Digital conversions
- This topic has 6 replies, 2 voices, and was last updated Mar 6, 2006 at 12:40 am by
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Bryan –
No. That conversion is strictly an angle-of-view conversion. It comes about because the digital CCD sensor is approximately the size of an old APS film box: 23.5mm on a diagonal if I remember correctly.
Conceptually this is a little difficult but remember that the image projected onto a 23.5mm “box” will be blown up to exactly the same size “on paper” as a 35mm sheet.
Thus, assuming you want to make a 4×6, a 35mm “box” would blow up less than a 23.5mm box, because it is already bigger. Because the lens mounted on the camera still projects an image that is the same actual size, this means the 23.5mm box image gets blown up more.
When you use a film lens, an old one meant for the 35mm window, on a new DSLR, you are losing part of the light circle the lens projects, the part around the edges, meaning the angle of view is not as wide. This doesn’t ultimately affect image quality at all, because the actual light projected by the lens isn’t any particular “size” or “compression.” Thus, when the light circle hits a 23.5mm box and is then blown up to 4×6, you get a perfect image with greater enlargement.
Whatever size you blow the image up to is also irrelevant, because the relative angles of view between the 35mm and the 23.5mm “boxes” remain constant.
Example:
A 50mm lens mounted to an old SLR will make an image that looks about like what the human eye’s field of view sees. There’s actually a degree count for this, but I don’t know what it is (might be, say, 60 degrees, just as an example).
The same 50mm lens mounted to a new DSLR will make an image that looks like a 75mm lens would if mounted on the old SLR because of the decreased angle of the light hitting the CCD. Thus, the available angle might be, say, 45 degrees (anyway it would be narrower).
Again because the actual light and the image remain perfect there’s no “quality loss” in this reduced angle, it just changes what the “zoom” of the lens looks like.
This works out to a big advantage for DSLR users on the really telephoto range, since a 400mm lens projects the same “perfect image” as a 600mm lens did on an old film SLR.
Unfortunately the flip side is where we DSLR guys get bit: DSLRs must use extra wide angle lenses to get the same wide angle shot which was relatively easy on an SLR. Old SLRs used to start going “wide” at about 24mm, whereas today with a DSLR you’d need a 16mm lens to get the same angle of view, which makes wide angles hard to get, and expensive.
Lens technology hits a wall at that real wide angle because light really wants to bend and curve when you gather it from nearly 180 degrees around and put it on a two dimensional surface (the picture). Thus, wide angle DSLR lenses have to have a lot of technology and wind up being real expensive. The Nikon pro standard is the 12-24mm f/4 DX AF-S G, a $1200 lens (and one that still can’t reach the usual f/2.8 standard of pro lenses).
However, none of this affects aperture to my knowledge. Because the light is still transfered through the lens as “100%” pure light, the aperture and the relative blur shouldn’t be affected. I’ll have to wrap my head around this to try and figure out if the reduced angle of view might impact aperture a little more but I think the answer is no.
Zach
Thanks, Zach.
That’s pretty sweet. I’m thinking of buying a Canon 20D in the next month or two. I still have a few lens for my EOS 35mm, those being: a 20 – 35/2.8, an 85/1.8, and a 200/2.8. If what you think is correct I’ll still end up with a 32-56mm, 136, and a 320 without sacraficing any speed. I might cut my losses on the low end for a while and eventually pick up another prime lens in the 14-17mm range and be set for a few years. But, I’d still love to have another 300 some day.
That is accurate, Splitshot.
I’ve never heard of anyone saying they lost anything aperture-wise, and you know that’d be a hot topic of conversation.
Do you use your LCD screen for composition and information? I have a Canon G5, a slightly more sophisticated point and shoot, and I use the screen because the viewfinder is little more than a sight.
Bryan
Bryan –
DSLRs work just like SLRs – the viewfinder is the only way to look through the prism at the image.
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