Below lens or above? (contrast filters)

tkamiya · Mar 9, 2011

This is one of those things I wish I can test it myself but I'm not willing to destroy my filter to see how it will change things....

Commonly held wisdom is that less below lens (in enlargers) the better the print image quality. In this wisdom, it is said that below lens multi-grade filters degrade images if not perceptibly - at least theoretically.

There are statements found on internet, some firsthand, some second or third, that in reality, one will be hard pressed to see the difference in actual products even when filters are somewhat scratched. One on this forum has said, because the filter is so thin and close to the lens, it won't affect images.

I have two enlargers that currently, both are fitted with below lens Ilford filters. Personally, I have no seen anything objectionable but since I cannot test the other, I have no idea if it could be better.

Do we have anyone who has actually done tests in this regard? That is, comparison between above and below, then for below, pristine filters and scratched? If so, I would very much like to know what the experiences were.

For this thread, I am not so interested in how it *should be* based on theory alone. I'm more interested in actual accounts.

Thanks much!

brucemuir · Mar 9, 2011

I haven't done a direct comparison but used to use the below flip out type on my DII condenser set up.

I never noticed image degradation but I guess it has to happen. Whether it shows in the print is the main issue and I was quite happy with up to 11x14 (largest I did then). My filters were in decent shape.

I later went to above the lens and I'm now using a Dichroic diffusion head. The diffusion head definitely isn't as crisp as my below lens period so I think it all comes out in the wash

I wouldn't sweat using below lens filters if you are happy with your prints. I doubt you'd gain much.

vpwphoto · Mar 9, 2011

Without any doubt ABOVE... unless B+W makes filters and a holder that keeps them right in line with the lens... even if they did I'd still say above with no doubt.

If you were to measure lines of resolution in the print you would be able to quantify it. I won't even argue the point. If you are happy that is fine. But I am not projecting an image through a nice Rodenstock lens and then throwing those image bearing photons though a dusty piece of plastic with micro scratches. The light will scatter and reflect within the plastic. I have been using above the lens plastic 6x6 filters just below the cold light for 25 years.

For the record, my Ziess T* lenses bought new never had a skylight filter, and still have no ill effects on the elements from heavy professional use. Oh I did buy a B+W skylight filter on when I shot in rolling steel mills.
Only color correction minus blue and Orange filters when needed.

Rick A · Mar 9, 2011

Any time you add anything below the lens you have a reflective surface that can degrade the image. Definitly use above the lens filters.

ic-racer · Mar 9, 2011

I almost spent over $200 on 12" above the lens Ilford MG filters for my 8x10 enlarger based on that "above the lens" dogma. If you read the instructions for the smaller Ilford MG filters it says it is ok to use them under the lens, and I can say that indeed that is the case. I wound up getting the six inch below the lens filter set and saw no loss of quality (with grain magnifier and in prints) using the filters below the lens.

tkamiya · Mar 9, 2011

I understand, theoretically, above the lens is better. The filter is removed from the image path. But, my concern is more of practicality. Do any of you seen any degradation by placing one under? As I said in my original post, I would like to know if anyone have seen any difference, and if so, at what point (amount of scratch) did you notice the difference.

MattKing · Mar 9, 2011

I don't have anything but anecdotal evidence to rely upon, but here goes.

I have worked in darkrooms with multiple enlargers, where some use below lens filters, and others use above negative filters, and have seen no difference in results.

I have gone from using a below lens filter enlarger to one with a multi-grade head, and can see no filter induced degradation in the earlier prints.

It doesn't surprise me that this would be the case - the role that a below lens contrast filter plays in an enlarger's light path is very different than the role that a camera filter screwed on to the front of a lens plays.

BetterSense · Mar 9, 2011

It doesn't surprise me that this would be the case - the role that a below lens contrast filter plays in an enlarger's light path is very different than the role that a camera filter screwed on to the front of a lens plays.

How? It seems exactly the same thing to me.

The only change I think that below-the-lens filters cause is slightly increased flare. Since you are adusting the contrast anyway, I don't think it matters. I was too cheap to buy the nice plastic Ilford below-lens filters so I bought the 3" above-lens filters and I use them below the lens. It works.

holmburgers · Mar 9, 2011

If you've ever shot through a dusty lens, or an old fungus ridden lens, you'll be amazed at how the imperfections in the image path, but not in the plane of focus, have very little effect. Undoubtedly, there is an effect, but for most work I'm sure you'll scarcely notice a difference.

MattKing · Mar 9, 2011

BetterSense said:
the role that a below lens contrast filter plays in an enlarger's light path is very different than the role that a camera filter screwed on to the front of a lens plays.

How? It seems exactly the same thing to me.

...

Well ...

You are working with a single light source, either collimated or close to it. The range of light intensities in the darkroom is much narrower than outside in daylight. The colour spectrum of the light source in the darkroom is relatively narrow, and very consistent. The paper has limited sensitivity to most colours, so chromatic aberration isn't as much of a potential problem.

All working distances are in the macro range, rather than ranging from macro to infinity.

There are a lot fewer sources of high intensity off axis flare in the darkroom as compared to outside in daylight.

