Unable to Focus Accurately with Wide Angle Lenses

[Michael Perlmutter]

With my eyeglasses removed (they are not bifocals), I am unable to get the ground side of the GG in focus with the loupe I have. It feels like I would need to turn the focus ring another 180 to 275 degrees beyond its stopping point to get it sharp. Does it mean I need a better quality loupe, or is it that my eyesight is too far gone!?

Is my fresnel misplaced? If it is, then why, when using a loupe (with eyeglasses on), does an object appearing sharp when the fresnel is on also look equally sharp when I remove the fresnel and look again? Shouldn't I see a clear difference right away? Furthermore, if I placed the fresnel on the lens side of the GG (which in practice means the GG and the fresnel would switch places), then it means the plane of the ground side of the GG would be moved towards me by an amount equal to the thickness of the fresnel, and thus differ from where the factory placed it. This seems strange to me.

[tim atherton]

might want to read this as well (although note that a somemanufacturers do actually produce a GG setup with the fresnel on the inside of the GG - Toyo for example, I think, but that is basically a factory set-up)


http://www.wisner.com/viewing.htm

[Gem Singer]

Michael,

Judging from discrepency you are describing, yes, you probably do need a better quality loupe. You should be able to sharp focus the ground side of the groundglass, with your eyeglasses removed, using only your loupe, and adjusting the diopter setting on the loupe. If your eyeglasses are able to correct the discrepency, your focusing loupe should also be able to make the correction. The combination you have been using, your eyeglasses plus your loupe, is probably the cause of the focusing discrepency you have been experiencing, not your Fresnel.

[Ralph Barker]

It sounds like your eyes are unable to focus on objects that are very close, Michael. That's not uncommon, I believe. For example, without my glasses (bifocals), I can see quite well at a distance of about 10-12" or so. Any closer is all a blur. The focal distance of the bifocal segment of my glasses are set so I can focus on very small print at a distance of 5" or so.

Thus, you might need a loupe that provides both built-in diopter adjustment and adjustable focus. Or, you might find the inexpensive "reading" eyeglasses available at drug stores (and targeted toward the "chronologically-enriched" buyer) helpful. They are available in several diopters, so you should be able to find a pair that enables you to focus at 3" or so. (It's only your "loupe eye" that is important.)

Another approach that has been recommended in the past is the spectacle loupes used by jewelers. These are set to work at very close distances, and clip onto your regular glasses.

[Claire Curran]

Michael says in his post that other people have also focussed his camera and had the same problem with the camera. I can't imagine they all have eye problems. Does this focussing problem happen only with the 58m and 72m lenses? Please let us know what other lenses you use and if the problem occurs with them.

[Leonard Evens]

This discussion has been very interesting. The others have suggested all sorts of possibilities. I focus regularly with a 75 mm lens at f/4.5 in 4 x 5 and a 65 mm at f/7 in 6 x 9. I haven't encountered what you describe, so it doesn't seem to me that this sort of thing is inevitable with wide angle lenses.. Let me suggest a couple of other things to think about.

First, don't trust measurements made with a caliper. Set up a target with lots of fine detail, like a page of newsprint, and photograph it at a 45 degree angle from about 15 times the focal length away. Use the highest power loupe you can find to focus carefully, and make an exposure. Repeat the process several times and then examine the negatives carefully. Do this first with a longer lens, and repeat with your wide angle angle lenses. You should see some shift but it should be quite small.

Second, as has already been pointed out, the shifts in the image plane for the lenses you are using at the distances you describe are pretty small, certainly less than 1 mm. The circle of confusion for someone with normal vision viewing an image from about 10-12 inches is supposed to be about 0.2 mm. With a 4 X loupe, it would be reduced to one fourth of that or 0.05 mm, but given that the surface of the gg is not perfectly clear and that you don't have the best vision, it might be more realistic to estimate the viewing circle of confusion as 0.1 mm. For sufficiently distant subjects (measured relative to the lens focal lenth), the depth of focus is twice the f-number times the circle of confusion. For f/5.6, that would come out to twice 0.56 mm or 1.12 mm. That means you might not be able to see any difference in the sharpness of the image over a range of 1.12 mm, or to be more precise within 0.56 mm on either side of the true position of the exact image plane. If we used .05 mm for the circle of confusion, that would reduce the depth of focus by half, but it would still be significant. Note that this is inevitable. By the laws of optics, there has to be a certain range in which the image looks equally in focus. Also, it doesn't really have anything to do with the focal length of the lens. The only way that enters is that the corresponding subject distances are much closer to the lens than for a longer lens.

