josh_ross1 Posted August 11, 2007 Share Posted August 11, 2007 This is a basic photography question. I put it in this forum because I didn't know where else to put it and I was trying to figure it out for use crosslighting in full sun. Someone asked me how much to underexpose the scene when using the flash. I said between one and two stops and then attempted to give a starting point scenario. That's where it kinda fell apart. This is what I said: Sunny 16 rule says you should theoretically be at f/16, iso 100, and 1/125. Since we want 1/200 [max x-sync on a 30D with a cheap radio trigger] I am guessing that might put the aperture around f/12.5 then from there we really want to go. That should put the aperture at about f/9; iso 100; and 1/200th. If someone could tell me if I worked this out correctly I would really, really appreciate it. My thinking was this: everything iso 100 f/16 = 1/125 [in theory] so f/11 = 1/250. I wasn't sure exactly how much aperture I needed to get from 1/250 to 1/200 so I guessed that 2/3 stop would do it. Hence the f/12.5 then from there 1 full stop should be f/9. So how did I do with photography 101? Link to comment Share on other sites More sharing options...
josh_ross1 Posted August 11, 2007 Author Share Posted August 11, 2007 This should read "put the aperture around f/12.5 then from there we want to go 1 stop under. That should put the aperture at about f/9; iso 100; and 1/200th." Link to comment Share on other sites More sharing options...
josh_ross1 Posted August 11, 2007 Author Share Posted August 11, 2007 Okay so I actually got my answer long before anyone is going to see this. I did make one mistake and that was going from f/12.5 to f/9 in order to underexpose the scene. That was just a tired mistake. You are supposed to go up in number not down so that would put 1 stop under exposed at f/18 with a shutter speed of 1/200 at iso 100. Link to comment Share on other sites More sharing options...
mr. sullen Posted August 11, 2007 Share Posted August 11, 2007 Hi, I just wanted to ask ya because I'm new and am just getting into this but I do know a little about it. 1) What exactly are you shooting? 2) wouldn't a light meter do the job? 3) Wouldn't you always keep the ISO at 100 unless you want noise or it's just too dark? 4)Isn't your aperture setting really dertermined by how much DOF you want unless you need it for lighting situations? Link to comment Share on other sites More sharing options...
mr. sullen Posted August 11, 2007 Share Posted August 11, 2007 Oh and lastly let's say you get really close to the exposure you want but your off just a liitle bit, can't photoshop help in that area? Link to comment Share on other sites More sharing options...
josh_ross1 Posted August 11, 2007 Author Share Posted August 11, 2007 Another correction. I think that wikipedia has an incorrect f stop scale. Another site shows 1 stop up from f/12.7 to be f/16 not f/18. They have f/12.7 f/13 f/14 f/16. Thats confusing as all hell but f/16 at iso 100 being 1 stop under seems more inline with what I have experienced. Link to comment Share on other sites More sharing options...
sam_thompson2 Posted August 11, 2007 Share Posted August 11, 2007 The general rule is that it is the inverse of the iso. So you take 100 and it turns into 1/100. 1/100 to 1/200 is one stop. f/11 is one stop from f/16. To underexpose the ambient you increase the f number to f/16. It's only an approximation and the math is easier with whole numbers. As far as Shay's questions. 1) generally you are shooting static people in harsh light between 10 am and 2 pm. 2) Light meter would work. You can use the built in meter and then use exposure compensation or manual mode to manually under expose the background. 3)If your camera has a high enough sync like some Nikon DSLRs and point and shoots, you can use ISO 400 to double the range of the flash. Inverse square law says you can double your range when you increase the amount of light by a factor of 4. 4) Aperture setting is determined by DOF you want, but because of the limited sync speeds, you can't have a wide open aperture, to get ISO 100, at f/4 you would need a shutter speed of 1/1000. However the syc speed of the 30d is 1/200 so that's not possible. You can use special modes and flashes to get that speed, but then you lose range among other things. So for our purposes, the maximum aperture you can use for fill flash is f/8. 5) Photoshop can't really help because you are trying to underexpose the background and lighting the subject at a proper exposure. You can do some post processing to emulate lighting, but trying to get lighting right is pretty difficult after the fact. This is because there are two exposures, one at the camera ambient level and one at the flash level. Link to comment Share on other sites More sharing options...
josh_ross1 Posted August 11, 2007 Author Share Posted August 11, 2007 Thankyou Sam those are all of the correct answers and you verified the f/16 f/18 question I still had. Link to comment Share on other sites More sharing options...
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