Re: lens questions

From: Harry Smart ^lt;[email protected]>
Date: 07/06/05-08:22:29 AM Z
Message-id: <000b01c58236$28bac1e0$0e00000a@harrynet>

I've tried shooting welding a couple of times, and, assuming you're talking about arc welding, the arc itself and the area around it are going to burn to solid black on the neg pretty much whatever you do. I'd consider metering from that area as completely pointless. How wide that area is and how bright it is at its core will depend largely on what's being welded, the size of the rods, the current the welder is using, etc. and will also vary substantially during the weld as the conductivity of the joint that's being formed changes substantially. If they're working along a seam, then once you've got over striking the arc, and if they're good (i.e. they keep a constant rate of movement to achieve a consistent depth of weld ... and most welding is actually pretty rough work, so don't count on consistency unless you know your guys are good and are working to close tolerances) then you should get fairly steady illumination. The falloff of light from the arc itself will be very steep, so, if you can protect your eyes, you should be able to use a spot meter to get values for areas close to the arc ... I'd suggest you equip yourself with one of the masks that are clear up to the moment the arc is struck, and which then darken immediately. I've used one when welding myself and they really are magical ... I got mine on ebay quite inexpensively ... they cost hundreds when they first appeared a few years ago.

However, even so, I think you'll find there's a lot of wasted shots. Quite apart from the consistency of the light output from the weld, your welders will probably be continuously varying the angle at which the present the rod to the joint, you'll find that sometimes the workpiece itself is masking part of the arc .. and you'll just have to shoot a lot to get the occasional good result.

Cheers,

Harry
  ----- Original Message -----
  From: Jeff Sumner
  To: alt-photo-process-l@sask.usask.ca
  Sent: Wednesday, July 06, 2005 2:35 PM
  Subject: Re: lens questions

  On Jul 6, 2005, at 2:31 AM, Barry Kleider wrote:

    Last week, I was shooting some welders.

    My ultimate goal was to shoot them with a Hasselblad and a 150 f4 lens. (No internal meter.)

    Since these exposures are obviously tricky, I started with my Nikon D-1 and a Tamron 28-70 f2.8

    I was getting some decent exposures - nothing great, but certainly within tolerance for a first attempt. So I decided to switch over to the Hassy using the same readings.

    Since the Nikon was running at ISO 200 and my provia is 100, I gave it a full stop. I got the lab results today: $%#^$%. (Translation: way too dark and looks like s**t.)

    So my question is: what's the relationship between a medium format Hassy with a 150 f4 lens, and a 35mm Nikon with a 28-70 f2.8 lens (running close to 70 if not full on)?

    I assume there's a very straight-forward (though possibly hard to follow) explanation having to do with the difference in lens designs rather than a format comparison or a digital vs. analog thing (as my lab guy surmised.)

    Barry

  Lens speed and shutter speeds are constants between the format. Measuring with a meter through one in any format will work fine in another, depending on what you are metering.

  You say the prints are too dark- how do the negatives look? Are they thin? Did you take pictures metering with that actinct light from the welder? That brightness can easily fool the folks doing the printing (or much more likely, the machine doing the printing)

  f-Stops is f-stops.
Received on Wed Jul 6 08:23:06 2005

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