I think I may have bit off more than I can chew. That being said, I am now the proud owner of a Turner Reich Convertable lens. I have read the previous threads regarding this lens However there are still questions looming. This lens is a 12" 19.7" and 25". How do you get 12 and 19.7 to equal 25? Why do you remove the front cell? Why not the back? How do you tell how many elements each cell has? This lens is mounted in a Betax Shutter #4 the speeds seem ok. I have not yet used the lens I will get the spare board filed out a bit in the morning. Do you think I should try to manufacture a hood that shades the shutter with no lens on the front to reduce flare? Is there a particular sweet spot regarding aperture? When you talk of focus shift should I look at the overall image once the lens is stopped down or just the center? I am confused about the focus shift it just doesn't make sense. I am not sure if i am confused because of the heat or my age. not a blonde. thanks in advance for your help in sorting this out. j

-- jacque staskon (jacque@cybertrails.com), July 16, 1999

Answers

Hi Jacque - I don't know that particular convertible, but I think one can apply the general rule about composite lens: 1/F = 1/F' + 1/F" where F is the total focal lengt, and F' and F" are the focal lengths of the single cells. So I think one cell has 19.7", the other has 25", and the two cells together have 12". In effect if you apply the formula you will obtain 11" for the total focus lenght, but the formula does not take account of the air space between the two cells. Always as a general rule, using the two cells together one should put the longer-focus in front of the aperture iris and the shorter one behind; using only one cell, it should be placed behind the iris. On assembling the lens in these ways, the optical aberrations (barrel and pin-cushion) should be reduced at the minimum. But for my opersonal experience with other convertibles, the first rule is all-right, the second depends from the lens. Regards, Franco

-- Franco Rallo (f.rallo@ele.uniroma3.it), July 16, 1999.

You don't get 12+19.7=25.

On my lens, the two cells, are marked Back 19.7" 500.4 MM and Fr 25" 635 MM.

When used in combination, with the 19.7" in the back and the 25" in the front you have a 12" f7.0 lens. Take the front off and use only the 19.7" in the rear and you have a 19.7" f12.5 lens. Place the 25" in the rear and you have a 25" f16 lens. Converted, the element that you choose foes in the rear and the other element foes in your pocket, camera bag, etc. As to why the element goes in the rear, it has to do with the optics of the lens as Franco states, I just know that I have tried it both ways and it works better doing it as recommended.

As to a lens hood, I don't think so. In my experience, the type of flare you ae concerned about comes from light directly hitting the front lens element, which is gone when converted. I usually hold a dark slide so as to cast a shadow on the front of the lens, this does the same thing and costs and weighs nothing.

I find that the entire image suffers from focus shift and that it is in relation to the f-stop used. The smaller the stop the more focus shift. I just refocus off of the center, say a little prayer and fire the shutter. The focus shift is real, I'm sorry I don't have the technical info to explain it.

I find that they work well at all the apertures, but at f45 and f64 they work a little better for closeup work. For scenics if I can shoot up to f22 or f32 I seem to like the results a little better. All very subjective and very dependant on subject matter and materials used.

The shutters don't cost much to get cleaned, lubed and calibrated, I have fellow tht does them for around $50.00 U.S. Mine is dead accurat on a bench test at the stated speeds, and it performs that way in the field.

I wouldn't get to hung up on the semantics, just take it out and use it. They have a lot of little pecadillos that take some getting used to, but trial and error, and a few good notes will steer you around the pitfalls.

-- Marv (mthompson@clinton.net), July 16, 1999.

Hi

Just another note re correcting for focus shift. Trying to correct the focus after stopping down can be a pain. The ground glass can be quite dark and you can barely see the stuff. Also your aperure is small and DOF can interfere with your ability to hit sharp focus. Here's what I suggest. Focus the camera on a bright object, a lamp or some specular highlights (I find a lamp works well). Mark the point on your monorail/bed/whatever. Now stop down. Even though the ground glass is dark, the lamp is very bright and you can focus on it quite sharply. I would suggest you actually move the standard out of focus and refocus (this seems to help with the DOF problem i.e., you are now looking again for the sharpest focus point - when i try to just refocus from what i know is a focussed position, I don't seem to do it accurately). Mark this on your monorail again. Check the difference between your two marks. That's your focus shift at that aperture. Make a note of this and toss it into your camera bag. Repeat for every aperture you think you will use. Now you have a list of how much you must correct for focus shift when you stop down. Makes life easier in the field to have these numbers on hand. You can just do a quick check on the groundglass to make sure its looking in focus as opposed to sweating under the blanket and squinting at a dim GG. Hope this helps.

DJ

-- N Dhananjay (ndhanu@umich.edu), July 16, 1999.

Many thanks to Franco and Marv. Your advice helped tremendously. Thanks. I did some shooting with the lens this weekend and am very pleased with the results. I am still working on the focus shift you are correct its not easy to see through the lens at smaller stops. The 8x10 I own is a Wista although I love it, it must not have the bellows draw to use the 25" all alone. Every thing I read about the large format seems to indicate that there are two camps here. The first camp is the mathmatical engineer type, they know exactly what to expect and do extensive testing on everything they own and the second camp works more on trial and error and seems to be less concerned with the absolutes of large format. I am grateful to both sides of the coin. This forum provides a wonderful balance. Thank you once again for your input.

