Focal Length of APO Nikkor 305 : LUSENET : Large format photography : One Thread

I recently ordered an APO Nikkor 360mm from a camera store, and got it today. The invoice says it's listed as a 360mm, but the front of the lens says 305mm. I held it up and measured the focal length; the image at infinity comes into focus 14" behind the front plane of the lens (360mm) and 12" behind the rear plane. My question is, do I have a 305mm lens, or a 360mm? Were the graphic arts lenses measured differently than ones used for LF photography?

-- Jonathan Smith (, May 28, 2002


i believe you have a future priced collectible there....

-- domenico (, May 28, 2002.

Sounds like the front ring was unscrewed and replaced by the wrong one.

-- Per Volquartz (, May 28, 2002.

ahhhh...then.. someone has 305mm with a make-believe 360 ring...LOL.. :)

-- dan n. (, May 28, 2002.

When you have the lens focused on an object at infinity, the focal length is measured from the rear nodal plane to the image. Normally the nodal planes are close to the center of the lens. The main exceptions in LF optics to the rule of thumb that the nodal planes are near the center of the lens are true telephoto lenses. I don't know of data specifically on the location of the nodal planes of Apo-Nikkors, but I have a datasheet for Schneider Apo-Artars, which are similar designs. These datasheets show the nodal planes as located near the center of the lens. Usually the aperture diaphram is located near the nodal planes.

Basically, you should focus on infinity and measure the focal length from the aperture diaphram. You should measure neither from the front nor the back. Your measurements suggest something around 330 mm for the focal length, which makes sense neither for a 305 mm nor 360 mm Apo-Nikkor. You probably should repeat the measurement more carefully.

-- Michael Briggs (, May 28, 2002.

I think the marking on the lens is more likely to be right than the invoice.
As Michael says; the only accurate way to measure the focal length is to find the position of the rear nodal plane of the lens. However, you may be able to verify the focal length from the aperture scale, because some process lenses have two scales for the aperture. One scale is the usual f-number marking, and the other is the actual aperture diameter, marked in millimetres.
A 360 mm f/9 lens will have a maximum aperture diameter of 40 mm, and a 305 will have a diameter of 33.9mm. At f/32, the corresponding diameters will be 11.25mm and 9.5mm - enough of a difference to distinguish them - if that second millimetre scale is there.In any case, you could measure the diameter of the front element with a pair of calipers. With this simple design of lens, the front glass diameter will be close to the theoretical maximum aperture size. If the front glass is around 34 mm diameter, then you know you've got a 305mm lens. If it's 40mm or more, then there's a good chance the lens is 360mm in focal length.

I'm betting you've been fobbed off with a 305mmm lens, BTW.

-- Pete Andrews (, May 29, 2002.

Better turn that accidental bold off.

-- Pete Andrews (, May 29, 2002.

The odds that Nikon mislabelled the front of the lens seem very slim. I wouldn't bother with measuring nodal points or anything else, you ordered a 360mm lens and you're entitled to get a lens that says it's a 360mm lens. I'd return it and demand that the store replace it. Even if by some weird chance the 305mm inscription on the lens is wrong and it's really a 360, you're going to have a terrible time explaining that to a prospective purchaser if you ever want to sell it.

-- Brian Ellis (, May 29, 2002.

I agree with Brian. Send it back, and keep a copy of the invoice.

Another good check is to measure the distance between the standards when focused at infinity, and measure the distance between the standards when focused at 1:1. The difference in these two measurements will be the focal length of the lens. Using this approach, you don't have to estimate the center of the lens.

-- neil poulsen (, May 29, 2002.

Yeah, it looks like the camera store was wrong - the lens aperture is the smaller 34mm diameter. Thanks for the info on measuring the focal length!


-- Jonathan Smith (, May 29, 2002.

It sounds like the lens is of the kind that Nikon used to sold as process lenses or in a shutter. I think that they have been made in the '60. They were made in different focal length and named APO- Nikkor.

Look at the B&H site, they probably have a photograph of the present telephoto 360 Apo-Nikkor

-- Dominique Cesari (, May 30, 2002.

Moderation questions? read the FAQ