Confusion with new lens

ahansen60

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Hey I have a Nikon D5100 and have been using the 55-300 mm DX Nikkor lens for my telephoto shots. I wanted to upgrade to a telephoto that was at least twice as powerful as my current lens. So my logic was that I could either buy another DX lens that goes out to 600 mm, or buy an FX (full frame) lens that goes out to 400 mm for the same amount of reach as the DX lens. Is this logic sound? I realize that the focal length will always be what it says on the lens, but if you have a 400 mm FX lens on a DX (crop sensor) camera won't the equivalent field of view be like a 600 mm DX lens? The reason I ask is because I just bought a new telephoto lens thinking that it would give me twice as much reach as my old one, and instead it is only just slightly better. I bought the Sigma 120-400 mm full frame lens and thought that it would give me twice as much reach as my Nikkor 55-300 mm DX but it doesn't. Not even close. Why is that?
 
Your focal length doesn't change DX to FX. An FX lens on your D5100 sensor will just crop the image to the equivalent field of view of a 600mm lens, not actually give you an extra 200mm of focal length.

In other words, a 400mm lens is a 400mm lens. You can't get more zoom just by putting it on a camera with a smaller sensor.
 
An FX lens on your D5100 sensor will just crop the image to the equivalent field of view of a 600mm lens.

I agree. It should have an equivalent field of view of a 600mm lens. But for some reason it does not. For example, if I take my DX lens and set it to 200mm and take a picture of an object, then I take my FX lens and set it to 200mm and take a picture of the same object the pictures and the field of view are exactly the same! The object should appear closer with the FX lens but it does not. I can't figure out why.
 
You will have the field of view of a 600mm lens, yes. It works the same way as digital zoom though. Take a picture with a 400mm lens and crop it to the field of view of a 600mm lens and blow it back up to the original size. You won't be able to distinguish more detail, it's still the same image. Just less of it and larger.
 
An FX lens on your D5100 sensor will just crop the image to the equivalent field of view of a 600mm lens.

I agree. It should have an equivalent field of view of a 600mm lens. But for some reason it does not. For example, if I take my DX lens and set it to 200mm and take a picture of an object, then I take my FX lens and set it to 200mm and take a picture of the same object the pictures and the field of view are exactly the same! The object should appear closer with the FX lens but it does not. I can't figure out why.

Are the peripheral details identical?
 
That is strange. I shoot Canon myself but I am fairly certain Nikon's DX lenses do not account for crop factor. I wish I could be of more help. You're positive the Sigma lens is for a full sized sensor?
 
Focal length of a lens is a optical property of the lens. It has nothing to do with the body nor the size of the recording medium.

So if you have a 50mm lens, the focal length is going to be 50mm in D5100. It is also going to be 50mm in a full frame camera such as D700. So for your lens that can zoom to 300mm, it will be 300mm on your D5100 or D700. Based on your original post, I assume you do not have experience with full frame DSLR nor a 35mm film camera. So the crop factor really don't mean much to you at all.

So back to your original question, what you gain jumping from 300mm to 400mm is 100mm.
 
You're positive the Sigma lens is for a full sized sensor?

Yes it says on their website that it is. But who knows. It doesn't really matter anymore anyways cause I realize now after your explanation that I could have just taken the photos from my old lens and cropped them on the computer to essentially do the same thing as the FX lens was supposed to do right? I just wasted a bunch of money. Lol. I hope they will let me return it!
 
Focal length of a lens is a optical property of the lens. It has nothing to do with the body nor the size of the recording medium.

So if you have a 50mm lens, the focal length is going to be 50mm in D5100. It is also going to be 50mm in a full frame camera such as D700. So for your lens that can zoom to 300mm, it will be 300mm on your D5100 or D700. Based on your original post, I assume you do not have experience with full frame DSLR nor a 35mm film camera. So the crop factor really don't mean much to you at all.

So back to your original question, what you gain jumping from 300mm to 400mm is 100mm.

Correct, but shouldn't the sensor on his D5100 crop the image circle from a lens made for a full framed camera? It should give him the field of view equivalent (not focal length equivalent) of a 600mm DX lens.
 
Correct, but shouldn't the sensor on his D5100 crop the image circle from a lens made for a full framed camera?
That's exactly what happens

FxDx.png
 
Some more FX/DX crop confusion. I guess this is not going away anytime soon.


