• The effect is similar for both plus lenses and telescopes but because of vergence amplification the effects will be much more severe for telescopes