Understanding focal length, sensor size and crop factors

[Peter Boughton]

This is something I thought I understood, but then I was reading something and got confused, so now I want to make sure I've got things straight...

I currently have a Nikon D300s - a DX (1.5 crop) sensor - and a DX 18-70 lens.
Because of the crop factor, the magnification is equivalent to what would be 27-105 on a full-frame body.

However, if that lens was actually put on a FF body (all Nikon SLRs have the same lens mount) the effect goes the other way, and it would theoretically act as a 12-46 - but with blurred edges because the lens doesn't fill the sensor.
Except Nikon FF cameras have an "Auto DX crop" setting - enabling this means that it'll just use the smaller sensor area, effectively acting as a DX sensor, so the crop factor is negated/re-applied, meaning it behaves as an 18-70?

I'm looking at getting a D750 which is full-frame and one of the kits is an FX 24-120 lens.
Using the FX 24-120 on the D750 will be roughly equivalent to the DX 18-70 on D300s, right?
And if the FX 24-120 was attached to the D300s it would work as 36-180.

So yeah, does everything I've written make sense, or are there any parts I've misunderstood?

Thanks.

[davidc]

It sounds about right. The focal lengths on lenses, even DX, are given as their FF ones. Your calculation for determining the field of view on a DX vs FX senesor is pretty much correct too.

So putting the 18-70DX on a DX camera you get 27-105 as you say. Likewise putting it on an FX camera you'd get the advertised 18-70 field of view EXCEPT you would lose some field of view because of the lens not filling the sensor (so not 12-46, it's 18-70, you just lose FOV). Finally yes, the field of view of the 24-120 FX on the D750 lens will be ballpark similar field of view to the 18-70 on your D300s and if you put it on the D300s your FOV will be 36-180.

One thing that will be different is the effective aperture and impact on DOF because you need to mulitply the aperture by the same multiplier to assess how the aperture and impact on DOF will change. So the 18-70 f3.5-5.6 on your DX is effectively 27-105 f5.25 - f8.4 whereas your new FX is f4 throughout the zoom range. This is one of the problems with smaller sensored cameras, you lose out on some ability to generate lovely bokeh because of the small sensor.

Basically, you are looking a 100% trade up Have fun with the D750, it looks like a cracking camera. Assuming of course you're not willing to take your photography seriously and invest in Canon

[Peter Boughton]

Thanks. I hadn't connected the dots on aperture being affected too, though it makes sense.

I did consider switching system a little while back, but I decided I didn't want a toy the Nikon shape just seems to fit my hands better.

[Mike Farley]

Thanks. I hadn't connected the dots on aperture being affected too, though it makes sense.

I have seen some heated and, frankly, pointless "discussions" on the Internet about the effective aperture for DOF on cameras which have smaller than full frame sensors. Just get out there and take pictures, guys*, that's what really matters.

I did consider switching system a little while back, but I decided I didn't want a toy the Nikon shape just seems to fit my hands better.

The least important component of any image is the camera provided it gives acceptable results. Go with what works best for you. I second David's comment about the D750.

* And it does always seem to be guys.

[davidc]

Why do you say they are pointless? It's a real consideration, especially if you are interested in using bokeh creatively. It's not about being smaller than FF per se, it's about being so small you get ugly/restricted bokeh or in some cases not at all that is a limitation.

The least important component of any image is the camera provided it gives acceptable results. Go with what works best for you.

Ah the quote always trundled out by people who get annoyed by the layman's "great photo, you must have a nice camera" comment For pleasant bokeh, smaller sensors give less acceptable results than larger ones

The camera certainly isn't the least important component though, it all depends on what you are trying to achieve. It's one of the corners of the "Great Image Triangle" (tm), along with photographer's skill & post-capture processing. It's as important to use the right tool as it is to have the right idea and the right approach to processing.

Otherwise to suggest the camera is of least importance we'd all still be using wet plates

[Mike Farley]

I did not say that the camera is unimportant. Clearly it isn't as it is the means by which we make photographs and the technical advances since the time of Fox Talbot and Daguerre have made it easier to obtain competent images. By which I mean in focus, well exposed etc. It's just that I value photographer input and vision more highly. What I really ask of a camera is that it does not get in the way of the image making process and some are better than others for that.

As for this argument about DOF and bokeh on snaller sensors, there are at least as many occasions when I find having increased DOF more useful than the other way around. I can also think of some judges who want to have everything in an image sharp, from back to front, although that is a different discussion.

[davidc]

And that is where your phone camera comes to the fore

[Mike Farley]

And that is where your phone camera comes to the fore

In some instances it will, but on other occasions it will fail the "getting in the way" test. There are some pros who are using phone cameras for their work.