Crop and perspective

Hello all.

 

There's a question I've been asked and I can't clearly answer it: we know that long lenses compress the elements in the picture; wide angle lenses separates

them. When we say that a sensor with a 35mm lens is equivalent to 50mm, say, because of its cropping factor, is it really true? Would the small sensor with

35mm lens produce the same image as a 35mm sensor with 50mm lens? Wouldn't it produce distortion due to the actual lens?

 

Thanks

<p>This tutorial might clear some things up for you... http://www.cambridgeincolour.com/tutorials/camera-lenses.htm</p>

<p>According to that tut perspective is influenced by distance of subject from the lens.</p>

<p>I found that link from this discussion off the same site... http://www.cambridgeincolour.com/forums/thread4459.htm</p>

Thanks!

<p>A photograph presents “correct” perspective when it is viewed from a distance about equal to the focal length of the taking lens multiplied by the magnification used to make the final image. This is just a rule of thumb with the emphasis on “about”. However this factorial is real and it is based on geometric optics.<br /> <br /> This “correct” perspective view from a particular viewing distance is true regardless of the focal length lens used. That being said, most pictures need not display correct perspective. Some images like portraiture and architecture need to show correct perspective others wise the subject looks weird.<br>

<br /> Say we are using a 35mm full frame. This camera sports an imaging chip that measures 24mm height by 36mm length. We mount a 50mm lens. This focal length is the industry standard “normal”, meaning it most likely will image with correct perspective (not wide-angle not telephoto). Now the 24mm by 36mm image made by the camera is tiny and thus useless unless enlarged. Typically we make an 10x12 inch print or display about this size on a computer monitor. The magnification applied to make this size display is 10.5x. This yields a viewing distance of 10.5 x 50 = 525mm = 20 inches.<br /> <br /> Now we procure a compact digital (APS-C format). This imaging chip measures 16mm height by 24mm length. We mount a 30mm lens, this is “normal” for this format. We make the same 10x12 print. Because the imaging chip is smaller, we must apply more magnification. The magnification works out to 16X. As to the viewing distance: The math is 30x16= 480mm = 19 inches.<br /> <br /> So we can now compare and say the 30mm on the compact digital delivers about the same perspective as the full frame with a 50mm.<br /> <br /> Nobody said this stuff is super easy. I call it gobbledygook.</p>

<p>I'm not sure that tutorial is very clear, but it is true: perspective is determined by subject distance. The only reason that we tend to associate focal length with perspective is that we tend to use longer focal lengths for things that are further away.</p>

<p>Here is the easiest way to understand it: Experiment with the widest lens you own by taking photos of a person facing you at different distances, then cropping and magnifying so that all you see is the face. Make sure one of the photos is taken at the minimum focusing distance of the lens. You will see different perspective effects with that one lens.</p>

<p>Hi Cedric<br /><br />Yes you can get very close to the same image with a few changes between a full frame and a cropped sensor. I will assume a 1.5x crop factor and skip the “why” detail on the math<br /><br />1) To get the same "perspective" in the image you must keep the distance from camera to subject the same<br /><br /><br />2) To get the same angle of view on the sensor of a cropped sensor, you need to move to a focal length 1/1.5x or 2/3 of what was used on the full frame camera e.g. 75mm on full frame you would need a 50mm on the cropped sensor camera<br /><br /><br />3) To maintain the same Depth of Field for the image you need to increase the aperture size (decrease the aperture size by 1.5X). If you were using f/6 on the full frame you would need to use f/4 on the cropped sensor camera. This works well for subject distance > 10x the focal length and for closer in subjects the aperture would need to be increased a bit more (not going there with the math though)<br /><br /><br />4) Since the aperture number is reduced by 1.5X on the cropped camera, you have to adjust either the shutter speed or ISO for the same expsosure as used on the full frame camera. You would either need to reduce the ISO by 1.5^2 = 2.25X or increase the shutter speed by 1.5^2 = 2.25X Reducing the ISO by the 2.25X factor with a similar sensor technology would yield about the same noise as the full frame camera because the full frame sensor is 2.25X the area of the cropped camera and to the very first order, the amount of noise at a given ISO setting scales with the area of the sensor. If you chose to change the ISO instead of the shutter speed, then the shutter speed could be left the same on both cameras.<br /><br /><br />5) Using the above steps 1 thru 3 interestingly enough keeps any diffraction noise the same as well.<br /><br /><br />The above is all the academic stuff, bottom line after that though is that camera bodies and different lenses have somewhat different characteristics and you will not get a perfect identical match yet you can get the main image characteristics to be very close.<br /><br /><br />Hope the above information was helpful.<br /><br /><br /></p>

<blockquote>

<p>"we <em><strong>know</strong></em> that long lenses compress the elements in the picture; wide angle lenses separates them."</p>

</blockquote>

<p>I think it is better not to think that way, because literally that statement is not correct and it causes confusion - as mentioned by others above, the "compression" or "separation" that you mention is determined by distance from Subject to Camera, and NOT the focal length of the lens.</p>

<p>The practical experiment which Hector suggested - I sincerely recommend you perform it to see for yourself</p>

<p>WW</p>

 

Thanks all that's exactly what i needed. That clarifies things.

<p>Here is another take on the subject: http://neilvn.com/tangents/full-frame-vs-crop-sensor-cameras-comparison-depth-of-field/</p>

 

<p>For the record, the clearest explanation I've seen.<br>

From the Life Library of Photography series:</p>

<p>Just keep in mind there are photographers who need this information for technically accurate captures for documentaton purposes and there are other photographers who just want to get 6 ft. tall Aunt Betty in the frame of a family reunion group shot without chopping her head off.</p>

<p>And then there are photographers who want to creatively compose by making elements within a scene overlap, spread apart or become large or small in relation to other elements and still fit in the frame so as to form negative & positive, light & dark shapes that make up a tastefully designed overall image. </p>

<p>One of these ways of capturing/viewing the scene requires technically accurate measurements and a complete understanding of lens optics while the others only requires you look through the viewfinder. If accuracy is not the goal then this crop factor and perspective stuff is just a waste of time because no one including myself is going to remember all that complexity out and about shooting.</p>

<p>In short use the viewfinder and adjust accordingly.</p>