is there a real difference between "digital" and "film" lenses?

Question

I'm looking at the Sigma 20mm F1.8 EX DG ASP RF lens right now to use on a 35mm film Camera (Nikon N80).

The DG stands for "Digital Full Frame" according to Sigma, but they don't say what the "digital" really means. (Related, when you look at the Nikon website they say "View all D-SLR Lenses" with no indication of Film)

Does the "Digital" really mean anything if it's a Full Frame lens (as opposed to a Crop or m43), or is this just marketing talk to make people buy new lenses with their digital cameras?

Answer 1

Yes, lenses designed for digital sensors have several differences from their older film based camera lens counterparts. One of the primary differences is that digital sensors are more reflective than film, so anti-reflective coatings are applied to the rear element of a digital lens. This helps prevent reflections off the sensor that could result in image ghosting.

Additionally, digital sensors require light to travel down a narrow tube produced by the stack of filters (color, AA, etc) that lie directly in front of the actual photosites that convert the light energy hitting the sensor in to an electronic signal. This alters the way that the light needs to be directed to the sensor (it needs to come in from more straight on) and digital lenses may be designed to handle this better.

Finally, as Michael Clark pointed out, film didn't generally lie perfectly flat, while digital sensors do, so there is more emphasis on extreme levels of sharp resolving power on good digital lenses than film cared about.

It isn't generally a problem to use an old film lens as long as it is compatible with your camera mount system, but it is good to understand the caveats that they can have ghosting problems when shooting in to light, they often have lower sharpness and may have additional chromatic aberrations. They also tend to be older, so they may lack some of the more recent advantages in terms of focus motors and control.

My general recommendation if you are buying new and can afford it, then buy a modern lens, but if you need the capabilities of an expensive lens (such as a fast lens) cheaply, then it may be worth picking up an old film lens and working around the limitations, just make sure to research the particular lens you are looking at and how it compares to other lenses of both similar prices and similar aperture and focal length.

Answer 2

I think you are putting too much emphasis on the "digital" part of the lens' DG designation. It seems to be more to differentiate them from "digital" lenses that are APS-C only. Sigma calls their current APS-C only lenses "DC". When digital SLRs first began to gain a foothold in the market, they almost all had sensors that were APS-C or similar sized. So new lenses designed to work with APS-C cameras that could be smaller, lighter, and cheaper than their film counterparts were often referred to as "digital" lenses. With the later introduction and adoption of more Full Frame camera options, and some of them much more affordable than in the past, "digital" no longer means APS-C or smaller as it once did.

Although not universally the case, most lenses designed and introduced during the digital age are better than their older film era counterparts, especially in the consumer and mid grade sectors. Manufacturers of the top tier lenses have also been forced to introduce newer versions of old classics. The new consumer lenses may not be as good as the old "L" glass (but sometimes they get close), but they are much better than yesterdays consumer lenses. Especially zoom lenses which have benefited tremendously from computer aided design and modeling. What used to take weeks or even months to test by making a physical prototype can now be accomplished in a few hours using supercomputer simulation.

Users of digital cameras tend to expect more out of their lenses due to primarily two factors:

• Digital sensors are perfectly flat. Film isn't. Some of the most expensive film cameras actually had mechanisms that created a vacuum behind the film to aid it in laying as flat as possible while being exposed. Even then, with color film the emulsion layer for each color was at a slightly different depth. So if focus was perfect for one color, it would be slightly off for the other two!
• Pixel peeping has raised expectations to a ridiculous level. Take a 20MP image and display it at 100% (1 pixel per screen pixel) on an ≈23 inch HD (1920x1080) monitor and the magnification is equivalent to printing at 56x37 inches! No one expected a 35mm consumer grade lens to be perfect at 56x37! But a lot of folks now seem to.

Using older film era consumer grade lenses on newer digital cameras usually means a bit of a performance hit compared to the newer consumer grade digital era lenses. My Tamron SP 17-50mm f/2.8 Di II, introduced in 2006 as an upper consumer grade "digital lens", compares very favorably to my slightly older design Canon EF 17-40mm f/4 L that was introduced in 2003 during a time when most non-professional photo enthusiasts were still using film SLRs. Go back a little further to the EF28-70mm f/3.5-4.5 (which yields roughly the same FoV with a 35mm camera that 17-50mm does with an APS-C camera) introduced in 1987 and the image quality difference, as well as the speed difference, is quite striking!

On the other hand, in most cases using newer digital era lenses with film cameras will not decrease the performance of the film camera compared to using the lenses' older film era counterparts. But your film camera (and the specific film you are shooting in it) may not be able to take advantage of the additional performance you are paying for with newer digital era lenses. If you use very fine grain film (high quality, low ASA speed) and are printing at rather large sizes or scanning the negatives or slides at very high resolutions it might be worth the difference to you. If you are only printing 4x6 or 5x7s, it might not. Of course, if you have both a digital and film camera in the same mount then the "digital" lenses you use on your DSLR will also work as well or better with your film camera than any older lens designs from the same grade of lenses in their respective eras.

Answer 3

There is one significant difference between film and digital sensor for lens optics - digital sensors have a bit of glass and some filters in front of them.

The lensrental.com blog has a pretty extensive series of posts on the effect of the sensor stack (short version, there is a very real effect for large aperture lenses) - so it is quite possible that the Sigma 20mm f/1.8 will actually be significantly worse on film than digital when shooting wide open.

This effect is minimal when you stop down to f/2.8 or less, so unless you like to shoot wide open this may not be relevant for you.

You can start reading at Sensor Stack Thickness Part III: The Summary

Of course, you may never be able to tell the difference because people don't usually "pixel peep" on film :-)

Answer 4

Some 'film' lenses designed for 35mm rangefinder cameras have a rear element which lies quite close to the plane of the film or sensor (mostly wide-angle lenses). These work fine for film, but when used on a digital camera cause quite noticeable colour shifts to the left and right of the frame. This is due to the extreme angle of the light from the rear element to the edges of the sensor.

Answer 5

Another factor: Digital sensors are typically smaller than 35MM film frames. A lens which only has to produce a good (reasonably flat-focus and evenly lit) image across a smaller area can be simpler/cheaper than one which has to cover a larger area, for the same focal length and light-gathering ability.

So some "digital" lenses are so marked as a warning that while they're fine for this purpose, they will be suboptimal for film.