Does Cropping Obviate Need for Different Focal Length Lenses?

[JimAscher]

The new Leica Q’s feature of permitting different frame lines to appear in its viewfinder to replicate 35mm and 50mm focal lengths poses a (pretty obvious?) question, some of the implications of  which to my knowledge have not yet been discussed in this forum.  Such viewfinder framing reduces the Leica Q’s file sizes from the native 24 MPs for its fixed 28mm lens to 15 MPs for the 35mm image cropping to 7 MPs for the 50mm cropping.

If one has a camera with a significantly higher native MP count than the Leica Q’s 24MP (i.e., 47 MP in the Sigma DP series or 36 MP in Sony’s a7r) the cropped size from these cameras’ sensors would result in MP file sizes also significantly higher than those of the Leica Q.

Instead of the need to employ lenses of different focal lengths to secure longer-lens croppings, why not just crop takes from larger MP sensors to secure photos of “sufficient” and equivalent MP size?  It might prove to be much cheaper and less cumbersome in the long run than the need to switch lenses frequently to achieve the same effect.  

I’m certain I’m missing something here, and am hopeful I’ll be corrected by others in this thread.      

[Telecaster]

Sure. So long as you stay within reasonable bounds, as in your example—and so long as the reduction in pixels isn't an issue—you can create pseudo longer focal lengths with a wider lens. (I'd probably opt for a 35mm lens with crops equivalent to 50 & 70/75mm.) One thing to note is that potentially you'd be giving up some depth-of-field control with the cropped fields-of-view relative to actually using longer lenses. That is: a 28/1.7 lens cropped to "50mm equiv." will give you deeper DOF than a 50/1.7 lens, both lenses used wide open. Whether or not this would matter would be up to you.

-Dave-

[MattBurt]

This is probably fine as long as you aren't looking to compress the scene or more easily generate bokeh.

Bokeh can be produced with wide lenses, but is often most pronounced with longer ones. Depending on your lens and settings a cropped wider shot may not have bokeh that is a blurry as a longer lens would make.

Compression is nice if you want to do something like make the moon look big over a foreground element like a building or mountain. It also makes people look better than the exaggerated perspective a wider lens brings.

So if these are not critical or you have a lens that allows you to work around the issues then it should be fine. 

[Telecaster]

Just to note: image compression is a function of camera-to-subject distance. We tend to imagine a longer focal length yielding a more compressed image, but that's because we typically use longer lenses to photograph more distant subjects. In fact the lens itself doesn't compress. You can verify this by using a zoom lens at its widest & longest focal lengths, with you remaining stationary, to photograph the same subject. Then crop the wider shot to match the framing of the longer one. The spatial relationships amongst objects in both frames will be the same. Depth-of-field effects may vary but that's a different thing (and is related to focal length, assuming the same aperture setting is used for each photo).

-Dave-

[JimAscher]

Thanks guys.  As I'm currently using (and very satisfied with) my Epson RD-1 -- which only has a 6 MP sensor! -- a reduction in MP's from cropping of a higher MP image would obviously not be much of a problem for me.  I'm continually being amazed at the apparently high resolution I'm getting from my RD-1.  And my standard print size is 13 by 19 inches.  Maybe it's the different type sensor -- CCD vs CMOS -- that accounts for the difference.

[Telecaster]

I'm continually being amazed at the apparently high resolution I'm getting from my RD-1.  And my standard print size is 13 by 19 inches.  Maybe it's the different type sensor -- CCD vs CMOS -- that accounts for the difference.

My first APS-C cameras were Canon SLRs. The R-D1, though, has the distinction of being my first APS-C camera to exhibit moiré effects—rarely objectionable but evident if you look closely—under average pic-taking conditions. Its per-pixel resolution is indeed high. The lenses help, of course, but that sensor and the relatively weak anti-aliasing filter in front of it make a fine combo.

-Dave-

[spidermike]

Thanks guys.  As I'm currently using (and very satisfied with) my Epson RD-1 -- which only has a 6 MP sensor! -- a reduction in MP's from cropping of a higher MP image would obviously not be much of a problem for me.  I'm continually being amazed at the apparently high resolution I'm getting from my RD-1.  And my standard print size is 13 by 19 inches.  Maybe it's the different type sensor -- CCD vs CMOS -- that accounts for the difference.

The 6MP you get from a cropped 20MP image will be quite different in some respect to a 6MP image from a 6MP camera with the same sensor size.
The individual pixels from the 20MP camera are noisier at the pixel level and while technological developments will reduce the difference the larger pixels from the 6MP camera will in some ways be easier to manage. So you won't have as much latitude on ISO with the cropped version.
Also, the smaller pixels in the '6MP cropped' image won't collect as much light so you will lose some of the tonal delicacies compared to the native 6MP image.