DX vs FX lenses question

Clint_001

Clint_001

Clint_001

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1 Jul 2013
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Quite new to this so apologies if this is a stupid question. I'm trying to compare the Nikon 35mm f/1.8g DX and 50mm f/1.8g FX for a d3200. I've got my head around the implications in terms of effective focal length but am wondering if this effects aperture size. Although the physical apeture of the FX lense would still be 50/1.8, does it have an effective aperture you should use when comparing to DX lenses on a DX body?

Really this comes down to the fact that although I'm only just starting out, I'm finding I really enjoy low light stuff. I've just started an 8 week evening class and won't make a decision until I've finished that and also got a lot more hours under my belt with the supplied 18-55mm lenses, however I do find myself wishing for lower f numbers when I'm using it.

Any advice much appreciated.
 
FX lens is fine on a DX body. DX body won't have enough coverage for full frame bodies.

The only difference between DX and FX lenses is that DX lenses produce a smaller image circle as they don't need to cover a large area.

That said, if the price is close, buy the full frame version and future proof your equipment, if you go full frame in future you will not be able to use your DX lenses (technically you could, but not really)
 
Thanks for your reply. Yes I understand you can use an FX on a DX body. I was really looking for some input on comparing the low light capability of the two. As I understand it a 50mm lenses with the same f number as a 35mm lenses would have a larger physical aperture. I'm making the leap to then assume the 50mm would be better for low light (not sure this is true). If that's true of an FX lense vs FX lense comparison can the same be said of an FX vs DX lenses?
 
aperture is a ratio, where f is the focal length.

So on a 50mm lens f/1.8 is 50/1.8 = 27.7mm aperture (remember it is area and not diameter)

35mm lens at f/1.8 is 35/1.8 = 19.44mm

But that doesn't really mean more or less light, the longer lens often has more elements and a longer distance to lose the light.

For example, take the extremes, a 400mm f/2.8 would have a massive aperture compared to a 35mm f/2.8 but the lens is much longer with many elements that will cause a reduction in light transmission, what is better for low light depends on the scenario. If you shot a wedding, 35mm, 50mm are going to be better than 400mm, conversely if you was shooting low light sports, a 35mm won't be better than 400mm in low light.

Choose the focal length for the purpose, and then get the widest aperture you can.
 
Clint_001 said:
Thanks for your reply. Yes I understand you can use an FX on a DX body. I was really looking for some input on comparing the low light capability of the two. As I understand it a 50mm lenses with the same f number as a 35mm lenses would have a larger physical aperture. I'm making the leap to then assume the 50mm would be better for low light (not sure this is true). If that's true of an FX lense vs FX lense comparison can the same be said of an FX vs DX lenses?
Click to expand...

The main difference between using a 35mm on DX body vs an FX body is that on the FX body you will have to get closer to get the same subject magnification. Moving closer will reduce the depth of focus, but the aperture itself is constant.

If you change the lens to a 50mm on FX then you will get about the same subject magnification. And since the lens is longer you will have shallower depth of focus. The aperture of a 50mm f1.8 lens is bigger than a 35mm f/1.8 lens.q.8 is just the ratio of front lens element diameter vs focal length (which is just the distance of the front element to the sensor (unless the distance is less than the flange distance then the lenses get very complex retro focus designs)
 
Clint_001 said:
Quite new to this so apologies if this is a stupid question. I'm trying to compare the Nikon 35mm f/1.8g DX and 50mm f/1.8g FX for a d3200. I've got my head around the implications in terms of effective focal length but am wondering if this effects aperture size. Although the physical apeture of the FX lense would still be 50/1.8, does it have an effective aperture you should use when comparing to DX lenses on a DX body?

Really this comes down to the fact that although I'm only just starting out, I'm finding I really enjoy low light stuff. I've just started an 8 week evening class and won't make a decision until I've finished that and also got a lot more hours under my belt with the supplied 18-55mm lenses, however I do find myself wishing for lower f numbers when I'm using it.

Any advice much appreciated.
Click to expand...

No, it would have no affect on the aperture although it does have an affect on the perceived depth of field due to the image circle being cropped away on a dx body, so you would effectively get a greater depth of field on a dx body.
 
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