Cropping photo to what size lens equivalent?

[jody_stowitts]

<p>I print 6x9's with my dye-sub printer @ 300dpi. Beautiful results with my Canon 5D. (Even though using all 12MP's, same results are possible with only about 6+MP's). My question is: If I took a picture with a 50mm lens at optimum aperture, and presupposing perfect focus, how far could I crop this image and still get the same 6x9 result if I used the new Nikon D800 (35MP) camera. In essence what I am asking is, "what size mm lens would be the equivalent of this new image cropped "as far as it could go with the same quality result". I.e., can I take a 50mm lens and via a 35MP sensor, crop the image down, and get the same result as if I had originally used a 100 or 200mm lens and not cropped it? I have not tried this yet empirically, but thought some "sharpies" here on p.net could tell me. Seems like a pretty nifty idea to use only one lens (50mm), and be able to produce shots that previously required a long (100 - 200mm or ??) lens. Thanx for your input!</p>

[Matt Laur]

<p>Personally, I wouldn't buy a $3,000 camera (and at least one new lens) in order to avoid buying a one or two different focal length lenses to use on a 5D I already owned. <br /><br />But in the spirit of the question ... remember that you're already throwing away a lot of pixels when you print 6x9 from your 5D. Whether you keep the composition the same and throw away (for example) every third pixel to down-size to that small print, or whether you throw away the outer third of the image and crop to that smaller print, pixels are getting the heave-ho.<br /><br />You'd be throwing away more pixels yet, using the D800, but you'd have more to play with, that's for sure. But the main thing to understand is that lenses aren't about how zoomed-in you can get. The main thing is to understand that a big part of your composition is the <em>perspective</em> changes you get by moving physically closer to or farther away from your subject and the background behind it. Moving your shooting position hugely impacts the way things look, no matter how zoomed-in or cropped you might also wind up.</p>

[luis_modesti]

<p>This is the way I see it; and I think eventually you may too. Throwing away pixels in the first place is not the right idea, maybe for artistic editing reasons but not for avoiding a Zoom Lens.<br>

The main reason one could argue you select different focal length lenses is to change perspective. There may be many more but this is a different subject. Now having said that, remember the beauty of photography is that the rules are to be broken and there are really no rules in art. <br>

To go back to your question, it really delves more into mathematical pixel formulas in order to see how much you can crop and how big you can enlarge, not going into that but eventually you will see the difference in quality. You can do trial and error or you can gather all your variables , file size, pixel dimenstion, printer output quality, etc and then test until you find that limit to your satisfaction.</p>

 

[jody_stowitts]

<p>Matt: OK then, notwithstanding the "perspective" issue, what focal length (mm lens size) would I be able to "simulate" using 35MP with an Xmm lens? 1.5X, 2X, 2.5X? Put another way, if I can make a beautiful 6x9" print from 6MP, what new 6x9" print of equal quality using a 35MP sensor can I get (by cropping) that would "appear" to have been made originally with another lens of longer focal length--expressed as a factor of "x- times the shorter focal length" lens that was actually used? My thinking is only in regard to being able to carry a modest (50mm) lens and being able to create a print that I couldn't have made without a longer lens. I.e., by what "nominal" size (power) have I now given my 50mm lens that it didn't possess before. Hope all this makes sense!</p>

[Dieter Schaefer]

<p>If I understand your question correctly, then the answer is: take the square root of the ratio of the two sensor sizes (36 and 6MP): sqrt(36/6)=2.45 and multiply your 50mm focal length with that number to get about 125mm.</p>

 

[jody_stowitts]

<p>Dieter: Wow, square root of the ratio. Sounds good. Thank you!</p>

[jody_stowitts]

<p>So, as far as my routine 6x9" prints go, with a Nikon D800's 35MP's, my 50mm lens is a 50-122mm zoom. I know this one narrow minded way to look at it, but was just curious to see what "in theory" could be done with just one lens. With an 85mm lens you could get a 85-208mm zoom. </p>

[cjk]

<p>A couple of things I can think of: <br>

- The D800 has already a DX mode with about 16MP, so that's a crop of of 1.5, giving you a 75mm lens. Going down to 6MP should indeed give you a 2.45x crop factor.<br>

- Quality wise, you are assuming that 6MP off <strong>part</strong> of the sensor of the D800 are as good as 6MP on the <strong>full</strong> sensor of another (I assume) FX camera, i.e. that the size of individual pixels won't influence quality. That might or might not be true as it might be offset by the more recent technology of the D800's sensor.<br>

- Lastly, isn't that though basically doing the equivalent of an "optical zoom", so often decried on P&S's and other bridge cameras???</p>

 

[jody_stowitts]

<p>All true.</p>

[cjk]

Correction: I meant the equivalent of a "digital zoom".