'Focal Length' crop mode

[catcher]

<p>How about this for a future Canon firmware update (one can dream)--In LiveView, the option to select a crop that is equivalent to a fixed focal length (or some proportion). For example, if I have a 28mm mounted on my 20MP 6D body, I could, through liveView 'crop' the field of view (and resulting image) to a 35mm or 50mm lens. There would of course be costs--a drop in MP and a change in depth of field. But, with 20mp there's some to spare; and for many subjects the depth of field change would be unobtrusive. Plus, for the possibility of very modest 'digital zooming' while carrying only one small, light lens (instead of bigger zooms or several primes), I'm game. </p>

<p>Anybody else think that sounds like a good idea? Any possibility for practically implementing that <em>now</em>?</p>

<p>(PS--I believe the newest Ricoh GR does this. It's a 28mm fixed lens but you can select it to automatically crop and display through the LCD the field of view of 35mm and 40-something mm lens--at a cost of megapixels, of course).</p>

[Marcus Ian]

<p>Of course you can do that now in post, you'll just have to do the math to figure out what the new sizes would be.</p>

<p>Alternatively you could do the math and frame a variety of FLs into in a .psd and overlay that as a layer to help you crop properly (so you can easily crop the images without figuring in from scratch each time. </p>

<p>What I'm having trouble figuring out is why you would bother?, Instead of simply cropping to a desired composition in post...</p>

[JDMvW]

<p>Nikon sort of does this on some FX models when a DX lens is mounted, but I second Marcus in asking why bother?<br>

I predict that the "feature" will soon, and certainly eventually, disappear wherever it is offered; hind pockets on a hog, and all that.</p>

<p> </p>

[eric_altherr]

<p>Lots of consumer cameras do this ("digital" zoom), but I don't see the point of it. You can't replicate the effect of a longer lens by cropping a shorter one. As an extreme example, if you shoot a 28mm at f3.5 and shoot a 200mm lens at f3.5, cropping the 28mm to display the same area as the 200mm won't give you the same image (DOF, etc.). <br>

The result is not carrying less lenses and getting the same images, it would be cranking out cropped images while missing out on all a good zoom or selection of primes have to offer, in which case you might as well buy a really nice consumer camera, really save on the weight and at least have a decent optical zoom.</p>

[catcher]

<p>All good points/questions. <br>

First, yes, one could obviously do it in post-processing; but then you couldn't see the composition as its happening. The purpose of doing it in live view is that you could see the crop in 'real time'. The live view image would digitally 'zoom' to the pre-determined crop area and then save only the crop area as the image. The 6d already does something like this with aspect ratio--you can select, for example 1:1 and then liveview only shows on the LCD the 1:1 crop ratio (or perhaps crop lines--can't recall off the top of my head) .The jpg image is outputted automatically by the camera in 1:1 ratio (RAW remains unaffected). My idea is simply to do a 'focal length' crop, but functioning like the aspect ratio crop does currently. </p>

<p>Second, of course cropping a 28mm lens to 200mm will mess things up. I'm only suggesting very moderate cropping--28mm to 35mm or 50mm. Yes, there will be some change in depth of field but from 28 to 35 it won't be much; and for any images with small apertures wouldn't matter much anyway. </p>

<p>Third, megapixels of course aren't everything--by digitally zooming you're also zooming into any imperfections in the lens itself, or cutting into apparent resolution. But, again, from 28 to 35, for example, it wouldn't be much. And, the loss of pixels wouldn't be much either. On my 20MP 6d digitally cropping to 35 would still give me a roughly 15MP image (I think--my math could be off). That's still more than my old 5D would do. </p>

[paddler4]

<blockquote>

<p>a drop in MP and a change in depth of field.</p>

 

</blockquote>

<p>If the camera would do this by cropping, there wouldn't be a change in depth of field, because you would not be changing how far you are from the subject. I would find this feature worthless. If I happen to want to crop, there is no reason to expect that it will be by the change in angle of view that happens to correspond to the dimensions of a crop sensor. If you want to crop, why not just crop in postprocessing? Then you can crop however you want. </p>

[Robin Smith]

<p>Yes, I'm afraid I just don't see the point for anyone who shoots raw or does any kind of post-processing. For someone who just takes JPEGs as they come out of the camera, and plans on doing no processing, then it has a point, but I don't think it is this audience who buys an 6D or up.</p>

[ed_avis2]

