Dear Friends,  My Nikon digital SLR has a 1.5 "crop" or in other words, a 30mm lens will read about like a 45mm on the camera.  This is due to the "crop factor" of the camera.   We go by film camera "mm" of focus but the digital works according to the sensor size.  

I want a good prime lens of about 50mm.  It seems one way to do that is get a prime lens of about 30mm, giving an "effective" focal of 45mm.     Seems sensible as to fl...but , do I inherit with this sort of choice all the problems sometimes associated with a wider angle lens?   Will this be an "efffective 45mm" that behaves like a 30mm?

The above example is just from a random look at lenses available.  Sigma has a new 30mm f/1.4 I saw and use this as an example lens to get effective fl of close to 50mm. Am I barking up the wrong tree or going down the wrong rabbit trail in this thinking?

Any input will be greatly appreciated.           TomDart.

Hi there! Good question. I hope you don't mind me adding a question to this thread, which may be relevant...

If you buy a lens that is specifically made for the Nikon digitals (specified by the initials DX on the lens i believe), does the crop factor become irrelivant? I would think the crop factor is for lens made for the 35mm cameras?

Yes thats right Tom the debate may also centre on why you wont want to use a mini Zoom that will cover the 50mm focal range a lot of recent zooms are almost as good as a prime and the 1.5 crop makes them a compromise anyway as  the optics unless a DX design will have been optimised for a full sized sensor or film.. The main advantage of a prime on a digital sensor that is not full frame is the speed and 1.4 will be great for those low light situations.. A more natural lens would be a 35mm and that will give you the 50mm spot on and probably have less chromatic abberations as well... I would not worry about the crop factor as the sensor picks more of the sweet spot on the lens

Attached Link: Sigma lens designations

http://www.sigma-imaging-uk.com/support/abbriviations.htm

35mm is the closest you will get to a FF 50mm on a canon 1.6 crop.

Yes you will inherit the distortions associated with wide angle. That is because you are not changing the actual focal length on the lens, all you are doing is changing the FOV

Using a 50mm lens will be your best bet if room is available.

Let us not forget this is not really a new issue, the difference in FF to crop censor can also be compared to a 35mm and medium format film, where a 50mm lens would be a 50mm lens, the same lens would be much wider on a MF body (if it fitted it)

If I remember correctly (long time since I used MF) an 80mm lens on a FF body is approx equivalent to 50mm on MF so the conversion factor is approx the same (from fading memory so I could be slightly wrong)

I think it is probably time to forget about the equivalents to a certain degree when buying lenses, and think more about what the apparent focal length is and the effects it will give you.

Lol yeah I  did mean DG DX is the best quality designation for Sigma

The perspective should be the same as if you had used a 50mm lens on a 35mm camera; the only real difference will be the matter of depth of field, which will be the same with ant 30mm lens, regardless of sensor size. You will have significantly greater depth of field.

One thing to consider about the distortion of a wide angle lens.  The distortion is relative to the distance the subject is to the camera not the focal length.   We associate the dog nose distortion effect to wide angle lens because we usually have to get closer to the subject to fill the frame.  Therefore we get more distortion. 

With the crop factor you will most likely not have to get as close to your subject therefore the distortion will be less.  Other factors such as Depth of Field will remain, as already mentioned.

One benefit of the crop factor with wide angle lens is, a lot of lens have distortion at the corners and are softer at the corners.  With the corners cropped out you get a better image.

Thankjs randy and cinema, this help clear up a few dog ears.  Tom.

That is right to a certain degree, however there will still be more distortion with a 35mm lens than with a 50mm lens because of the way the lens is constructed (the lens elements in a wide lens are more convex) that said because of the conversion factor (small sensor only using part of the FOV) it will be less noticeable.

The DOF will be the same with any lens, it is the perspective that changes, for example if you shoot a subject with a 200mm lens at a given aperture, then shoot the same subject with a 50mm lens at the same aperture but move closer so that the subject is the same size in the frame, the DOF will be the same. Don’t forget that the DOF is also relative to distance from subject as well as aperture and focal length.

Thanks, Enola.  This is much what I thought.  I am experimenting with a shorter fl lens which will give an effevtive crop of about 50mm on my Nikon.  If aberration is not a problem, either in barrel or color, I will likely get the lens.  I like the field of view of the effective 50mm crop with my camera.

So, I try simple tests on the "correctness" of the lens and try to determine what is really seen and what is not.... oh my.   Well, this is part of the fun of  photography.           TomDart