question about focus (which receives little scrutiny)

[David Lyga]

I have a 'kit' Ricoh lens that is K mount but was made for use with the KR-5. Its specs are 2.2/50 and there is something different from this lens that confuses me. The rear element set (behind the aperture) is rigid, does not move when lens is being focused. Only the front element set (in front of the aperture) moves. The closest focus is about 25 inches, certainly not impressive. At infinity the lens is tack sharp, even at f2.2. At the CLOSEST distance, the viewfinder indicates that 29" is in 'focus' when, on the negative, 25" is in focus. At infinity, the viewfinder indication matches the reality on the negative. That discrepancy, alone, surprises me, as always test lenses for both infinity (I have wonderful, distant skyscrapers outside my window) and closest focus.

The overwhelming number of prime SLR lenses are not like this, as all elements are moving as the lens is being focused (because the entire element set is housed in one set). My question is this: there has to be an advantage to the rear elements moving, right? But what is this advantage, in detail, and what is the disadvantage to the rigid rear element set that I have? Many early rangefinders lenses also had this 'fixed rear set' also. - David Lyga

 

[E. von Hoegh]

Focussing a lens by changing the element spacing disturbs the corrections of the lens. It sounds like you also have a screen that is not properly aligned.

 

[David Lyga]

OK, I thought that the screen was properly aligned because infinity was properly matched. I will look into this. - David Lyga

 

[E. von Hoegh]

OK, I thought that the screen was properly aligned because infinity was properly matched. I will look into this. - David Lyga

The screen may be aligned well enough to be within the DOF at infinity, but the mis-alignment will show up at closer distances due to the restricted DOF at close up reproduction ratios. Try it with a different lens.

 

[ic-racer]

Your test for infinity focus is off. How did you test it?

 

[E. von Hoegh]

Sometimes, SLR lenses will focus "beyond" infinity. But if there is a four inch discrepancy between the viewfinder and film plane at two feet, the problem is not in his infinity test.

 

[Rol_Lei Nut]

When a lens focuses by moving only some of its elements it's:

1) a good thing ("floating elements"), aimed at correcting aberrations at closer distances
or
2) it's a cheap approach, making the lens easier to build but will probably negatively affect close up performance


How did you actually measure the distance which gave sharpness on the negative?