Front cell focusing: optimal distance setting ?

I wonder at what setting of distance a front cell focusing lens

(Tessar type)equals its normal brothers at image quality. Is it at

infinity, the hyperfocal distance, etc ? There must have been a rule

based on experience and optical laws that lens designers used to rely

on. I'm thinking of lenses of 6x9 folders mainly and the possibility

to use a front cell focusing lens with the front cell fixed at the

right distance on another folder camera where the lens in total is

brought into focus by moving the lens panel.

 

Ernst Dinkla

May be there is some scientifically based approach, but practically I would think of using a high quility focusing screen (say, a spare one for any of 35mm SLR with interchangable screens) and a good loupe. The good thing would be to attach the screen at a proper distance to the loupe first. Then with the lens fitted on folding camera and the shutter on T one can point camera on some contrasty object and move this screen/loupe combo inwards/outwards in a focal plane to focus. Based on my experience one can judge the resolution quite efficiently in this way. And it really differs for different front lens element positions! One can do this conveniently indoors with a light off and a tripod mounted camera and nicely illuminated texbook as an object.

 

From what I've discussed with from other people and basing on what I've seen, normally the best resolution is achieved at middle distances, say 5-10m (group snapshot distance).

 

Well, probably, you can't judge abberations and center/corners quality so well in this way, but the central resolution can be seen efficiently.

 

Hope this might be of some help,

This whole discussion should be in the context of what the cameras were used for ~ snapshots rarely enlarged more than 2 times. The advantage of front element focussing is primarily the small distance the front element has to move to focus. Think of it as a short focal length lens (positive front group) and a tel-extender (negative rear group). The short focal length requires little movement for focus. In all probability the thing is set up for sharpest performance in the middle of its range.

Dear Ernst,

 

according to J. Stüper, Photographische Kamera, 1962, a lens with front focusing is designed to have optimum performance at a distance of 40x focus_length, hence 2m for an 50mm Triplett or approx. 4m for a 6x9 camera. Thus, good performance for such a design is then provided between 20x focus_length and infinity. If a Tessar design is set to 4m and fixed then, the quality will be much the same for far distances, because Tessar's are robust in variation in object distance.

 

On the other hand, a lens with front focusing which is optimised for 40x focus_length the spherical aberration will be overcorrected for far distances, this is not a problem because for landscape photographie the lens is used with smaller stops most times and the increase is small. If you fix the front lens at infinity setting, the overcorrected spherical is (not exactly) fixed for all distances and subsequently reducing the quality. Best regards,

Good answers guys,

<P>

However, I would bet on a tessar type lens that you would see far greater results by setting the lens at infinity and always stopping up to F16 or above. The f:stop will be more ciritcal than where the front element is set. Remember, when shooting at high f numbers that there is an increase in deth of field at both the subject and film plane.

Thank you all for the answers.

 

Before I asked the question I tried to find an answer in Cox's book on optics but couldn't find it. I will try the theoretical options and check them with a groundglass etc.

Maybe the rule was adapted to the expected use of the camera.

 

I'm collecting some Polaroid 150, 110 A, 120 models and convert them to use either the Polaroid sheet film holders, Grafmatic or the Fuji Quickchange 4x5 holder permanently. That makes them a kind of point and shoot large size cameras. The smaller Polaroid film holder with a size of 3.25 x 4.25"(665 film etc) will be too big for a 6x9 folder 105 mm Tessar lens but I do not mind some fall off etc at the sides. The sheetfilm is at least a lot more plane than in most 6x9 folders.

That's what I've seen with the normal Rodenstock Ysarex lens of the 110 A. And getting B&W film instantly is fun so far. This is a low budget experimental thing. First lens I will try to use is the Color Skopar 105 mm of a Bessa 1. If it works the next step will be to change the cam of the Polaroid rangefinder to the 105 mm lens used. Parts seem to be big enough to use the blacksmith skills that I have. The Ysarex then can be used for Polaroid 110 A project with a Fuji Quickchange holder.

 

Are there any other good folder lenses that have a wider angle at approx. 100 mm than the Skopar ? If that stage has gone well I may go for a Wide Field Ektar 100 mm but first I have to see whether I'm qualified for the job.

 

Good answers, all of them, again thank you, Ernst Dinkla

You will get great results with a Skopar. On my page on this site, I have a glamour head shot that was done with a 135mm Skopar on a Voigtlander Avus. It is the highest quality portrait I've ever done. The lens is quite sharp, but gives a long, smooth gradation. Nothing quite like it.