<p>Certainly useful for deciding whether to buy a new lens. You have a 100mm lens, and you want to see what things would be like with the 200mm... On the other hand you might decide that the image quality from cropping out the centre part of the image is good enough you have no need to buy the longer lens!</p>

[Marcus Ian]

<p>Yes, I think the OP has a point, there is one advantage to doing this in-camera. Compositional discipline. But frankly, limiting yourself to that composition (w/ a precropped image) is going to negatively impact your output since you will loose a large part of the ability to 're'crop when you have the image displayed across a 24" screen (instead of a 3.5" one). It seems like it would defeat the purpose...</p>

<p>... Then again, if your post consists of plugging in/wirelessly transmitting the pictures and uploading to FB, there may be a decided advantage...</p>

[robin_sibson1]

<blockquote>

<p>If the camera would do this by cropping, there wouldn't be a change in depth of field, because you would not be changing how far you are from the subject.</p>

</blockquote>

<p>The circle of confusion defining depth of field needs to scale with the image size, so this sort of in-camera cropping does imply a DoF change.</p>

<blockquote>

<p>I would find this feature worthless.</p>

</blockquote>

<p>Agree.</p>

[catcher]

<p>I mentioned above the aspect ration crop option in LiveView on the 6D. Does anyone use or find that feature helpful for any application? That seems like a fair analogy to me--if that's a useful feature, then I don't see why a 'focal length' crop option might not be useful either (again, keeping in mind I mean moderate cropping--not 28 to 200). I don't use it often, but I've found that seeing and composing in 4:5 or 1:1 instead of 3:2 all the time has its uses. So, maybe a moderate digital crop would as well. </p>

<p>Yes, this kind of feature would be most useful to .jpg shooters; and yes, I would imagine that 6D owners are perhaps more likely to shoot in RAW. However, personally I'm finding that I want to spend less time in front of a computer screen (even a 24"!). If I can nail exposure the first time, as well as white balance, etc. then I don't have to post-process. </p>

[paddler4]

<blockquote>

<p>The circle of confusion defining depth of field needs to scale with the image size, so this sort of in-camera cropping does imply a DoF change.</p>

</blockquote>

<p>Oops. Yes.</p>

[dcstep]

<p>Digitally zooming doesn't change DOF or focal length, it merely narrows the field of view and uses less of the file. Nikon does provide for digitally changing the sensor's effective size on its D800, with the added advantage of a smaller file, allowing more frames per second. This is not a problem with the 5D MkIII while the 6D might gain from this, if firmware were updated to allow it.</p>

<p>Cropping (no different from digitally zooming in-camera) does not necessarily magnify the imperfections of a lens, so long as your using the middle of the image, where most lenses actually perform better. If you crop to show a corner of an image, then the lens's imperfections will be amplified. </p>

<p>I crop a high percentage of my wildlife shots, even those shot with 1,000mm, and see little ill effect until I start nearing pixel-levels. A 20% crop is no big deal, but 50% and above can be problematic when the image is viewed full-screen. Any focus imperfection is amplified, as is noise and other pixel-problems. Optical focal length is always superior to digital zoom, or cropping. </p>

[robin_sibson1]

<blockquote>

<p>Digitally zooming doesn't change DOF or focal length</p>

</blockquote>

<p>Read my earlier post to understand why it does change DoF assuming the final image size is kept constant.</p>

<blockquote>

<p>Cropping (no different from digitally zooming in-camera) does not necessarily magnify the imperfections of a lens</p>

</blockquote>

<p>Of course it does if the final image size is kept constant; it magnifies everything. Equally, it may also crop away some imperfections and the fact that it magnifies what is left does not necessarily result in an unacceptable outcome.</p>

[dcstep]

<p>"Implying" a DOF change and actually making one are two different things. DOF does not change with cropping, you merely look at a smaller portion of the same image.</p>

[robin_sibson1]

<p>David, I am sorry you do not understand this basic point. Let me make one more attempt. DoF is defined in terms of a "circle of confusion" based on how much out-of-focus blur you are prepared to tolerate in your final image, say, a 36cm×24cm print (or equally a 36cm×24cm display on your screen). That circle of confusion on the print corresponds to a circle of confusion on the sensor; if the sensor is 36mm×24mm it is one-tenth of the size, and if the sensor is 18mm×12mm (whether by design or cropping) it is one-twentieth of the size and thus the DoF is different in the two cases. It would be the same only if your print size was also reduced with the print viewing distance kept constant.</p>

[philip_wilson]

You can always crop an image in post processing so I am not sure what you gain by doing it in the camera. As a Leica

shooter I always like the "preview"capability of the viewfinder - you could see the field of view of usually 2 different lenses

in the viewfinder and by moving a lever could switch between three different sets of lenses. In this way you could see the

image that lenses from 28mm to 135mm would produce (of course it varied depending on which model of camera body

you were used as the system evolved over decades). Unfortunately this is the one feature that was lost with the new

M240 where you only get the frame lines from two lenses depending on the lens mounted (e.f. 28mm and 90mm).