With a very few exceptions, in the darkroom you don't have to worry about your lens having to zoom through a wide variation of angles of view, so your filter doesn't have to be concerned with that.

tkamiya · Mar 9, 2011

Sounds like the difference is really not enough to worry about....

Photo Engineer · Mar 9, 2011

All filters come in two types. One type type is used above the lens and another below the lens. It used to be that they were marked with identifying notation such as "CC" and "CP" which is what Kodak used.

Your filter instructions should telll you.

PE

Saganich · Mar 9, 2011

When using my Focomat IIA I tried putting a filter in the negative mask slot below the negative but above the lens and below the condenser above the film. I also compared prints using the same filters under lens on top of condenser. No difference with the above condenser and below the lens but I can say that the filters under or over the negative, between the condenser and lens, without a doubt caused the image to degrade.

ath · Mar 10, 2011

I've used the "above" Ilford filters under the lens. I didn't notice any degradation but additionally did a test:
- ultra high resolution negative (USAF1951 on Orthopan UR / Adox CMS) with 180lp/mm (sic!)
- enlarged to 45*65cm
- analysed with a loupe: 180lp/mm are there.

So my first hand evidence is: as long as the filter is clean and not scratched you are fine. Common sense says scratches and dirt will only lower contrast.

jerry lebens · Mar 10, 2011

Having interviewed some of the best printers in the UK in my series 'Master Printers' for B&W Photography magazine, I can confirm that several of them use filters below the lens without any obvious degradation in image quality. It's especially helpful when you're split grade printing, if you haven't got a multigrade head there's less risk of knocking the enlarger as you change filter drawers if you hold the filter in your fingers beneath the lens.

Yes, there's a theoretical chance that you may degrade the image with a very scratched, dusty, filter but it remains mostly theoretical. If you've got that much dust on your filters then, in putting them into your enlarger, you're probably dumping a shed load of dust onto your negative anyway...

Do whatever suits you best.

Regards
Jerry

Ian Grant · Mar 10, 2011

Having used both, commercially and at home, I have to agree with Jerry and others that below the lens filters are fine, just treat with care.

There's no compromise in quality with below the lens filters, the Ilford ones are in good holders and will easily last a few years.

Ian

Neal · Mar 10, 2011

Dear tkamiya,

A quote by Ctein from Post Exposure, 2nd Edition, p144: "According to my tests, it makes no difference. I set up a high-resolution target with my 55mm Computar lens at optimum aperture and examined the projected aerial image with no filter under the lens, with modern thin filters under the lens, and with older cast plastic filters under the lens. In all cases, I could see a clean, 320 line pairs per millimeter (lp/mm) in the center of the field and more than 280 lp/mm at the corners. I could not convince myself that I saw any degradation in image quality with the filters in place, no mater how hard I looked. That surprises even me, but it's true. As long as your VC filters are not scratched enough to create serious flare (see Chapter 5) I can see no reason for avoiding below the lens filters."

Neal Wydra

Ian Grant · Mar 10, 2011

i think Greg Davis has already said that

However thanks, you added more details, and that backs up what many of us have said

Ian

Neal said:
Dear tkamiya,

A quote by Ctein from Post Exposure, 2nd Edition, p144: "According to my tests, it makes no difference. I set up a high-resolution target with my 55mm Computar lens at optimum aperture and examined the projected aerial image with no filter under the lens, with modern thin filters under the lens, and with older cast plastic filters under the lens. In all cases, I could see a clean, 320 line pairs per millimeter (lp/mm) in the center of the field and more than 280 lp/mm at the corners. I could not convince myself that I saw any degradation in image quality with the filters in place, no mater how hard I looked. That surprises even me, but it's true. As long as your VC filters are not scratched enough to create serious flare (see Chapter 5) I can see no reason for avoiding below the lens filters."

Neal Wydra

holmburgers · Mar 10, 2011

Nothing to see here folks... go home to your families.. nothing to see here.... let's pack it up and call it a night....

oliceman:

Ian Grant · Mar 10, 2011

holmburgers said:
Nothing to see here folks... go home to your families.. nothing to see here.... let's pack it up and call it a night....

oliceman:

Hasn't that been the US Policy on helping the anti Gaddafi Libyan's for the past 2 weeks?

tkamiya · Mar 10, 2011

Thank you everybody.... I'm curious, how badly the filter has to be scratched to start affecting images.... any volunteers?

holmburgers · Mar 10, 2011

Ian Grant said:
Hasn't that been the US Policy on helping the anti Gaddafi Libyan's for the past 2 weeks?

Who's Gaddafi? What's Libya? All I know about is Charlie Sheen....

Ian Grant · Mar 10, 2011

holmburgers said:
Who's Gaddafi? What's Libya? All I know about is Charlie Sheen....

One day Hamburger it'll be your butt on the line There's no jokes in the real world.

Ian

tkamiya · Mar 10, 2011

Moderator, moderator, Ian said "butt!!!"

holmburgers · Mar 10, 2011

I know.. I'm actually mocking my own country.

It's an interesting situation and I'm not really aware of how the US is responding compared to other nations. 'Mums the word' I take it?

Below lens or above? (contrast filters)

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