So here are some suggestions for improving the situation.

First, consider using the highest powered loupe you can find. That would reduce the viewing circle of confussion and also the depth of focus for critical focusing. Unfortunately, that is likely to create problems with a normal gg because you will tend to focus on imperfections on the gg surface rather than on the image. so get the finest, brightest gg you can find.

Second, consider using the method of focusing described in the large format photography web page. In this method, you first focus on the nearest point you want in focus, note the position on the rail or bed, then on the furthest point, noting that position, and then place the standard halfway in between. Of course, you will also have the same depth of focus issues in focusing on the near and far points, but there are tricks for having the focusing errors cancel out. For example, you can approach the two points from opposite directions, so if you overshoot, you are likely to overshoot in compensating directions. You can also do the procedure several times, and take a rough average for the final position of the standard.

[Paul Fitzgerald]

Hi Michael,

As an aside check ebay item #3857124892, it may be useful to you.

"When focused on an object 6 meters in front of the lens, the only thing sharp on film when shot at f/5.6 was an object 4 meters in front – 2 meters off."

You just said that your ground glass is closer to the lens when focused than your film is. Your using an American camera with a Graflok back, the fresnel should be in front of the ground glass with the grooves towards the lens. The spacing difference would fix your problem.

A dial indicator is a type of caliper for measuring depth and run-out. I just measured a set of 12 new Fidelity Elite 4X5 holders. The max. difference between the shallowest & deepest was .035". The circle of confussion for 4X5 is .004" so tolerance should be .002". The only way to fix this that I have found is the shave down the deep side to match the shallowest side in the set and adjust the ground glass to match. A lot of fussy work BUT who wants to blow a once in a lifetime shot for a bad film holder.

The lenses you mentioned are new, high quality lenses design to fit Copal shutters that are now standard in size. There could be no problem with them being swapped, they would still work fine. This 'swapping' is only a problem with older lenses that were in barrels or re-shuttered. Lens spacing needs to be correct BUT the iris placement must be correct also or the lens will focus shift when stopped-down.

Happy holidays.

[Michael Perlmutter]

Claire: I also use 90mm, 110mm and 210mm lenses. As I recall, I only did a test with the 110mm (though for some reason I cannot find that test sheet just now ... it was all done 7 years ago, as I mentioned). As I remember it, I had the same type of problem with the 110mm, but it was not nearly as extreme as the wider lenses. Hence my concern that the problem gets worse as the lens gets wider.

I am nearsighted, not farsighted, which means I can see things up close quite clearly without glasses. With that in mind I shouldn't have trouble seeing the ground glass through a loupe with my glasses removed . . . that is, when I get ahold of a loupe that I can focus given my particular eyesight. I think this shall be my first area of investigation.

Many thanks.

[Henry Ambrose]

I ground the bottom of one of my loupes so that it would focus for my eyes. The normal screw adjustment was not enough. If you can see acutely at close range you should be able to focus these two lenses adequately without a loupe or get very very close. But the difference in visual focus on the GG and actual focus on film you write of seems huge.

[David Karp]

Hi,

I think that Eugene is on to something here. I am nearsighted and must remove my glasses to use the loupe, and to frame and rough focus. My loupe allows me to focus, and for me, focus is at the extreme of my loupe's ability to adjust. (Unfortunately, I can't give you a brand to try since it is an old loupe that I found somewhere, and the brand name and power information have long been rubbed off.)

I do not think the problem is with your Fresnel lens. If you are using the Cambo 45SF with a Cambo Fresnel, it is designed as the snap on, snap off type, and properly sits on the photographer's side of the groundglass when it is in place. This is to allow its use while composing, while giving the photographer the option of removing it for fine focusing, just as you described above.

I have a Cambo 45SF and a Calumet 45NX (both with similar backs - one is reversible, the other revolves), and have not experienced this problem with my 75mm and 90mm lenses. I do not think it is an inferior product, as others suggested.

A while ago people reported having problems rough focusing 80mm XLs wide open. Was this a problem with the 58mm and 72mm also? I know you photograph interiors, but perhaps rough focusing stopped down a bit might make a difference. Or perhaps try focusing on a small flashlight pointing back at the camera. (Along these lines, do you have the same problem with photographs taken outside in bright sunlight?) I don't know if any of these suggestions are useful, I am just grabbing at straws to find something that might help.