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AndrewL9
27-Sep-2008, 05:55
I have just purchased a Nikor 300mm F9 from keh. The shutter says T 360 Nikor on it and has a maxium aperture of F8. When moving the scale to the F8 position the lens is wide open. When moving the scale to F9 it closes in a bit. For metering should I consider F8 to be F9 and it to move consitiently like that through the range? Maybe some testing will be the only way to determine that. Thank you for your input.

Nathan Potter
27-Sep-2008, 09:14
The difference between f/8 and f/9 is only a third of a stop. Moving from f/8 to f/9 will very slightly change the diameter of the aperture as you describe. Sounds like everything is as it should be.

Nate Potter, Austin TX.

AndrewL9
27-Sep-2008, 10:22
I will keep that in mind then thank you.

Jon Shiu
27-Sep-2008, 15:12
Hi, the f stop for a lens is proportional to focal length of a lens:

f number = focal length/aperture diameter

So, to calculate your new f stops for a 300mm lens, just multiply by 300/360, call it .83

example, for the f11 mark: 11 x .83 = 9.11, which would be your actual f stop for 300mm lens.

Jon

AndrewL9
28-Sep-2008, 15:06
Thank you for your reply Jon. I am confused how you figured out the aperture diameter was 360. The numbers make sense next to Nathans answer providing about a third of a stop difference between the fstop indicated and the actual one. Just curious how you determined the aperture diameter.

Jon Shiu
28-Sep-2008, 16:04
Hi, sorry I didn't do a good job explaning it. The f-stop is relative to the focal length of the lens. F stop for a given lens = focal length/aperture diameter. So, when your shutter is at F8 the actual aperture diameter is 45mm (360/8). This 45mm diameter aperture (hole) would give a different f number with a 300mm lens, ie 300/45 = F6.6. I hope this makes sense. Here is a table of approximate values:

Marked F-stop (360mm) -- Actual f-stop for 300mm lens
f8 -- f6.6
f11 -- f9
f16 -- f13
f22 -- f18
f32 -- f26
f48 -- f40
f64 -- f53

I hope you can see that this message is actually contradicts your understanding of the other answer.

Jon

AndrewL9
28-Sep-2008, 16:47
That cleared it up perfectly guess now I can consider my options on getting a new aperture scale or making one myself. Thank you for your help!

Nathan Potter
28-Sep-2008, 18:15
Jon and Andrew, I apologize. I totally misread Andrews original question. I take it you purchased a 300 mm lens but with a 360 mm shutter/aperture between the front and rear elements. First I hope that the shutter/aperture mechanism was installed with the proper spacing between front and rear elements.

Jon indicated how to determine the correction factor for f/number. Another method that I've used is by comparison to another lens or lenses.

Set up your camera with a different lens, one that you are sure of the f/no and as close as possible to the FL of the 300 mm. you need to calibrate. Use a large piece of white mat board or equivalent and fill the GG frame with it. Illuminate the white board as evenly as possible and set your bellows extension to infinity for the lens you have chosen. Cut a black cardboard piece that covers the GG completely. In the center of the black cardboard cut a round hole slightly smaller than the spot meter front filter size. Next set an aperture in the lens, say f/8 and using the spot meter tight up against the hole in the cardboard measure the light intensity from the GG. You'll read it in EV; write it down. You may want to measure all the apertures in that standard lens.

Now repeat the measurements with the lens in question noting the EV values. The difference between the EV values at the same aperture with the two lenses indicates the correction needed.

Errors:

1. Keep both lenses focused on infinity - you don't want to introduce bellows factors.
2. Keep the FL of the two lenses similar since scattering from the GG from off axis rays may alter the intensity to the spot meter.
3. Measuring the intensity on the optical axis will further improve the reliability of the readings between the two lenses. Hence the hole for the spot meter needs to be centered on the GG.

Nate Potter, Austin TX.