Modifying minimum focus distance

[pshute]

Many old long lenses, e.g. 400mm f6.3 Spiratone, can only focus as close as 20 feet or more. I know I can add macro rings, etc, to decrease this, but it would be good to still be able to focus to infinity.

I thought this was impossible till I read an old posting at:
http://photography-on-the.net/forum/archive/index.php/t-43941.html
"If you are mechanically inclined it is quite easy on some of these to modify the focus limiter to get closer focus."
(second last posting on that thread)

This makes it sound like you could open the lens up, remove some obstruction, and the barrel will then be able to turn a little further and focus a little closer. I can't find any other discussions anywhere about this. The term "focus limiter" seems to mostly refer to a switch on AF lenses to restrict the AF range to reduce focus hunting.

I'm not trying to turn a lens into a macro lens, just get the distance down from, say, 23' to 20'.

Does this sound like a practical idea? Has anyone tried it? I can't imagine the manufacturers limited the distance without a reason, so is the image quality likely to suffer from this?

[cooltouch]

Based on my not terribly comprehensive experience in dismantling (and even re-mantling) lenses, I'd have to say that the stop is placed there for a good reason. More often than not, if it is removed, there is a strong chance that if the focusing ring is turned too far, the helical will become decoupled to its threads, and the lens might even come apart. Can't say specifically if this is a possibility with this type of optic, but I would be inclined to think it would.

[Arkku]

Sometimes there is a little extra room available in the focus range, whereas the original stop plays it safe and keeps the focusing helicoid from coming off its threads. But if your particular lens happens to be closer to one end of range for register distance adjustment, it may be that you can get away with modifying the stop for better close focusing.

However, usually the changes in focusing range are very small and not worth the effort, unless some other modifications accompany them. The best I've been able to gain in a regular lens is when I re-adjusted infinity focus position on a Canon FL lens to focus to infinity with Minolta AF mount, which required a rather extreme infinity focus position (does not hit the stop but rather the end of the helicoid). Even this extreme case gave only 10 cm more close focus distance (from 60cm down to 50cm)… With a longer lens you need more extension to get a given amount of close focusing, so I doubt it's worth trying.

[pshute]

OK, thanks, I won't expect it to help much, but I feel I ought to give it a try.

Can anyone give me some tips on how to get at the stop? I've never pulled a lens apart.

[kds315*]

Close up lenses, easy.

http://fuzzcraft.com/achromats.html

[rick_oleson]

You can do it, but to gain much on the Spiratone 400 you'd need to add a lot of travel - I think if you add 40mm it will get you down to 13 or 14 feet - and the chances of you having that much extra thread on the focus helical beyond the stop is not very great. I removed the close focus stop on a cheap mirror lens once and got it down to a few feet before the front end fell off. But, of course, that's the other issue ... the front end WILL fall off.

I have not tinkered with the stops on my Spiratone. My guess is to look under the rubber grip on the focus ring, but I'm not sure.

[estudleon]

You can see:

http://www.pentaxforums.com/userreviews/showproduct.php?product=411&cat=88

5th post.

Refer to J-9 (it's a job that may be done over my lens too, 'cos it focus from 0,65 m and I pirchased as it).

Rino.

[egidio]

why not use shallow macro extension ring

[pshute]

[cooltouch]

The optical formula typically used by these lenses, which is usually an achromatic doublet, requires a lot of extension to achieve close focus. Probably more than 360 degrees of rotation of the focusing ring, so . . .

[rick_oleson]

The amount of extension required to achieve close focus is proportional to the focal length of the lens. A 400mm lens like the Spiratone needs about 40mm of extension to get to its minimum focus of about 27 feet, and another 40mm will cut that in half. To cut it in half again (to get to about 6 1/2 feet) would require doubling the extension again, so you'd now be 160mm out from the infinity position.

There are ways of shortening the extension required - the simplest is front-element focusing, which actually shortens the effective focal length of the lens rather than increasing its distance from the film (the close-focus capability being the difference between the focal length and the distance from the film). Generally speaking, unit focusing gives better optical performance, so that's the basic trade-off.