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10-12-2015, 06:43 PM   #1
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auto focus and iso

Hi
I was recently at a wedding where i had an opportunity to use my 50mm F1.8 SMC DA on my k5iis.

so i have a couple of questions...
i set the aperture to F2.8 and shutter to 1/160 and opened up the iso range to 25000 max....
should i of set the auto focus mode to AF C or left as AF S? i take it that AF S is more for stationary subjects correct?

also, i don't like to use that flash on the camera as it washes everyone out., hence i set the iso range from 100 to 25000. a little grain added some character and grittiness to the shots, so was actually very pleased with the majority of the images.and some of the bokeh was pretty cool.
.....my question is....other than opening the aperture to the full 1.8, how can i get a decent exposure if i reduce the iso to say a max 3200 and kept the shutter at 1/160 ( any lower and i believe the images would not be that sharp.) bearing in mind that it was pretty dark....should i be afraid to under expose and then bring back in post processing?
cheers
jon

10-12-2015, 07:19 PM   #2
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QuoteOriginally posted by jonnytheweb Quote
.....my question is....other than opening the aperture to the full 1.8, how can i get a decent exposure if i reduce the iso to say a max 3200 and kept the shutter at 1/160 ( any lower and i believe the images would not be that sharp.) bearing in mind that it was pretty dark....should i be afraid to under expose and then bring back in post processing?
Either underexpose and fix in post, or somehow get more ambient light.

QuoteOriginally posted by jonnytheweb Quote
should i of set the auto focus mode to AF C or left as AF S? i take it that AF S is more for stationary subjects correct?
AF-C will continuously change the focus setting whereas AF-S locks on and then stops. Just depends on what you're trying to accomplish.

Adam
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10-12-2015, 08:16 PM   #3
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This set of photos is right out of the camera without any post processing. The good and bad shots are there showing full exif data.

https://flic.kr/s/aHskeMw1df
This was a dance recital, the purple are moving, 1/100s seemed to give good results at f2.8 ISO 1600 most of the time. Obviously your lighting was different.

Looking again at your numbers, you are far from your goal. You need three stops to get to 3200, and you can recover 1 and 1/3 on the aperture. Maybe if you settled for 6400 at f1.8, and 1/80s?
10-12-2015, 09:58 PM   #4
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underexposing and brightening the image afterwards will just give you a noisier image than shooting at a higher iso to begin with.

10-13-2015, 06:41 AM   #5
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QuoteOriginally posted by jonnytheweb Quote
i don't like to use that flash on the camera as it washes everyone out
if you flash can rotate then you can bounce flash onto the wall or the ceiling, it will mix well with the ambiance light
10-13-2015, 07:11 AM   #6
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You can reduce power on the flash and/or improvise some sort of diffuser to soften the light from the flash. It could be as simple as a piece of white paper held on with a rubber band.
10-13-2015, 09:11 AM   #7
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QuoteOriginally posted by jonnytheweb Quote
also, i don't like to use that flash on the camera as it washes everyone out.,
Some already mentioned this, but flash can be controlled. You can add a diffuser (very popular, fairly cheap), or bounce it, or adjust its strength, or all of the mentioned. There are many tutorials, blog posts, and youtube videos about this

QuoteOriginally posted by jonnytheweb Quote
.....my question is....other than opening the aperture to the full 1.8, how can i get a decent exposure if i reduce the iso to say a max 3200 and kept the shutter at 1/160
Were you using SR correctly? You have to half hold the shutter for a little bit before the SR activates and compensates appropriately. When it does this, a little icon lights up (I think in the viewfinder and on the LCD, but check the manual for your camera). Secondly, work on your posture and hand holding technique. This can help a lot with slow shutter. And finally, think about a tripod or monopod.

QuoteOriginally posted by jonnytheweb Quote
should i be afraid to under expose and then bring back in post processing?
To some extent. If some parts are too dark, then you cannot recover/brighten them, because there just isn't any colour information there. And adding brightness in post will always increase the perceived noise. It is usually better to use appropriate in-camera ISO to get as close as possible to what you want to accomplish.

What aperture were you using, btw? At f1.8 or f2 and 1/100, you should be able to go with ISO a little lower than 25600. True, at that aperture the DoF will be really shallow, but its a compromise.

Regarding AF: AF.S and AF.C are just different. You can try live view for yet another type of AF (but this one is usually slower). But in super dark conditions, AF will not have enough light, so you might be better off trying manual focus. With a little practice, it can be as fast as AF. With an f1.8 lens, live view and focus peaking+digital zoom should be pretty good for MF, too.
10-13-2015, 10:22 AM   #8
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When you use a flash, think of it as two exposures combined into one image. One exposure is the existing ambient light - what you'd get with the flash turned off. The second exposure is with the flash supplying all the light. Your final image is a mixture of the two.

Mostly everyone dislikes the look when the flash supplies all the light and it's facing directly forward. Often the flash doesn't reach the background resulting in black corners. Diffusing or bouncing the flash helps spread out the light and make it less head-on directional. The flash duration is really short, 1/1000 sec. or shorter, so it freezes people effectively, just like a very fast shutter speed. Its color temperature is closer to daylight, though if it's bounced off a tinted surface, it gets tinted.

Ambient light can help balance the light from the flash, or it can cause trouble. Trouble is when the ambient light is from a source that doesn't match the flash color temperature well. The ambient light tints part of the image. Sometimes this is an interesting effect but if you don't like it, the tint is difficult to change. Subject motion can also be trouble with a mix of ambient and flash light, because you can get motion blur with the ambient portion and freezing with the flash. Again this can be interesting but not always desirable. So sometimes you want to use settings so the flash supplies a lot of the light to overpower the ambient light. Sometimes you want to use settings that let in a lot of ambient light and use flash to just fill in some shadows. This is where you might use high ISO, because your other settings are already as high as you want - aperture close to or at f1.8, shutter speed just fast enough to freeze motion.

Take a couple of test shots indoors at night and you'll see what I'm talking about right away. Pop up the onboard flash. Take one shot at ISO 80 and another at ISO 25600 or whatever high number. You should see mostly direct flash at ISO 80, mostly ambient at the high ISO. Then you just have to figure out what balance between these images works best for your situation.

10-13-2015, 05:09 PM   #9
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Thanks everyone, some very useful tips and advice there....some good take aways...
originally i didnt want to open up the aperture right up to 1.8 as DOF could of been to shallow, the 2.8 gave me a little lee way...i will definitely check out use of a diffuser for the flash...
thanks for your input!
Jon
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