Nikkor T ED 360 to 500 Aperture Scale Conversion

[Rafal Lukawiecki]

I have just bought this lens, second hand. The 500mm element was a separate add-on, purchased by the original buyer at a later stage than the main 360mm set. The 360 set is an older type, and its Copal 1 shutter does not have the triple aperture scale for the different focal lengths, there is only one scale, for 360.

Would anyone be able to share with me the aperture conversion information for this lens, perhaps as a table? Alternatively, a photo of the newer 360 shutter should enable me to figure out the 500mm apertures, I would hope. If you happen to have this lens or if you could point me in the right direction, I would be very grateful.

Thank you.

[Jeff Keller]

Settings for 500 are one stop slower:
f8 -> f11
f11 -> f16
f16 -> f22
...
f45 -> f64

Jeff Keller

[biedron]

Now that you know the conversion, you just have to remember to use it :) I've got all 3 scales on my shutter, and I'd guess about 3 times out of 4 when I put use the 500mm cell instead of the 360mm one, I forget and just use the 360 aperture scale. Doh!

Bob

[Nathan Potter]

Now that you know the conversion, you just have to remember to use it :) I've got all 3 scales on my shutter, and I'd guess about 3 times out of 4 when I put use the 500mm cell instead of the 360mm one, I forget and just use the 360 aperture scale. Doh!

Bob

Rafal, I accidentally cured myself of the same mistake. I had a great scene framed of Mt. Robson in Canada including a Grizzly with cubs along the upper Robson River. Used the Nikon 500 but the 360 aperture with Astia so overexposed by a stop. Grizzly gone so no second chance. I've not made that mistake since. Every time I pull the 500 that vivid memory returns.

Maybe if you wait for an extraordinary scene, then make that mistake, you will be cured!

Nate Potter

[Rafal Lukawiecki]

Thanks, I am wonderfully pleased to see that the conversion is so simple. I expected something non-linear, this is a nice surprise.

Grizzly bear point duly noted. Just arrived in Wyoming for a two week photo tour, will be watching out for those cuties.

[Rafal Lukawiecki]

Used the Nikon 500 but the 360 aperture with Astia so overexposed by a stop.

Not sure if I got the scale right, but wouldn't it be a case of an underexposure?

[Emmanuel BIGLER]

Thanks, I am wonderfully pleased to see that the conversion is so simple. I expected something non-linear, this is a nice surprise.

Hello from France !

As strange at it may sound, the problem is solved in one second if you remember the general definition of the f-number for a thick compound lens, actually almost all lenses in actual use except the Single Lens Element Wollaston Meniscus or the astronomical doublet ;)

The f-number as engraved on aperture scales is equal to the ratio between the focal length of the lens and the diameter of its entrance pupil, i.e. the diameter of the image of the iris as seen from the front side of the lens, through the front group ; this is not the actual, physical size of the iris.
The nice thing in the Nikon telephoto convertible lenses is that you swap the rear element only.
Hence the entrance pupil, as seen from the unchanged front group, does not change when you swap the rear element.
The conclusion is immediate, for a given setting of the iris, the f-number exactly scales like the focal length of the combined (front + rear) groups. Not only is it perfectly linear, but you could compute it by hand on the back of an envelope without any pocket calculator, paper and pencil suffice ;)
500/360 = 1.39, which is extremely close to the theoretical ratio for one f-stop = sqrt(2) = 1.414.

In the Wollaston meniscus, the diaphragm is located in air at a certain distance in front of the single lens element, therefore the entrance pupil, the image of the iris as seen from the front side is the iris itself, and in this case, the f-number is computed like in a single lens element with the iris located close to the lens.

Conversely, in many convertible lenses like the good old Schneider Symmar Convertible, you remove the front element to get a longer focal length, hence the entrance pupil changes when you unscrew the front element: the image of the iris in the compound lens is seen through a kind of a loupe and is bigger; with the front element removed, you get the iris itself playing the role of the entrance pupil, which exhibits a smaller diameter. And eventually you have to rely on Schneider's engravings for "converted" f-stops, but the correspondence is still perfectly linear.