Question/Idea using crop function for longer focal length

Keoki

Senior Member
If you use a 200mm lens on a crop frame you multiply it by approx 1.6 you get a focal length of 320mm, right? So if you go into the menu on a D600/610 and choose DX, then does that increase your focal length to 320 also even though it's a full frame camera? Also would you need to use a DX lens or can you use a FX lens and get the same effect? I'm curious.
 
You can use any lens. All it is doing it cropping the shot in the camera and it gives you a smaller file size. So if you are only interested in getting a longer focal length then you are better off cropping in post production. If you are interested in smaller file sizes/faster frame rate in the camera then you the crop function.
 

WayneF

Senior Member
If you use a 200mm lens on a crop frame you multiply it by approx 1.6 you get a focal length of 320mm, right? So if you go into the menu on a D600/610 and choose DX, then does that increase your focal length to 320 also even though it's a full frame camera? Also would you need to use a DX lens or can you use a FX lens and get the same effect? I'm curious.

Nikon DX sensors are typically near 1.5x crop factor, Canon might be 1.6x.

It does not matter which lens... the lens never changes in any way.... it projects the same image regardless of sensor size. So assuming using the same lens, the lens does not actually play any part in this. Only the scene view that the cropped sensor captures is different (the sensor is smaller, so its view is smaller).


A crop is just a crop. the image gets cut smaller, and so subsequently it has to be enlarged more later. This greater enlargement is a necessary part of the whole. It is the magnification that causes us to perceive a telephoto effect.

So, you can
A. use a DX camera sensor to crop the lens image (smaller than if an uncropped sensor didn't),
or B. you call tell a D600 camera menu to crop it smaller,
or C. you can crop it smaller any time later in a photo editor... any image any time later, cropped is always smaller.

All crops make it smaller, simply meaning you have to subsequently enlarge it more to see it larger later.... which is a magnification making it look bigger, comparable to as if a longer lens took an uncropped copy. You can see this in your photo editor by simply zooming in on the view of the image. Same thing then.

If assuming all other things are equal (i.e., 24 megapixel sensors, etc), then

The real difference is that
A. the smaller sensor crop with a 24 megapixel sensor will still have all the pixels, 24 megapixels.
The B. larger 24 megapixel D600 crop only has about 10 megapixels left.
the C. image crop later to same DX size would also only have about 10 megapixels left.

But the telephoto effect is true of any crop (crop smaller and then enlargement back to comparable size). It is just an apparent visual effect, but yes, your image does appear magnified (because the smaller image does have to be enlarged more). If you refused to enlarge it more, you would just have a smaller image (smaller cropped frame borders) still containing the same size subject object (not enlarged).

Of these crops, the way to bet would be A., still with 24 megapixels. Or a better way to bet would be an uncropped full FX sensor with a lens 1.5x longer, or standing at 2/3 the distance (same smaller views, but all the FX pixels).

This is NOT saying all these images are identical images. Distance changes Depth of Field, and where you stand changes perspective, and enlargement changes perceived sharpness (CoC). It is however saying the included scene borders are the same (same crop, same view of scene).
 
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Thanks Don,
Darn, I thought I was on to something.


My D7100 has a crop mode too so I did a little checking when I got it. Also one really bad part it that the viewfinder does not show the cropped area so you have to guess at it.

I just crop in post and then I can get the results I want. I also shoot RAW and use ACR in Photoshop so if you crop in RAW you can maintain the full 24MP to do your editing in. Great way to get a little better quality when you do need to crop for one reason or the other.
 

mikew_RIP

Senior Member
The D7100 should show a reduced size frame in the viewfinder when using the 1.3 crop to make composition easier,as Don said though unless you want smaller files or like me on occasion more frames before the buffer fills your better cropping in PP.
 

aroy

Senior Member
Cropping is cropping, it just reduces the frame size, nothing else is changed. In camera cropping is preferred when you want a smaller file size, of if the camera supports it, faster and longer burst. Otherwise it is better to shoot RAW and crop out what you do not want during post processing.

As the 24MP DX sensors have more resolution (pixels/mm) compared to even 36MP FX sensor, a smaller object will look magnified as it has more pixels to it than FX sensor, otherwise cropping to DX in an FX sensor of 24MP will yield around 12MP.
 
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