A question regarding focal length, cropping, and resolution

Discussion in 'Open Discussion' started by ijm5012, Feb 4, 2016.

  1. ijm5012

    ijm5012 Mu-43 Legend

    Oct 2, 2013
    Pittsburgh, PA
    We have some intelligent individuals on this forum who I figured would be able to help me out with this, because I haven't been able to find a way to calculate this on line.

    I own a little Canon S120 P&S that I take with me to sports car races when I'm helping out on the pit crew, so that I can kind of document the work that goes on "behind the scenes". The camera has a 1/1.7" sensor with a native resolution of 4000x3000 (12MP, 4:3 aspect sensor). A lot of times I'll crop the image to 3:2, resulting in in ~10.5 MP (4000 x 2667). The lens is a 24-120 f/1.8-5.7.

    I'm looking to replace the S120 with something that is roughly the same size (can fit in my pocket), yet will give me better low light performance with regards to noise, better video quality, and can give me some more reach (via cropping).

    The two cameras I'm looking at are a Canon G7X, which has a 1" sensor (3:2, 5472 x 3648) and a 24-100 f/1.8-2.8 lens, and a Sony RX100M3 with a 1" sensor (3:2, 5472 x 3648) and a 24-70 f/1.8-2.8 lens.

    I'm wondering, if I crop each of the above cameras to say, 120mm, 150mm, and 200mm, what would the resultant resolution be for each?

    Please note, I selected these cameras because of their sensor size and lens speed. I figure that I can crop more due to the greater resolution and lower noise (lower ISO due to faster aperture), and achieve better results).

    Thanks for your help!
  2. dwig

    dwig Mu-43 Top Veteran

    Jun 26, 2010
    Key West FL
    There are too many variables involved to make it possible to calculate which of the 3 cameras will yield "better results", not to mention that "better" is a value judgement that only you and no one else can make.

    That said, the G7X will yield a roughly 13mp image when zoomed to 100mme (equivalent of 100mm on 35mm FF) and then cropped to the same FOV as 120mme at 3:2 ratio. Given that the cropped area is still larger than the S120's sensor and that the lens is faster, allowing lower ISOs, it will likely meet your requirements for "better".

    The RX100M3 suffers substantially more due to its max of 70mme. When zoomed to max and then cropped to the equivalent FOV of 120mme the effective sensor area is a bit small and it is yielding somewhat less than 7mp. If (a BIG IF) its lens is enough sharper than the S120's this may still yield "better" images as it is likely a cleaner sensor (noise, ...) and the lens is 2 stops faster reducing the need for as elevated an ISO. It depends to a very large degree what you do with your images. 7mp is a bit of a challenge for large prints, but is far more than enough for any web display.
  3. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    If you are likely to crop on an ongoing regular basis, I would strongly recommend using a longer lens camera. The new Panasonic ZS100 has a slower lens, but the same 1" sensor and goes from 25-250mm. No cropping needed, full 20MP resolution. It also has a touchscreen and an EVF.

    However, if that won't fit in your pocket, I would pick the G7X. The shorter lens, lack of a touchscreen and the price point on the late-model RX100s are dealbreakers for me.

    To figure out the resolution, you just divide by the crop factor squared. So with an RX100 with a 70mm lens at the long end:

    20/2.92=6.8MP @ 120mm

    4.3MP @ 150mm

    2.4MP @ 200mm

    For comparison, the 100mm lens on the G7X gives 5MP at 200mm crop.

    If you can swing the ZS100, you get 20MP at 250mm, obviously. But it may not be pants-pocketable for you. I don't consider the RX100 IV pants-pocketable either, but it obviously depends on the size your pockets and the mobility that you need (I prefer to get around by bicycle).
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  4. ijm5012

    ijm5012 Mu-43 Legend

    Oct 2, 2013
    Pittsburgh, PA
    Thanks for explaining how to solve for the resulting resolution for a given FoV crop, that's what I was looking for.

    Yes, the things that draw me towards the Canon are the longer lens (43% more optical reach), the menu system (Canon P&S menu system is very simple and quick, just what you want in a P&S), a faster lens throughout the range, and Canon's color science. It is also cheaper than the RX100M3

    However, the RX100M3 also has a number of intriguing features that keep making me think about it. Timelapse functionality, better video codec, faster burst rate when shooting RAW (6.5 fps), click-less aperture ring to pull focus manually in videos, tiltable flash, up/down tilt screen (vs. up only on Canon).
  5. Klorenzo

    Klorenzo Mu-43 All-Pro

    Mar 10, 2014
    Not strictly about your main question but when considering compact cameras noise I find dpreviews charts about equivalent aperture quite useful for a comparison (even if in this case you have the same sensor size so it is easier):

    Canon PowerShot G7 X Review

    The G7X up to 50mm remains brighter then the RX100.

