"Digital" lens?

Hi,

I have just run into a confusing issue here, so any replies would be welcomed.:)

What exactly is a "digital lens"? I have seen many of the lenses for D-SLRs described as "digital".

I do know that their optical elements are optimized somehow, I also know that this lenses produce a smaller image circle, but if this is so, then there would be no crop factor, right? For example:

~Canon EF-S 55-250mm f/4-5.6 IS

This particular lens has a 35mm film equivalent of 88-400mm, why is this so, if its image circle is APS-C?:confused:

A 50mm lens is a 50mm lens no matter what camera you put it on. As you stated, the "digital" lenses produce a smaller image circle. Because of this, the lenses can be made smaller, and lighter, but they are still XXXmm lenses. The crop factor still applies because your sensor is still smaller than a 35mm film frame. I'm probably making this more confusing.

Check out this thread. Loupey has a visual example that should help clear things up for you.

http://forums.photographyreview.com/...ad.php?t=41711

There are two concepts here.

First is the size of the image circle. In Nikon's line-up :
- a "DX" lens has a smaller image circle and can be used ONLY with an APS-C size sensor (or with the D3/D700 which knows about cropping the image)
- a "FX" (or unspecified) lens has a larger image circle and can be used with cameras with an APS-C sensor and a sensor the size of a piece of 35mm film

In practice a "DX" lens is only used with digital cameras because there are no film cameras with APS-C sized films. An "FX" lens could be used with film or digital

Second the idea that a lens is optimised for digital rather than film. There are a number design parameters that could be affected:
1. High resolution. The light-sensitive elements on a sensor are much smaller than the grains in a film grain. It takes four elements to make one pixel and they are spread out across the sensor not piled up as in a film
2. Low chromatic aberation. This is the purple fringing you get on the edge of bright lines that destroys the impression of sharpness in the image
3. Light hits the sensor head-on rather than at an angle. Film can accept light at any angle but on a sensor the corners of the image tend to go dark if the light is arriving at too steep an angle (this is less of a problem nowadays)
4. Rear lens element is treated to avoid reflections. The sensor is shiny. You can get a ghost image in extreme conditions from light coming out of the lens, being reflected off the sensor then back off the rear lens element onto the sensor again

Thanks for your responses everyone.

I followed your link mjs1973, and was getting more mind-boggled as I read down the replies to that thread, but when I saw Loupey’s photo, it sure cleared everything up.:)

Guess I should have searched the forums more thoroughly.:blush2: