Lens testing: help with geometry

[Darin Cozine]

Hello all.

I am going to start testing some of my lenses, but my geometry is a bit rusty, it's been in an old cardboard box in the garage for 15 years, and I didnt pack it with a silicone beads.

So.. If I want to use a target that is 20x the size it appears on the negative, can I just multiply the focal length of the lens by 20 and set the target at that distance?? I dont think it is that simple.

Thanks!

[Leonard Evens]

It's not quite that simple.

The magnification is defined to be the ratio of the image size to the subject size. That assumes the subject is confined to a plane parallel to the film plane at a certain distance from the lens. But the magnification can also be calculated as follows. Take the distance from the subject plane to the lens and divide it by the focal length. Subtract one, and then take the reciprocal of that to get the magnification.

Alternately, you may specifiy the magnification and then get the distance. You take the reciprocal of the magnification and add one. That gives you the ratio of the subject distance to the focal length. Multiply by the focal length and you get the proper distance.

So if you want the magnification to be 1/20, take its reciprocal to get 20, and add one to get 21. So if the focal length is 150 mm, you should have the subject at 21 x 150 = 3150 mm (i.e., 3.15 meters). To convert to inches, divide the mm by 25.4 to get about 124 inches or about 10.335 feet.

There is one quibble. For most lenses, the distance should be measured to the lensboard, but for lenses of telephoto design, you need to use a point some distance in front of the lens. But if you just measure to the front of the lens, you won't be too far off for a projected magnification of 20.

You can double check what you are doing by carefully measuring the image on the gg.

[Darin Cozine]

Thanks Leonard!

[Gene Crumpler]

The way I do it is multiply the focal length by 20 and measure fr0m the film plane. All fixed lenses (hasselblad, etc)are calibrated for focus that way.

Do you know how to do meaningful lens tests? I use the same method as Carl Zeiss, except I don't mount the camera on a 2 ton block of granite, instead I use a Bogen 3046/3047. I found out this several years ago from Dr. Kornelus Fletcher (sp?), lens designer with Carl Zeiss.

Contact me off line if you want more information.

[Gene Crumpler]

BTW- The highest resolution I have ever obtained was with a P67 55mm, f-4 on gigabitfilm. I was able to read 156 lp/mm without the use of a microscope. The 55mm is absolutely remarkable, exceeding the performace of my 55mm f2.8 micro nikkor or any of my Zeiss lenses. It would be the last lens/body that would leave my cold, dead hands :>)

Gigabitfilm is quite remarkable also. Barely detectable grain looking at a negative with 200x Nikon scope. I've done some experiementing and plan to publish a report on the net when I get-a-round-twit. My comparison is with Tech Pan(sigh!), Tmax 100 and Delta 100, same camera, lens and subject.

[Tracy Storer]

It's getting to the point where there's so much info posted, it's hard to find!
Formula posted halfway down the post:
http://www.largeformatphotography.info/lfforum/topic/497633.html#507401

[Emmanuel BIGLER]

The way I do it is multiply the focal length by 20 and measure from the film plane. All fixed lenses (hasselblad, etc) are calibrated for focus that way.

Gene. I know what you mean but let me precise a few things.

Manufacturers of lenses & cameras where an helical or similar focusing device is attached do engrave the focusing device with distances measured from the film plane. This is not really obvious and was discussed on the French MF/LF forum and the consensus after several independant tests on various MF cameras was that, yes (and it should have been well-known) MF cameras engraved with a distance scale on the lens refer to the distance measured between the object plane and the film plane.

Here we are dealing with another issue, we want to determine from a USAF-like test target the minimum legible image of the famous 3-bar target images.

Actually if we have access to a calibrated microscope we coulnd directly read from film and forget about reading the group numbers on the target itself. Most of us do not have a calibrated microscope and actually derive the finest period of bars recorder on film from the group numbers on the object target and from magnification ratio. So we need to estimate the magnification ratio. As Leonard mentions, we could do it by measuring the image size of a ruler on the ground glass but it is easier to measure the distance to the object.

The magnification ratio can be related to several measurements between the object, the lens and the image plane ; the best procedure is to follow Leonard's advice. Actually you do not need a very precise evaluation of the actual magnification.

If you want the formula, here it is, very simple where M is defined as

M = (image size)/(object size)

M is small and usually it is easier to deal with its inverse

1/M = (object size)/(image size)

If "u" denotes the distance between the object and the lens ; actually : the distance "u" has to be measured from the front principal plane of the lens H ; it is located usually about 1- 2 cm behind the front lens element, depending of course on lens designs !! ; we get

1/M = (u/f) - 1 or u = f (1 + (1/M))

if u = 51 times the focal length, 1/M = 50 and M = 1/50.

The relationship between the total object-film distance "D" and the object distance 'u' is quite complicated ; let us neglect the separation HH' between principal planes and forget about telephotos...

(u/f) = (1/2). (D/f + SQRT( (D/f)^2 -4.D/f) ) ; 1/M = u/f - 1

... but fortunaly when D is large the approx value is simple ; again neglecting the separation between principal planes

u = D-f ; 1/M = D/f - 2

So there is just a factor 2 to subtract : if lens-to-film distance D = 52 times the focal length, 1/M= 50.

Now what happens with a thick lens ? simply replace D by D - HH' in the above formulae and you are all set even in macro provided that you are brave enough to compute a square root ;-)
HH' being the principal plane separation, HH' is documented in manufacturer's datasheet and is usually positive for quasi-symmetric view camera lenses. It can be negative for some, but actually far from all telephotos. For example in the 250mm Zeiss Sonnar for MF, HH' is positive, about +19mm ; for the 400mm Schneider tele xenar and recent apo-tele xenar, HH' is negligible (+0.5mm).

[Gene Crumpler]

Emmanuel;

I did a lot of math during my 40 year career as an engineer. These days I like to keep it simple.
All of the tests I've done over the last 10-15 years were done measuring from the film plane. So when I tested my LF lenses, I did the same. After all, other than trying to get bragging rights, the only point of lens testing for us guys in the field is to determine what is the optimum aperture for the center and the corners, and how much the image quality on film strays from the optimum(Also to weed out the true dogs)

When I set to shoot whether 35mm, MF or LF, I always set the optium aperture on the camera as the starting point and reluctantly stray from it. Right now I have the optimum aperture for my 150 lens posted on my camera bed! F-16 is optimum for both the center and edges. (Bragging rights now-my 150 performs better than the same lens on the tables here )

[Gene Crumpler]

Btw, I used 4x5 gigabitfilm and the tests on this site were done with Tmax 100. This may explain why mine were better. For those not familiar with gigabitfilm, the 4x5 version will resolve up to 900 lp/mm. Zeiss recently announced that they got 400 lp/mm with their new Zeiss Ikon lenses on the 35mm gigabitfilm emulsion.

[Dan Fromm]

Uh, Gene, 900 lp/mm requires f/1.8 or larger. 400 lp/mm requires f/4 or larger. So tell me, do you shoot on larger formats, or even 35 mm, at those apertures?

I think we're seeing mindless pursuit of resolution without regard for practicality. Yeah, sure, my good macro lenses -- for work around 1:1, I use a 100/6.3 Neupolar -- really are best wide open, but I get more satisfying flower shots at magnifications of 1:2 to 2:1 at f/20 than at f/6.3. And around f/22 at 1:1 on the emulsions I shoot, the difference between my best lens and my third best is nil.

Either that or we're seeing another manifestation of the male "my thing exceeds your thing" syndrome.