ND filters on Fuji 100-400mm lens

[bravocharlie]

A friend and I both have XT2s and 100-400 lenses. We have both tried to use a 10 stop ND filter at 400mm and it is impossible to get a sharp image at any aperture. At 100mm it seems ok and both lenses perform superbly without filters. Fitting a 10 stop ND is not something I would often want to do but I would like to know why this is a problem. Is there some technical reason? Has anyone else experienced this?

[mdotson90]

A friend and I both have XT2s and 100-400 lenses. We have both tried to use a 10 stop ND filter at 400mm and it is impossible to get a sharp image at any aperture. At 100mm it seems ok and both lenses perform superbly without filters. Fitting a 10 stop ND is not something I would often want to do but I would like to know why this is a problem. Is there some technical reason? Has anyone else experienced this?

There are a few things to consider here, a few of which aren't exactly related to the physics of optics, but we'll start there.

 

A lot of engineering has to go into a lens to give it the highest resolving power. Ultimately, aberration and diffraction limit this.

 

A lens' job is to focus light into a single point. As light strikes the surface of glass, or any other medium, some of it is reflected and some of it travels through, but it refracts (changes angle). In an ideal lens, the change in angle would be as close to 0 as possible, and the reflections kept to a minimum. Lens manufacturers try to mitigate these problems with differently shaped lens elements and fancy coatings, but they will never completely eliminate them.

 

It is easier to tune the glass elements for a specific focal length, which is why primes tend to be sharper than zooms. You have to make sacrifices when designing the lens elements to be used across a large focal length range.

 

Now that that's out of the way, on to your issues:

 

1. The filter

 

Any time you add a filter of any sort, you're adding an element to the lens that wasn't accounted for when the actual lens elements were designed for optimal light transmission. A high quality filter with high quality glass and coatings can minimize the image degradation beyond what your pixel peeping eyes might make out. However, I believe it is unsafe to say that there is any filter available that completely eliminates it.

 

2. The zoom

 

Like I said above, manufacturers must make compromises when designing zoom lenses. Some do better than others. Most (I say most...) consumer lenses do have some degree of softness at their longest setting. Obviously, a piece with superior optics will do better than a cheaper lens. Lenses with large zoom factors are worse than ones with smaller ones. For example, my 18-135mm (7.5x zoom) is annoyingly mushy at 135mm. The 100-400 (4x) is not nearly as bad in this respect, but still, physics.

 

3. The atmosphere

 

Fog, dust, smog, smoke, pockets of warm air, all these things scatter and refract light as it passes through the air. The effect is much worse at telephoto focal lengths.

 

4. The f stop

 

Everyone knows that most lenses perform best anywhere from 1-3 stops down from the largest aperture due to the minimization of aberration. The sweet spot is different for each lens but the idea is the same. Stop down too far, and diffraction becomes an issue. Diffraction is the disturbance of a wave when it encounters an obstacle -- think of a ripple in a pond encountering a rock in the water. When waves of light travel through your aperture, the Iris itself causes some disturbance in the light. At smaller apertures, diffraction becomes very noticable depending on the lens/camera, but it could begin degrading an image as low as f/11. At 400mm, the largest aperture on the XF 100-400mm is f/5.6. This gives you about two stops from wide open until you're in potential diffraction territory. It's a fine balancing act.

 

5. What are you actually doing?

 

Aside from the limits of your lens, the environment, and physics... What could you be doing better? I'm not going to lie, I've been thinking the whole time ive been writing this about the possible application of 10 stops of ND filter @ 400mm, I guess not everything is here for me to understand.

 

Even in broad daylight, a 10 stop ND requires stupid long exposure times (I guess that's the point). And, not to insult your skill/intelligence in any way but I ask you:

 

Are you using a quality tripod? Any wind? That could affect very long exposures. Do you have OIS turned off? Are you using the timer or a remote to start the exposure?

 

All of these things are probably working against you. Sorry for the length, but I hope this helps.

 

 

Sent from my Pixel XL using Tapatalk

[bravocharlie]

Thank you so much for your well considered and comprehensive reply. I was fairly sure that this problem was more to do with asking this lens to do what was probably technically impossible but nevertheless I was still a bit mystified. I would not normally use this filter at such extreme focal lengths but we all want to know the extent of the capabilities of our gear so I thought it worth a try. In answer to your last point, solid Gitzo tripod wth quality head, minimal wind, OIS off and self timer set. Interestingly I have had superb results with the 50-140mm lens using the same filter.

Thanks again for your response..

[fugu82]

I was able to get very sharp images of the sun during the August eclipse with the 100-400 @ 400, with a 1.4X, and an 18 stop solar filter. There really shouldn't be an issue with your 10 stop. 

 

Can you try shooting again with a really high-contrast subject, to ensure good focus?

[bravocharlie]

Thanks for your reply. I have no experience of solar filters so I don’t know how they compare with normal ND filters but this problem has been found on two separate XT2s with different 100-400 lenses. In both cases they were focused firstly with the filters in place and then with no filter, the filter being added after focusing. The point here is that in the second examples the cameras had achieved perfect focus before carefully adding the filter whilst ensuring no change in focus or zoom. The result was a VERY unsharp image in both cases. I therefore do not believe that this is a focusing problem and having read the post by mdotson90 above I think that it may be that placing a dense ND filter in front of the lens renders it unable to produce a sharp image even if it achieves focus.

[fugu82]

A solar filter is simply a very dense ND filter with additional IR filtration. 

 

But I tried to produce a sharp image with a 10 stop filter, also. It worked fine. There is nothing wrong with the 100-400 in regards to dense filters.

 

It does not auto-focus well with a dense filter and relatively dim light, though; remember that it is a 5.6 lens at 400mm. In order to achieve sharp focus you will need to make sure the switch is set on manual focus, and either focus without the filter or manually focus with preview picture mode turned off. 

[bravocharlie]

Tried all the suggestions here to no avail. Interestingly, when I locked focus on a high contrast subject without the filter band then slid the filter into place the image immediately went very soft. Pressing the autofocus button showed the green "in focus" light, yet the image was still extremely soft. Trying to manually focus made no difference. 

fugu82, I would be interested to know what make of 10 stop filter you used.

[fugu82]

Wow, that must be frustrating.

 

My 10 stop is a Breakthrough. But if you get a sharp image with your filter on another lens, then your filter is fine.

 

The fact that it went soft right away means that the lens is trying to automatically re-focus, which it doesn't do well with a dense filter. It shouldn't do that unless it's on S or C focus mode with Pre-AF enabled.

 

Make sure the focus mode selector switch on the front of the camera is set to M [manual], not C or S. 

[mdotson90]

Bravocharlie, what brand of filter are you using?

 

Sent from my Pixel XL using Tapatalk

[bravocharlie]

Its a Hightec Firecrest 10 stop ND

Edited December 4, 2017 by bravocharlie

[Pavlos Euthymiou]

I'm having the exact same problem. I already had it with other lenses and cheap nd filters on my XT2, but decided to buy this ND 2-400 KeF filter for my 70-300. I cannot get one photo in focus. Actually it is kind of in focus but with a terrible haze/halo effect. Even though the filter may not be the best quality in the world, it seems my XT2 cannot get proper focus/definition with an nd filter on. I can get good pictures with uv and cpl filters.