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Sanity check: calculating field of view for diff focal lengths

Discussion in 'Help Team' started by Barney, Apr 18, 2012.

  1. Barney

    Barney Well-Known Member

    I'm trying to work out whether a particular shot is feasible or not with my kit.

    I have an image taken at 24mm from the the spot I'd be shooting from. For the new shot I'll need a much narrower field of and I'm trying to calculate the effect of various focal lengths.

    I'm assuming that if I reduce an image by 50% on both height and width that will give me the equivalent field of view for a lens with twice the focal length. Is this right?

    If not, how can I can work this out?

    My method so far has been to reduce the original image size as above, double the focal length for an equivalent field of view, note the new pixel dimensions, and then repeat.

    Returning to the original, I've used the above dimensions to create white boxes at 15% opacity to give me an idea of how the shot would look with a longer focal length.

    Also, I need to work out how far I'll be from the subject so that I know whether my remote triggers will work. I can't simply measure it as the shooting position is in an elevated spot with restricted access).

    In my original 24mm shot, the subject was a certain size in the frame. How can I easily work out what the distance would be to have the subject the same size in the frame at a longer focal length?

    I was roughy 2m from the subject in the first shot, but it would be much better if I had a more precise figure. is it me or did Lightroom used to show the distance to subject (based on focus) as part of the EXIF information? If not, is it recorded in EXIF and if so what's the best way of reading it (for a Mac OS).

    Right, after all that I need a lie down! :cool:
     
  2. NosamLuap

    NosamLuap Rebmem Roines

    Re: Sanity check: calculating field of view for diff focal lengt

    Close enough!

    If you double the focal length, you'd need to double the camera/subject distance so that the subject was the same size on the image - but I'm confused as this seems to contradict your earlier statement "I have an image taken at 24mm from the the spot I'd be shooting from" - does this mean you can't alter the distance from which you're shooting?? :confused:

    No idea about Mac specific software, but if you upload an image to Flickr with Exif intact, you can view it there. (assume your Exif is similar to that of my D300s, you're looking for 'Focus Distance' - assuming (again) that you focussed on the subject in question!)
     
  3. P_Stoddart

    P_Stoddart Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    Yeah, it should as said give a close enough feel of a 48mm. Just think of MFT sensor. They are around 1/4 of the size of a 35mm FF sensor and double the focal length any lens. You are doing the same you turn your FF sensor into a MFT.
     
  4. Barney

    Barney Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    Fantastic! :cool:

    I'll be in the same spot but the subject will be further away. Maybe I'm rubbish at judging distances but if the above's right I must have been further away than I thought. I'll have to get an accurate reading to be sure then.


    I don't want to go through the hassle of uploading anything to the web, but just want to read it straight from the RAW file if possible.

    I'll just downloading a couple. Phil Harvey's ExifTool is supposed to be the most comprehensive one, but as I found out it has to be started from a Terminal window and if I wanted to have to fart around with DOS like prompts I wouldn;t have bought a Mac. :mad:

    The second one was Exif Editor form the App store, but that doesn't even show what lens was on at the time so I'm promptly message the developer for a refund.
     
  5. Barney

    Barney Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    Thanks, but that's needlessly over complicating it somewhat.

    As long as I was thinking along the right lines I know what focal length I'll need.
     
  6. NosamLuap

    NosamLuap Rebmem Roines

    Re: Sanity check: calculating field of view for diff focal lengt

    Ken Rockwell (I know, I know...) suggests that there is something built in to Macs that will show it: http://www.kenrockwell.com/tech/exif-reader.htm

    (and if you can't bear to increase his page views, the interesting bit is "Almost every Mac comes with Graphic Converter already installed, free. Find it in your Applications folder. Open an image and click the EXIF tab. It's in the listing!")

    :)
     
  7. Barney

    Barney Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    After all that, I've found out that Preview displays EXIF information including focus distance.

    It seems like my memory is ok, focus distance was 1.5m (set on the subject who is sharp at f/3.2) so the distance works out at c96m which might be on or over the limits of my triggers.
     
  8. NosamLuap

    NosamLuap Rebmem Roines

    Re: Sanity check: calculating field of view for diff focal lengt

    I think I need a diagram here! if focus distance was 1.5m, and the distance you want is ~96m, that means you're (roughly) 60x further away. You said you want to maintain subject size in the frame... my back-of-cake*-packet maths suggest that means you need to replace your 24mm lens with a 1600mm lens?!

    (My thinking:
    Start with a 25mm lens (to simplify the maths!).
    2x is 50mm
    4x is 100mm
    8x is 200mm
    16x is 400mm
    32x is 800mm
    64x is 1600mm, which will give you the same subject size at ~96m as you got with the 24mm lens at 1.5m)

    But, if distance is an issue for your triggers, do you have enough flashes to 'chain' them optically? (or are you shooting in mid-day sun?).

    Or couldn't you bodge a PC connector cord so it was 100m? (i.e. get a cheap roll of two core speaker cable, cut a PC cord, splice in the speaker cable... no idea if it'd work at that distance but I'd have a gamble for ~£10 :eek:)

    Or just fork out for some different triggers - many of the 2.4ghz models claim >200m range (and I assume you're outdoors, so limited obstructions?)

