Digital vs Anaolgue lens equivalency

jiffy

Hi,

I've just bought an Eos 350D which comes with the 18-55 EF-S lens. I was looking for a 75-300 USM lens to go with it and I found one but the seller says that it is equivalent to a 120-480mm with Canon digitals.

Why is that, it's not the first time I've heard this but I'm not sure why apart from that it's something to do with the shorter distance from the back of the lens to the sensor.

Is there a calculation I can do to convert analogue EF lenses so that I know what they'll be like with the 350D?

Cheers,

Matt

whyulittle

Multiply it by 1.6 I think.

JackKelly

A lot of the the Canon Digitals have a crop factor, because the sensor isn't the same size as a 35mm film slide. It's a 1.6 crop factor AFAIK on the 350D. Check here for a load of threads on the subject.(x-factor being the 1.6 crop not the program;))

edit; this is probably the best way of looking at it.

DotOrg

if you've not used film SLR's then you needn't bother thinking about the 'equvalent' stuff as it is only relevant if you are used to and understand lenses on film cameras

jiffy

DotOrg wrote:

if you've not used film SLR's then you needn't bother thinking about the 'equvalent' stuff as it is only relevant if you are used to and understand lenses on film cameras

I have used Canon Eos 35mm lenses for 10 years which is why I want to make sure that the lenses I'm buying are what I think I want :eek:

Matt

DotOrg

look at this image, the 5D images is what you would have had with your film camera, the 20D image is what you'll get with your 350D



Pros of the cropping factor:

• Uses best part of the lens (center)
• Not as lens-sensitive
• Advantage for telephoto (Field Of View crop)
• Compact, light 'digital only' lenses
(however these are seldom any better from a performance point of view than a normal 35 mm lens on a FF camera.)
• Increased depth of field

Cons of Cropped images of Canon 350D
• Wide angle requires even wider lens
• Viewfinder view is smaller & darker


also, taken from dpreview.com
"Potential optical disadvantages

As you can see from the 'Cons' list above the primary disadvantages of full frame appear to be related to lenses. The simple truth is that most digital photographers who have used cropped sensor cameras (virtually all digital SLR's) have lived with the luxury of always using the best part of their lenses. This means that you are avoiding the corners and edges of the elements of your lens which are more prone to aberrations and softness. A full frame sensor is far less forgiving, in fact it's not forgiving at all, it 'reveals' a lens fully, which means good lenses and good practice (such as stopping down; using a smaller aperture) are important on a full frame camera.
Depth of field differences

Depth of field is the amount of depth in the image that will be 'in focus' (acceptably sharp) as a distance around the focus position, typically more behind the subject than in front. A shallow depth of field can deliver the soft 'bokeh' background that is the signature of SLR photography, by contrast a larger depth of field can be useful for getting more of the image in focus. Depth of field is a function of the absolute focal length, aperture, subject distance and film / sensor format.

Using the same 105 mm focal length on a cropped sensor camera (say 1.6x, the EOS 20D) and a full frame sensor camera we would have to move 1.6x further away (subject distance) with the cropped sensor camera to get the same field of view. Imagine our subject is 1 m away from our EOS 5D and is perfectly framed, we would have to step back 0.6 m (1.6 m subject distance) to get the same framing with an EOS 20D. The depth of field at F4 in the EOS 5D shot would be approximately 2 cm, for the EOS 20D shot it would be 3 cm.