digital mm to film mm

[tinman0]

Could someone tell me what the difference is?

I'm assuming that the ccd has a smaller area and is thus affecting the calculation on the lense.

If thats true - how do you tell the difference between a digital lense and a film lense?

[rj_vaughan]

Probably varies from camera to camera as they use different sensors, but most of the cheaper DSLRs are about 1.6x focal length. They just crop the image from the lens so appear to give a longer focal length, the angle of the view of the lens isn't changed, just the portion you can see on the neg/sensor.

Digital lenses and Film lenses are the same focal length, just that newer lenses tend to be 'optimised' for digital use.. i.e higher quality (to compensate for the crop) and sometimes wider angle, again to compensate.

[simpo two]

tinman0 said:

I'm assuming that the ccd has a smaller area and is thus affecting the calculation on the lense.

Yep!

tinman0 said:

If thats true - how do you tell the difference between a digital lense and a film lense?

RJV has it exactly, though I'd be a bit wary of the word 'optimised' when it comes from a marketing department! It's simply that most DSLRs ave a chip about 2/3s the size of 35mm film. So if you use a lens that was designed with film in mind, a chunk of the image is wasted because it falls beyond the chip. A 'digital' lens simply throws a smaller area of light - and cannot therefore be used with 35mm film or a DSLR with full-frame sensor.
The fact that the lens doesn't have to throw such a large image means they can get way with less glass and a more compact design. But there's nothing digital about the lens itself, it's still glass, plastic and maybe a bit of metal if you're lucky!

[CVP]

tinman0 said:

Could someone tell me what the difference is? I'm assuming that the ccd has a smaller area and is thus affecting the calculation on the lense. If thats true - how do you tell the difference between a digital lense and a film lense?

As has been said the focal length of the lens is exactly the same in both cases.

The difference comes from the difference is the size of the CCD or CMOS sensor v's a piece of 35mm film.

What happens is the light goes through the lens and forms the image. However, as the image circle is a lot bigger than the sensor the sensor only picks up the centre of the image circle and so you only get a portion of the image. This then lokks like you have been using a longer lens than you were.

The only thing that changes is the angle of view, the focal lenght and the maximum and minimum apertures remain exactly the same as these are governed by the lens not the sensor.

The key advantage some camera manaufacturers have realised is that as the digital image sensor is smaller than the old piece of film was, if you make a lens specifically for the digital sensor it does not need to produce such a large circle of light and so it can therefore be smaller and lighter but still have a big enough circle to cover the entire digital sensor. The downside of these is they do not produce a lrge enough image circle to cover a 35mm frame of film and so if you use them you get the dark corners to your images.

The manufacturers have specific designations for lenses designed for the smaller format, for instance Nikon calls theirs "DX".

HTH

Chris

[simpo two]

Duet anyone

[CVP]

simpo two said:

Duet anyone

You're just too fast on the keyboard

[tinman0]

CVP said:

The key advantage some camera manaufacturers have realised is that as the digital image sensor is smaller than the old piece of film was, if you make a lens specifically for the digital sensor it does not need to produce such a large circle of light and so it can therefore be smaller and lighter but still have a big enough circle to cover the entire digital sensor. The downside of these is they do not produce a lrge enough image circle to cover a 35mm frame of film and so if you use them you get the dark corners to your images.

surely one of the advantages of digital photography is that because the image that falls onto the ccd is that much less, you need less lense.

less glass area must mean that the quality must be higher?

another stupid question* - but when you have ccds of greater resolution like the top canon, is the ccd bigger or more populous?



*v drunk, shouldn't be anywhere near a keyboard....

[srider]

tinman0 said:

another stupid question* - but when you have ccds of greater resolution like the top canon, is the ccd bigger or more populous? *v drunk, shouldn't be anywhere near a keyboard....

In the case of the 1Ds mk2, both. The chip is 35mm sized, and the pixels are packed in there. The Nikon D2x uses a 1.5x sensor.

[simpo two]

srider said:

The Nikon D2x uses a 1.5x sensor.

Shouldn't that be 0.67x? I know what you mean though
Don't forget pixel quality as well as size. Normally the smaller the pixel, the less sensitive to light it is. Canon have obviously overcome this somehow with their top models.