Digital Lenses

[dmm96452]

Digital lenses (as I understand it) are made specifically to correct for the size difference between digital sensors and 35mm film size. Since there are already DSLRs with full size sensors out there and since cameras with full size sensors are likely to become more affordable as time goes by what would be the point of buying digital lenses? Have I missed something?

[another view]

There isn't actually any correction going on. I think you're talking about some lenses that are designed only for use on cameras with smaller sensors (with the 1.5x and 1.6x crop factor). What happens is these lenses don't project an image that would completely cover a frame of 35mm film. This is something that only large format photographers had to think about until a few years ago, but it's not anything complicated. Nikon is pretty well planted in the 1.5x format, and I thought I heard recently that Kodak is discontinuing their full-frame DSLR's. That only leaves one very expensive Canon.

Some day things might change and full-frame 35mm sized sensors will become common, but the 1.5x format does have some advantages. Adding a 12-24 lens gives you the wide angle range again and even the Nikon version at almost $1k with the DSLR of your choice is pretty inexpensive compared to the $8k Canon.

[Franglais]

Digital lenses (as I understand it) are made specifically to correct for the size difference between digital sensors and 35mm film size. Since there are already DSLRs with full size sensors out there and since cameras with full size sensors are likely to become more affordable as time goes by what would be the point of buying digital lenses? Have I missed something?

35mm film has a "sensor size" of 24x36mm. The lens focal length needed to give a "normal" view on a given sensor is approximately the diagonal of the sensor i.e. about 42mm in this case.

Most current digital SLR's have a sensor size of about 24x16mm (APS-size) i.e. a "normal" view is given by a 29mm. Set the zoom on a film SLR to 42mm and on an APS-sized digital SLR set it to 29mm and you won't see any difference in the viewfinder.

The important thing about optimised-for-digital lenses is not the focal length. They are specifically optimised for the digital sensor in terms of definition, chromatic abberation and light path.

Perhaps one day the 24x36 sensor size will make a comeback in the mass market but I'm not convinced about that. I keep on hearing people complaining about how big film SLR's used to be and how pleased they are with their digital cameras because they're so much smaller.

Charles

[dmm96452]

Ok. I understood that the digital lenses were optimized for use with a digital sensor but I also thought they were made to correct for the "crop factor". For example, my 20D with a 50mm lens with it's 1.6x crop factor would produce the same image size as a 35mm film camera with an 80mm lens. Do the digital lenses not correct for that so that there is no need to correct the focal length of the lens by multiplying by 1.6?

[Franglais]

Ok. I understood that the digital lenses were optimized for use with a digital sensor but I also thought they were made to correct for the "crop factor". For example, my 20D with a 50mm lens with it's 1.6x crop factor would produce the same image size as a 35mm film camera with an 80mm lens. Do the digital lenses not correct for that so that there is no need to correct the focal length of the lens by multiplying by 1.6?

No digital lenses do not "correct the focal length". The focal length of the lens is an absolute value connected with the optical configuration of the lens.

People are used to expressing the view (what you call image size) of a lens on a given camera in terms of the focal length of a lens that would give the same view on a 35mm camera. This thing about multiplying by 1.6 is only a mental trick to express the focal length of a lens fitted on your 20D in the 35mm terms that everyone is familiar with. Over time it will die out and be forgotten about because nobody is buying 35mm gear any more.

For instance, my F10 point-and-shoot has a much smaller sensor than the 20D. The real focal length of the lens is 8mm-24mm which I know is the equivalent of a 35-110mm in 35mm terms.

Charles

[JeffHall]

We should clarify, there are "digital lenses" such as the Tamron Di lenses that cover the 35 mm focal plane but have special coatings and designs to reduce ghosting from the sensor reflections, and then there are the EF-S lenses Canon makes specifically for the 1.6 cropped cameras. The EF-S lenses have elements farther back into the camera body that would clip the mirror as it flips up if you mount them to incompatible bodies.

Note how much smaller and lighter the Canon Rebel XT is compared to a 1 series camera. They know where the market is going. An APS-sized sensor is MORE than capable of producing ample and EXCELLENT 4x6s with the occasional bump up to 8x10, 11x14, etc. that the 35 mm film camera used to occupy.

My opinion stays with the fact I WILL upgrade to a full-frame sensor camera one of these days (when I win the lottery ), as in times past someone would upgrade from 35 mm film to a medium format camera--much more expensive, but more sensor/film area and theoretically better resolution/finer grained pictures. Thus I prefer full-frame intent lenses. Then again, resale values are pretty good and losing a couple hundred is no biggie if you have to sell off an EF-S lens.

[another view]

There's nothing to correct; the sensor is just picking up a smaller area than a frame of 35mm would see. It's like this - print an 8x12 (from 35mm film) and cut (literally, like with scissors) a couple inches off each side. That's pretty much how it works with the same lens on a DSLR that has that 1.5 or 1.6 crop factor. Focal lengths never change regardless of what the lens is attached to. It's the angle of view that does.

Nikon has their line of DX lenses made especially for DSLR's. Because they've standardized on 1.5x crop factor sensors, these lenses don't need to cover as big an area as they would have to on a 35mm camera. What happens if you use a DX lens on a film camera is that the image doesn't (usually) extend all the way to the corners of the film. 35mm lenses would have to be much bigger, heavier and expensive than their DX counterparts.

Comparing 16x20's from 35mm and my Fuji S2 (6mp, 1.5x crop factor sensor), the Fuji blows film away. Both examples from shots on a tripod, good technique, sharp glass, etc.

[Franglais]

Ok. I understood that the digital lenses were optimized for use with a digital sensor but I also thought they were made to correct for the "crop factor". For example, my 20D with a 50mm lens with it's 1.6x crop factor would produce the same image size as a 35mm film camera with an 80mm lens. Do the digital lenses not correct for that so that there is no need to correct the focal length of the lens by multiplying by 1.6?

We've all answered this one in the same way. Something's been bothering me from the start though. What do you mean by "correct for the crop factor" or "no need to correct the focal length of the lens"? How do you think it all works? I can't understand what you think is happening.

I promise we won't make fun of you.

Charles

[dmm96452]

I'm afraid my question wasn't very clear. I'll try to clarify. The image that hits the focal plane from a non-digital lens will cover a frame of 35mm film, but because the digital sensor is smaller some of the image around the edges is lost on a DSLR.

My question was whether or not that is still the case with the made for digital lenses. That question was answered by "another view".

If the digital lenses were projecting a smaller image then they would become useless if, in the future, I upgraded to a camera with a larger sensor.

Thanks for your patience. Hope this was clearer.

[another view]

You've got it. 24x36mm (aka full frame) sensors may or may not become standard or at least more common in the future. I'd assume it's the price of making a big sensor that's keeping that from being the case right now.

The image coming out of the lens is focused on the sensor or film, and that image shows up as an upside down and backwards circle. That image circle has to be large enough to cover the intended medium (35mm film, full-sized sensor, 1.5x sensor etc). Like I said above, some of the "made for digital" lenses project a smaller image circle than a 35mm lens. Ifyou had lenses that don't cover 35mm film (like Nikon DX lenses) and a full sized sensor, you'd need to replace those lenses. At least some of Tamron's DI lenses (maybe all of them) will cover full frame/35mm but have other reasons for making them (supposedly) better for digital, like Jeff said. I don't think DX lenses be worthless any time soon because I'm gonna guess that 1.5x sensors will be around for a long time. There are advantages to them, as well as disadvantages - like everything else in the world...