digital lens vs 35 mm lens

[Franglais]

I'm getting ready to invest in a 20D. I have a couple of Sigmas and a Tamron for my Rebel 2000 right now, but none quite as fast as I'd like (fastest is 28-80/f3.5-5.6). I really like what I've read about the Sigma 70-200/f2.8, but it's not "made" for digital. Sigma sells an 18-125/f3.5-5.6 DC that is "made" for digital but I'd like something a little faster.

Am I correct in that the image circle is designed to match the size of the image sensor of digital SLR cameras on Sigmas DC? What, if anything, do I lose by not going with a DC?

Thanks.

Its not the size of the image circle that's important. Designed-for-film lenses have a larger image circle than designed-for digital ones, so there's no problem with coverage. There are two other problems:

1. A film image is made up of grain set out in three coloured layers on top of one another. To make a white spot, the "target" is fairly large. In digital, on the other hand, the three colour sensors that make up each pixel are set out side-by-side. In order for a white spot to come out white, the lens has to render it very precisely, with no blurring caused by chromatic aberration. Designed-for-digital lenses have high definition and low chromatic aberration..

2. Film is flat and light hitting it at most angles will expose it correctly. Digital sensors are often in the form of pits and light hitting the sensor at an oblique angle might not reach the base of the pits. Designed-for-digital lenses are usually designed so that the light comes out of the rear of the lens so it hits the sensor squarely.

Bear in mind that the 8Mpix 20D has small sensors and is particularly sensitive to these problems.

Hence - the lenses that give you satisfaction on your Rebel might not be satisfactory on the 20D. To do it justice you may need high-end lenses.

Charles

[peted56]

So really to be fair to consumers (all people that buy cameras, pro and amateur) manufacturers should abandon small sensors and only use film size sensors.

[Franglais]

So really to be fair to consumers (all people that buy cameras, pro and amateur) manufacturers should abandon small sensors and only use film size sensors.

The cost of the sensor increases exponentially with its size. The few cameras out there with film-size sensors are so expensive that only professionals can afford them.

If you want to put everyone on the same foot then you drop film-sized sensors and only use the small ones, with lenses that are optimised for them. But personally I like the idea of having a wide choice. Small sensors, 24x36, digital backs for medium format, film - let the consumer decide.

Charles
BTW Put a low-end film lens on a camera with a film-sized sensor and small pixels like the Canon 1DS Mark II and it's still not going to perform. This sort of camera needs excellent lenses (not that I've got one..)

[Liz]

First let me say, that I don't always perfectly comprehend the technical aspects - so in layman's terms. ;)

I'm planning on purchasing the new Rebel 300 XT. It has a new kind of sensor. I have 3 Canon lenses: 17-40mm/f4L - 50mm/f1.4 - 85mm/f1.8. Will they do as well with this camera?

Thanks.

Liz

The cost of the sensor increases exponentially with its size. The few cameras out there with film-size sensors are so expensive that only professionals can afford them.

If you want to put everyone on the same foot then you drop film-sized sensors and only use the small ones, with lenses that are optimised for them. But personally I like the idea of having a wide choice. Small sensors, 24x36, digital backs for medium format, film - let the consumer decide.

Charles
BTW Put a low-end film lens on a camera with a film-sized sensor and small pixels like the Canon 1DS Mark II and it's still not going to perform. This sort of camera needs excellent lenses (not that I've got one..)

[jph]

[QUOTE=Liz]First let me say, that I don't always perfectly comprehend the technical aspects - so in layman's terms. ;)

I'm planning on purchasing the new Rebel 300 XT. It has a new kind of sensor. I have 3 Canon lenses: 17-40mm/f4L - 50mm/f1.4 - 85mm/f1.8. Will they do as well with this camera?

Thanks.

Liz[/QUOTE

Yes. Good choices. I have the 17-40. Worked great on my 10D and my 1ds.

[Liz]

Thank you for your quick reply. The 17-40 is my favorite lens. Actually it's my all time favorite lenses - and I've had a few others. ;)

Liz

[/QUOTE Yes. Good choices. I have the 17-40. Worked great on my 10D and my 1ds.[/QUOTE]

[Franglais]

First let me say, that I don't always perfectly comprehend the technical aspects - so in layman's terms. ;)

I'm planning on purchasing the new Rebel 300 XT. It has a new kind of sensor. I have 3 Canon lenses: 17-40mm/f4L - 50mm/f1.4 - 85mm/f1.8. Will they do as well with this camera?

Thanks.

Liz

It looks like the Rebel XT sensor is very similar to the 20D sensor and I haven't seen any tests indicating these lenses are not suited to the 20D. The 17-40 does about as well as the designed-for-digital 18-55 and 17-85 EF-S lenses.

Sorry I took so long in replying. I'm travelling a lot at the moment.

Charles

[Ultra Magnus]

It looks like the Rebel XT sensor is very similar to the 20D sensor and I haven't seen any tests indicating these lenses are not suited to the 20D. The 17-40 does about as well as the designed-for-digital 18-55 and 17-85 EF-S lenses.

Sorry I took so long in replying. I'm travelling a lot at the moment.

Charles

I was wondering about these digital specific lenses too. I am also looking at the 20D, but in the future would like to get a new film SLR too, so not being able to use the same lens back a forth is a drag.

Do these digital specific lenses still have the 1.5x focal length multiplier or are the focal lenths already corrected for 35mm equivlents?

Edit: Also, how good or bad is the kit lens that comes with the 20D?

[SmartWombat]

I'm using only one of the short focus EFS lenses, the 10-22.
All the others are EF L series and work pretty well.

I'm surprised at the image quality of the kit lens, given that it felt rubbish in my hand.
Manual focussing on the front element it slopped up and down so much I could move the focus point from below to above someone's eye at 15 feet.

But a friend of mine bought the kit and no other lenses and it's working out fine.

Your existing Canon lenses should serve you well, if you like really wide angle thouugh you might want to consider the EFS 10-22 because of the 1.5x multiplier.