Thomas78
Well-known
As I was trying to check the focusing of my Super Ikonta 531/2 with a Tessar 105 mm f/4.5 I observed that it shows front focusing at full aperture - for a focus at 1.50 m I had to turn the scale to 1.65 m while it look as if it is spot on at f/8 and 1.50 m.
Is this ammount of focus shift normal for front element focusing tessar type and triplet lenses?
Is this ammount of focus shift normal for front element focusing tessar type and triplet lenses?
Roger Hicks
Veteran
Fascinating! Dunno the answer but I'd love to hear from others.
Thanks,
R.
Thanks,
R.
sevo
Fokutorendaburando
Hard to tell for Super Ikonta Tessars, there are so many variables to their focusing that you can't trust the scale or rangefinder either. But it sounds perfectly possible.
But I doubt it can be generalized. The effect of the same phenomenon on unit focused Tessars (or if we assume it to be a Triplet legacy, anything from Triplet to Sonnar) would be that the front element would be the spot to adjust for minimum focus shift. But I cannot find any reference to that, instead it seems to be sometimes used to adjust the focal length. So the behaviour seems to vary, at any rate across multiple design generations and brands.
But I doubt it can be generalized. The effect of the same phenomenon on unit focused Tessars (or if we assume it to be a Triplet legacy, anything from Triplet to Sonnar) would be that the front element would be the spot to adjust for minimum focus shift. But I cannot find any reference to that, instead it seems to be sometimes used to adjust the focal length. So the behaviour seems to vary, at any rate across multiple design generations and brands.
Roger Hicks
Veteran
Hold on. Doesn't changing separation necessarily change focal length, which is how it changes focus? Or have I for decades been labouring under a misunderstanding?
Cheers,
R.
Thomas78
Well-known
Yes, the front element ist used for focusing the lens, it should change the focal lengt of the lens so that the relation of actual focal lenght and lens to focal plane distance (which is fixed) is in the right relation for the object to lens distance to be in focus.
(I hope someone can understand what I am trying to say.)
Roger Hicks
Veteran
Yes, I think I understand you, and if I do, your understanding is what I understand too.
That's if ANYONE can understand my construction...
Cheers,
R.
sevo
Fokutorendaburando
Of course it does. My point was that manufacturers won't recommend procedures for focus fine tuning that involve front element spacing if the penalty is significant focus shift, as the alternative, shimming the unit distance, is bound to be free from the latter.
Roger Hicks
Veteran
Sorry for misreading your post. I'm quite busy -- buzzing around like a blue-arsed fly, comme on dit en anglais -- and any time spent on RFF is pure displacement activity. Hence my laziness and puzzlement.
Cheers,
R.
Thomas78
Well-known
I did no fine tunig, but checked if I put the lens together the right way as there are ony three different ways to put the lens back together as there are three holes in the front element to fit the screws from the focusing ring.
The fine tunig is done by shims between the lens and the lens mount at the body.