Tele Raptar . . .Infinity and Extension Calculation

[Drew Bedo]

I have a Graflex 156" Tele Raptar. Havn't used it much. Now Its mounted on a board and I want to shoot.

Being a true telephoto, infinity focus is ~8" not 15". How do I measure extension and calculate the correction for exposure with this lens?

[Leigh]

I think the lens focal length is 15", not 156" :D

You must measure the distance from the film plane to the front of the lensboard at infinity focus.
There's no way to "figure this out" unless you have the complete optical design of the lens.

The focus position at 1:1 (full-size image) will be that distance plus 15".

Magnification and exposure calculations are standard once you have that information.

- Leigh

[Lou Baleur]

Determine where the exit pupil is and that will determine the bellows and flange position. This can be done by experiment. Alternatively, utilize the (1+M) x focal length or just M x focal length to determine intermediate flange positions once the infinity position is known.

Exposure with a telephoto is very much different than with a normal lens. There is a parameter called pupil magnification factor which is like an additional "bellows factor" that must be utilized. See point number 5 on this article which shows the calculation of proper exposure for a telephoto lens for a Hasselblad camera:

http://www.hasselbladhistorical.eu/HT/HTComp.aspx

It is likely you will not find that parameter for your lens and you must determine it by your own experimental measurements. This can be done without any special tools with a precision sufficient for practical use.

[Drew Bedo]

Liegh: right you are15 inches not 13 feet.
Thanks for the clarification. I had thought the extension was to be measured from the infinity position just as you describe.

Just thinking about it now . . .from the infinity position; each 2.5 inches of extension will bneed ~1/3 stop addiional exposure.

Lou: Thanks for the link. It’s a lot of reading for me as I am visually impaired. I’ll have to wade through it later on. I need to be able to work it out in my head when shooting in the field and the light is changing. For now, I’ll try out the focal plane probe that I am still trying to sell!

Is anyone else using this method of calculating exposure compensation in LF photography with any of the Tele Xenars for example?

[Kevin Crisp]

Does this mean the Quick Disc does not work with telephoto lenses? I stopped worrying about calculations when I discovered that.

http://www.salzgeber.at/disc/disc.pdf

[Leigh]

Hi Drew,

The focal plane probe (sounds obscene) will certainly give you the most accurate exposure reading
for any lens and any bellows extension.

It's cumbersome to use as compared with a hand-held meter, but that's the price of accuracy. :D

- Leigh

[Drew Bedo]

Leigh:

You are so right (or is it "Spot-On"?) about the Booster-1/ Minolta combination being inconvenient , clumsy etc—which is why it is now For Sale on the FS/WTB forum!

I will be shooting T-Max 100 with this lens in Colorado sometime soon and I think I'll just use a tape measure and allow 1/3 stop extra for every 2.5 inches beyond infinity. We'll see how that works out.

My thanks to all contributors to this thread.

[Bob Salomon]

Leigh:

You are so right (or is it "Spot-On"?) about the Booster-1/ Minolta combination being inconvenient , clumsy etc—which is why it is now For Sale on the FS/WTB forum!

I will be shooting T-Max 100 with this lens in Colorado sometime soon and I think I'll just use a tape measure and allow 1/3 stop extra for every 2.5 inches beyond infinity. We'll see how that works out.

My thanks to all contributors to this thread.

1/4 life size = 1/2 stop, 1/2 lifesize = 1 stop, lifesize = 2 stops.

Just put a ruler in a close up scene and measure how big an inch is on the GG. Then you will know the required exposure extension.
Or get the Rodenstock DOF/Scheimpflug calculator. It will determine the amount of correction needed based on the difference in lens extension between the near object in focus and the far object required in focus. It will also tell you where to reset the lens position to hold that DOF at the largest useable aperture.

[Dan Fromm]

Bob, you're as right as could be and no more precise than necessary. I've never understood why the third digit matters in photography ...