Why Does A 400mm Lens Need Re-Focussing When Orange Filter Attached?

[simon_fallon]

<p>Hi all, I recently took some b/w pictures on my newly acquired 400mm F5.6 Nikkor through an orange filter and I realised that the image went out of focus when the filter was attached, necessitating a re-focus. And obviously I then had to re-re-focus when the filter was removed. Does anyone know why this should happen? I have used fiters extensively for many years and never encountered this before on any other lens. Also, the filtered negs were drastically less sharp than the unfiltered ones; this too has never been noticeable with shorter lenses. Grateful for any suggestions.</p>

[umesh_bhayaraju]

<p>Normally, for a given lens, each wavelength of light focuses at different points. This is called chromatic abbreation. But, for all photographic lenses this abbreation is corrected. Otherwise you will get really messy pictues when aperture is opened fully. Since you used orange filter, there shld be a small shift in the focusing point than without filter (technically). But, the nikkor lens should and will handle this shift. Probably, the orange filter is not a plane glass, but comes with a small curvature and acting like a lens which obviously is not corrected for this abbreation. If it is a cheap filter, it will be a broadband filter, so there will be different wavelenths passing through with less transmission efficiency than the orange waveleght light and hence the images can be bit less sharp. Thats my guess. May be wrong.<br />Just a question and also test : what was the aperture? is it fully open? see if you reduce the aperture will the effect go down? See, whether there is small curvature in the orange filter.<br>

If you used this orange filter with other lenses and it is ok, since you are saying it is a new 400 mm nikkon lens. you need to check it. could be problem with some coatings. (I assume this line is a bad guess)</p>

[kelly_flanigan1]

<p>The orange filter cuts off the blue; thus one is just allowing the longer wavelength non-blue light thru the lens. Light is roughly 400 to 700 nanometers. An orange Nikon filter is typically a O56; O is the letter "O" ; 56 is the cuttoff; it stands for 560 nanometers. Thus your lens with a O56 filter passes light from about 560 to 700 nanometers to the film. The lens is not perfect; thus there is some chromatic shift</p>

[ronald_moravec1]

<p>Nikon APO are not really APO. <br>

There is also a focus shift whenever another piece of glass is introduced even if it is clear and perfectly flat and when it goes inside ithe lens, there is more change than if you put it on the front.</p>

[conrad_hoffman]

<p>My guess is along Ron's line. The color has almost nothing to do with it. A flawlessly flat and parallel piece of glass does not have zero power when placed in front of a lens system. The thicker it is, the worse the shift will be. I think it's mostly an issue with longer lenses. That's one reason that the thin Wratten gelatin filters are preferred for precision lab tests and such- they're so thin that they have almost no optical power.</p>

[alex_lofquist]

<p>I don't know of any 400mm f/5.6 Nikkor with a drop-in filter, so focus shifting due to it's thickness is quite unlikely. Since there were several 400 f/5.6 Nikkors, which one do you have? Does it have the gold ring signifying ED glass? If so, you have an exceptionally fine lens with absolutely minimal chromatic aberrations. Are you focussing visually on the screen, or using a "rangefinder" center spot? With the longer focal length lenses switching to another screen especially matched to the lens focal length might help. Otherwise I suspect that the filter might just be of inferior flatness which can adversely affect longer lenses more than short ones. </p>

[bob_sunley]

<p>Well a good test would be to take a few frames of different subjects with and without the filter and not adjust the focus with the filter in place.<br>

.</p>

[ronald_moravec1]

<p>A few examples. Some lenses furnish a clear filter that is supposed to replace colored internal ones so there is always one in place.</p>

<p>A few teles have a protective clear filter in front. If you remove it, you lose sharpness. It is clearly designed in.</p>

<p>The very old 50 2.0 Summicron R needs the UV filter to perform best. I have checked about a half dozen samples and this was true of all.</p>

[walterh]

<p>My 300mm F4 AF Nikkor has a drop in filter that is supposed to be populated by a clear glass or a filter.</p>

[frank_skomial]

<p>Different light color wave length and different focusing point distance should not be an excuse, but an indication of a poor quality lens.</p>

<p>Lens design corrections for this were either not taking place or were insufficient. </p>

[dan_brown4]

<p>Because the air-glass-air interfaces causes light to refract, moving the point of focus. The lens is designed to have a filter in place at all times.</p>

[keith_b1]

<p>If the focus shift was due to the color of the filter, it wouldn't have occured, since you were focusing by eye through the filter. <br>

Very long telephoto lenses are extremely sensitive to any thickness of glass being added to the light path; you may need a better(flatter, thinner?)filter, or this particular lens may not react well to any filters being placed in the light path. BTW all the 400/5.6's were ED, the first run of them didn't include 'ED' in the name because NIkon hadn't come up with the 'ED' marketing designation at that time. </p>

[alex_lofquist]

<p>Since the inquiry was concerning a 400mm f/5.6 Nikkor without a drop-in filter, much of the later posts seem irrelevant. Besides, this has been the subject of much discussion in the past.</p>

[simon_fallon]

<p>Thank you all for your responses so far. To clarify a couple of points: the lens is indeed the ED AIS version. There is no provision for any type of drop-in filtration. I should explain that it is only "new" to me; it is actually 2nd hand but in near mint condition. Shots taken without a filter in place look crisp and sharp, even with a 14b teleconverter attached. Images taken through a protective skylight filter likewise appear to show no sign of image degradation, nor indeed any focus shift at the taking stage. The filter was, I believe, a Cokin resin type (may be Hi-Tech or Cromatek, I can't now remember as I dont have the original container). Not the very highest pro quality perhaps, but the very same filter used on shorter lenses seems to give excellent results. There is definitely something unusual happening, it's almost as if the apparent need to re-focus once the filter is in place is actually an optical illusion, and that it is the re-focussing itself that is actually throwing the final image out of focus, although this in itself would be even stranger. FWIW I noticed the same filter de-focus phenomenon a few years ago when using an old Sigma 400mm, but I put this down to cheaper, older optics. Have any of you experienced the same problem(s)?</p>

[alex_lofquist]

I've used Hoya (glass) filters on my 400 f/5.6 with no focus shift. I'd guess that replacing the resin filter with a glass one will solve your problem.<br />

[simon_fallon]

<p>Bob, that's a sensible suggestion. It goes against instinct to shoot when the subject appears out of focus but it's the most logical next step to try. Alex, from your last post it would seem that the orange colour is irrelevant, so it must be something else. Could imperfect flatness really be the cause, though? I've used these resin filters on 70-300mm zooms without seeing the same issue, and polyester filters can't possibly be perfectly flat, (although my experience with those is limited).</p>