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Meister

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Oct 10, 2013
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WHAT??? Nikon has an extensive selection of prime lenses that fit the D3100.

EVERY FX (full frame) lens will fit your D3100. You don't need to by the DX version of the lens.
You are right about that.
BUT: If you stick an FX lens on a DX body you effectively change the f-stop by the crop factor.
For example the classic 24-70 f2.8 on FX will end up being an effective 36-105 f4 on DX.
Feel like spending 1500,- on a slow f4 lens?

I am not aware of any lenses on DX that would be fast enough to match FX lenses on an FX body.
 
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Apple fanboy

macrumors Ivy Bridge
Feb 21, 2012
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Behind the Lens, UK
You are right about that.
BUT: If you stick an FX lens on a DX body you get effectively double the f-stop.
For example the classic 24-70 f2.8 on FX will end up being an effective 48-140 f5.6 on DX.
Feel like spending 1500,- on a slow f5.6 lens?

I am not aware of any lenses on DX that would be fast enough to match FX lenses on an FX body.

Can you explain that to me? I realise the focal length changes on a crop sensor, but the f number still dials all the way down to 2.8 on my selection below.
 

Meister

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Oct 10, 2013
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Can you explain that to me? I realise the focal length changes on a crop sensor, but the f number still dials all the way down to 2.8 on my selection below.
It doesn't matter what your dial says.
Mathematicaly you have to apply the crop factor to the focal length and the aperture to get equal results.
A lot of folks seem to not know this. Some manufacturers like sony and pentax use this to falsley advertise their cameras with tiny sensors.
 

MCH-1138

macrumors 6502
Jan 31, 2013
448
543
California
You are right about that.
BUT: If you stick an FX lens on a DX body you get effectively double the f-stop.
For example the classic 24-70 f2.8 on FX will end up being an effective 48-140 f5.6 on DX.
Feel like spending 1500,- on a slow f5.6 lens?

I am not aware of any lenses on DX that would be fast enough to match FX lenses on an FX body.

Not to nitpick, but DX is a 1.5x crop factor, so you would have an equivalent field-of-view of 36-105mm.

Also, f-stop math isn't my strong-suit, but I think doubling f/2.8 would be f/4. I understand that, all else being equal, you will have a shallower depth-of-field on FX than DX, but does that affect the "speed" of the lens in terms of setting your exposure?
 

Meister

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Oct 10, 2013
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Not to nitpick, but DX is a 1.5x crop factor, so you would have an equivalent field-of-view of 36-105mm.

Also, f-stop math isn't my strong-suit, but I think doubling f/2.8 would be f/4. I understand that, all else being equal, you will have a shallower depth-of-field on FX than DX, but does that affect the "speed" of the lens in terms of setting your exposure?
yes it does. The lens itself doesnt change, but you have to adjust it's specs according to the crop factor.

The f-number is given by dividing the focal length through the diameter of the entrance pupil. With a crop factor your focal length changes and for the equation to stay equal, so does your aperture. Now do the math ;)

You are right it's f4 for the 24-70 on dx. I was thinking of M4/3 sensors. My bad maths. :)


I understand that, all else being equal, you will have a shallower depth-of-field on FX than DX, but does that affect the "speed" of the lens in terms of setting your exposure?
The depth of field is not shallower on FX, it merely appeares shallower, because the lens will appear slower on a smaller sensor.

Let me give a real example: Sportsprotogs usually spend 6k on a d4s or 1dx and then another 30k on fast long lenses.
Why would they do this if they could get the same result by just sticking let's say a 70-200 2.8 on a nikon1 system camera with an adapter? They would have an incredible reach with f2.8!
They don't do this, because on the Nikon1 the same lens does not have f2.8 anymore, it's more like f16.
Ever tried to freeze fast action with f16?

I find it hard to explain in a forum. Watch the video I linked to below, tony explains it perfectly.
 
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MCH-1138

macrumors 6502
Jan 31, 2013
448
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California
yes it does. The lens itself doesnt change, but you have to adjust it's sepecs according to the crop factor.

The f-number is given by dividing the focal length through the diameter of the entrance pupil. With a crop factor your focal length changes and for the equation to stay equal, so does your aperture. Now do the math ;)

But the focal length doesn't change. The equivalent field-of-view changes, no?
 

Apple fanboy

macrumors Ivy Bridge
Feb 21, 2012
55,097
52,701
Behind the Lens, UK
It doesn't matter what your dial says.
Mathematicaly you have to apply the crop factor to the focal length and the aperture to get equal results.
A lot of folks seem to not know this. Some manufacturers like sony and pentax use this to falsley advertise their cameras with tiny sensors.

Interesting. I've never read that before. Of course the big advantage of buying FX lenses is when you upgrade to your new body, your good to go!
I should point out I didn't pay 1500 for my 24-70 mm f2.8 (or should that be 36-105 f4):confused:

Anyway, I'm sure they will fit nicely on my D750!

http://nikonrumors.com/2014/08/18/t...ra-will-be-called-nikon-d750.aspx/#more-79382
 

Meister

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Oct 10, 2013
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But the focal length doesn't change. The equivalent field-of-view changes, no?
and so does the amount of photons gathered by the sensor. How would you get the same amount of light with a smaller sensor but same lens?
 