-- jacque staskon (jacque@cybertrails.com), July 19, 1999.

I definitely fall with a resounding thud into the latter camp. N Dhananjay, are you telling me that the shift can be corrected by the same amount of correction in extension at any focus distance for a given aperture? Or did I misunderstand? This seems hard to believe, but then....I..uh..THUD!

I just became the proud(?) owner of own of these babies for not much more than the Betax 4 is worth (my reason for buying it).

-- Wayne (wsteffen@skypoint.com), May 07, 2001.

Here is what I know about focus shift. Somebody please chime in if I'm off base on any of this.

Focus shift is supposedly caused by uncorrected zonal spherical aberration of a lens. All lenses suffer from this defect to some extent but often it is small enough not to be noticeable. Some types of lenses are particularly prone to it, Dagors and related designs supposedly being notorious. The idea behind spherical aberration is that rays passing through the edges of the lens are brought to focus at a different point than the rays passing close to the center of the lens.

Spherical aberration, in effect, makes the lens have a band of focal lengths near the design focal length, in other words, an object at a fixed distance will produce a series of focussed images as the lens is moved back and forth around the point of apparent best focus. This last is important - focus shift also depends somewhat on how you judge focus -we usually focus visually, the eye will judge the point of best focus mostly as the point of highest contrast.

The actual point of focus from the area near the center of the lens doesn't change, it is just overlayed with out of focus light from other parts of the lens. If you judge focus mainly by image contrast the point of best focus will seem to wander as the lens is stopped down - this is because local contrast will seem to change and also spherical aberration is being reduced as you stop down. If you look at the core image to judge focus you may not be aware of focus shift except as an increase in image contast. This is why some people think their lenses do not suffer from this effect. Generally, if you see a halo of light around highlights as the lens is opened up the lens is likely to have focus shift.

So, the point of apparent best focus will seem to change as the lens is stopped down. Depending on how you interpret maximum sharpness you may or may not experience as much focus shift as someone else - its partly an optical illusion. The focus shift of lenses which have it will be pretty much gone by the time they are stopped down about three stops from maximum opening. The worst lenses are supposed to be single elements of convertible lenses.

Folks have suggested that the focus shift would tend to be in the same direction at all stops, depending upon whether the lens was under corrected or over corrected. To my suggestion about using the bulb as a subject. As the above indicates, it is perhaps more correctly described as a way to figure how you judge focus i.e., do you vary focus as the lens is stopped down or not. Keep in mind that a lot of sperical aberration is actually eliminated as you stop down. The bright bulb allows you to actually find 'a point of best focus' (whatever that means) when you are stopped down. That is, what you are actually worried about is whether you judge the focus point at a different place when the lens is wide open for focussing. I would hesitate to comment on whether the focus shift is the same at all apertures or not, given the fact that some people seem immune to the effect in the first place. I would speculate that it should vary since the sperical aberration is being reduced as you stop down but there's probably complex relationship based on the contrast inherent in the subject etc. The exercise will tell you a) whether you should worry about focus shift and b) if you should worry about, in what direction and roughly by how much should you worry about it.

Sorry to be so long-winded. Cheers, DJ.

-- N Dhananjay (ndhanu@umich.edu), May 07, 2001.

I've always believed that a picture is worth a thousand words :-)

The JPEG shows the typical situation for a singlet lens. Since line drawings often look terrible online, it is linked to a PDF which should at least print nicely.

The point is that the outer parts of the lens focus light at a different distance to the parts at the centre - in this case more closely to the actual lens. Instead of a single point of focus you get a sort of tube of light near the focus point. 'Best focus' is usually chosen as the place where the tube is thinnest. Note that this has nothing to do with bad manufacture: a lens with perfectly-ground spherical surfaces will have this problem - it's inherent in the geometry.

If you stop the lens down the whole tube becomes narrower (which is why apparent depth of field still increases) and the point where its cross-section is smallest moves away from the lens. This is the cause of focus shift in most LF lenses like Dagors and the single cells of convertables.

Note that other aberrations will complicate the shape of the tube, especially off-axis where coma and field curvature come into play. Even spherical aberration will vary with the distance to the thing you are photographing, so a simple, one-time measurement of the shift can be useful but in general it is best to squint at the ground glass if you can.

Note also that there is also room for subjectivity when deciding where the 'best focus' actually is, because the pattern of light within the tube changes. Look carefully at the wide-open diagram. If the film is a bit closer to the lens than the marked 'best focus' position you will get a bright ring of light because of the way the rays converge. This will make edges sharp at the expense of doubling very small features and turning point sources into rings. If the film is further away than the marked 'best focus' you will have a bright point surrounded by a halo, which is the classic flattering diffusion used by portrait lenses. Which is 'best' depends on what you are photographing and your personal tastes.

Here endeth the lesson.

-- Struan Gray (struan.gray@sljus.lu.se), May 08, 2001.