Use of ND filters at small apertures and resolution

[davidfung]

Hi. It is a given that when using the very small apertures (f/22 or less) in most 35mm lenses the resolution and contrast is not ideal. It seems from general concensus that it is sometimes performing worse then wide open. I gather the problems mainly stems from diffraction effects, and the difficulty in producing a good aperture opening that is so small. Especially for shorter focal lengths. It is also assumed that regardless of the quality of a filter, it will add negative effects to resolution and contrast to an image.

The question is this. If the shutter speed had to remain the same (that is, it cannot be faster), is using a ND filter to reduce exposure from (say, a maximum aperture of) f/22, to f/11 be better then using it at f/22? Another words, is the effect of the filter worse then the effects of using a lens at its maximum aperture?

Anyone done any research or tests on this matter? Thanks for your comments.

 

[ou1954]

>Posted by David Fung (Davidfung) on Monday, October 27, 2003 - 11:36 pm: > >Hi. It is a given that when using the very small apertures (f/22 or >less) in most 35mm lenses the resolution and contrast is not ideal. It >seems from general concensus that it is sometimes performing worse >then wide open. I gather the problems mainly stems from diffraction >effects, and the difficulty in producing a good aperture opening that >is so small. Especially for shorter focal lengths. It is also assumed >that regardless of the quality of a filter, it will add negative >effects to resolution and contrast to an image. > >The question is this. If the shutter speed had to remain the same >(that is, it cannot be faster), is using a ND filter to reduce >exposure from (say, a maximum aperture of) f/22, to f/11 be better >then using it at f/22? Another words, is the effect of the filter >worse then the effects of using a lens at its maximum aperture? > >Anyone done any research or tests on this matter? Thanks for your >comments.

I've heard the same thing all my life, that is that lenses seem to do best at f/5.6, f/6.3, or f/8. (I know that's not a normal sequence but I've had cameras with at least 1 or two of those settings.)

On the other hand, there is a French optics professor on the RolleiGroup who, I think, said that f/22 was not small enough to cause diffraction problems. I'll Bcc: him on this and see how he responds (being too lazy to work it out for myself). Besides, who can work in this smoke and heat?

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DAW

 

[sem]

>

The effects of diffraction aren't so difficult to estimate. For a lens operating at a given f#, the diffraction-caused spread of an ideal "point image" at the film is about 2.5 * Wavelength * f#. If you take the Wavelength as 0.5 micrometer (kind of an average value for visible light), then the spread at f22 is in the range of 10-15 micrometers. This will have a measurable effect for a lens that could otherwise deliver 100 lp/mm resolution, which is only the very best lenses, operating under ideal conditions, and near the center of the field.

In other words, diffraction may visible at f22 if you go out of your way to look for. It is probably fair to say that it is not important in practice until you go to even small apertures.

 

[mikel]

Well, actually, each lens performs at its best at some "golden-middle" f/stop setting. It all depends on the type of lens and its unique characteristics. For most lenses, that peak performance f/stop is usually either f/4.0, f/5.6 or f/8.0. As a rule, you can usually take f/8.0 as best-optical performance f/stop. Actual figures can be found from manufacturer's spec sheets and Zeiss is doing a very good job at it.

Suppose we take Planar 85mm f/1.4 as ex&le. MTF data published includes measurements made for f/1.4 and f/5.6 and at f/5.6 it delivers the best performance. [ NOTE: Long time ago I have actually asked Zeiss how do they choose what f/stops to show on their graphs and was told that they usually show wide-open setting and the best-overall performance setting. But it's from my memory, so you may want to verify it ]

Now, performance of all 35mm lenses goes down quite drastically at any f-stop settings beyond f/8.0 due to diffraction. It works differently in large-format lenses though and I know that many of them are actually designed to deliver stellar performance stopped down to f/22 or even more. I think it's all in the design/purpose basically.

Anyway, getting back to issue of "using ND or stopping down". My opinion on this (but as usual, there are many schools of thought on it) is use ND. Why? Very simple. Majority of filters were designed for specific purpose and with serious amateurs/pros in mind (when was the last time you seen some amateur care about things like neutral density filters?) and if you're getting Contax, Heliopan or B+W, they're all made using Schott optical glass (same glass used in Zeiss instruments and lenses). They're all multicoated to eliminate reflections and increase light transmission and all are designed to have little or no effect on overall MTF. [It would be definitely nice to see some MTF tests of lenses with filters though, but I doubt someone will ever do that. Unless someone asks Zeiss and ships them their lens and some filters and covers their expenses ]. Now, I personally also rarely shoot without any filters. I always keep UV filter on for protection and also to eliminate dust/oil buildup on the surface of the lens. With air that is quite polluted these days with carbon dioxide and G-d knows what, any lens will get covered by a thin layers of oily junk pretty quickly. And by keeping filter on, you essentially eliminate the problem and make your life easier - it's a lot easier to clean filter than cleaning the lens itself. And if you damage it - no big deal, it's always cheaper to get new filter than repairing your lens.