 

This preview capability and the ability is something I am surprised has not been added to a DSLR. Of course it seems

most lenses used these days are zooms where this feature is not needed. The other interesting capability (but almost

impossible to add to a DSLR) is the ability to see outside the image area.

[dcstep]

<p>Robin, you're saying that changing the print size changes DOF, I'm saying it doesn't. Let's disagree.</p>

[robin_sibson1]

<p>David, no, I am saying that cropping the digital image but keeping the print size constant changes DoF. Please do not put words in my mouth. Everyone is entitled to their own opinions, but not to their own facts. I am disappointed that you do not understand the elementary geometrical optics involved, but there seems to be nothing more that I can do about it.</p>

[dcstep]

<p>If you take a digital image has 50% of the pixels of another image, taken with the same lens at the same focal length, and you make the prints the same size, you are enlarging the smaller image, or, "changing the print size." No words are put into you mouth. Just trying to provide clarity to other readers. Changing print size does not change DOF.</p>

[ruben leal]

Back to the original cropping question, here's my opinion:

1- If a DSLR user wants to crop, they will probably be using a zoom lens, so no cropping would be needed,

2- Pre-configured cropping, in conjunction with prime lenses is quite limiting, why would I crop from 24 to 28, if I could

crop to 25.5mm in post? If I'm carrying primes, I may have the next one anyway.

3- If this is for visualization, I would use a pre-visualization App in my smartphone before taking the camera from the bag,

and decide what fixed focal length lens I need for the composition at hand. If I'm doing all this, I'm so careful in my

photography that cropping 4 or 6mm is worse to me than just using a L grade zoom lens instead of a prime lens.

 

By the way, I believe the cropping ratios are a different story: even while they won't show in the optical viewfinder of

current Canon DSLRs, a quick chimping will let you know that your subject properly fits in an 8x10 or 6x8, something very

useful when doing event photography and being paid to produce a specific print size (I used to have visual references in

my film cameras' viewfinder back when I did wedding photography and delivered traditional albums with 5x7s).

[catcher]

<p>Ruben, <br>

What previsualization app do you use? I didn't know there was such a thing, but it sounds like it could be useful. </p>

<p>I still think the 'focal length' crop could be useful. My 28 IS is much smaller and lighter than my 24-105. If I know I'm not going to need to zoom way out, or if light weight and small form factor are more important to me for some reason than zoom, then having the ability to crop in camera on the fly would be useful (at least, to me). </p>

<p>Also, I suppose another way to do it would be instead of having fixed 'focal length crop' have a 'zoomable' crop set to 3:2 aspect ratio. So, in live view, you can digitally zoom, take the photo and have the camera do the crop in-camera. Sure, this is limiting if compared to cropping on the computer. But, at least for me, I'd like to spend less time in front of my computer. </p>

[dcstep]

<p>Isn't a DSLR equipped a zoom lens"previsualization" hardware? Why would we need an app when it's quick and easy to put our camera to our eye and zoom?</p>

[catcher]

<p>David, <br>

I can think of a number of reasons. Perhaps I'm going about my usual work-a-day life and don't have my large SLR with me. Perhaps using an app could help me compose, then return at another time with the camera. Or, perhaps I'm schlepping around lots of gear in a bag or case and am trying to decide whether or not it's worth unpacking the camera for a shot. Or, perhaps Im' trying to be relatively inconspicuous, and could compose relatively unnoticed using a smart phone instead of putting a big camera to my face. Then use the camera quickly at just the right time to remain unnoticed. </p>

<p>Or maybe not. I've not used one of these apps. I'm curious what Ruben uses. I'd like to give it a try. </p>

[dcstep]

<p>Aaron, I think that might be useful for the noob, but with time and experience, you'll know which lens to grab simply by looking at the scene. Really.</p>