DX lenses.

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I'm getting tied in knots over on the Lens Lust Forum - see "Any votes for DX primes?"
The suggestion has been made that a telephoto prime for DX could not be made smaller than one for FX, given the same spec, because the governing factor is the size of the front element, and that will be the same whether for DX or FX.
Could a technical member here put me straight on this?
I simply want to know if, for example, a 300mm F4 designed to cover the DX format could, theoretically at least, be made smaller and lighter than one for FX?
 
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Not for telephoto (in the optical sense of the word, not just "long" lenses) designs. With those lenses the aperture is the limiting factor anyway. Although it's possible to have a pupil size (the apparent aperture size as seen through the front element) that is larger than the front element diameter, you will just not see this with long lenses. 300/4 will pretty much require a front element Ø75 or more.

Wide angle lenses are a different story, because of the retro focus design. Take a look at your average 12-24/4 zoom. Logic dictates a front element size of Ø6, right? Obviously that's not the case. The challenges to have a register distance of 46.5 mm with a focal length of 12-24mm means that some serious optical hurdles have to be taken, and the image circle that is cast now is a limiting factor. Which is why you see most specialized DX lenses at the wide end of the spectrum, not at the short end.

The short answer: no, a 300 ƒ/4 cannot be made smaller and lighter than one for FX. Once you go under 50 mm the story changes and that's where we see DX specific lenses.
 
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Thank you, Lurker. So my dream of a 300mm/4 no bigger than a tube of Smarties will not come to pass. (Can you still get Smarties?)
 
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Thank you, Lurker. So my dream of a 300mm/4 no bigger than a tube of Smarties will not come to pass. (Can you still get Smarties?)

I know of these yard-long sticks of Toblerone chocolate. If you manage to find the right size of a tube of smarties your 300/4 will be no bigger than that :wink:
 

Growltiger

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Although the answer given is exactly correct, it could still be misleading.

To get the equivalent of a 300 FX lens on DX you only need a lens of 200mm. A 200mm f/4 lens is smaller than a 300mm f/4 lens.

The equivalent of a 300mm lens for a phone sized sensor is very small.
 
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Although the answer given is exactly correct, it could still be misleading.

To get the equivalent of a 300 FX lens on DX you only need a lens of 200mm. A 200mm f/4 lens is smaller than a 300mm f/4 lens.

The equivalent of a 300mm lens for a phone sized sensor is very small.

Yes, a 200mm f/4 is smaller than a 300mm f/4. But it's not the equivalent lens on DX!

In order to achieve the same DOF, and hence the same image, you need a 200mm f/2.8 lens - which is approximately the same size as a 300mm f/4.

Cheers

Mike
 

Growltiger

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Yes, a 200mm f/4 is smaller than a 300mm f/4. But it's not the equivalent lens on DX!

In order to achieve the same DOF, and hence the same image, you need a 200mm f/2.8 lens - which is approximately the same size as a 300mm f/4.

Cheers

Mike

It gives you the same number of pixels on the bird.

And in some cases more DOF can be very helpful, like getting all of the bird into focus. So the lens is smaller and gets more of the subject into focus.
 
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Thanks for the input, guys.
I'm well aware that the FOV of a 200 on DX is equivalent to 300 on FX. And I'm not concerned with DOF. I'm after an actual focal length of 300 at f4, not a DX equivalent. I already have the 70-200 f4. I can dial in the 1.3 crop on the D7100 and get an "equivalent" 390mm - but that's just cropping by another name.
Nothing beats "real" focal length.
 
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It gives you the same number of pixels on the bird.

And in some cases more DOF can be very helpful, like getting all of the bird into focus. So the lens is smaller and gets more of the subject into focus.

But they're not equivalent, as you stated. True equivalence means same perspective, same DOF. For that, you need approx. 1 stop wider aperture on DX vs FX, and 2 stops wider on Micro 4/3 vs FX.

Your other argument doesn't quite make sense to me - I could simply stop down the FX lens 1 more stop to match the DOF of the DX lens, right? Then I'd have the same amount of bird in focus :smile:

Cheers

Mike
 
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Thanks for the input, guys.
I'm well aware that the FOV of a 200 on DX is equivalent to 300 on FX. And I'm not concerned with DOF. I'm after an actual focal length of 300 at f4, not a DX equivalent. I already have the 70-200 f4. I can dial in the 1.3 crop on the D7100 and get an "equivalent" 390mm - but that's just cropping by another name.
Nothing beats "real" focal length.

Yes - but at some point, real focal length gets "real expensive" :frown: That's where the pixel density of the smaller sensor helps...

Cheers

Mike
 

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