Flat Lens Filters v Curved Lens Filters

[VidThreeNorth]

For digital photography, there is less need for filters than for "film photography". We generally do not use them for simple colour correction. Moreover, for many cameras, we do not need them to block infrared light. My Sony and Lumix camera bodies block near-infrared by default. I would need to send my camera to a repair and pay extra to remove the infrared blocking filters.

The picture I am uploading was taken on my Sony a6400 with a Sigma 30mm F2.8 lens. It was random chance that I had a Tiffen "Hot Mirror" filter mounted. It was not necessary, and I did not expect it to make a difference. However, due to the high contrast of "post-sundown" lighting with the moon reflecting the sun, I found a reflected "ghost" image of the moon showing up in the frame. I could have taken off the filter, but on a whim, I decided to include it in the composition and see how it turned out.

The ghost image is the result of the moon's light reflecting on the sensor, back out through the lens and reflecting on the inside surface of the filter, back through the lens again and re-focusing again on the sensor in another position. If I pointed the camera straight at the moon, the two images would be at the same place. My moving the camera slightly, I offset the reflected "ghost moon". Then later I cropped the picture for best composition.

It got me thinking about Pentax lens filters. Pentax brought out a special filter that used curved glass in stead of flat glass. The intention was to stop this type of "internal reflection" from happening. It focussed the light inside the body of the camera so that the light was unfocussed on the "film plane/sensor". I don't know if Pentax still makes these filters, but I hope they do.

 

[Thiophilos]

Yeah, I remember those filters. I don't think they ever caught on.

Back in the olden days, it helped to put a UV filter on a lens to filter out haze, and also to protect the front element. Those front elements were coated with an antireflection coating but that film was fragile. And the front elements were made of soft glass that scratched easily. The filter kept the front element clean, meaning less cleaning was needed and there was less abrasion. But using a regular flat filter caused those ghosts if there were strong bright patches in the photo.

The curved filters purportedly solved the problem. I don't think they ever caught on, and I never owned one or even saw one.

Today front elements are strong, and the coatings are really tough. There isn't that need to protect the front element any more. Besides, it has been proved that filters shatter easily when they suffer an impact.

Good quality filters are coated now. So if you use one, that greatly reduces reflections and ghosting.

Optical designs have changed a lot over the years, with most lenses now being zooms and internal-focusing. The complicated optical designs have had an effect on bokeh, and they also sometimes produce the ghosting that we saw before with filters, without a filter.

Today with the tough front elements and coatings, and with UV-proof sensors, we don't need filters any more (except for special situations). But habits die hard and lots of folks still put protective filters on their lenses, and many are still called UV filters.

 

[VidThreeNorth]

Yeah, I remember those filters. I don't think they ever caught on.

. . .

Only Pentax knows how many they sold. I bought one (49mm) and used it for years. I don't know where it is now. How much difference did it really make? To know that, I'd have to take a bunch of real pictures with a flat filter and the curved filter and look at the difference. I never really did that test. A really distinct ghost like this moon picture is actually not that common. But if I could buy one for one of my current cameras, I would probably buy it.

 

[petrochemist]

I can remember taking a shot of the moon through clouds & getting a distinct ghost image of the moon. I'm fairly sure I wasn't using a filter on that occasion (though I often do).
In my case the real moon was somewhat overexposed & the ghost image had a strong blue colour. The blue moon effect is why it sticks in my memory

It would seem there was another internal reflection causing this image unless my memory of not having a filter mounted is faulty!

 

[VidThreeNorth]

I can remember taking a shot of the moon through clouds & getting a distinct ghost image of the moon. I'm fairly sure I wasn't using a filter on that occasion (though I often do).
In my case the real moon was somewhat overexposed & the ghost image had a strong blue colour. The blue moon effect is why it sticks in my memory

It would seem there was another internal reflection causing this image unless my memory of not having a filter mounted is faulty!

After midnight (this morning) I took my camera out and confirmed that the "blue moon reflection" was caused by my filter.

 

[Golem]

I can remember taking a shot of the moon through clouds & getting a distinct ghost image of the moon. I'm fairly sure I wasn't using a filter on that occasion (though I often do).
In my case the real moon was somewhat overexposed & the ghost image had a strong blue colour. The blue moon effect is why it sticks in my memory

It would seem there was another internal reflection causing this image unless my memory of not having a filter mounted is faulty

I recall those pentax filters, called "Ghostless".

I dont think front filters are especially to blame for ghosts since removing the filter usually doesnt bust the ghost. Theres plenty more reflecting surfaces, and one that can definitely be trouble is the last (rearmost) surface combined with the shiny surface of the sensor.

I suspect that one aspect of lens designs marketed as "digitally optimized" is avoidance of rear surface curvatures that exacerbate ghosting between the rear surface and the sensor.