Meister

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Oct 10, 2013
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It's the exact same light that would be hitting the center portion of the FX sensor...
yes, only the center portion = less total light gathered.

It's not really a matter of opinion, it's a fact. If you claim that 70mm(fx) = 105mm(dx) then f2.8(fx) = f4(dx), because N=f/D. Otherwise you are saying that basic algebra is wrong somehow. Also you can observe the results in real life.
 
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Menel

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Aug 4, 2011
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I want to upgrade from my D3100, any ideas?

Oh well, almost three years passed since I got my first DSLR, the D3100 and I kinda want to upgrade to something better. Over the years I got a few things, I also had a telephoto which I sold since I didn't really find much use to it and I still have a SB-910, 35mm f/1.8 lens and (obviously) the 18-55mm kit lens. Today, it's battery is failing,



I'm not sure if I should go full frame since it's very expensive plus that I need to invest in new lenses... The single reason I am considering this is because the "crop" in crop sensors is really frustrating me, the prime lens selection is just sad and I'm really looking into low f stops. So I don't know if more megapixels a higher ISO or more overall settings of a D7100 would help much.


D7100 is still another entry level spec DSLR. Will still have a crop factor.

Buy a replacement battery. Keep chugging along,

Save your money for a D800 or 5D.
 

Apple fanboy

macrumors Ivy Bridge
Feb 21, 2012
55,097
52,701
Behind the Lens, UK
D7100 is still another entry level spec DSLR. Will still have a crop factor.

Buy a replacement battery. Keep chugging along,

Save your money for a D800 or 5D.

There is a lot of difference between a D3100 and a D7100. They may both have a crop sensor, but the D7100 has a much better usability factor as all your adjustments aren't hidden away in menus and submenus.

There is some thought regarding going straight to a FF camera. Personally, I think I'll always have a DX and FX camera as they both give you things the other doesn't.
 

Meister

Suspended
Oct 10, 2013
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Interesting. I've never read that before. Of course the big advantage of buying FX lenses is when you upgrade to your new body, your good to go!
I should point out I didn't pay 1500 for my 24-70 mm f2.8 (or should that be 36-105 f4):confused:

Anyway, I'm sure they will fit nicely on my D750!

http://nikonrumors.com/2014/08/18/t...ra-will-be-called-nikon-d750.aspx/#more-79382
this is a long video, but it explains it very well, with real life examples!
http://www.youtube.com/watch?v=DtDotqLx6nA
 

MCH-1138

macrumors 6502
Jan 31, 2013
448
543
California
Hopefully our banter is providing some entertainment for others... :)

The depth of field is not shallower on FX, it merely appeares shallower, because the lens will appear slower on a smaller sensor.

Let me give a real example: Sportsprotogs usually spend 6k on a d4s or 1dx and then another 30k on fast long lenses.
Why would they do this if they could get the same result by just sticking let's say a 70-200 2.8 on a nikon1 system camera with an adapter? They would have an incredible reach with f2.8!
They don't do this, because on the Nikon1 the same lens does not have f2.8 anymore, it's more like f16.
Ever tried to freeze fast action with f16?

I find it hard to explain in a forum. Watch the video I linked to below, tony explains it perfectly.

Sorry, my choice of words ("all else being equal") probably confused matters. I think the point is that if you want to frame the photo similarly (i.e., to keep an equivalent field-of-view), if you keep the camera-to-subject distance and f-stop the same, you will end up with a shallower depth-of-field on FX than DX. So if you shot the same subject from 10 feet first with a 35mm @ f/1.8 on a D7000 (DX) and then with a 50mm @ f/1.8 on a D800 (FX), the FX image would have a shallower depth-of-field (1.29 feet on the FX setup, as opposed to 1.77 feet on the DX setup, according to http://www.dofmaster.com).

But my point is that the lens is not "slower" on a DX body with regard to exposure. I'm not familiar with Tony Northrup, but I watched the video and I think he acknowledges this at 16:25 when he says that crop factor does not affect the camera settings (while showing similar exposures from DX and FX shot with the same shutter speed, f-stop, and ISO settings).

I'm not a sports protog, so I can't speak with much (any) authority to their buying decisions, but I suspect it has to do with auto-focus speed, auto-focus accuracy, shutter delay, frames-per-second, image quality, noise levels, and perhaps depth-of-field -- not necessarily in that order).

yes, only the center portion = less total light gathered.

It's not really a matter of opinion, it's a fact. If you claim that 70mm(fx) = 105mm(dx) then f2.8(fx) = f4(dx), because N=f/D. Otherwise you are saying that basic algebra is wrong somehow. Also you can observe the results in real life.

I'm not disputing algebra, I'm questioning the inputs or underlying assumptions. The focal length and f-stop numbers in that equation are a function of the lens, not the body.

Yes, there is less total light gathered, but there is also less sensor area. With regards to exposure, if you shoot the same subject with the same lens from the same position with the same settings on FX and DX, the DX image is basically a crop from the center portion of the FX image. The amount of light hitting that portion of the FX sensor is the same as what hits the DX sensor. Resulting noise in the image may be a different issue...
 
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