Filter vs Image Quality

[Nathan Potter]

Without going off thread too much I hope - - What about a filter on the rear side of the lens? Would one expect more image degradation with rear mounted filters than front mounted. I have done this occasionally out of desperation and I believe seen no significant adverse effects.

Nate Potter, Austin TX.

[Bob Salomon]

Without going off thread too much I hope - - What about a filter on the rear side of the lens? Would one expect more image degradation with rear mounted filters than front mounted. I have done this occasionally out of desperation and I believe seen no significant adverse effects.

Nate Potter, Austin TX.

Of course. Not only will any defect or mark on the filter affect the image quality the filter will also shift the focus by about 1/3rd the thickness of the filter material.

Any filter or glass or plastic place inside or behind the lens has to be computed by the lens designers to be part of the optical system. Filters belong in front of the lens with very few exceptions.

As to seeing degradation. The lenses most affected by filters are long focal length lenses. The longer the focal length the more critical that the best quality filters are used.

[neil poulsen]

I have a lot of respect for Bob Salomon's knowledge of photography and equipment. It's always reassuring to know that he's there, ready to bring some particular photographic insight to the conversation.

With that said (), I don't have a problem with placing filters behind the lens. I think that the most important consideration is for the photographer to compose and focus the image with the filter in place. In that way, he or she can take into consideration any negative effects that the filter might have on the optical system.

For example, Ansel Adams recommended placing two filters on either side of the camera, versus placing them both in front of the lens. This avoids inter-reflections between the filters that can negatively affect the image.

Since I don't have data, I'll lay claim to a belief. I believe that, because of the large quantity of light, reflections off the lens front can manifest themselves on the rear of the filter and negatively impact the image in way that won't be nearly the case with the filter mounted behind the lens. I would be interested in other photographers' perspectives on this point.

With all this said, I tried a comparison once where I enlarged an image with and without a Kodak Wratten neutral density filter placed below the enlarging lens. I was quite surprised at the difference in image quality. Of course, I adjusted the f-stop to offset the lightening of the image by the ND filter.

So, maybe better to use filters only if needed, regardless of whether they're placed before or behind the lens.

[Bob Salomon]

Neil,

Thank you for your comments. However, it is important to note that this is not Ansel's days. And filters, lenses and coatings are many times improved from those days.

While many filters and lenses are still manufactured with older coating technology and some filters are still not coated when they should be, modern filter coatings like Heliopan's SH-PMC ensure that 99.9% of the light that strikes the filter passes through the filter to the image plane. Lesser filter technologies do not pass that much through to the image plane and that is flare.

Using modern technology coatings flare is not an issue with filters.

As to observing degradation with a filter behind the lens - it won't work. If you have a light fingerprint, for example, on the filter you won't see the degradation on the groundglass but in a comparison of two shots, with and without the filter with the fingerprint.

[Toyon]

Neil,

Thank you for your comments. However, it is important to note that this is not Ansel's days. And filters, lenses and coatings are many times improved from those days.

While many filters and lenses are still manufactured with older coating technology and some filters are still not coated when they should be, modern filter coatings like Heliopan's SH-PMC ensure that 99.9% of the light that strikes the filter passes through the filter to the image plane. Lesser filter technologies do not pass that much through to the image plane and that is flare.

Using modern technology coatings flare is not an issue with filters.

As to observing degradation with a filter behind the lens - it won't work. If you have a light fingerprint, for example, on the filter you won't see the degradation on the groundglass but in a comparison of two shots, with and without the filter with the fingerprint.

On the B&H website Heliopan filters (67mm) for black and white do not indicate any coating. Are these uncoated, single-coated, or multi-coated?

[Neal Shields]

Several years ago, I think in Phototechniques magazine there was an article showing a compairison in resolution between a B&W photo without a filter and one taken with a "band pass" filter. The difference was dramatic.

Point is there should be some resolution increase anytime we reduce the spectrum that a lens has to deal with.

[Brian Ellis]

Several years ago, I think in Phototechniques magazine there was an article showing a compairison in resolution between a B&W photo without a filter and one taken with a "band pass" filter. The difference was dramatic.

Point is there should be some resolution increase anytime we reduce the spectrum that a lens has to deal with.

That's interesting (I assume you mean "resoluion decrease").

I've considered performing some tests myself but it's always struck me as a difficult test to perform with b&w filters since the effect of a filter, if any, presumably varies depending on the colors in the scene. Any chance of being more specific about the time frame of the Photo Techniques article? I subscribed for many years and still have some back issues somewhere. Thanks.

[Bob Salomon]

On the B&H website Heliopan filters (67mm) for black and white do not indicate any coating. Are these uncoated, single-coated, or multi-coated?

All Heliopan filters, except diffusion, soft focus types are fully coated.

Heliopan offers their standard coating and their SH-PMC 16 layer (8 per side) multi-coating. With filters for B&W the SH-PMC coating is a special order and dealers do not stock them. With UV, Protection and Circ Pols dealers do stock the filters with SH-PMC coating. B&H is a smaller Heliopan dealer. Samy's is a much larger Heliopan dealer.

[Nathan Potter]

Neal is theoretically correct - there should be an increase in the resolution of a lens when the bandwidth of visible light is reduced. The principle has been utilized for years in the IC industry where imaging optics can be designed for single emission peaks in mercury high pressure bulbs (430, 365, 290 nm) for instance. Currently laser sources have even narrower peaks so yield very high resolutions (due also to shorter wavelengths of course). However narrowing the bandwidth also can drastically reduce the light intensity since the total illumination from any subject source is the sum of the individual wavelengths from that source as seen by both lens and film. Most photographic sources are a very mixed color spectrum so selecting a bandpass filter for a B&W representation of a scene could produce pretty wild results. The only predictable illumination source is a classic black body radiator which can be run at a known degree Kelvin temperature. Using bandpass filters is a bit of an intrigueing idea though - anybody here fooled around with this?

Nate Potter, Austin TX.

[Neal Shields]

No I meant increase. With a band pass filter you almost have a single wavelength of light. That makes it very easy to focus in one spot. This is why you use a yellow filter on a convertable lens when only using a single element. The single element does well for everything but cromatic aberation. So to get the quality back you use a yellow filter to get rid of the blue end of the spectrum.

Just to keep you digging through you old magazines there was an excellent article about the compromises in lens design in the Leica magazine that got into this.