Cropped Sensors and Lenses

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toto8462

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OK, I have a question bugging me about how DSLRs and in particular those with cropped sensors work.

I have a Canon 450D, so have a 1.6x crop. I have mainly been using a EF-S kit lens, but have recently diversified into an EF and most recently a Tamron 28-75mm.

Now, I know the EF-S cant be used on a full frame, presumably due to the fact the optics of the EF-S series are optimised for a 1.6x crop. Using a lens like the Tamron which is designed to be used with a full frame, I come across a question I can't answer.

If there is a 1.6x crop, why is this not visible as a difference between what you see through the view finder and the picture itself? I was expecting the image through the view finder to appear wider than the image recorded by the sensor. But in a few shots where I have lined up the image in the viewfinder and find that why I see there is reproduced in the captured image itself pretty nicely.

Maybe this makes no sense to you, because I don't think I follow where the 1.6x crop comes in. The only way I can explain it is that perhaps the viewfinder itself has been optimised for a 1.6x crop? Can anyone shed any light on the matter?


(Sorry for the newbie question, perhaps i should have googled it, but i thought i would throw it open) Thanks :)
 
fabs said:
Not sure of the technicalities, but whatever you see through the viewfinder should be what the sensor will record, or thereabouts as I think some viewfinders don't get it all (Maybe 95%). If you had a wider view through the viewfinder then you may well have a large portion of your photo missing.
Click to expand...

OK, so its kind of like what I thought, but does that mean that the viewfinder (or the mirror i guess) is optimised for the sensor, such that lenses designed for full frames can be used on cropped?
 
toto8462 said:
OK, so its kind of like what I thought, but does that mean that the viewfinder (or the mirror i guess) is optimised for the sensor, such that lenses designed for full frames can be used on cropped?
Click to expand...

That would be my understanding. I only tend to use two lenses most of the time, a 24-70 and a 70-200, both designed for full frame and what I see through the viewfinder is what I get as a final image.
 
toto8462 said:
OK, so its kind of like what I thought, but does that mean that the viewfinder (or the mirror i guess) is optimised for the sensor, such that lenses designed for full frames can be used on cropped?
Click to expand...

Yes. No reason to make a pentamirror or pentaprism larger than it needs to be. Even EF-S lenses can actually 'see' more than the sensor but you don't want that showing in the viewfinder else you'd end up with people having their feet chopped off etc.
 
toto8462 said:
OK, I have a question bugging me about how DSLRs and in particular those with cropped sensors work.

I have a Canon 450D, so have a 1.6x crop. I have mainly been using a EF-S kit lens, but have recently diversified into an EF and most recently a Tamron 28-75mm.

Now, I know the EF-S cant be used on a full frame, presumably due to the fact the optics of the EF-S series are optimised for a 1.6x crop. Using a lens like the Tamron which is designed to be used with a full frame, I come across a question I can't answer.

If there is a 1.6x crop, why is this not visible as a difference between what you see through the view finder and the picture itself? I was expecting the image through the view finder to appear wider than the image recorded by the sensor. But in a few shots where I have lined up the image in the viewfinder and find that why I see there is reproduced in the captured image itself pretty nicely.

Maybe this makes no sense to you, because I don't think I follow where the 1.6x crop comes in. The only way I can explain it is that perhaps the viewfinder itself has been optimised for a 1.6x crop? Can anyone shed any light on the matter?


(Sorry for the newbie question, perhaps i should have googled it, but i thought i would throw it open) Thanks :)
Click to expand...
To answer the first part the view finder is made to suit the AOV of the lens. Put it another way, if you fit the view finder of a canon 10d (cropped sensor) to a canon 5D (full frame sensor), the image you see would look exactly like a 10d
 
My reply above might not be clear. The viewfinder is made to suit the image that is on the sensor.If it`s a 10d with a cropped sensor the image on the focal plane is the equal to the full frame image, but only part of the image is used, so the angle of view that you see is equal to this using a 50mm lens H 25.58 V 17.17 D 30.5. Using the 5d with the same lens, the Angle of view is H 39.68 V 26.99 D 46.79. If you look at the spec sheet of a prime lens, it always gives you diagnol AOV
 
toto8462 said:
OK, so its kind of like what I thought, but does that mean that the viewfinder (or the mirror i guess) is optimised for the sensor, such that lenses designed for full frames can be used on cropped?
Click to expand...

It would make sense, wouldn't it? That what SLR's are all about.

WYSIWYG. (or 95% of WYSIWYG anyway):)
 
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