To infinity and beyond... or how to calibrate my long lens?

[Laurent]

Hi all,

sometimes ago, I gave up to a GAS itch I've had for 40 years... I now am the proud owner of a Novoflex telephoto set, with the Pigriff, Balu, 280 and 400 lenses... This is really a kid's dream becoming real, these lenses were out of reach when I was a teenager.

I discovered today that I could calibrate the Pigriff to make sure that I focus on infinity when the grip is fully pressed.

The manual recommends using an object at 1500m or beyond, and I'm wondering what would happen if I use a closer object (say 500m).

I'd expect the DOF would cover for this, but am unsure.

What's your advice?

 

[Chuck1]

I didn't know this could be done, curious to eavesdrop

 

[Sirius Glass]

I would use the 1500m length. You will find using 500m could leave infinity in less then perfect focus. I have used over a mile for adjusting a Speed Graphic range finder.

 

[xkaes]

I'd use the "or beyond". Long lenses have very little DOF.

 

[r_a_feldman]

The Moon is a standard infinity subject.

 

[Xylo]

I usually just reverse collimate the lenses...

 

[reddesert]

The thin lens formula is: 1/f = 1/d_i + 1/d_o, where d_i and d_o are the image and object distances.

For an f=400mm lens, the extension past infinity is 0.10mm for an object at 1500 meters.
The extension past infinity is 0.32mm for an object at 500 meters.

Assuming the lens is f/5.6 and the circle of confusion for 35mm is 0.03mm, the one-sided depth of focus is 5.6*0.03mm = 0.17mm. This 0.03mm c-o-c value is for "just good enough" focus, one would like it to be a little more accurate.

So the answer is no, you can't use an object at 500 meters. The manufacturer probably specified 1500 meters or greater by doing a similar calculation to get the extension (0.1mm) well inside the depth of focus (0.17mm).

With this information you can do similar calculations yourself.

 

[snusmumriken]

Having lusted after one of these when I worked on wildlife, I am curious about the answers too.

Isn't the whole point of a Novoflex that you squeeze or relax the grip until the object is visibly in focus through your SLR? So the thing you don't want to happen is that you hit the infinity stop before the lens has reached optimum focus for your distant object. Surely that is what would happen if you calibrate it against 500m and then photograph the moon? If you only ever photograph objects within 500m, I suppose it may be convenient to 'blank off' about greater focussing distances, but at the moment I don't really see how. Would it give you finer control over distances up to 500m?

 

[Laurent]

Thanks for your answers!

I need to think more about this, as these lenses are only used for wildlife, so I don't *think* I need "real" infinity.

My initial thought was that I could have a setup that could make me more reactive for subjects at some distances, but since I 'm not sure what it would be, I may simple be overthinking.

At the moment I'm lusting over a 600mm, they are so cheap I'm not sure I can resist.