Focal Length of APO Nikkor 305

[jonathan_smith2]

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?

[domenico_foschi]

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

[per_volquartz1]

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

[dan n.]

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

:)

[michael_briggs2]

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.

 

<p>

 

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.

[pete_andrews]

I think the marking on the lens is more likely to be right than the

invoice.<br>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.<br>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.<b>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.<p>I'm betting you've been fobbed off with a 305mmm lens, BTW.

[pete_andrews]

Ooops!</b><br>Better turn that accidental bold off.

[brian_ellis3]

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.

[neil_poulsen1]

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

 

<p>

 

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.

[jonathan_smith2]

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!

 

<p>

 

Jonathan

[dominique_cesari]

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.

 

<p>

 

Look at the B&H site, they probably have a photograph of the present

telephoto 360 Apo-Nikkor