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I'm theoretically confused when it comes to the question; upgrade to FF, or get very wide lens?

I have a 70D and mostly use the canon 24-105mm f/4 L lens. The issue I have with this lens is the crop on the 70D, which gives me (x1.6) a focal length of 38-168mm and fixed aperture of +/- f/6.

What is (and why) the better option (don't mind the price, Im looking for a theoretical answer): get a FF camera which loses the crop, and thus gets wider shots. Or, get wider lenses: to get a 'real' 24mm shot, I would need to have a 15mm lens...

So, a focal length of approx. 10mm wouldn't even by viable...

What are your thoughts?

EDIT: Reformulation of question

Suppose I'm positioned and have a camera pointing into a city street. Now, I know that theoretically, I could obtain a shot with a specific angle with (1) an aps-c camera with some lens on focal length X, or (2) a ff camera with some lens on focal length X'. Both delivering me the same shot. (where the focal length X is cropped by factor 1.6 in case of my 70D)

Aside from the price, and the lens inherent properties such as sharpness etc. Is there any significant theoretical reason for one te better then the other?

Would the same shot by (1) and (2) have the same perspective? FoV? and/or other properties?

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    There are a few false assumptions expressed in your question. When you use your 24-105mm f/4 lens on your 70D it is a 24-105mm f/4 lens. It is only "equivalent" to a 38-168mm lens in terms of Field of View if the 36-168mm lens were mounted on a FF camera. It is also only "equivalent" to f/6.4 in terms of Depth of Field, but is not in terms of Exposure Values. – Michael C Apr 29 '16 at 22:21
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    To accurately answer your question, you need to tells us exactly what type of wide angle shots you wish to take. As it stands now the question is way too broad. Do you wish to take wide vista landscapes? Interior architectural/real estate shots? Mind bending close perspective images? Meticulously lit studio scenes? Or natural light only in dim light? Etc... – Michael C Apr 29 '16 at 22:23
  • To clarify, are the choices buy full frame and use current lens, or keep current camera and buy wider lens? Is buy full frame camera with wide lens also an option? – tittaenälg Apr 29 '16 at 22:27
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    How wide exactly do you want to go? Not enough info here to make any recommendations. – dpollitt Apr 30 '16 at 2:22
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Six of one, half a dozen of the other in terms of FoV. Not so much in terms of expense and the character and size/weight of the individual lenses.

Just me, but if you want your 24-105 to be the wide-angle walkaround it was designed to be, then get a full-frame body to mount it on. When I added a 5DMkII to the arsenal, my usage of the 24-105 changed drastically.

If you just want to shoot ultrawide angle shots without shelling out a lot of cash, then get an EF-S 10-18 lens. If you want a lens that yields similar FoV on crop that the 24-105L does on full frame, possibly grab an EF-S 15-85 IS USM.

Basically, the main issue here is that your 24-105 wasn't designed for crop--it was designed for full frame, and the reason most EF-S walkaround zooms have a 15-18mm focal length on the wide end is to have FoV equivalency with 24-28mm on full frame.

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ADDENDUM: Reformulated answer :)

... Is there any significant theoretical reason for one [to be] better then the other? ... Would the same shot by (1) and (2) have the same perspective? FoV? and/or other properties?

Assuming all the reported number (focal length) etc. are accurate [not always the case with lenses], then the FoV should be identical. The magnification, however, may be different, with different focal lengths, and the DoF will differ, albeit slightly.

The full frame shot will yield a smaller DoF (depth of field), assuming you are only changing focal lengths, and not the vantage point of the camera; or only vantage points and not focal lengths. Changing neither, the DoF would be identical, but the FoV, of course, would not.

Also, inherent lens characteristics, aside from sharpness, such as vignetting, chromatic aberration and distortion are all up for grabs, and depend on the specific individual lenses you're comparing. Crop may have disadvantages in these arenas because of the sharper lens design, but full frame may have a disadvantage because of the larger image circle the lens has to project causing more extreme angles to reach the corners.

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Canon has realeased a new EFS 10-18mm f4.5-5.6 IS STM for about $300. Its reletively cheap and the picture quality is ehh but still quite sharp. Its cheaper than getting a ff camera and also gives you a good range now that you have a 10-18 and a 24-105, give this a thought.

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It is possible to go both ways but you would have access to a wider angle-of-view by going full-frame using a rectilinear lens. On full-frame, the widest lens currently is the EF 11-24mm F/4L USM which has a 126° maximum angle-of-view. For APS-C, the widest rectilinear lens is a Sigma 8-16mm F/4.5-5.6 which gives an angle of 114° (Actually, I think it less on Canon APS-C but that is the number stated by Sigma). The difference is not huge but notable.

The other point to consider is that wide-angle lenses for APS-C will rarely be usable on Full-Frame, so if you consider upgrade some day for another reason, any such lens will not be compatible with your new system.

  • Might want to mention that going the 11-24L+FF route is liable to cost about $5k ($3k for the lens, $2k for the body). vs. a $700 Sigma lens. Also that the entire full-frame (135) film era was pretty much restricted to 24mm or longer (with 18mm lenses being a rare exception). A 10mm EF-S lens at 16e is wider than that. – inkista Apr 30 '16 at 18:30
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    That depends how far back you go. I still have a Minolta 17-35mm which seemed extremely wide but several 14mm were also produced before digital cameras came about. The 11-24mm is costly but it is just to show the limit, you can get a Sigma 12-24mm FF or Tamron 15-30mm to get an extreme-wide-angle on a full-frame for less. – Itai Apr 30 '16 at 20:38
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You can go both ways. Both have their pros and cons. But moving to FF has some consequences.

  1. Do you ever want to expand to the wildlife/sports territory and how much money you can afford to pay for the equipment?
  2. Do you travel/walk a lot and how much do you want to carry heavy equipment?
  3. If you have a limited budget, are you willing to sacrifice some features, like flash, shooting speed etc.?
  4. Are you interested in wide angle small DOF photography?
  5. Do you print large, i.e. do you need the highest available resolution?
  6. Is high ISO quality important for you?

As a starting point, try answering these questions for yourself and see if making an upgrade to FF makes sense for you.

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I own Nikons and Leicas so am not a Canon shooter but I switched to a full frame Nikon (D3s) several years ago and have been very pleased although I have bought 3 FX Nikkors to take full advantage of the camera's capabilities. I have also bought a F610 as a backup - Based on my experience I would go for the full frame Canon body and build from there. *

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