    * I don't smoke, so can't do back-of-fag-packet maths... ;)
     
  9. Barney

    Barney Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    I see where you're coming from, but I only wanted to calculate the distance and so I used the size of the subject in the original 24mm image to use as a reference to scale against the proposed shot. In the actual shot the subject will be full length and so smaller in the frame, so I'll be looking at around 400mm with my D3, or I might get away with 200mm with my D300.

    Or couldn't you bodge a PC connector cord so it was 100m? (i.e. get a cheap roll of two core speaker cable, cut a PC cord, splice in the speaker cable... no idea if it'd work at that distance but I'd have a gamble for ~£10 )

    Or just fork out for some different triggers - many of the 2.4ghz models claim >200m range (and I assume you're outdoors, so limited obstructions?)[/quote]

    I have 2.4GHz Phottix II triggers which claim a range of 150m+. Large pinch of salt to hand, I reckon 60% is more accurate. If I make sure all batteries are fresh and fully charged I should be okay, but it's still too close for comfort to not do a test run first.

    Also, there's a good chance l'll want to trigger the camera remotely from the ground so that I'm on hand to take alternative shots, pose the subject, adjust the flash etc so wireless triggers are the only option.

    The good news is, I think it's all feasible. When I first had the idea I was thinking that I might have to hire a 600mm bazooka. I still wouldn't mind shooting it with the D4 and WT-5(?) trigger just so that I could control it wirelessly from my iPad. I may yet contact Nikon to see if they'd loan me one. :cool:

    Now, after all that, all I need is for the shoot to actually be confirmed. :eek: Even if it isn't though, it's still an idea that I'll shoot at some point, plus it's given me an idea for another shot. Thanks for your help Paul. :cool:
     
  10. NosamLuap

    NosamLuap Rebmem Roines

    Re: Sanity check: calculating field of view for diff focal lengt

    Dumb question, but do you have more than one set? You could probably 'piggy back' them to get roughly double range (if you're not working right up at your sync speed). i.e. put trigger on hotshoe, walk 50m with receiver, add 2nd trigger to that receiver, walk 50m with receiver and add flash to that receiver

    Ah - that implies you do have more than one pair of triggers/receivers, but you'll be using both! :( (Make sure if you're using one set to trigger camera and the other set to trigger flash that they're set on different channels/groups - otherwise you pressing the button to fire the camera will also fire the flash, but will do so before the shutter is open ;) )

    You're welcome :D (And if you need an assistant, let me know - I'm intrigued by this one!!)
     
  11. Barney

    Barney Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    I have one trigger and five receivers so I'd be triggering the second camera with the wireless trigger and the flashes with either the other receivers, a Speedlight/SU-800 or combination of both (the Phottix trigger has TTL pass through)

    If it comes off then the likelihood is that I will. How's your head for heights? :D

    Seriously, given the subject and location it's not a shoot I'd want to take chances on. Even something as daft as second body going to sleep would cause a massive disruption as there's no quick way up.

    The shoot would be in Leeds and would probably be a week day. I'll let you know as things progress. :cool:
     
  12. NosamLuap

    NosamLuap Rebmem Roines

    Re: Sanity check: calculating field of view for diff focal lengt

    hmm - not sure you can do that... If you trigger the camera with a transmitter/receiver, you can't then put the remaining receivers on flashes and have them sync - the ~40ms shutter delay will be enough to throw out sync. The flashes will fire, but not when you need them to...


    Heights! Waah - terrible!I can't bear heights (can't even clean the upstairs windows from a ladder
    Without sweating!
     
    Last edited: Apr 18, 2012
  13. Barney

    Barney Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    That could be true enough. I'll be testing it thoroughly to check the range so any failings should come to light then. If needs be I'll get a second trigger, which could be worth doing anyway to have as a spare.

    In that case you'll be fine - I refuse to go ladders like that full-stop!
     
  14. willie45

    willie45 Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    I was going to suggest Preview but you've got it already :cool:

    But I have a question now. Can you tell me why I get subject distance coming up as "unknown" when I look?

    I'd like to know how to get this info

    Willie
     
  15. NosamLuap

    NosamLuap Rebmem Roines

    Re: Sanity check: calculating field of view for diff focal lengt

    What camera/lens are you using? I'd guess that you'd need to be using a CPU-enabled lens for the distance info to be passed back to the camera, so perhaps if you're using an older/manual lens, this field will remain blank?
     
  16. willie45

    willie45 Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    I've been looking at pictures taken with a few different bodies and lenses. I will check again tonight :)

    Willie
     
  17. Barney

    Barney Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    Willie, it showed as "unknown" for me too, but I used the focus distance data instead. HTH.
     
  18. willie45

    willie45 Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    Hah! Thanks, Barney. It certainly does help. Sorted :)

    Willie
     
  19. Barney

    Barney Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    Glad to help Willie! It still doesn't explain the "subject distance" anomaly though.
     
  20. willie45

    willie45 Well-Known Member

    Re: Sanity check: calculating field of view for diff focal lengt

    No, I'm a bit perplexed by that too, but I'm perplexed by an awful lot of software - type stuff ;)

    Willie
     

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