Effective focal lengths with full and small frame sensors

[necigrad]

I need a little help straightening my mind out. To keep things simple, let's assume we are talking about a D7000 (it's what I've got, but this should apply to all small frame) cameras and lenses. I understand that a small frame sensor can take pictures at an apparent different focal length then a full frame, but where I'm confused is when.

Taking my D7000, if I put on a 35mm f/1.4G AF-S FX SWM I should get an effective focal length of 52.5mm or so, correct? Obviously it would be 35mm on a full frame camera.

What about a 35mm f/1.8G AF-S DX on a D7000? Will that also be an effective 52.5mm? Or will it be 35mm because it's designed for that? What if I put this same lens on a D3s (besides the vignetting)?

Appreciate the help. This popped into my head and I really haven't seen anything on it.

[RichardTaylor]

The focal length does not change.

However the field of view does.
So your 35mm lens on a crop camera has approx the same field of view as a 50mm lens on a full frame camera.

Quote
What about a 35mm f/1.8G AF-S DX on a D7000? Will that also be an effective 52.5mm?
End quote.

Yes.

[jdepould]

Focal length rating is determined by the physical design of the lens, irrespective of what camera you put it on.

Crop factor is just a handy way of thinking about the resulting images.

[inkista]

What he said. Focal length is a physical property of a lens. It does not change just because you mount a different-sized sensor behind it. But it will look kinda like you did.

Visual aid:



Same lens. Same focal length. Different FoVs, due to the sensor size.

[necigrad]

Got it. This leads to another question then. Why have small frame lenses (like the Nikon DX) at all? Are they just lower quality or capability? Cost?

[RichardTaylor]

I am a Canon shooter, however the principal reasons for the crop frame lenses is that they can be made smaller and cheaper ( Canon has them also, EF-S lenses)

[inkista]

Originally Posted by necigrad

Got it. This leads to another question then. Why have small frame lenses (like the Nikon DX) at all? Are they just lower quality or capability? Cost?

Actually, they're often higher quality at lower cost. The hardest thing in lens design is to get the corner performance as good as the center performance of the image circle. The smaller image circle of a crop sensor means that the lens needs less correction to achieve the same performance. Less correction means a simpler design, fewer elements, and lower cost and smaller size. In addition, some of these crop lenses can sit closer to the sensor than the full-frame lenses (smaller sensor => smaller mirror box => shorter mirror) and, just like moving a slide projector closer to the screen, you can end up with a sharper (smaller) picture.

One class of lens that pretty much didn't exist until crop-body dSLRs came on the scene is the ultrawide zoom lens. It wasn't feasible to create a lens THAT wide for a full-frame image circle. It is for a crop body. If you look at full-frame lenses that are in that focal length range, you'll note they're (almost) all in the $1000+ price range, while a crop-body ultrawide lens is around $600.

[necigrad]

Awesome, thanks all.

[jdepould]

Also, lenses designed for a crop sensor tend to be smaller/lighter, since they use less glass.

[kcoppock]

Originally Posted by inkista

Actually, they're often higher quality at lower cost. The hardest thing in lens design is to get the corner performance as good as the center performance of the image circle. The smaller image circle of a crop sensor means that the lens needs less correction to achieve the same performance. Less correction means a simpler design, fewer elements, and lower cost and smaller size. In addition, some of these crop lenses can sit closer to the sensor than the full-frame lenses (smaller sensor => smaller mirror box => shorter mirror) and, just like moving a slide projector closer to the screen, you can end up with a sharper (smaller) picture.

One class of lens that pretty much didn't exist until crop-body dSLRs came on the scene is the ultrawide zoom lens. It wasn't feasible to create a lens THAT wide for a full-frame image circle. It is for a crop body. If you look at full-frame lenses that are in that focal length range, you'll note they're (almost) all in the $1000+ price range, while a crop-body ultrawide lens is around $600.

So, I'm curious then -- and this might be a stupid question -- why wouldn't full-frame lenses just be made larger, and effectively still "cropped" to the full-frame? Would the larger glass not be more cost-effective than the lens corrections?