Still my Caltar 90mm closeup focusing saga continuing...
As per advise of other LF fellow, I drew the bellows of my Shen-Hao 4x5 to their maximum (about 30cm) and instead of fidling with front standard looking for focus on the subject, I grabbed the tripod with the camera with bellows fully extended and started slowly getting closer to teh subject watching the GG.
That did the trick - was able to find focus at about 10-13cm from the subject ! (i.e. this is the distance between the front surface of the lens and the subject).
The magnification was quite large, about 5:1 or something like that...
Then, fixing the tripod location at this point, I intended to get a feeling of close focusing by pulling the subject very slightly away from the lens and trying to find focus again by means of front standard (pulling the front backwards). To my surprise I wasn't able to focus at all, moreover, the more I pulled the front standard backwards, the more out fo focus diffusion I got on GG. I was confused, however, then went once again fully extending the bellows and trying to focus by pushing the back standard towards the lens - BANG - caught up the focus pretty quickly ! That was a real revelation for me, so far I have never used back standard for focusing, though till now have never shoot anything but landscapes (i.e. distant views).
Now I learnt improtant thing of the basics of close distance focusing technique.
Trying to base that onto some theory, I apparently made another personal discovery (for me, of course):
Utilizing front focusing I actually alter both simmultenously, lens-to-subject distance and lens to film plane distance which, according to lens formula isn't going to help WA focusing in closeup and in such case the total of film plane -to - subject distance remains the same:
1/L + 1/l = 1/f
where L - lens-to-subject distance
l - lens to film plane distance
so in this case the sum L + l = constant
However, calculating L for full bellows extention (say, 300mm) using f=90mm lens we obtain L ~= 13cm which makes L+l = 43cm.
Now, the similar calculation for l = 20cm of bellows extention yields L = 16.4cm, i.e. lens-to subject distance must be 16.4cm which makes L+l = 36.4cm.
As we see, in order to keep focus we cannot keep film-to-subject distance constant (i.e. sum L+l). Thus, the obvious solution in such case of WA close focusing is utilizing back focusing in which case we only alter lens-to-film plane distance (l) keeping L constant, thereby the sum of L+l is changing int such case and this is what we wanted.
Since in both cases (front focusing and back focusing) the l/L relationship is altered, in both case magnification is altered also.
Now, the obvious question is why front focusing works well in non-close range situations with WA lenses (such as landscapes for instance) ? The answer is clear: for WA lens the lens-to-subject disatnce very quickly approaches to an equivalence of infinity as we pull off the subject, for instance the necessary bellows extention for 90mm lens for subject 1m away is almost at the same as for infinity subject.
Now, if will yet consider the issue of film plane - to - subject distance, it is approaching to infinity due to lens-to-subject distance (L) and if so, even though by front focusing we still alter both lens-to-film and lens-to-subject distances, this doesn't matter because subtracting anything from infinity still yields infinity...
Well, my luck Shen-Hao offers back focusing, albeit a bit stiff and the back moment isn't as smooth as the front, but at least provides and that option which I found useful. Now I learn to appreciate the bulk and weght of monorails allowing full back momevents including as smooth back focusing as the front one.
Sorry for rambling, just feld I had to share my excitement ot revelation
Alex
As per advise of other LF fellow, I drew the bellows of my Shen-Hao 4x5 to their maximum (about 30cm) and instead of fidling with front standard looking for focus on the subject, I grabbed the tripod with the camera with bellows fully extended and started slowly getting closer to teh subject watching the GG.
That did the trick - was able to find focus at about 10-13cm from the subject ! (i.e. this is the distance between the front surface of the lens and the subject).
The magnification was quite large, about 5:1 or something like that...
Then, fixing the tripod location at this point, I intended to get a feeling of close focusing by pulling the subject very slightly away from the lens and trying to find focus again by means of front standard (pulling the front backwards). To my surprise I wasn't able to focus at all, moreover, the more I pulled the front standard backwards, the more out fo focus diffusion I got on GG. I was confused, however, then went once again fully extending the bellows and trying to focus by pushing the back standard towards the lens - BANG - caught up the focus pretty quickly ! That was a real revelation for me, so far I have never used back standard for focusing, though till now have never shoot anything but landscapes (i.e. distant views).
Now I learnt improtant thing of the basics of close distance focusing technique.
Trying to base that onto some theory, I apparently made another personal discovery (for me, of course):
Utilizing front focusing I actually alter both simmultenously, lens-to-subject distance and lens to film plane distance which, according to lens formula isn't going to help WA focusing in closeup and in such case the total of film plane -to - subject distance remains the same:
1/L + 1/l = 1/f
where L - lens-to-subject distance
l - lens to film plane distance
so in this case the sum L + l = constant
However, calculating L for full bellows extention (say, 300mm) using f=90mm lens we obtain L ~= 13cm which makes L+l = 43cm.
Now, the similar calculation for l = 20cm of bellows extention yields L = 16.4cm, i.e. lens-to subject distance must be 16.4cm which makes L+l = 36.4cm.
As we see, in order to keep focus we cannot keep film-to-subject distance constant (i.e. sum L+l). Thus, the obvious solution in such case of WA close focusing is utilizing back focusing in which case we only alter lens-to-film plane distance (l) keeping L constant, thereby the sum of L+l is changing int such case and this is what we wanted.
Since in both cases (front focusing and back focusing) the l/L relationship is altered, in both case magnification is altered also.
Now, the obvious question is why front focusing works well in non-close range situations with WA lenses (such as landscapes for instance) ? The answer is clear: for WA lens the lens-to-subject disatnce very quickly approaches to an equivalence of infinity as we pull off the subject, for instance the necessary bellows extention for 90mm lens for subject 1m away is almost at the same as for infinity subject.
Now, if will yet consider the issue of film plane - to - subject distance, it is approaching to infinity due to lens-to-subject distance (L) and if so, even though by front focusing we still alter both lens-to-film and lens-to-subject distances, this doesn't matter because subtracting anything from infinity still yields infinity...
Well, my luck Shen-Hao offers back focusing, albeit a bit stiff and the back moment isn't as smooth as the front, but at least provides and that option which I found useful. Now I learn to appreciate the bulk and weght of monorails allowing full back momevents including as smooth back focusing as the front one.
Sorry for rambling, just feld I had to share my excitement ot revelation
Alex