    About the Panasonic ZS100 (25-250mm equiv. f2.8-5.9) the lens gets dark quickly (f4 by 50mm, and is at f5.1 by 100mm). On the dpreview chart this mean f10.8 at 50mm and 13,8 at 100mm.

    But if you crop you end up using only part of the sensor so you get more noise and much less resolution. So for "cropped focal lengths" it may actually be faster then the 2.8 of the other cameras. It's about 1.5/2 stops slower so it should match and surpass the noise performance about at a 7MP crop that means 120mm and above according to TurboFrog calculations.
    Last edited: Feb 5, 2016
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  6. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    Yep, that's a good observation. I used the same argument to demonstrate that a smartphone with a optical zoom (the new Asus Zenfone Zoom) will still give you better quality than a 3x cropped iPhone in any light, even though the aperture slows to f4.7 compared to f2.2 (which everyone was saying would give you worthless images).
  7. Listener

    Listener Mu-43 Regular

    The Panasonic FZ1000 is another bridge camera option you might consider. The 25-400 mm (35 mm eq.) zoom range is very useful for a covers-it-all walking around camera with f4 max. aperature at 400mm. It records very good video including 4K video.
  8. tkbslc

    tkbslc Mu-43 Legend

    If you want more reach, just wait for the ZS100 to hit soon. It's cheaper than a RX100 M3 and has 4x the reach.

    Yes the lens is slower when you zoom, but cropping a 20MP 1" sensor from 70 to 200mm leaves you with roughly a 3x crop factor and 1/9 the resolution. 20MP 1" sensor at f5.6 is probably going to slaughter a 2.2MP 1/3" sensor at f2.8. Even compared to the G7X, we are still talking a 2x crop factor, so 1/2" 5MP sensor at f2.8 vs 1" 20MP sensor at f5.6.

    And remember you are upgrading an S120 which already has a f4.9 lens at 100mm, just like the ZS100. So you get basically the same aperture through the standard range, plus another 150mm of zoom past that, and a sensor that is over 4x larger than you have now.
  9. ijm5012

    ijm5012 Mu-43 Legend

    Oct 2, 2013
    Pittsburgh, PA
    Yeah, that's why I was looking at the G7X and RX100M3 and not the M2, because of the aperture at the long end. f/4.9 is slow to begin with, and it's made even worst by the smaller 1/1.7" sensor. I could go pick up an RX100M2 for around $300, but the lens is slow, it isn't 24mm on the wide end, and it doesn't have a touch screen.

    The TZ100 might have been compelling if they had omitted the EVF. By doing so, the camera would have been shorter, and the EVF isn't good for much to begin with. The focal range is indeed impressive, but as mentioned above, the lens slows pretty quickly (f/4 by 50mm).

    The more I think about it, the more I'm leaning towards a Sony. I would use the camera to shoot video more so than telephoto images, and I found a review between the two specifically looking at video performance, and the Sony absolutely kills the Canon. Between the full sensor read out, and S-Log profiles, the Sony is MUCH better suited to video, which is of interest to me. Yes, the Sony's cost more, but I think it'll be worth it in the long run, especially when shooting video and putting it together.

    Now, having gotten a taste of 4K and slow-motion with my GH4's, it now has me comparing an RX100M3 vs. an M4. I can find used M3's for around $500, and right now used M4's are about $700. I figure I could move my S120 kit for $225 net, helping alleviate the cost. Now the only question is, how bad to I want 4K? Having worked with it on my GH4, it's amazing how detailed the footage is, and the ability to crop/pan/punch-in is really nice.

    Fortunately I've got some time to think about it, as I won't be going to work another race for a number of months.
  10. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    To add a bit more fuel to the fire, the TZ100 also shoots 4K, and is considerably cheaper than an RX100 IV. Unless you're buying used, as you say, which is obviously an advantage to the camera being slightly old. The RX100 does have more high FPS modes, which could be interesting for some race footage, though. But if it's too big for you, it's too big, so that's that.

    As a slight digression, I'm glad you brought up the whole cropping discussion though. I'd never really thought about it in that much detail, but it's reinforced to me that it's almost never a winning proposition to crop a larger sensor with a shorter focal length compared to going with a native focal length on a smaller sensor. The resolution and SNR penalties are just through the roof for even relatively small crops, to the point where you're quickly throwing away any advantage you once had.

