cropped sensor

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mrgrimsby

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Conversation at work today relating to cropped sensors and lenses.....If I have a canon camera with a 1.6 sensor and use a sigma apo 70-300mm lens on it at the telephoto end is it actually 300mm? Conversely if you have a canon 50mm 1.8f lens on the same camera is that the same...and the 17-85 canon lens....very confusing to me...please help...an idiots guide would be nice.
Pete
 
is it actually 300mm?

No. Its 300mm. You just don't see the edges. They are "cropped".
 
mrgrimsby said:
Conversation at work today relating to cropped sensors and lenses.....If I have a canon camera with a 1.6 sensor and use a sigma apo 70-300mm lens on it at the telephoto end is it actually 300mm? Conversely if you have a canon 50mm 1.8f lens on the same camera is that the same...and the 17-85 canon lens....very confusing to me...please help...an idiots guide would be nice.
Pete
Click to expand...


Hi,

You do not physically change the focal length of the lens, that always remains the same. All lenses (still) are designed for use on full frame 35mm cameras. Digital camera sensors are smaller than a 35mm neg, so they only use the light falling from the centre of the lens. This effectively means that they are ENLARGING the picture by the 'crop factor'.

On a camera with a crop factor of 1.6, a 100mm would effectively deliver a picture which LOOKS LIKE it was taken with a 160mm lens, ie 100mm times the crop factor of 1.6 eg 100x1.6=160mm.

a 200mm lens would be 320mm.

In most cases, with DSLRs it is easier to make a quick conversion using a crop factor of 1.5, which in every case means adding 50% to the focal length
The math is much simpler ie a 500mm lens becomes a 750mm, a 400 becomes a 600 etc etc.

I hope this makes sense!!
 
Doug, does this mean when people say "effective focal length on a cropped sensor" they are really talking about the "cropped" length?
 
yes

but bear in mind the DOF will be per the 35mm length, adjusted slightly by the smaller COC on the crop sensor.
 
circle of confusion - look it up on wikipedia

Basically because the sensor is physically smaller, producing a print of the same size means blowing it up more (than a 35mm frame). Hence for a point to look sharp it must be more sharp (follow!?).

Pretty crap explanation, sorry.
 
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