UNDER EXPOSURE WITH EXTENSION

Paul Sanders · Post by **Paul Sanders** » Mon Jul 03, 2006 5:19 pm Etc/GMT-1+01:00

Hi,

Now I know you will all chuckle at my inexperience with the LF but is there a simple rule of thumb for exposure compensation when you extend over the focal length of the lens. Basically I was trying for a close-up in the landscape, and after remembering to allow for the polariser, warm-up wind and everything else I realised I had no idea what to do about the extension so I guessed at doubling it?????? Tech spec - 90mm lens the extension was about 140ish give or take a thumb, and no I didn't have a atape measure with me - one more thing to put in the bag. Sorry being a bit stupid.

Paul

Post by **Charles Twist** » Mon Jul 03, 2006 7:21 pm Etc/GMT-1+01:00

You'll get the hang of it after a while so that you'll know to add 1/3, 2/3 or a full stop, just by gauging the magnification on the ground glass. It's rare it gets to more than one stop unless you get in really close. The usual rule (for non-tele lenses) is extension divided by focal length, the whole thing squared. This is an exposure compensation, not an aperture compensation; ie a factor of 2 equates to one stop. You only need apply this rule if the distance between the film plane and the object is less than 10x the focal length of the lens (ie 1 yard for a 90mm lens). No need for a tape measure, just guestimate using your hand - you're not within 20%.
Charles

Tim Myers · Post by **Tim Myers** » Mon Jul 03, 2006 8:55 pm Etc/GMT-1+01:00

You can always give the 'QuickDisc' a go.

It's a circular target you place on the subject, and a ruler you place on the GG, measure across the target and read off your compensation.

Just download the PDF, print, laminate, cut out!

gari · Post by **gari** » Mon Jul 03, 2006 10:51 pm Etc/GMT-1+01:00

I have one of these and I find it works great, nice price too!!
Gari

Paul Sanders · Post by **Paul Sanders** » Tue Jul 04, 2006 1:09 pm Etc/GMT-1+01:00

Thanks chaps, I'll have a look at the quick disc.

dennis · Post by **dennis** » Wed Aug 16, 2006 9:38 am Etc/GMT-1+01:00

Hi Paul,
If you send me your email address I'll send you a photo of a simple to make card calculator complete with rule to measure extensions. Dennis

Bobbo · Post by **Bobbo** » Wed Aug 16, 2006 2:28 pm Etc/GMT-1+01:00

Paul,

You don't really need a calculator what you need is the easlily memorised method/formula;

Lens to image distance X same,

Divide by;

focal length X same,

Gives increase factor.

E.G.; lens extention in inches 12"...12x12=144

focal length 150mm.......6"....6x6= 36

144/36=4x...ie 2 stops

The best way to measure lens extention is the old time photographers method of sewing white stiches into one edge of the black focussing cloth to use as a measure,...you only need to put a white stich at every inch to suffice

Ole Tjugen · Post by **Ole Tjugen** » Fri Aug 18, 2006 9:40 am Etc/GMT-1+01:00

Another way is to measure the extension (see caution at the bottom) at infinity, and note this down. Then measure the extension when focussed close.

Now try to think of these two numbers as points on an f-stop scale.

If extension at inf. is 16cm and close focus is 22cm, that's a one stop difference. 32cm in the same case is a 2 stop difference. 16 - 22 - 32 - 45...

The "caution" is: Ideally, you should measure the distance from film plane to front node of the lens which may not be anywhere near the lens board. Most lenses are close enough, in LF only telephoto lenses have significant differences. measuring flange dstance can put you a whole stop off with a Tele-Xenar!

To be very, very precise with wide-angles (especially modern ones) and some Tessar-type lenses (especially some old ones), there can be as much as 1/3 stop difference. But if you want better than 1/3 stop precision, only a focal plane meter probe can help you (not even hte Quickdisk is any good with unsymmetrical lenses).

Brian P · Post by **Brian P** » Sun Mar 04, 2007 11:03 am Etc/GMT-1+01:00

Another option to work out the exposure increase is the formula

, where S= the scale. So if you have a magnification of, say, 0.5 then that would work out as

= 2.25x

But I love the QuickDisk - such a simple idea!

buze · Post by **buze** » Mon Mar 05, 2007 1:24 am Etc/GMT-1+01:00

Ole Tjugen wrote: If extension at inf. is 16cm and close focus is 22cm, that's a one stop difference. 32cm in the same case is a 2 stop difference. 16 - 22 - 32 - 45

I beg to differ here.

If your lens at infinity is at 160mm, and you are focused at 320mm, thats ONE stop in my book. Double the distance, half the light.

Inverse square law applies, not "f stop number law" that are just there for convenience.

It makes the measure a LOT simpler. You just need to know the rough infinity "distance to focus board" and measure it when in focus.

If for that 160mm lens you were at 240mm, add 1/2 stop. 320, 1 stop etc.

For a "100mm" lens, 150mm is 1/2 stop, 200mm is 1 stop

etc.

Ole Tjugen · Post by **Ole Tjugen** » Mon Mar 05, 2007 8:32 am Etc/GMT-1+01:00

buze wrote:
Ole Tjugen wrote: If extension at inf. is 16cm and close focus is 22cm, that's a one stop difference. 32cm in the same case is a 2 stop difference. 16 - 22 - 32 - 45
I beg to differ here.

If your lens at infinity is at 160mm, and you are focused at 320mm, thats ONE stop in my book. Double the distance, half the light.

Inverse square law applies, not "f stop number law" that are just there for convenience.
...

Then you will underexpose.

As you said, inverse square law applies. And 16 squared is 256, 22.6 squared is 511 - one stop difference, half the light. 32 squared is 1024.

Double the distance, quarter the light. That's what "inverse square" means!

buze · Post by **buze** » Mon Mar 05, 2007 10:04 am Etc/GMT-1+01:00

Yeah I got my rule of thumbs wrong, with only one thumb

Note to self: Don't post past 1am