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Monday, 21 April 2008

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This is an excellent piece, Ctein. It's certainly also a timely piece in the wake of the mini-revival of rangefinders ala the Leica M8. A casual stroll through the Leica M8 forum on any given day is likely to reveal at least one "my camera is back-focusing" thread. No doubt some are, as the rangefinder is a mechanical linkage that can become misaligned on a lens-by-lens basis.

But I've also no doubt that many, maybe most, of these complaints are actually due to the owner's vision. Getting accurate rangefinder focus can be rather tricky in nearly any lighting situation other than bright daylight with a high edge contrast subjects. For someone with astigmatism or in need of dioptric viewfinder adjustment focus would be mostly a matter of luck in any other conditions. Even with good vision it's easy to muff rangefinder focus by putting your finger over the rangefinder window.

My rangefinder cameras have mostly quite accurate focus (knock wood). But your opening remark, "The complex connection between what you see in the rangefinder window and what you get on film is prone to errors. Most rangefinders aren't very precise." is very correct. When precision is required I use another camera.

I always wondered how Zeiss managed to focus every internal mount 50mm Contax lens w/o a custom-cut focusing cam as part of the lens mount (like Leica). I decided that when manufactured they probably optically adjusted every lens to the same focal length.
Recently I found out the truth: their in-body rangefinder profine was based on the AVERAGE focal length of their 50mm lenses, each of which is shimmed at the factory to be correctly focused at only one distance (I don't remember if it was at infinity or somewhere closer), and let DOF cover their arss. So much for the famous ZEISS CONTAX precision!
I don't know about the external mount lenses; they may be individually cammed (or not?).

I have an aspheric loupe for looking at 35 mm negatives. What you say about astigmatism applies fully to users of such loupes. If you are astigmatic (like me) and don't use your eyeglases, then it is a waste of money to buy such an expensive item.

Rangefinders are not the only camera with this problem. In the 1970s I sold cameras at Altmans on Wabash street in Chicago(now long closed). Most sales I handled were then current SLRs. When my customers had problems with unsharp pictures at least half the time it was because they could not see the screen clearly. I sold more eyepiece diopter lenses than any other salesperson. I got to the point that when I sold a camera I made sure they could focus it accurately before they left the store. I suppose now, with auto focus so common and highly developed many new DSLR owners don't even notice if they have the built in diopter adjusted correctly for their eyesight. A pity really.

Ctein, this is very interesting indeed.
Maybe I face a similar problem.

What I don't understand: why does one need a diopter adjustment in order to use the camera with glasses on? I thought the diopter adjustment is for eyeglass wearers to use the camera without glasses - which's sense I never understood anyway...

thanx in advance
Andreas

John Robison,
You were a good salesman. Lack of diopter correction is why I don't use more older cameras more often. Also, my standard for a camera now is that it have built-in diopter correction (as many better cameras now have) not because I object to custom eyepieces, but because I know from experience that after the cameras are discontinued the diopter correction eyepieces will become steadily harder to find, making the cameras less useful to people who need the correction but can't find the proper eyepieces. (If anyone out there has a -2 for a Spotmatic, name your price....)

Mike J.

Dear John,

Rangefinders *are* the only cameras where astigmatism matters. The problem I'm talking about is not merely the difficulty in focusing when the image in the viewfinder is unsharp, it's that astigmatism produces an erroneous indicator of correct focus. To put it another way, a simple lack of sharpness in the viewfinder reduces precision -- sometimes your focus will be on and sometimes it will be off, but the average will be correct (for the fat lot of good that does us). Uncorrected astigmatism reduces accuracy -- regardless of how precise you're focusing is (or isn't), the indicated point of best focus will be incorrect.

Many of us have learned to deal with viewfinders that don't put the projected image at the most comfortable distance review. If we squint a bit, or learn to adapt, we can still focus accurately with a modest amount of blurring in the view, even though it isn't as comfortable as focusing with a sharp image. But if you throw astigmatism into the mix, that is impossible. You may think you're focusing correctly, but you will not be.

That is the import of my column-- a systematic error in focus is most typically a mechanical problem with the camera... unless you suffer from astigmatism.


~ pax \ Ctein
[ please excuse any word salad. MacSpeech in training! ]
======================================
-- Ctein's online Gallery http://ctein.com
-- Digital restorations http://photo-repair.com
======================================

Dear Andreas,

As you get older and presbyopia sets in, which I guarantee it will, you may find that you cannot comfortably focus on the image in the viewfinder even while wearing your glasses. That's where diopter eyepieces come in. Conversely, if you have astigmatism then using only a diopter eyepiece is likely to leave you with uncorrected aberrations that will throw off your focus.

I would also note that astigmatism can develop or worsen with age. When I was young I was extremely nearsighted but I had no astigmatism. Now I am less nearsighted, but I have slight astigmatism.


~ pax \ Ctein
[ please excuse any word salad. MacSpeech in training! ]
======================================
-- Ctein's online Gallery http://ctein.com
-- Digital restorations http://photo-repair.com
======================================

John: Altman's Camera! Wow, that's a real blast from the past! That was a huge store as I recall. It was a bit before I became a shutterbug but I do recall wandering through in the late 1960's / early 1970's. "Long gone" is right. Central Camera is the last camera store on Wabash today, and it's only propped-up by nearby schools that still use film.

Sorry for the digression, Mike.

On the advice of my optometrist I used +2 correction screw-in diopters on my manual Nikons.

My Nikon D200 has a built-in diopter range from -2 to +1. It focuses clearly for me, but given what my optometrist had recommended I always feel unfairly prejudiced against by camera manufacturers.

