PDA

View Full Version : Infinity Focus



Terry Hull
12-Jun-2009, 08:37
I have used a Fujinon 180MM successfully, and wonder if there is a longer focal length lens that I might use on my Ebony SW 45 (190mm bellows). More specifically do all general purpose 210MM lenses focus similarly, e.g., (without getting into specifically designed telephotos) do 210MM lens from different manufacturers require the same bellows extension to focus at infinity? I have no interest in adding back extenders, etc.

Thank you.

Vaughn
12-Jun-2009, 08:55
That would be a safe assumption -- you would have to find a telephoto type of 210mm (if there are any) to significantly reduce to the film-to-lens distance.

A "top hat" type of lensboard might be possible -- one that extends the lens forward. It would not have to extend the lens very far...a couple inches. But what one gains going from 180mm to 210mm may not be worth the hassle.

Vaughn

Eric James
12-Jun-2009, 08:55
Your 180mm lens is the limit if you are opposed to using an extension back or "top hat". The Nikkor 200mm M comes close to focusing at infinity on your rig. Do you know the exact maximum extension of your particular SW45 - there is some variation within models.

http://www.ebonycamera.com/articles/lenses.html

Gem Singer
12-Jun-2009, 09:15
Ebony makes extension lens boards. When I had an Ebony, I purchased one from Midwest.

They come in various lengths. You should be able to add enough front extension to use a 210 instead of a 180.

Wouldn't it be easier to move the camera a few feet closer to the subject?

percepts
12-Jun-2009, 09:22
Yes a top hat lens board would do it. But how often do you focus on infinity? Mostly you focus closer which means you need even more extension than 210mm. I think you will run out of extension very quickly. Your camera is simply not designed for lenses of 210mm or longer.

Ed Richards
12-Jun-2009, 09:42
You would also hardly notice the difference in field of view.

Terry Hull
12-Jun-2009, 10:44
Thanks.

ic-racer
12-Jun-2009, 11:18
do 210MM lens from different manufacturers require the same bellows extension to focus at infinity?

When you look at a lens datasheet the "flange focal length" tells you the exact distance from the front surface of the lensboard to the film plane when the lens is focused at infinity. It is also called "flange back."

Terry Hull
12-Jun-2009, 13:32
So all 210MM have the same flange back distance? (other than teles, etc.) I thought lens construction and design differences by manufacturer might cause similar focal length lenses to have different flange back distances

Dan Fromm
12-Jun-2009, 13:36
ic-racer, like all values published for lenses, flange-focal distances or tirages mechaniques or whatever we want to call them are nominal. Real lenses as made don't always exactly meet their specifications, so the published flange-focal distances can't be relied on.

For example, I once bought 20 38/4.5 Biogons made to be fitted to AGI F135 aerial cameras. The lenses are bolted to the body's main casting, which is very precisely machined; the lens is focused to infinity by a spacer that sits between the rear of its flange and the body. Each of my little Biogons had its own focusing spacer with its thickness, to 0.01 mm, and the lens' serial number marked on it. Each lens was marked with its measured focal length (nominal, per Zeiss, is 38.5 mm), to 0.1 mm; the range was 38.3 to 38.8 mm, and the spacers varied accordingly.

Many, not all, process lenses have similar calibrations marked on them.

You really can't believe everything you read.

Cheers,

Dan

ic-racer
12-Jun-2009, 14:58
ic-racer, like all values published for lenses, flange-focal distances or tirages mechaniques or whatever we want to call them are nominal. Real lenses as made don't always exactly meet their specifications, so the published flange-focal distances can't be relied on.

For example, I once bought 20 38/4.5 Biogons made to be fitted to AGI F135 aerial cameras. The lenses are bolted to the body's main casting, which is very precisely machined; the lens is focused to infinity by a spacer that sits between the rear of its flange and the body. Each of my little Biogons had its own focusing spacer with its thickness, to 0.01 mm, and the lens' serial number marked on it. Each lens was marked with its measured focal length (nominal, per Zeiss, is 38.5 mm), to 0.1 mm; the range was 38.3 to 38.8 mm, and the spacers varied accordingly.

Many, not all, process lenses have similar calibrations marked on them.

You really can't believe everything you read.

Cheers,

Dan

Good point that, in general, view camera lenses wouldn't need a critical flange focal distance (because you focus on the ground glass). Like you I have a set of view-camera lenses where the exact flange focal distance is critical. As you may know, in the Horseman 6x9 Rangefinder system, two lenses can share a single infinity stop, so they need an exact flange-focal distance. In the case of the Horseman lenses I think they may have exactly milled the 4 contact points on the semi-permanently attached lensboards (rather than the shims used by your Biogons).

Dan Fromm
12-Jun-2009, 15:23
Interesting. So a board can be used with the RF with only one lens?

Archphoto
12-Jun-2009, 18:24
As I read it: one specific board is made for one specific lens to match the RF.

This is an interesting option I hear for the first time of.

Peter

falth j
13-Jun-2009, 07:35
"As I read it:

one specific board is made for one specific lens to match the RF.

This is an interesting option I hear for the first time of."


and, Perhaps...

this is why leica rangefinder camera owners get so out-of-sorts, about having difficulties focusing their various lenses on their camera bodies without first coordinating the rangefinder and the film plane with the proper spacer for each lens...

and, linhof has specifically ground rangefinder 'cams' to match a specific serial numbered lens.

Steve M Hostetter
13-Jun-2009, 08:44
I never focus on infinity,, If you use hyper focal distance you will show apparent sharpness at infinity which is more commonly used practice
so yes you will need even more bellows

Terry Hull
20-Jun-2009, 07:56
So if I have a 200MM Nikon lens, that won't quite focus at infinity, due to 190MM bellows maximum length, how does the 10MM bellows shortfall translate into distances where I can focus? How far from the film plane towrd infinity can I focus?

Dan Fromm
20-Jun-2009, 08:00
If it won't focus to infinity it won't focus anywhere. Flange-to-film distance is shortest at infinity, longer for shorter distances.

Terry Hull
20-Jun-2009, 08:13
Thanks. It was a dumb question!

Terry Hull
20-Jun-2009, 08:36
Last dumb question. there is a Nikon 270MM T for sale on EBay. Seller says 7.5" of bellows extension is need for infinity focus. I make that to be 190.5 millimeters bellows. I presume that is cutting it too close?

GPS
21-Jun-2009, 04:49
Terry,
the 270mm T Nikon lens is a true telephoto lens so its flange focal length is much shorter than its nominal focal length. In this case the FFL of this Nikon lens is only 187.6mm.

aduncanson
21-Jun-2009, 07:51
taking the 187.6 mm FFL for the Nikon from GPS and your stated 190mm bellows draw, the Nikon 270 would focus no closer than about 100 ft. With a top hat lens board extension of only 1/2 inch, you could focus to just under 17 ft.

Be sure to check the image circle of the Nikon tele. Many teles have no extra coverage for movements on 4x5.