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bellows factor on film speed

 
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Kim



Joined: 10 May 2001
Posts: 44
Location: upstate NY

PostPosted: Sat Jun 16, 2001 3:01 pm    Post subject: Reply with quote

I would be interested in a discussion of film speed and bellows extension. In general I seem to need about a three stop adjustment to my meter readings from the specified film speed. By this I mean that 3000 asa seems correctly exposed when corrected to a film speed of around 400...now this is for medium
depth focus 8-25 feet.

Does this seem about right?

Kim Hartshorn
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Les



Joined: 09 May 2001
Posts: 2682
Location: Detroit, MI

PostPosted: Sat Jun 16, 2001 3:58 pm    Post subject: Reply with quote

Something's wrong. Bellows factor doesn't really have a noticable effect until you are a couple of feet away with a normal lens. at 8 to 25 feet, the only way bellows factor plays in is if you have a very very long lens.

I'd check the iris and make sure that it's moving properly, then the shutter. If you can hear a decernable difference from shutter speed to shutter speed that should be okay.

Is the underexposure consistant at different shutter speeds? Try variing the shutter speeds with different apertures.

Just so we are on an even footing, what camera ,lens and meter are you using? Is it possible you are using an old meter, that like me, isnt' as responsive as it's supposed to be?

[ This Message was edited by: Les on 2001-06-16 09:01 ]
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hartwell_a_m



Joined: 04 Jun 2001
Posts: 84
Location: Northern California

PostPosted: Sat Jun 16, 2001 9:12 pm    Post subject: Reply with quote

The bellows factor only aplies when the bellows is extended past the lenth of the lens and is about 1/3 f stop per inch. The bellows factor usualy coms into play at distances under about 3-4 ft in my experiance
though this does depend on the lenth of the lens.
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LelandRay



Joined: 24 May 2001
Posts: 115
Location: Mississippi

PostPosted: Sat Jun 16, 2001 9:37 pm    Post subject: Reply with quote

There's a formula for computing exposure increases necessary as bellows length increases:

bellows length2
----------------
lens length2

=exposure factor

Thus, with the bellows extended to 12" and using a 6" lens, the formula would be

122
----
62

or...

144
---
36

equals 4x

So two stops of increase.

[ This Message was edited by: LelandRay on 2001-06-16 14:40 ]
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hugo13



Joined: 06 Sep 2001
Posts: 14
Location: Georgia

PostPosted: Thu Sep 27, 2001 12:31 am    Post subject: Reply with quote

Check out Calumet's Bellows Extension Calculator - by far the easiest way to get it right.
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LelandRay



Joined: 24 May 2001
Posts: 115
Location: Mississippi

PostPosted: Sat Sep 29, 2001 9:55 am    Post subject: Reply with quote

Quote:

On 2001-09-26 17:31, hugo13 wrote:
Check out Calumet's Bellows Extension Calculator - by far the easiest way to get it right.


I don't do all that much closeup work, so exposure change with bellows extension isn't a big problem for me, but I don't do math well at all, so a calculator would be a big help. I tried to order a Speed Stick through B&H, but they're out of stock and probably will stay that way. I'll check with Calumet.
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Henry



Joined: 09 May 2001
Posts: 1439
Location: Allentown, Pennsylvania

PostPosted: Sun Sep 30, 2001 11:34 pm    Post subject: Reply with quote

I've shot chromogenic b/w (Ilford, Kodak), also Gold 100 and Ektachrome 64 and 100 on the Century Graphic with 65, 101, and 203 mm Optars, never compensated for bellows factor with any of these film/lens combinations, and exposure for me has been right on---no surprise with the very tolerant chromogenics, and a pleasant surprise on the Ektachromes. Have you tried it both ways to see if there really is a difference? I might add that my main meter for outdoor (sun-lit) subjects is the venerable GE DW-68, and for indoor (dim-light and flash) subjects I rely on the Gossen Luna-Pro F.
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LelandRay



Joined: 24 May 2001
Posts: 115
Location: Mississippi

PostPosted: Tue Oct 02, 2001 4:37 am    Post subject: Reply with quote

I haven't had any occasion to factor in bellows extension, so I just trust my meter, a high tech Sekonic L508. There's no reason to worry about exposure compensation unless you shoot closeup work or for some reason are working with totally unforgiving film. What change would 1/20th of a stop make to Ektachrome? My guess would be that you'd be hard pressed to see it with the naked eye.
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Les



Joined: 09 May 2001
Posts: 2682
Location: Detroit, MI

PostPosted: Tue Oct 02, 2001 8:20 pm    Post subject: Reply with quote

Not to mention that it's a rare shutter that's consistantly accurate to a 1/20 of a stop.

Les
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Dan Fromm



Joined: 14 May 2001
Posts: 1882
Location: New Jersey

PostPosted: Tue Oct 02, 2001 10:42 pm    Post subject: Reply with quote

Quote:

On 2001-06-16 08:01, Kim wrote:
I would be interested in a discussion of film speed and bellows extension. In general I seem to need about a three stop adjustment to my meter readings from the specified film speed. By this I mean that 3000 asa seems correctly exposed when corrected to a film speed of around 400...now this is for medium
depth focus 8-25 feet.

Does this seem about right?

Kim Hartshorn

Pmfji after everyone else has put in his 2 cents. Kim, something is horribly wrong. Your meter is way off or you dialed in the film speed incorrectly. Try comparing its reading with a known good meter. When my old LunaSix went berserk, I confirmed it by comparing with a Nikon that was working well. At the distances/magnifications you're shooting, there's no need to correct for extension and that's not the problem.

Good luck, and keep us posted,

Dan
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