I think there is a general trend in dSLRs (not just Nikon) to have more diopter points in the negative range than in the positive.

I wonder sometimes why we "plus people" are given fewer diopters than are "minus people".

Ctein, sorry, but I still don't get it.
Blue-eyed as I am (pun intended), I think that I don't need a diopter adjustment on the camera as long as I wear the glasses, contact lenses or whatever I need to correct my sight.

Is the problem that you are both near- and far-sighted at the same time? But how can a diopter-adjustment on the camera compensate for this?

best always
Andreas

PS: thank you for the reminder, I know that I am getting older ;-)

Dear Andreas,

Okay, let me give an example. Camera viewfinders create a virtual image in space at a distance that is fixed by the optics in the viewfinder. Let's say it's one meter (a commonly used value). And let's say you're nearsighted, so you commonly wear glasses to give you good long-distance vision.

When you're young, and your eyes can focus well, there is no problem focusing on anything between about half a meter and infinity while you're wearing those glasses; you can see the virtual image in the viewfinder quite sharply. As you get older, your eye's ability to focus over a wide range of distances decreases (what really happens is that the lens of the eye gets stiffer with time, so the muscles in the eye can't change its shape as much). At some point, your comfortable near-focus distance will pass beyond one meter. You'll be able to see things closer than that, but they won't be super-sharp. At that point, assuming you're still wearing mono-focal glasses, you'll need a diopter lens for your camera to move the virtual image distance further out.

And in a sense you're right; another way to look at presbyopia is that it does make you both near-and far- sighted at the same time.

~ pax \ Ctein
[ please excuse any word salad. MacSpeech in training! ]
======================================
-- Ctein's online Gallery http://ctein.com
-- Digital restorations http://photo-repair.com
======================================

I bought Contax G1 and G2 bodies with their Zeiss 28, 35 and 90mm lenses a few years ago. (Well, in 2000 actually.) Wow, I thought, rangefinder with AF. Zeiss lenses. Great travel outfit. My worries are over, I thought.

What a disappointment. I haven't had a single shot that I'd call super sharp and I don't know what I'm doing wrong! There's no way to see focus with these cameras, not even superimposed squares. Not even in manual focus. No focus ring on the lens. The viewfinder is always sharp. No focussing aid at all except the electronic rangefinder readout, and that's so coarse as to be guesswork.

Yet people rave about the lenses! The 90mm is supposed to be legendary, but I haven't had a sharp image yet from it, even when I try. I've just bought a Ricoh GX100 and the sharpness I'm getting is just fantastic. $599 vs about $3500 for the set!

I use glasses and have astigmatism, but I wear them and use the dioptre correction.

The Contaxes sit on the shelf most of the time. I bought 3 rolls of Fuji colour neg the other day to have another attempt, but when you're fighting to find focus all the time, double and triple checking the LCD rangefinder figures, trying to estimate the distance to look for error, it kinda distracts from the photographing. Very disappointing. I've looked at the Contax fan web sites and there seem to be many examples of fantastically sharp images, but the technique eludes me.

Pete

A thousand thanks for this post - I just realised that the finder of the F3 I received last week isn't very sharp because there was a diopter screwed in the back! Duh!

Ctein, your right of course, I'm almost 60 and have only very slight astigmatism so I forget how debilitating it can be. Of course, at my age presbyopia has now become a problem. Always been left eye dominate, still use film SLRs with manual advance so therefor have a permanent dent in my right eyeball from my thumb on the advance lever....nah, just kidding.

My explanation of my desire for a diopter adjustment is:

I wear progressive bifocals - but I can't use them with my camera, because the part of the lens I look through into the camera viewfinder is nearer the top, so it's set up for distance focusing. But if I tilt my head a little, everything changes, and I can't be sure whether it's my bifocals or my camera that's out of focus.

So when out shooting with my camera (or scoped rifle for that matter), in order to avoid getting head or neck-aches, I wear my mono-focal glasses. But now I can't focus clearly at the (virtual) distance imposed on me by the optics.

By dialing in a diopter adjustment, all becomes clear.


Thanks for the thought-inducing post.

Ctein, many thanks to you for that elaborate answer.

Btw, the whole post and the following comments are a worthy contribution to the things about photography that really matter. These are the things that drive so many people nuts and make them buy another (supposedly better) lens or camera - without ever reaching salvation.

It is exactly as you said: everyone thinks that focussing is easy. So if one doesn't get correct focus repeatably, (s)he tends to doubt his/her own abilities or the equipment one uses. But things can be very different.

I always thought that I better shoot with contact lenses, but now I understand that my eyeglasses may be better since they correct my astigmatism. And this exactly matches my experience.

As I remember there are many tech-related posts on TOP that are hardly discussed elsewhere. Maybe we should build some searchable or otherwise organized knowledge database with let's say "exotic insights to photography".

best always
Andreas

This is a really excellent thread. The astimatism issue explains why I find focusing easier with glasses, though I can see more of the 28mm frame with contacts.

It also helps explain why focusing became much easier once I got a magnifier for my M eyepiece that has an adjustable diopter correction built in.

Another wrinkle on the issue. Those of us who are over 40 (ahem, OK, over 50) often find that our eyes change significantly between appointments with the eye doctor. It is more noticeable with a rangefinder camera than anything else. With reading or the computer, you often just move a little forward or back and don't notice. But with the RF, focusing becomes just a little more difficult, until one day you wonder what the heck happened. The adjustable diopter makes things right again.

--Peter

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