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08-30-2012, 06:07 PM   #1
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Optics Question

In laypersons terms, how do the optics of a film lens differ from that of a digital lens and what are the practical implications of using a film lens on a digital camera APS-C or FF?

08-30-2012, 06:38 PM   #2
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There isn't really much inherently different, except for the general advancements over the years, like better zoom formulas and better coatings on newer lenses. Many of the lenses today use optical formulas from the film era, especially primes, because not as much has changed in prime formulas. Most differences aren't related to the optics, but the mechanics and electronics of the lenses.
08-30-2012, 07:17 PM   #3
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One issue is that while film can pick up very oblique light rays,
the wells of digital sensors can not,
so lenses for digital (especially wide angles)
need to be designed to avoid oblique rays
(i.e., to be "telecentric").
This is not so much a problem with DSLRs,
because of the long register distance,
but you can sometimes get color shifts and other aberrations
using certain film type wide angles on mirrorless cameras (except the K-01).

Another issue is that digital sensors reflect more light back than film,
so digital lenses should be designed not to send that light back to the sensor again.
Some old film lenses can be prone to doing this.

Here's a weird image sample, taken with an M200/4 on a K-x,
that may be showing this sort of effect, although in a negative way.



Ahead of the plane, there's a kind of dark "extension" of the vapor trail.
It may be hard to see on the web image,depending on your monitor.
It's not an optical illusion,
because it can still be seen if you block off the real vapor trail.
I'd be grateful for a coherent explanation of the effect.

BTW, I have a similar image where the vapor trail
does not cut close to the center of the frame,
and there is no dark prolongation on that one.

Last edited by lytrytyr; 08-30-2012 at 08:01 PM.
08-30-2012, 07:21 PM   #4
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I think the lens coatings have seen the biggest change, along with the electronics.
QuoteOriginally posted by CMG Quote
practical implications of using a film lens on a digital camera APS-C or FF?
I use about half and half DA and film era lenses. From Takumars through FA. I think individual lenses have different characteristics but not sure there is anything different between film and digital that you could point to and say that looks different. I do think many of the Takumars have a distinct 'look' but I don't think that it is because they were designed for film.

08-30-2012, 08:50 PM   #5
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There are a few lens designs that interact badly with the sensor, which is more reflective than film. I tried once to use a 25mm extension tube with an FA 35/2, assuming I'd have a 35mm macro. There was a large spot of overexposure in the center of the frame no matter what I did. The Tamron model 52 90/2.5 macro lenses have a large flat rear element that might do the same thing, far less obvious. At f16 or smaller apertures I get a small purple spot in the center of the frame. It could be something else; the lens makers say just buy our latest designs and everything is fine. I haven't seen any issues with other lenses.
08-30-2012, 09:56 PM   #6
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I've done a lot of shooting using what can only be described as "dirt cheap budget lenses" and I honestly havenb't seen anything that would sway me away from using them. About the only reall effect so to speak is that the contrast and such suffers a bit (which works as it lends a bit of a vintage feel to the shots). This is also easily corrected in camera to offset it if I want to avoid it so its a non-issue.
08-30-2012, 10:23 PM   #7
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I use lens designed for film about 99% of the time.Have only two lens designed for digital,the two DA's that came with the camera that I don't care much for,the optics are just OK but am used to having an aperture ring.Several lens designed for film are still being made and sold and are very popular
08-31-2012, 05:05 AM   #8
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lytrytyr perfectly described the differences. Some lenses, like the Vivitar series 1 70-210, exhibits more chromatic aberrations on digital than on film. For many, many lenses, the lack of a telecentric design will not show but in absolute terms it's a limitation of digital sensors.

The newer lenses, starting with the F line, also have much more advanced electronic boards in them, allowing more complete information to be carried to the camera body. For instance, Pentax lenses can communicate their sharpest settings, and the body can take advantage of that. They can also communicate their focal length, which allows the SR system to act accordingly.

Regarding optical flaws, lenses must be tested ona case by case basis. Refering again to the Vivitar, apart from CA it's an excellent lens, sharp and well contrasted.

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