Any Real Reason To Favor One Particular Lens Line?

[Mark J]

1. Filters behind the lens, even gels, are a no-no unless the lens has been specially designed with that in mind.

2. Gels smudge and crinkle and attract dirt and grit easily; and rear mounting is generally a bad idea.

3. Have you ever seen some of AA's classic 8x10 shots blown up more than a 3X enlargement? Well, I've seen a lot of those, and nearly all of them are quite unsharp by modern standards.

1. Gels are quite thin, and LF lenses are used at very slow apertures. The potential risk we expect from a filter behind the lens is spherical aberration, but that's really mainly a problem for 35mm lenses which are faster, I can't see an issue at f/16 or less with a gel filter much less than 1mm thick. You still have the ghost image problem from bright sources eg. the sun in the picture, that appears diametrically opposite the main image, though.

2. I agree that gels can be less than ideal in surface quality or durability, but behind the lens, at least the pupil size for any point in the image is smaller (hence the transmitted wavefront damage is less) , and you avoid the problem of direct sunlight from outside the field of view, which can cause scatter from an imperfect gel. They are safer behind the lens than in front, in my opinion.

3. Yes, I saw some of Ansel's 'Mountains and Rivers' shots in London a few years ago, and was surprised at the variability in technical quality and the patchy focus across the frame in some cases. I still prefer the Desert SW shots ... but that's another well-worn subject.

[Mal Paso]

Maybe you smudge and crinkle gels but I've never had that problem. Gels are to thin to adversely affect the light path and inside the camera there are fewer problems with reflections. Your argument is FUD.

[Drew Wiley]

If you sometimes make very precise big expensive color enlargements, like I tend to do, it can make a significant difference. So do precision sheet film holders versus the regular kind. And per gels- they fade. And new ones certainly aren't cheap; often more than multicoated glass ones. I do have a lot of esoteric gels in the lab for technical purposes, where they are pampered; but that is another story. In the desert on mountains, I gave up on gels decades ago. If they get wet, you can't just wipe them off. And I'll only use uncoated glass ones if there is no other choice in a particular flavor I need.

Many of AA's classic mountain shots were taken in what was almost my back yard growing up; some of it our front porch view. So I can appreciate his sensitivity to the light, as well as his contribution to our National Parks and Wilderness protection movements. But his darkroom was rather primitive, and until late in life, the cameras, lenses, and films he used were less precise than what we take for granted today.

[Mick Fagan]

Is was reasonably common with our product photography, all lit with flash and using transparency film, to use colour correction (CC) filters stuck on the rear of the lens using Blue Tack.

Mostly the CC filters we used were to take the coldness away from sterile white objects. This was the late eighties through to the early nineties where pretty much anyone used a CC filter to get the look on the lightbox that the advertising account executive was looking for.

I cannot remember there ever being any hoopla about lost definition.

[Alan Klein]

Filters behind the lens, even gels, are a no-no unless the lens has been specially designed with that in mind. It's an utter myth that rear-mounted gels are going to give sharper images than modern coated glass filters mounted in front of the lens. Just the opposite. Gels smudge and crinkle and attract dirt and grit easily; and rear mounting is generally a bad idea. Yes, uncoated glass filters, perhaps routine back in Ansel's Day, do attract condensation and need cleaning way way more often than coated and multicoated glass ones.

In terms of position, graphics process lenses were designed for gel filter placement at the nodal point between the two cells, and often came with a slot to accept those along with optional Waterhouse stops. However, I get superb result with front mounted glass ones. Some truly big telephoto lenses have a built-in rotating contrast filter set installed at a specific internal position. But any lens actually optically engineered for rear-mounted filters would be a rarity.

Have you ever seen some of AA's classic 8x10 shots blown up more than a 3X enlargement? Well, I've seen a lot of those, and nearly all of them are quite unsharp by modern standards.

Isn't it true that dust and scratches on rear elements are worse than on front elements because the back elements focus stuff better on the film? So that would account for why filters are better on the front?

[Mark J]

If you sometimes make very precise big expensive color enlargements, like I tend to do, it can make a significant difference.

I've been pondering this. I realised that ripple in a filter can cause distortion ( image shape distortion ) when placed at the back, but wouldn't in front.

[Mark J]

Isn't it true that dust and scratches on rear elements are worse than on front elements because the back elements focus stuff better on the film? So that would account for why filters are better on the front?

There are two competing issues here.
1 Scratch or dig on the front lens can be illuminated by bright source ( usually the sun ) which is outside the field of view of the image. In that case, better that a fault is on the rear lens which only receives light for the image field of view.
2. The pupil size : the bundle of light going from a point in the subject to a point in the image is significantly smaller on the rear element than on the front element. If there's a dig of significant size on the rear element then it would be more likely to block or scatter light from a bright source in the picture.

So it can't be a hard & fast rule. My feeling is that there are more cases when a defect on the front lens would matter, than a defect on the rear lens.

[Mal Paso]

Gels have 4 handles, the center is never touched, there is no defect in the image area. They are only .005" thick but if your tolerances are that tight you can always refocus, you have to with glass (behind the lens). I wouldn't open the camera in a dust or rain storm but I probably wouldn't have the 4x5 out in that either. I'd shoot with my weatherproof digital and do filters in post.

[Drew Wiley]

Too bad no actual optical engineers are chiming in at the moment. I do know what they've said in the past, which certainly contradicts the previous two posts. But in this era when very high quality multicoated glass filters are routine, gels are almost antique by comparison. Much of what filters do simply can't be done in "post", not either well or not at all in many cases. That applies to both color and black and white film work except for minor color temp corrections. My own "studio" for several decades by now is everywhere but indoors. Weather comes with the territory. But how would I handle a situation with a scratch or ding on the front lens? - throw that lens away to start with. That's just common sense.

[Mark J]

Too bad no actual optical engineers are chiming in at the moment.

You must have missed posts 41 and 47.