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Cameras, Lenses and Accessories: Discuss Understanding lenses...Hi all Could anyone point me in the direction of a thread to Ďunderstanding lensesí? Iíve had a look around ...
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Old 17-06-2012, 22:40   #1 (permalink)
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Understanding lenses

Hi all

Could anyone point me in the direction of a thread to Ďunderstanding lensesí?
Iíve had a look around the forum but didnít find anything.

My existing kit lens is an 18-105 DX Lens, fitted to a Nikon D90.
Iím assuming that the focal length is 18-105 when fitted to a camera with a DX (CMOS?) sensor.

If I were to buy this, a 50mm, f1.8 prime lens
Nikon 50mm F1.8D AF Nikkor Lens: Amazon.co.uk: Electronics
Would it be equivalent to the 50mm setting on the kit lens or equivalent to ~75mm because the 50mm relates to full size sensors on FX cameras?

Basically, itís not obvious that the lens is for DX or FX. How can you tell which camera the focal length applies to? Is there a general rule? Do you assume FX unless it says otherwise?

Thanks, Chris
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Old 17-06-2012, 23:08   #2 (permalink)
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Re: Understanding lenses

The markings on the lens do indicate the actual focal length range. However, the cameras that do not have full size sensors (which are the same size as a 35mm negative or slide) will give the effect of a longer lens. The easy way to see what the effective range of the lens is to multiply by 1.5 as you have done with the 50mm lens.

You correctly say that this lens on your camera will give the same field of view as a 75mm lens on a full frame camera. Your kit lens has the range of about 27mm - 158mm in full frame terms and setting this lens at 50mm will give the same field of view as the prime 50mm lens.

Lens designation varies from make to make. For instance Nikkor use DX and FX. Sigma use DG to indicate a lens suitable for full frame and DC for cameras with the smaller (crop factor) sensors and Tamron Di and Di II.

DX lenses are smaller and lighter than FX lenses as internally they are smaller as they only have to fill the smaller sensor.

Just to add to your confusion, you can use either type of lens on full frame or crop factor cameras! However, there are some drawbacks to not matching FX lenses to a full frame camera. If you use a lens designed for use with crop factor cameras on a full frame camera you will not be able to fill the sensor with the image as the internal diameter of the lens is not big enough. My D700 will detect that a DX lens is fitted and will automatically crop down to 5 megapixels to enable the lens to be used, so I'm not using the full 12 MP of the sensor. Conversely, you can use a lens that is designed for full frame cameras on a crop body as the lens gives an image that is bigger than the sensor. This means that the image you get is not as big as the scene that the lens 'saw', but is just as if you had a telephoto lens on the camera giving a tighter field of view. This is where the 1.5x multiplication comes in to give you an idea of how the lens will perform.

HTH
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Old 17-06-2012, 23:23   #3 (permalink)
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Re: Understanding lenses

A related thread

http://www.pixalo.com/community/came...sor-25440.html
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Old 20-06-2012, 21:48   #4 (permalink)
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Re: Understanding lenses

Thanks for the replies. Makes sense now.
Chris
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Old 27-06-2012, 21:52   #5 (permalink)
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Re: Understanding lenses

Quote:
Originally Posted by Dabhand16 View Post
The markings on the lens do indicate the actual focal length range. However, the cameras that do not have full size sensors (which are the same size as a 35mm negative or slide) will give the effect of a longer lens. The easy way to see what the effective range of the lens is to multiply by 1.5 as you have done with the 50mm lens.

You correctly say that this lens on your camera will give the same field of view as a 75mm lens on a full frame camera. Your kit lens has the range of about 27mm - 158mm in full frame terms and setting this lens at 50mm will give the same field
HTH
Earlier I said I understood but having given it more thought I'm not so sure.

My thoughts are:
My 18-105 DX lens (it says DX on the lens) on my D90 is the same as a normal 18-105 FX lens on an FX camera.

Therefore, a 50mm prime lens (mentioned in previous post) I assume is an FX lens will be the equivalent of a 75mm on my camera.

So, is a 50mm f1.8 (FX) lens on my DX camera (actually 75mm because the camera is a DX) still a good lens to have? Or should I be looking for a 30-35mm f1.8?

Does this make sense?

Chris
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Old 27-06-2012, 22:25   #6 (permalink)
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Re: Understanding lenses

Quote:
Originally Posted by wigs View Post
My 18-105 DX lens (it says DX on the lens) on my D90 is the same as a normal 18-105 FX lens on an FX camera.
No. The lens on an FX camera to get the same field of view as the 18-105 would be a 27-160mm

Quote:
Originally Posted by wigs View Post
Therefore, a 50mm prime lens (mentioned in previous post) I assume is an FX lens will be the equivalent of a 75mm on my camera.
Correct

Quote:
Originally Posted by wigs View Post
So, is a 50mm f1.8 (FX) lens on my DX camera (actually 75mm because the camera is a DX) still a good lens to have? Or should I be looking for a 30-35mm f1.8?
It depends on what you want to do. The 50mm f1.8 is a cracking lens and on your camera is good for portraits. It can also be used for when you need a mild telephoto lens, but whether it will fulfill your needs only you can tell. A 30mm f1.8 will give a normal human eye view and for general use will probably be more useful.

The standard zoom on your camera is the 18-55. I had the 18-70 on my D70s and found it to be very good. You have the 18-105 which will cover 75% of your general purpose use, so the question is what are you trying to achieve by getting a 30mm or 50mm f1.8? The quality will be better and they will work in low light but at the expense of the flexibility that a zoom offers. If you want a good quality standard zoom lens the Nikkor 17-55 f2.8 is fantastic and has been welded onto my D300 ever since I got it.

Quote:
Originally Posted by wigs View Post
Does this make sense?

Chris
Yes. hope my answer does too
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