    It's an extreme example, but I'd bet you'd get better results out of something like a Nikon P900 @ 2000mm with it's 1/2.3" sensor than by cropping from any larger sensor camera with a similar size. Maybe a Canon G3X could compete, but the smallest kit you'd be able to work with on M4/3 would be the 45-175mm. You couldn't even fit a slow, soft 70-300mm on an FF camera at anywhere near the same size.
  11. tkbslc

    tkbslc Mu-43 Legend

    @ijm5012@ijm5012 Hey, you know what you like to shoot. You just opened the thread talking about 100-200mm focal lengths so it would seem odd to pick a 24-70 for that. If those are not really important, then I think the RX100 3 is a great choice, and yes noticeably smaller: Compact Camera Meter

    The RX100 cams do have that special in camera "clear image zoom" that works pretty well as a 2x digital TC using interpolation.
  12. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    Wow, I thought the difference in size was bigger than that.

    The TZ100 definitely is bigger, but both of them would fit just fine in a little belt holster, which is probably a more pleasant way to carry such a thick little camera than in a pocket.

    I'd put the upper limits of pocketability at the Canon G9X, or maybe Ricoh GR at a stretch, which fits better despite having an APS-C sensor.
  13. tkbslc

    tkbslc Mu-43 Legend

    I honestly don't think even the s120 is pocketable. I had an S95 (same size) and I kept it in a belt pouch because it was not only a bit thick, but pretty heavy bouncing around in my pocket. The ZS100 (It's not going to be called the TZ100 in N.America) is going to take the same size belt pouch as an RX100 or G9X so they are the same to me.
  14. ijm5012

    ijm5012 Mu-43 Legend

    Oct 2, 2013
    Pittsburgh, PA
    Yeah, I mean the whole discussion of focal length came up because I was recently in Daytona for the ROLEX 24 hour endurance race, and managed to get some snaps from pit lane of the cars up on the banking. But this required max telephoto reach plus cropping, all on a small sensor camera with a slow aperture. Here's what I managed to capture from pit lane:

    24762830385_b246204a56_b.jpg IMG_0457-E7M by Ian Menego, on Flickr

    24136023513_d2524c0587_b.jpg IMG_0360-E7M by Ian Menego, on Flickr

    You can see in that last image, I was at max telephoto, topping out on my auto ISO (set @ 1600), and was stuck with a shutter speed of 1/15s.

    Once there was some day light, things got a bit better, but I was still shooting at a relatively high ISO:

    24135992043_ecd2ca6b62_b.jpg IMG_0456-E7M by Ian Menego, on Flickr

    But there were a couple occasions where I was able to get closer to the track, and shoot at either 50mm or 85mm equiv.:

    24645041102_e757401488_b.jpg IMG_0427-E7M by Ian Menego, on Flickr

    24467359220_9bc8e14df9_b.jpg IMG_0185-E7M by Ian Menego, on Flickr

    24136011823_8e570a4904_b.jpg IMG_0381-E7M by Ian Menego, on Flickr

    But in that last photo, you again see the limitation of aperture (resulting in a 1/20s shutter speed) and poor high iso performance (ISO 1600).

    As for video use, here's the video I put together using my S120. You can see that a number of the scenes are a bit soft, or noisy, and it just comes down to the sensor size, how much of the sensor is used, and the codec. This is where I want the Sony for video, because it's so good.

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  15. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    I think the slow shutter speed does those images favours as long as you can nail the pan - the motion blur rocks.
  16. ijm5012

    ijm5012 Mu-43 Legend

    Oct 2, 2013
    Pittsburgh, PA
    Oh no doubt, and that's what I was going for. But the problem is trying to pan and shoot race cars going 100+mph (for the close up shots) or 180+mph (for the banking) shots is damn difficult to do with a shutter speed of only 1/15 or 1/20 of second. And even when I do nail it, the ISO is at 1600 on a 1/1.3" sensor. ISO 1600 on a RX100 M3/M4 looks really quite good, MUCH better than the S120.

    That's why I'm looking at larger sensor compacts with faster lens apertures, so that I'm not being forced in to a corner with a high ISO and small sensor.
  17. Turbofrog

    Turbofrog Mu-43 Hall of Famer

    Mar 21, 2014
    Yeah, I used to go to endurance races fairly regularly when I was younger. The only camera I had was a 10x Konica-Minolta superzoom with a 4MP sensor and about a second of shutter lag. I got a few shots I was really happy with back then, but I wouldn't say they've particularly stood up well to the test of time now that I look at them again.

  18. ijm5012

    ijm5012 Mu-43 Legend

    Oct 2, 2013
    Pittsburgh, PA
    That's still a great shot though turbofrog. I loved the DB9.R from Prodrive, as well as the 550/575 that they ran before the Astons.

    As for the images and video that would be shot with my compact, it would be basically web viewing and possibly printed in to a normal size photo book at the end of the season. I don't envision printing these images to frame them on a wall. I simply don't have enough time when I'm at the races to get the the right positioning and take the time to get the right shot. All of the above images were shot either in-between pit stops, or while waiting for my next set of tires to be ready to be picked up to go back down to the pits.
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