.... if I take my DX lens and set it to 200mm and take a picture of an object, then I take my FX lens and set it to 200mm and take a picture of the same object the pictures and the field of view are exactly the same! The object should appear closer with the FX lens but it does not. I can't figure out why.


The focal length of a lens, in very simple terms and for very simple lenses, is the distance from the front element to where the rays converge at the focal plane. This does not change weather the lens is FX or DX. So what you have described is perfectly normal. ANY 200 mm lens will give you the same image on your camera when taken from the same distance. Were you to put a MF 200 mm (which covers a 60 mm by 60 mm frame, and larger) lens on your camera and repeat the exercise you would again get the same image. Nikon lenses marked DX show the true focal length of the lens (remember, the distance from the front of the lens to the sensor) they do not mark what some other equivalent field of view would be. Heck, you might want to mount that lens on a System one camera and then the crop factor is something entirely different again, but only because the sensor size is different.

DX and FX only refer to the size of the image circle projected onto the focal plane.

Imagine if you will you made a drawing on a piece of letter size printer paper (280 mm by 216 mm). Now that could be considered the image formed on the sensor of a very large digital view camera with lets say a 100 mm lens. Now in the very centre of this draw a rectangle that is 35 mm by 24 mm, which is roughly the size of the sensor of a Nikon FX camera. The part of the image inside this little rectangle is only a fraction of the original. Cut it out and enlarge it until it is the same size as the original. This enlarged cutout image is now 8 times larger than it was originally, so in effect the crop factor of that little rectangle cut out from the larger image is 8x. So the enlarged image now appears to have been taken with a 800 mm lens. Only because it is being viewed at the same output size as the original full sensor image.

Now look at the sensor from an FX camera, it is about 24 mm by 35 mm and has a diagonal size of about 42 mm. Obviously an FX lens must be constructed to project an image circle big enough to cover it from corner to corner. Your DX camera has a sensor about 16 mm by 24 mm with a diagonal of about 28 mm. (42 mm of the FX sensor - is 1.5 times the 28 mm of the DX sensor, hence the 1.5x crop factor) Stick it in the back of that FX lens and you'll notice that the image circle is a fair bit larger than the sensor diagonal distance. That wasted image cost money to produce and requires more glass, so if all you'll be using is a DX sensor why bother with such a large circle? Reducing the size of the image circle does not affect in any way the image included within the rectangle of your sensor, varying the focal length does that, not the size of the circle.

Crop factor only has a semblance of meaning when the image is viewed or printed to the same dimensions as one from a larger sensor.

Just remember that a 200 mm lens is always a 200 mm lens no matter what you mount it on. Does not matter who made it and for which camera it was made, a DX, a FX, a MF or a LF - a 200 mm lens will always give you the same image on your camera if taken from the same distance.
 
You're positive the Sigma lens is for a full sized sensor?

Yes it says on their website that it is. But who knows. It doesn't really matter anymore anyways cause I realize now after your explanation that I could have just taken the photos from my old lens and cropped them on the computer to essentially do the same thing as the FX lens was supposed to do right? I just wasted a bunch of money. Lol. I hope they will let me return it!

I hope so! Best of luck.
 
Just remember that a 200 mm lens is always a 200 mm lens no matter what you mount it on. Does not matter who made it and for which camera it was made, a DX, a FX, a MF or a LF - a 200 mm lens will always give you the same image on your camera if taken from the same distance.

The image you get depends on the sensor's crop factor. A 200mm lens should give you "different" images on all of the cameras you mentioned. Yes, it's the same image circle you're projecting, but the sensors crop those circles in varying ways. Technically, ahansen was correct, even if he was doing with an expensive piece of glass what computer software could have done for free.
 
Just remember that a 200 mm lens is always a 200 mm lens no matter what you mount it on. Does not matter who made it and for which camera it was made, a DX, a FX, a MF or a LF - a 200 mm lens will always give you the same image on your camera if taken from the same distance.

The image you get depends on the sensor's crop factor. A 200mm lens should give you "different" images on all of the cameras you mentioned. Yes, it's the same image circle you're projecting, but the sensors crop those circles in varying ways. Technically, ahansen was correct, even if he was doing with an expensive piece of glass what computer software could have done for free.

Just want to clarify so that everyone are on the same page. The images produce with the 200mm DX lens and a 200mm FX lens should be the same with the D5100 camera.
 

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