Lenses Designed for Digital LF

[Gib Robinson]

I've recently seen a number of very good lenses up for sale because the sellers say they are "going to digital". What are they buying and why? Is there a significant diffrence between lenses designed for film and lenses designed for digital LF? In 35mm a good lens designed for film works just fine for digital.

[Frank Petronio]

Most of those guys probably just need the cash to purchase the latest Canon-Nikon dSLR gear.

But at the higher end of the market, if you're going to get a 39 mp Leaf back to use on a small view camera like a Sinar P3, the sensor size is only 38 x 46mm -- smaller than medium format cameras. So a digital lens can be designed to need less coverage and more resolution. In the case of some wide-angle designs, they are actually designed to increase the working distance between the lens and the sensor for practical working concerns.

We're actually on the second and third generation of digital specific view camera lenses, as the market has decided on small >6cm sensors for the foreseeable future. Ten years ago when large format digital was first viable, the cameras were using scanning backs and the lenses needed 4x5 coverage.

Of course there is no reason why you can't use an 8x10 wide angle lens with a digital back, even if it might not be optimal. In a lot of cases professional photographers are selling off their view cameras because the aren't using them for jobs anymore, which means great bargains for people like us who still do.

If you frequent the digital forums, you'll see a lot of frustration from professional digital users forced to use 35mm wide-angle lenses. The current dSLRs so outperform 35mm film that the lens don't hold up as well as hoped, especially for architectural applications.

[paulr]

There's the performance issue that Frank mentions, and also the simple fact that all your lenses will behave like like much longer ones when you're using the smaller format size. If you want your familiar angles of view, you'll need need different focal lengths.

There's one other performance issue that I've heard ... digital sensors are extremely sensitive to lateral chromatic aberation. An aberration that might cause a barely noticeable color fringe on film can be exagerated by aliasing (or some similar phenomenon) on a sensor, and make an image look horible. So extra attention goes into correcting lateral color on the digi lenses.

[Gib Robinson]

So, essentially, the higher quality of digital sensors is revealing the limitations of LF lenses, just as it does in every other format. In 35mm the current generation of top-quality lenses are the best for digital as well as film (Leica APO lenses come to mind). Is that also true of the top-quality LF lenses? For example, I assume that the Schneider 110mm XL would be likely to perform well as a "digital" lens.

[Oren Grad]

So, essentially, the higher quality of digital sensors is revealing the limitations of LF lenses, just as it does in every other format. In 35mm the current generation of top-quality lenses are the best for digital as well as film (Leica APO lenses come to mind). Is that also true of the top-quality LF lenses? For example, I assume that the Schneider 110mm XL would be likely to perform well as a "digital" lens.

It's not so simple. The physical structure of a CCD or CMOS imaging chip is very different from that of a sheet or roll of film. A design that's optimized for one isn't necessarily best for the other. That doesn't mean that the digital lenses are "better" in an absolute sense, only that they need to be optimized for different design criteria.

This becomes especially apparent when you look at wide-angle designs for 35mm. The latest generation M-Leica wide-angle glass is superb. In a properly-configured bench test, these lenses can put an image on fine-grain film that contains more information than even the best 35-format digital sensor can manage. But these lenses weren't designed for depositing photons into the wells of imaging chips, with the geometric constraints on angle of incidence that that entails. You can see the result in Leica's current struggle to build a digital M camera that can preserve the image quality of the existing M lenses.

More generally, there's an engineering constraint that becomes very apparent in designing large format lenses: the bigger the "sensor" (chip or film) area you want to cover, the harder it is to maintain image quality across the field. It's not correct to assume that just because a LF lens design is newer, it's necessarily "better for digital". The example you cited, the 110 SS-XL, is designed for very large coverage in a compact package. I wouldn't be surprised at all if, say, the 120 Apo-Symmar, a design that's maybe 10-15 years older and much less expensive, did better across the small size of a "medium format" imaging chip.

Finally, there are other subtleties in design for digital image acquisition. For example, the Rodenstock Apo-Sironar-Digital HR series design assumes the presence of a cover glass on the imaging chip. Many other "digital" designs don't. This isn't a question of "better" or "worse" overall, just a matter of what's the best match to the particular imaging device you're using.

[Neal Shields]

I can't really see that digital sensors will ever be able to capture as much information as film, so I agree that the diferent design of digital lens is more to account for the limitations of the sensor and the small sensor size.

In tests Zeiss is getting up to 400lp/mm to film and if you accept the standard idea that it takes at least 3 rows of pixels to replicate a line pair then it is easy to see that it is mechanically impossiable to pack that many useful pixels in such a small space.

http://www.zeiss.com/C12567A8003B8B6F/GraphikTitelIntern/Gigabitfilm/$File/gigabit.jpg

In real world testing the FBI lab found that it takes about 16 meg pixels to duplicate a frame of 200 ASA 35mm film

http://www.fbi.gov/hq/lab/fsc/backissu/april2002/swgitfield1.htm

I don't mean to be disagreeable but more and more I see asumptions being made that new technology is superior to old and that is not always the case. (CRTs are better than plazma, for example, in almost every measureable critera except that my wife likes flat.) Digital is not better or worse but different. However it has intrinsic technical limitations that prevent it from every capturing as much detail as a given amount of film that is the same size.

[Gordon Moat]

One thing not yet mentioned in this thread is that the reflectance of an imaging CCD is different that that of a sheet of film. If I recall correctly both Schneider and Rodenstock make comments about coating differences on the last element of their digitally optimized lenses.

Imaging chips have a long way to go to match the better B/W films just in outright resolution. However, I think such comparisons often miss an important aspect of photography: unless you are doing photogrammetry, there are usually far more compelling or interesting aspects of many images than their outright resolution. As long as people still enjoy seeing images that were captured on film, there is nothing inherently wrong with using film. Whether film or digital capture, it is the flaws and shortcomings of either that are often cited as examples of superiority of one choice over another; though when either method of capture is used properly and within their limitations technical merits tend to disappear for the average viewer.

Ciao!

Gordon Moat
A G Studio (http://www.allgstudio.com)

[Oren Grad]

Before this turns into another digital vs analog debate, here's a quick recap:

1. Are there differences between lenses designed for film and lenses designed for digital? Yes.

2. Do those differences have a visible impact in typical real-world use? Sometimes yes, sometimes no.

3. Is performance in digital image capture a more sensitive indicator of the overall quality of a lens than performance on film? Not necessarily.

4. Are the best ("top quality") film lenses also the best available for digital applications? Sometimes yes, sometimes no.

[Gib Robinson]

Very interesting discussion. Thank you.

It does seem as if digital image capture is going to have a continuing impact on lens design but the technology is so new, the design prameters aren't completely settled. Fortunately for me, I'm just using film for LF . . . at least for now :-)

[neil poulsen]

I know the rear coating on wide angles can make a difference. Sigma has been upgrading their wide lenses to a DG grade that includes a coating that isn't as prone to reflecting light back onto the imaging chip.

[Frank Petronio]

If the essence of the original question was "Should I buy nice expensive modern lenses now, because I can use them for digital someday when I can afford a digital back?" Then I think it would be poor bet. Get the nicest modern lenses that you can justify for today's film applications. And in a few years when or if you finally get your digital back, get digital lenses optimized for that.