So, when you take diffraction into account and the fact that your filter is usually mounted in front of the front element, its impact on image quality and MTF is far less significant than stopping down and thus losing MTF (and thus quality!) due to diffraction (and besides, you will get extreme depth of field, which is often undesirable).

So, imagine a situation. You're outside on a very bright sunny day and you're carrying your camera with TMAX 400. You stop by to take a picture, you want to get some nice shot at f/2.8 and oh, my, there is still too much light and the only think you can do is go to f/4.0 or put ND filter on.
If depth of field is not a problem in this case - no need for ND. If it's a problem - you have basically no other neat solution. Another situation. Another bright sunny day. You have NPH 400 and want to shoot at f/8.0, but you're two stops in overexposure in this case. You think to stop down to f/16 or to get ND filter. If you stop down to f/16, you're definitely losing, since performance degradation due to diffraction is significant. If you use ND - you're losing very little (if anything at all).

My 2 cents worth.

Mike.

 

[ou1954]

>Posted by Don Williams (Ou1954) on Tuesday, October 28, 2003 - 2:12 am: > > >Posted by David Fung (Davidfung) on Monday, October 27, 2003 - 11:36 >pm: Hi. It is a given that when using the very small apertures >(f/22 or less) in most 35mm lenses the resolution and contrast is not >ideal. It seems from general concensus that it is sometimes >performing worse then wide open. I gather the problems mainly stems >from diffraction effects, and the difficulty in producing a good >aperture opening that is so small. Especially for shorter focal >lengths. It is also assumed that regardless of the quality of a >filter, it will add negative effects to resolution and contrast to an >image. The question is this. If the shutter speed had to remain the >same (that is, it cannot be faster), is using a ND filter to reduce >exposure from (say, a maximum aperture of) f/22, to f/11 be better >then using it at f/22? Another words, is the effect of the filter >worse then the effects of using a lens at its maximum aperture? >Anyone done any research or tests on this matter? Thanks for your >comments. >DAW writes: >I've heard the same thing all my life, that is that lenses seem to do >best at f/5.6, f/6.3, or f/8. (I know that's not a normal sequence but >I've had cameras with at least 1 or two of those settings.) > >On the other hand, there is a French optics professor on the >RolleiGroup who, I think, said that f/22 was not small enough to cause >diffraction problems. I'll Bcc: him on this and see how he responds >(being too lazy to work it out for myself). Besides, who can work in >this smoke and heat? > >

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> >DAW

Here is his response regarding f-stops:

Don

The question is rather simple. Each lens designed to cover a given format has a best aperture. In 35 mm photography the best aperture is what you say, i.e. 5.6/6.3/8.

However for larger formats, the best aperture is smaller. In 6x6 it is 8->11, in 6x9 11-16, in 4"x5" 16-22.

Where does this come from ?

Residual geometrical aberrations define a minimum spot size. Engineers have to make severe compromises between cost, performance, max aperture, maximum image circle etc.. and this yields a minimum aberration spot. This spot scales in proportion of the focal length or with the diagonal of the format, for a given type of lens. For ex&le consider a 5-6 element standard lens, say a 50mm, or a 80mm planar/xenotar or a 100mm or a 150 standard for 4"x5". If we do not take into account the fact view camera lenses have to cover a wider angle than a 50mm for 24x36mm pictures, the minimum aberration spot size roughly increases proportionally to the diagonal of the format.

The best aperture is the one for which residual aberrations and diffraction contribute equally to a combined aberration/diffraction spot. The diffraction spot does not scale with the focal length, it depends only on the relative f-number : something like d_diff(in microns) ~= N microns where N is the f-number. At f/16 the diffraction spot is about 16 microns whether you use a 35mm focal length or a 350 mm telephoto. So for larger formats you can stop down more until diffraction is equal to the residual aberration spot, since you'll not enlarge the image as much as in 35mm when you use 4"x5" film.

In 35 mm photography as soon as you stop down beyond f/11 you degrade the quality of your image, but the degradation is very slow.

Now if you insist on having the maximum image quality, forget about 35 mm photography and use 6x6cm or larger formats on a tripod ;-);-)

I home that this is clear enough. People should not be afraid of stopping down a good lens for 35mm photography at f/16 ; simply with a Rollei 6x6 TLR stopped down at f/16 and a final image enlarged 5x (instead of 10x from a 35mm neg) will be much better. About 4 times in terms of equivalent number of pixels.

-- Emmanuel BIGLER