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11-24-2006, 09:03 AM   #1
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Flash for toddler's pics needed

Hi,

90% of the photos I take are of my 1.5-year-old daughter. Often she is inside, and as I don't really like ISO 1600 or ISO 3200 photos noise-wise, I got myself a used AF280T flash some months ago.

So far I've been quite happy with it, using it in TTL mode at ISO 400, using M-mode set to 1/180 sec. and mainly aptertures between f/4 and f/8.

However, as my little daughter almost never keeps still, it is a bit like taking sports photos, isn't it? Only that I am much closer to the subject, so I use my FA 50mm f/1.4 most of the time when I take pics of her. The thing is, 1/180 sec. most of the time don't seem to be fast enough to prevent motion blur and get her eyes really sharp. What time would I need to really freeze her movements - 1/500?

That gets me to the next question - would let's say a Sigma 500 Super be able to let me take a photo at 1/500 using this high-speed sync feature? Or would it not work, because I read somewhere that the high-speed burst flashes are quite week and more intended for use as fill-flash during daylight?

As I see it, at 1/500 sec. as opposed to 1/180 the flash output would have to be stronger, or maybe I could use f/2.8 instead of f/5.6... For portraits that wouldn't be a problem...

What are your thoughts on this...? Is a flash with high-speed sync a good option for me...?

gattofix

11-24-2006, 09:54 AM   #2
Ole
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The guide number for normal flash for my Pentax AF360FGZ is 30 at ISO 100 for a 50mm lens.
At 1/500 it is 10.6, let's say 10. So there is a factor of 3 in difference.

If we can assume that the ratio between normal flash and high speed flash at 1/500 is the same for the Sigma, then we get:

To achieve the same exposure you'd have to open up by a factor of 3, that is from 5.6 to 5.6/3 = 1.9 (use 2.0) .
11-24-2006, 03:12 PM   #3
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Not sure that exposure time depends on GN...

First of all thanks a lot, Ole, for the explanation...

Yes, in case the high-speed flash was weaker than the flash in normal mode, I could always open up aperture - that's quite reassuring.

But then I looked up how the GN is calculated, and found the following equation:

GN = f/stop x Flash-to-subject distance

So exposure time seems not to be part of the equation.

I guess I really need a flash with high-speed sync to be able to do what I want to do - use the flash inside at speeds between 1/250 and 1/500.

I just would have to be sure that the high-speed sync flash would be strong enough - I am not sure if the normal GN applies to the high-speed sync mode, as well.

By the way - I always bounce my flash off a wall, not sure if that would be a problem with high-speed sync mode...

Any ideas?

gattofix
11-24-2006, 06:19 PM   #4
Ole
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QuoteOriginally posted by gattofix Quote
First of all thanks a lot, Ole, for the explanation...

Yes, in case the high-speed flash was weaker than the flash in normal mode, I could always open up aperture - that's quite reassuring.

But then I looked up how the GN is calculated, and found the following equation:

GN = f/stop x Flash-to-subject distance

So exposure time seems not to be part of the equation.

I guess I really need a flash with high-speed sync to be able to do what I want to do - use the flash inside at speeds between 1/250 and 1/500.

I just would have to be sure that the high-speed sync flash would be strong enough - I am not sure if the normal GN applies to the high-speed sync mode, as well.

By the way - I always bounce my flash off a wall, not sure if that would be a problem with high-speed sync mode...

Any ideas?

gattofix
The GN goes down when you exceed the flash sync speed. That's becasue the flash now strobes instead of firing just once.

So, at 1/500 (on my flash) the GN has dropped to one third.

The shutterspeed is not part of the GN = f/stop x Flash-to-subject distance equation becasue the GN varies with shutterspeed when the speed exceeds the sync speed.

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