39mm drop in filter holder - how work?

[mike_smith2]

Bit of a silly question, but I have just acquired a s/h 300mm F2.8

AIS ED lens for evaluation and the rear filter holder appears to be

jammed.

 

Should the locking knob pull up and twist, or just twist - I don't

want to force it, but nothing seems to budge either way.

[mdunker]

I'll assume it is similar to the 300mm f/4. You have to push down (toward the lens body) slightly, then twist the knob one way or another (the lens isn't in front of me at the moment), then lift up. It shouldn't be hard to turn.

[HuskyMason1]

push down and turn CCW

[michaelging]

By the way the 39mm filter is a part of the optical system and need to be left in the camera.

[alex_lofquist]

While I often hear that the drop-in filter is part of the optical system (true), it is not imperative that it must be in place. If you photograph without the filter, focussing without it will compensate for any focus shift. (The actual focus shift amounts to the thickness of the filter multiplied by the index of refraction minus one. A very small amount.) Technically, there is a very slight difference as to wavelength (color), but this can be disregarded in all but the most severe cases. If presence or absence of a filter were so critical, a gel holder wouldn't be allowed. Its refractive index and thickness is not identical with the filter's crown glass matrix.

[joseph_wisniewski]

Alex - "While I often hear that the drop-in filter is part of the optical system (true), it is not imperative that it must be in place"

 

It's not all that "true". Nikon sells a gel filter holder that fits the same filter drawer, and the thickness of the gel is a small fraction of the thickness of the glass filter. The only problem will e a small shift in the focus scale, which won;t affect you if you're either using autofocus or focusing manually through the viewfinder.

 

What is "imperative" is that the filter holder (either glass or gel) must be in place to close off the filter holder slot and keep dust out of the insides of the lens. Dust in the lens is a pain to clean.

[alex_lofquist]

Good point! It is certainly useful for protection of the rear surfaces of the lenses. (I wish my 70-200 VR had a drop-in.)